Power for a grid dip oscillator.

[Peter Howard]

This is a question about a piece of vacuum tube test equipment commonly used
by amateur radio
operators but as all the ham radio newsgroups I've looked at seem rather
inactive I thought I'd ask it here. I live in a 240Vac mains country and I
have several USA spec GDO's. They are tube types, a Millen, a Lafayette and
a
Heathkit and are of course meant for 110Vac mains which is why I've only
just
got around to doing something to make them usable on 240Vac. A step
down transformer is not the answer. The one I tried put 8.8v on the 6.3V
tube heater. Maybe because the transformer is marked 230V-110V to suit the
nominal 230V Australian mains but in practice the local juice is anywhere
between 240V and 250V.
Each GDO is very similar with a small power transformer and a single half
wave rectifier diode with two electrolytic caps and a resistor for
filtering.. I
replaced electrolytics where necessary and also a defunct selenium stack
rectifier (shows how old the Millen is) and brought them all up on a
borrowed variac to the point where the heaters had 6.3v on
them. The B+ line on all is around 130Vdc, hardly surprising when two are
designed around the same 6AF4 triode and the other a very similar 9002 type.

I have it in mind to power these dippers from a step down power supply
consisting of two identical back to back transformers in a box. They have
multiple taps on their secondary windings and in practice I can achieve a
wide range of AC voltages by selecting the connections between the tapped
secondaries. When transformers with high voltage secondaries are almost
unavailable new, this arrangement is often used by experimenters with tube
equipment of modest current requirements. The heater supply will be from a
separate 6.3V heater transformer in the same box

I have two choices. One is to remove the transformers from the dippers,
leave the rectifier and filter caps in place and bring in the heater supply
and a higher voltage AC feed for the rectifier through a multiwire
connecting cable. The connecting cable with plug and socket are suitably
rated for the voltages and currents involved.
The other is to do the rectifying and smoothing in the transformer box and
make it a DC feed direct to the B+ line in each dipper.

I'm trying to choose between these alternatives and both seem equally good.
Can anyone think of a reason to prefer one method over the other?

PH

 

[Michael Black]

On Fri, 19 Jul 2013, Peter Howard wrote:

This is a question about a piece of vacuum tube test equipment commonly used
by amateur radio
operators but as all the ham radio newsgroups I've looked at seem rather
inactive I thought I'd ask it here. I live in a 240Vac mains country and I

But if you'd posted in the most obvious, you would have got an answer.
The reality is, a newsgroup thrives on people actually posting, and while
some groups have completely died, the lack of postings doesn't
automatically mean that, it just means nobody is posting to provide
activity.

have several USA spec GDO's. They are tube types, a Millen, a Lafayette
and a Heathkit and are of course meant for 110Vac mains which is why
I've only just got around to doing something to make them usable on
240Vac. A step down transformer is not the answer. The one I tried put
8.8v on the 6.3V tube heater. Maybe because the transformer is marked
230V-110V to suit the nominal 230V Australian mains but in practice the
local juice is anywhere between 240V and 250V. Each GDO is very similar
with a small power transformer and a single half wave rectifier diode
with two electrolytic caps and a resistor for filtering.. I replaced
electrolytics where necessary and also a defunct selenium stack
rectifier (shows how old the Millen is) and brought them all up on a
borrowed variac to the point where the heaters had 6.3v on them. The B+
line on all is around 130Vdc, hardly surprising when two are designed
around the same 6AF4 triode and the other a very similar 9002 type.

All GDOs were relativley low power, and used nothing special in the way of

a transformer. You might consider just scrounging up new transformers and
repalcing the old.

Another consideration that has come up in the antique radio newsgroup is
that line voltage used to be lower than it is now (all those "110vac"
references has now become "120vac". So older equipment might run at a
higher voltage now precisely because it's getting a higher voltage from
the wall.

Replacing the selenium rectifier may require the additon of a series
resistor, because there is a loss of voltage through the selenium
rectifier while a silicon diode will put out a higher voltage.

I have it in mind to power these dippers from a step down power supply
consisting of two identical back to back transformers in a box. They have
multiple taps on their secondary windings and in practice I can achieve a
wide range of AC voltages by selecting the connections between the tapped
secondaries. When transformers with high voltage secondaries are almost
unavailable new, this arrangement is often used by experimenters with tube
equipment of modest current requirements. The heater supply will be from a
separate 6.3V heater transformer in the same box

I have two choices. One is to remove the transformers from the dippers,
leave the rectifier and filter caps in place and bring in the heater supply
and a higher voltage AC feed for the rectifier through a multiwire
connecting cable. The connecting cable with plug and socket are suitably
rated for the voltages and currents involved.
The other is to do the rectifying and smoothing in the transformer box and
make it a DC feed direct to the B+ line in each dipper.

The cheaper GDOs tended to have the power supply built in. But the better
ones had the power supply separate. All with a separate power supply had
the rectifier and filter capacitor in the box with the transformer.

That said, leaving a solid state diode and filter capacitor (assuming they
still work) inside the GDOs won't cause an issue if the external supply
puts out DC. A slight voltage drop from the diode, but the filter
capacitor will just be an extra filter capacitor.

Michael

I'm trying to choose between these alternatives and both seem equally good.
Can anyone think of a reason to prefer one method over the other?

PH

 

[Phil Allison]

"Peter Howard"

I live in a 240Vac mains country and I have several USA spec GDO's.
They are tube types, a Millen, a Lafayette and a Heathkit and are of
course meant for 110Vac mains

** The US mains supply is typically 125VAC at 60Hz.

Back in the 1960s, it was 117VAC.

Have to go back to the 1940s to see 110VAC.

A step down transformer is not the answer.

** Of course it is.

You can get them rated at 100V for Japanese market equipment.

OR buy a transformer with twin 55V secondaries.


..... Phil

 

[default]

On Fri, 19 Jul 2013 13:11:26 +1000, "Peter Howard" <user@bigpond.com>
wrote:

This is a question about a piece of vacuum tube test equipment commonly used
by amateur radio
operators but as all the ham radio newsgroups I've looked at seem rather
inactive I thought I'd ask it here. I live in a 240Vac mains country and I
have several USA spec GDO's. They are tube types, a Millen, a Lafayette and
a
Heathkit and are of course meant for 110Vac mains which is why I've only
just
got around to doing something to make them usable on 240Vac. A step
down transformer is not the answer. The one I tried put 8.8v on the 6.3V
tube heater. Maybe because the transformer is marked 230V-110V to suit the
nominal 230V Australian mains but in practice the local juice is anywhere
between 240V and 250V.
Each GDO is very similar with a small power transformer and a single half
wave rectifier diode with two electrolytic caps and a resistor for
filtering.. I
replaced electrolytics where necessary and also a defunct selenium stack
rectifier (shows how old the Millen is) and brought them all up on a
borrowed variac to the point where the heaters had 6.3v on
them. The B+ line on all is around 130Vdc, hardly surprising when two are
designed around the same 6AF4 triode and the other a very similar 9002 type.

I have it in mind to power these dippers from a step down power supply
consisting of two identical back to back transformers in a box. They have
multiple taps on their secondary windings and in practice I can achieve a
wide range of AC voltages by selecting the connections between the tapped
secondaries. When transformers with high voltage secondaries are almost
unavailable new, this arrangement is often used by experimenters with tube
equipment of modest current requirements. The heater supply will be from a
separate 6.3V heater transformer in the same box

I have two choices. One is to remove the transformers from the dippers,
leave the rectifier and filter caps in place and bring in the heater supply
and a higher voltage AC feed for the rectifier through a multiwire
connecting cable. The connecting cable with plug and socket are suitably
rated for the voltages and currents involved.
The other is to do the rectifying and smoothing in the transformer box and
make it a DC feed direct to the B+ line in each dipper.

I'm trying to choose between these alternatives and both seem equally good.
Can anyone think of a reason to prefer one method over the other?

PH

Another option is to get a low voltage (step down from 240) and employ

the low voltage output to "buck" and in series with the isolation
transformer's primary winding to lower the input from 240 to what
gives you the right numbers. The buck transformer's low voltage
winding is just sized to carry => the primary current, so it is not a
large expensive transformer.
--

 

[Michael Black]

On Sat, 20 Jul 2013, default wrote:

On Fri, 19 Jul 2013 13:11:26 +1000, "Peter Howard" <user@bigpond.com
wrote:

This is a question about a piece of vacuum tube test equipment commonly used
by amateur radio
operators but as all the ham radio newsgroups I've looked at seem rather
inactive I thought I'd ask it here. I live in a 240Vac mains country and I
have several USA spec GDO's. They are tube types, a Millen, a Lafayette and
a
Heathkit and are of course meant for 110Vac mains which is why I've only
just
got around to doing something to make them usable on 240Vac. A step
down transformer is not the answer. The one I tried put 8.8v on the 6.3V
tube heater. Maybe because the transformer is marked 230V-110V to suit the
nominal 230V Australian mains but in practice the local juice is anywhere
between 240V and 250V.
Each GDO is very similar with a small power transformer and a single half
wave rectifier diode with two electrolytic caps and a resistor for
filtering.. I
replaced electrolytics where necessary and also a defunct selenium stack
rectifier (shows how old the Millen is) and brought them all up on a
borrowed variac to the point where the heaters had 6.3v on
them. The B+ line on all is around 130Vdc, hardly surprising when two are
designed around the same 6AF4 triode and the other a very similar 9002 type.

I have it in mind to power these dippers from a step down power supply
consisting of two identical back to back transformers in a box. They have
multiple taps on their secondary windings and in practice I can achieve a
wide range of AC voltages by selecting the connections between the tapped
secondaries. When transformers with high voltage secondaries are almost
unavailable new, this arrangement is often used by experimenters with tube
equipment of modest current requirements. The heater supply will be from a
separate 6.3V heater transformer in the same box

I have two choices. One is to remove the transformers from the dippers,
leave the rectifier and filter caps in place and bring in the heater supply
and a higher voltage AC feed for the rectifier through a multiwire
connecting cable. The connecting cable with plug and socket are suitably
rated for the voltages and currents involved.
The other is to do the rectifying and smoothing in the transformer box and
make it a DC feed direct to the B+ line in each dipper.

I'm trying to choose between these alternatives and both seem equally good.
Can anyone think of a reason to prefer one method over the other?

PH

Another option is to get a low voltage (step down from 240) and employ
the low voltage output to "buck" and in series with the isolation
transformer's primary winding to lower the input from 240 to what
gives you the right numbers. The buck transformer's low voltage
winding is just sized to carry => the primary current, so it is not a
large expensive transformer.

Just find a transformer with the right voltage, and then the secondary can
subtract from the primary. You end up without isolation, but since the
GDOs have their own transformers inside, the equipment is isolated. This
frees up a variac from adjusting the voltage.

I made a big mistake a few months ago. I spot a stepdown transformer,
nice and heavy, can't figure out what I'd use it for, so I leave it. But
the box on the sidewalk had a nice electric sander
(the newfangled ones with a sort of triangle at the front for the
sandpaper), so I bring that home.

Once I get home (it was dark out), I realize the sander is for overseas
use, it has an odd plug and runs off 240vac. That explains the stepdown
transformer, the two were used together. It was too late by then to go
back, the garbage would have been picked up. So in not looking things
over carefully, I ended up with a nice sander that can't be used, unless
inside there's a means of jumpering it for 120vac.

MIchael

 

[Phil Allison]

"default"

Another option is to get a low voltage (step down from 240) and employ
the low voltage output to "buck" and in series with the isolation
transformer's primary winding to lower the input from 240 to what
gives you the right numbers.

** Fraid that is wrong.

The OP needs to get down to 110V from 250V.



.... Phil

 

[defaullt]

On Sat, 20 Jul 2013 10:52:02 -0400, Michael Black <et472@ncf.ca>
wrote:

On Sat, 20 Jul 2013, default wrote:

On Fri, 19 Jul 2013 13:11:26 +1000, "Peter Howard" <user@bigpond.com
wrote:

This is a question about a piece of vacuum tube test equipment commonly used
by amateur radio
operators but as all the ham radio newsgroups I've looked at seem rather
inactive I thought I'd ask it here. I live in a 240Vac mains country and I
have several USA spec GDO's. They are tube types, a Millen, a Lafayette and
a
Heathkit and are of course meant for 110Vac mains which is why I've only
just
got around to doing something to make them usable on 240Vac. A step
down transformer is not the answer. The one I tried put 8.8v on the 6.3V
tube heater. Maybe because the transformer is marked 230V-110V to suit the
nominal 230V Australian mains but in practice the local juice is anywhere
between 240V and 250V.
Each GDO is very similar with a small power transformer and a single half
wave rectifier diode with two electrolytic caps and a resistor for
filtering.. I
replaced electrolytics where necessary and also a defunct selenium stack
rectifier (shows how old the Millen is) and brought them all up on a
borrowed variac to the point where the heaters had 6.3v on
them. The B+ line on all is around 130Vdc, hardly surprising when two are
designed around the same 6AF4 triode and the other a very similar 9002 type.

I have it in mind to power these dippers from a step down power supply
consisting of two identical back to back transformers in a box. They have
multiple taps on their secondary windings and in practice I can achieve a
wide range of AC voltages by selecting the connections between the tapped
secondaries. When transformers with high voltage secondaries are almost
unavailable new, this arrangement is often used by experimenters with tube
equipment of modest current requirements. The heater supply will be from a
separate 6.3V heater transformer in the same box

I have two choices. One is to remove the transformers from the dippers,
leave the rectifier and filter caps in place and bring in the heater supply
and a higher voltage AC feed for the rectifier through a multiwire
connecting cable. The connecting cable with plug and socket are suitably
rated for the voltages and currents involved.
The other is to do the rectifying and smoothing in the transformer box and
make it a DC feed direct to the B+ line in each dipper.

I'm trying to choose between these alternatives and both seem equally good.
Can anyone think of a reason to prefer one method over the other?

PH

Another option is to get a low voltage (step down from 240) and employ
the low voltage output to "buck" and in series with the isolation
transformer's primary winding to lower the input from 240 to what
gives you the right numbers. The buck transformer's low voltage
winding is just sized to carry => the primary current, so it is not a
large expensive transformer.

Just find a transformer with the right voltage, and then the secondary can
subtract from the primary. You end up without isolation, but since the
GDOs have their own transformers inside, the equipment is isolated. This
frees up a variac from adjusting the voltage.

I made a big mistake a few months ago. I spot a stepdown transformer,
nice and heavy, can't figure out what I'd use it for, so I leave it. But
the box on the sidewalk had a nice electric sander
(the newfangled ones with a sort of triangle at the front for the
sandpaper), so I bring that home.

Once I get home (it was dark out), I realize the sander is for overseas
use, it has an odd plug and runs off 240vac. That explains the stepdown
transformer, the two were used together. It was too late by then to go
back, the garbage would have been picked up. So in not looking things
over carefully, I ended up with a nice sander that can't be used, unless
inside there's a means of jumpering it for 120vac.

MIchael

All may not be lost. I ran into something similar with a power tool,
rectified and filtered the 120 and it ran like a champ. (RMS to peak)

Not much different than running it on 240 when all is said and done.
Maybe the AC inductance of the windings was causing current to be
lower?

I don't think a variable speed tool would work.

 

[defaullt]

On Sun, 21 Jul 2013 10:30:29 +1000, "Phil Allison" <phil_a@tpg.com.au>
wrote:

"default"

Another option is to get a low voltage (step down from 240) and employ
the low voltage output to "buck" and in series with the isolation
transformer's primary winding to lower the input from 240 to what
gives you the right numbers.

** Fraid that is wrong.

The OP needs to get down to 110V from 250V.



... Phil

I do realize this, but he already has a step down transformer just not

quite the right ratio to take his 240 to 120 (for some/whatever
reason).

My idea was to employ the low voltage xformer as a buck on the primary
of the auto/isolation transformer he already has, to take it down a
few percent. Efficient compared to a resistor and load doesn't affect
voltage (very much).

 

[_defaullt]

On Sat, 20 Jul 2013 10:52:02 -0400, Michael Black <et472@ncf.ca>
wrote:

On Sat, 20 Jul 2013, default wrote:

On Fri, 19 Jul 2013 13:11:26 +1000, "Peter Howard" <user@bigpond.com
wrote:

This is a question about a piece of vacuum tube test equipment commonly used
by amateur radio
operators but as all the ham radio newsgroups I've looked at seem rather
inactive I thought I'd ask it here. I live in a 240Vac mains country and I
have several USA spec GDO's. They are tube types, a Millen, a Lafayette and
a
Heathkit and are of course meant for 110Vac mains which is why I've only
just
got around to doing something to make them usable on 240Vac. A step
down transformer is not the answer. The one I tried put 8.8v on the 6.3V
tube heater. Maybe because the transformer is marked 230V-110V to suit the
nominal 230V Australian mains but in practice the local juice is anywhere
between 240V and 250V.
Each GDO is very similar with a small power transformer and a single half
wave rectifier diode with two electrolytic caps and a resistor for
filtering.. I
replaced electrolytics where necessary and also a defunct selenium stack
rectifier (shows how old the Millen is) and brought them all up on a
borrowed variac to the point where the heaters had 6.3v on
them. The B+ line on all is around 130Vdc, hardly surprising when two are
designed around the same 6AF4 triode and the other a very similar 9002 type.

I have it in mind to power these dippers from a step down power supply
consisting of two identical back to back transformers in a box. They have
multiple taps on their secondary windings and in practice I can achieve a
wide range of AC voltages by selecting the connections between the tapped
secondaries. When transformers with high voltage secondaries are almost
unavailable new, this arrangement is often used by experimenters with tube
equipment of modest current requirements. The heater supply will be from a
separate 6.3V heater transformer in the same box

I have two choices. One is to remove the transformers from the dippers,
leave the rectifier and filter caps in place and bring in the heater supply
and a higher voltage AC feed for the rectifier through a multiwire
connecting cable. The connecting cable with plug and socket are suitably
rated for the voltages and currents involved.
The other is to do the rectifying and smoothing in the transformer box and
make it a DC feed direct to the B+ line in each dipper.

I'm trying to choose between these alternatives and both seem equally good.
Can anyone think of a reason to prefer one method over the other?

PH

Another option is to get a low voltage (step down from 240) and employ
the low voltage output to "buck" and in series with the isolation
transformer's primary winding to lower the input from 240 to what
gives you the right numbers. The buck transformer's low voltage
winding is just sized to carry => the primary current, so it is not a
large expensive transformer.

Just find a transformer with the right voltage, and then the secondary can
subtract from the primary. You end up without isolation, but since the
GDOs have their own transformers inside, the equipment is isolated. This
frees up a variac from adjusting the voltage.

I made a big mistake a few months ago. I spot a stepdown transformer,
nice and heavy, can't figure out what I'd use it for, so I leave it. But
the box on the sidewalk had a nice electric sander
(the newfangled ones with a sort of triangle at the front for the
sandpaper), so I bring that home.

Once I get home (it was dark out), I realize the sander is for overseas
use, it has an odd plug and runs off 240vac. That explains the stepdown
transformer, the two were used together. It was too late by then to go
back, the garbage would have been picked up. So in not looking things
over carefully, I ended up with a nice sander that can't be used, unless
inside there's a means of jumpering it for 120vac.

MIchael

All may not be lost. I ran into something similar with a power tool,
rectified and filtered the 120 and it ran like a champ. (RMS to peak)

Not much different than running it on 240 when all is said and done.
Maybe the AC inductance of the windings was causing current to be
lower?

I don't think a variable speed tool would work.

 

[John Fields]

On Fri, 19 Jul 2013 13:11:26 +1000, "Peter Howard" <user@bigpond.com>
wrote:

This is a question about a piece of vacuum tube test equipment commonly used
by amateur radio
operators but as all the ham radio newsgroups I've looked at seem rather
inactive I thought I'd ask it here. I live in a 240Vac mains country and I
have several USA spec GDO's. They are tube types, a Millen, a Lafayette and
a
Heathkit and are of course meant for 110Vac mains which is why I've only
just
got around to doing something to make them usable on 240Vac. A step
down transformer is not the answer. The one I tried put 8.8v on the 6.3V
tube heater. Maybe because the transformer is marked 230V-110V to suit the
nominal 230V Australian mains but in practice the local juice is anywhere
between 240V and 250V.
Each GDO is very similar with a small power transformer and a single half
wave rectifier diode with two electrolytic caps and a resistor for
filtering.. I
replaced electrolytics where necessary and also a defunct selenium stack
rectifier (shows how old the Millen is) and brought them all up on a
borrowed variac to the point where the heaters had 6.3v on
them. The B+ line on all is around 130Vdc, hardly surprising when two are
designed around the same 6AF4 triode and the other a very similar 9002 type.

I have it in mind to power these dippers from a step down power supply
consisting of two identical back to back transformers in a box. They have
multiple taps on their secondary windings and in practice I can achieve a
wide range of AC voltages by selecting the connections between the tapped
secondaries. When transformers with high voltage secondaries are almost
unavailable new, this arrangement is often used by experimenters with tube
equipment of modest current requirements. The heater supply will be from a
separate 6.3V heater transformer in the same box

I have two choices. One is to remove the transformers from the dippers,
leave the rectifier and filter caps in place and bring in the heater supply
and a higher voltage AC feed for the rectifier through a multiwire
connecting cable. The connecting cable with plug and socket are suitably
rated for the voltages and currents involved.
The other is to do the rectifying and smoothing in the transformer box and
make it a DC feed direct to the B+ line in each dipper.

I'm trying to choose between these alternatives and both seem equally good.
Can anyone think of a reason to prefer one method over the other?

PH

---
They both seem excessive.

There's another certainly less expensive alternative, and that's to
use a capacitor in series with the primary of the dipper's power
transformer in order to use its reactance to drop ~ half the mains
voltage across itself - with very little loss - and put the rest
across the primary of the transformer.

If you're willing to measure, and post, the current into the various
dippers when operating at their rated input voltages, I'll be happy to
show you how to figure out the value of the capacitor.

--
JF

 

[Michael Black]

On Sun, 21 Jul 2013, John Fields wrote:

On Fri, 19 Jul 2013 13:11:26 +1000, "Peter Howard" <user@bigpond.com
wrote:

This is a question about a piece of vacuum tube test equipment commonly used
by amateur radio
operators but as all the ham radio newsgroups I've looked at seem rather
inactive I thought I'd ask it here. I live in a 240Vac mains country and I
have several USA spec GDO's. They are tube types, a Millen, a Lafayette and
a
Heathkit and are of course meant for 110Vac mains which is why I've only
just
got around to doing something to make them usable on 240Vac. A step
down transformer is not the answer. The one I tried put 8.8v on the 6.3V
tube heater. Maybe because the transformer is marked 230V-110V to suit the
nominal 230V Australian mains but in practice the local juice is anywhere
between 240V and 250V.
Each GDO is very similar with a small power transformer and a single half
wave rectifier diode with two electrolytic caps and a resistor for
filtering.. I
replaced electrolytics where necessary and also a defunct selenium stack
rectifier (shows how old the Millen is) and brought them all up on a
borrowed variac to the point where the heaters had 6.3v on
them. The B+ line on all is around 130Vdc, hardly surprising when two are
designed around the same 6AF4 triode and the other a very similar 9002 type.

I have it in mind to power these dippers from a step down power supply
consisting of two identical back to back transformers in a box. They have
multiple taps on their secondary windings and in practice I can achieve a
wide range of AC voltages by selecting the connections between the tapped
secondaries. When transformers with high voltage secondaries are almost
unavailable new, this arrangement is often used by experimenters with tube
equipment of modest current requirements. The heater supply will be from a
separate 6.3V heater transformer in the same box

I have two choices. One is to remove the transformers from the dippers,
leave the rectifier and filter caps in place and bring in the heater supply
and a higher voltage AC feed for the rectifier through a multiwire
connecting cable. The connecting cable with plug and socket are suitably
rated for the voltages and currents involved.
The other is to do the rectifying and smoothing in the transformer box and
make it a DC feed direct to the B+ line in each dipper.

I'm trying to choose between these alternatives and both seem equally good.
Can anyone think of a reason to prefer one method over the other?

PH

---
They both seem excessive.

There's another certainly less expensive alternative, and that's to
use a capacitor in series with the primary of the dipper's power
transformer in order to use its reactance to drop ~ half the mains
voltage across itself - with very little loss - and put the rest
across the primary of the transformer.

That would work. It's not like the GDO requires a lot of power. And

unlike some of the times I've seen a capacitor used to drop AC voltage,
there's an actual transformer between the AC line and the circuit.

Michael

If you're willing to measure, and post, the current into the various
dippers when operating at their rated input voltages, I'll be happy to
show you how to figure out the value of the capacitor.

--
JF

 

[Phil Allison]

"John Fields"

There's another certainly less expensive alternative, and that's to
use a capacitor in series with the primary of the dipper's power
transformer in order to use its reactance to drop ~ half the mains
voltage across itself - with very little loss - and put the rest
across the primary of the transformer.

If you're willing to measure, and post, the current into the various
dippers when operating at their rated input voltages, I'll be happy to
show you how to figure out the value of the capacitor.

** The load current for those dippers is gonna be nothing like a sine wave.

Firstly, there is half wave rectifier in the DC supply - so the iron core
of each AC transformer will be off set creating a highly asymmetrical
current wave.

Then there is the issue of 50Hz operation in lieu of 60Hz - driving the
cores hard into saturation. Normally this produces a spiky current wave
peaking around each zero crossing.

The combination of the above currents will look just horrible on a scope.

Putting a series cap in the primary circuit will not behave predictably.

The simplest solution is a resistor in series with each heater winding, to
bring the voltage close to 6.3.



.... Phil

 

[Peter Howard]

John Fields wrote:

On Fri, 19 Jul 2013 13:11:26 +1000, "Peter Howard" <user@bigpond.com
wrote:

This is a question about a piece of vacuum tube test equipment
commonly used by amateur radio
operators but as all the ham radio newsgroups I've looked at seem
rather inactive I thought I'd ask it here. I live in a 240Vac mains
country and I have several USA spec GDO's. They are tube types, a
Millen, a Lafayette and a
Heathkit and are of course meant for 110Vac mains which is why I've
only just
got around to doing something to make them usable on 240Vac. A step
down transformer is not the answer. The one I tried put 8.8v on the
6.3V tube heater. Maybe because the transformer is marked 230V-110V
to suit the nominal 230V Australian mains but in practice the local
juice is anywhere between 240V and 250V.
Each GDO is very similar with a small power transformer and a single
half wave rectifier diode with two electrolytic caps and a resistor
for filtering.. I
replaced electrolytics where necessary and also a defunct selenium
stack rectifier (shows how old the Millen is) and brought them all
up on a borrowed variac to the point where the heaters had 6.3v on
them. The B+ line on all is around 130Vdc, hardly surprising when
two are designed around the same 6AF4 triode and the other a very
similar 9002 type.

I have it in mind to power these dippers from a step down power
supply consisting of two identical back to back transformers in a
box. They have multiple taps on their secondary windings and in
practice I can achieve a wide range of AC voltages by selecting the
connections between the tapped secondaries. When transformers with
high voltage secondaries are almost unavailable new, this
arrangement is often used by experimenters with tube equipment of
modest current requirements. The heater supply will be from a
separate 6.3V heater transformer in the same box

I have two choices. One is to remove the transformers from the
dippers, leave the rectifier and filter caps in place and bring in
the heater supply and a higher voltage AC feed for the rectifier
through a multiwire connecting cable. The connecting cable with plug
and socket are suitably rated for the voltages and currents involved.
The other is to do the rectifying and smoothing in the transformer
box and make it a DC feed direct to the B+ line in each dipper.

I'm trying to choose between these alternatives and both seem
equally good. Can anyone think of a reason to prefer one method over
the other?

PH

---
They both seem excessive.

There's another certainly less expensive alternative, and that's to
use a capacitor in series with the primary of the dipper's power
transformer in order to use its reactance to drop ~ half the mains
voltage across itself - with very little loss - and put the rest
across the primary of the transformer.

If you're willing to measure, and post, the current into the various
dippers when operating at their rated input voltages, I'll be happy to
show you how to figure out the value of the capacitor.

I've read a lot of interesting replies with gratitude but your idea of using
a series capacitor sounds the most promising. When I come to think of it, I
have seen LEDs powered direct from mains with a capacitor and a half wave
rectifier.
So, here are some figures I took.
Millen GDO. 115Vac transformer primary current 39mA
Heathkit GDO 115Vac transformer primary current 44mA
Do those sound about right? The figures were taken with the only AC ammeter
I have, an analogue multimeter inherited from the Australian Post Master
General's telephone branch. It was allegedly a good instrument in its day.

In both cases 115Vac from the variac was the voltage that gave me heater
voltage of around 6.3V. I suppose I have a bit of latitude when ancient tube
manufacturers data sheets quote a heater supply tolerance of +/- 10 percent.
So, whats the procedure for getting the capacitor value? And of course, what
sort of capacitor?

The other Lafayette GDO is already operating from AC delivered by cable from
my back to back transformers in a box using the diippers own internal
rectifier and filtering. Works fine. The transformers inna box don't even
get warm. Not surprising considering the modest current requirments of a
little triode.
The Millen though is built like a tank. While I was able to renew the filter
caps and rectifier, removing the power transformer will require some
disassembly to get at it. On the plus side, the transformer windings passed
the megger test okay so it seems a good candidate for the capacitor trick.

PH

 

[Michael Black]

On Mon, 22 Jul 2013, Peter Howard wrote:

The Millen though is built like a tank. While I was able to renew the filter
caps and rectifier, removing the power transformer will require some
disassembly to get at it. On the plus side, the transformer windings passed
the megger test okay so it seems a good candidate for the capacitor trick.

The Millen is the best of the bunch. It cost more, but I think it had

more to it than just the name. About 1972 there was an article in one of
the ham magazines about their effort to solid state it, basically using
the same tuning capacitor and coils (and box) and they put in a lot of
effort to get the same sort of operation, so I assume they put effort into
the original.

The top of the line is surely the Measurement Corporation's Model 59, not
likely to go cheap today. I had one for years, and it didn't false dip
(which some of the lesser GDOs are supposed to, though the Model 59 is the
only one I've had experience with).

So you'd probably want to leave the Millen intact anyway, If you have a
sudden need for money, that's something that probably can be sold fairly
rapidly and at a decent price.

Michael

 

[Phil Allison]

"Peter Howard"

I've read a lot of interesting replies with gratitude but your idea of
using a series capacitor sounds the most promising. When I come to think
of it, I have seen LEDs powered direct from mains with a capacitor and a
half wave rectifier.

** Not even faintly relevant.

So, here are some figures I took.
Millen GDO. 115Vac transformer primary current 39mA
Heathkit GDO 115Vac transformer primary current 44mA
Do those sound about right?

** The idea of using a series cap in the AC supply is WRONG !!

To make sure, I tried it with a small ( ie 8VA) 120V, 60Hz transformer
today on the bench. With 120V input and using a mix of resistive and half
wave rectified load with filter electro, it worked as expected with a
current draw of 86mA.

With a 2.2 uF, 250V film cap added in circuit, a BIG problem occurred due to
*series resonance* with the transformer primary inductance. The primary
voltage INCREASED by 20% and the current draw by 90% !!

With a 1uF cap instead, it was not too much better.

With 0.57uF (0.1 & 0.47) it became possible to apply 240VAC to the circuit
BUT the secondary voltage wave was now *grossly* asymmetrical causing the
DC voltage to drop by 45%.

But the worst outcome was with no load (same as with valve removed from your
dippers ) where the DC voltage on the filter electro rose by 80% and the
0.57uF cap was showing 375VAC across it !!

Substituting a 1.5kohm, 10W resistor in lieu of a film cap restored
conditions close to normal.

So FORGET the series cap idea COMPLETELY !!



.... Phil

 

[John Fields]

On Mon, 22 Jul 2013 11:26:48 +1000, "Phil Allison" <phil_a@tpg.com.au>
wrote:

"John Fields"

There's another certainly less expensive alternative, and that's to
use a capacitor in series with the primary of the dipper's power
transformer in order to use its reactance to drop ~ half the mains
voltage across itself - with very little loss - and put the rest
across the primary of the transformer.

If you're willing to measure, and post, the current into the various
dippers when operating at their rated input voltages, I'll be happy to
show you how to figure out the value of the capacitor.


** The load current for those dippers is gonna be nothing like a sine wave.

---
Doesn't matter, as long as the RMS current is there for the heater and
there's enough HV voltage generated and smoothed to keep the tube
plate current reasonable.
---

Firstly, there is half wave rectifier in the DC supply - so the iron core
of each AC transformer will be off set creating a highly asymmetrical
current wave.

---
Doesn't matter, as long as it doesn't drive the core into saturation.
---

Then there is the issue of 50Hz operation in lieu of 60Hz - driving the
cores hard into saturation. Normally this produces a spiky current wave
peaking around each zero crossing.

---
That shouldn't be a problem since He's in Oz.
---

The combination of the above currents will look just horrible on a scope.

---
Sure, but so what as long as the GDO works properly?
---

Putting a series cap in the primary circuit will not behave predictably.

---
I've got an EICO 710 with 1.2ľF in series with its power transformer's
primary, and it works great!
---

The simplest solution is a resistor in series with each heater winding, to
bring the voltage close to 6.3.

---
Different strokes, yes?

--
JF

 

[Ian Field]

"Peter Howard" <user@bigpond.com> wrote in message
news:ksaabh$b8f$2@dont-email.me...

This is a question about a piece of vacuum tube test equipment commonly
used
by amateur radio
operators but as all the ham radio newsgroups I've looked at seem rather
inactive I thought I'd ask it here. I live in a 240Vac mains country and I
have several USA spec GDO's. They are tube types, a Millen, a Lafayette
and
a
Heathkit and are of course meant for 110Vac mains which is why I've only
just
got around to doing something to make them usable on 240Vac. A step
down transformer is not the answer. The one I tried put 8.8v on the 6.3V
tube heater. Maybe because the transformer is marked 230V-110V to suit the
nominal 230V Australian mains but in practice the local juice is anywhere
between 240V and 250V.
Each GDO is very similar with a small power transformer and a single half
wave rectifier diode with two electrolytic caps and a resistor for
filtering.. I
replaced electrolytics where necessary and also a defunct selenium stack
rectifier (shows how old the Millen is) and brought them all up on a
borrowed variac to the point where the heaters had 6.3v on
them. The B+ line on all is around 130Vdc, hardly surprising when two are
designed around the same 6AF4 triode and the other a very similar 9002
type.

I have it in mind to power these dippers from a step down power supply
consisting of two identical back to back transformers in a box. They have
multiple taps on their secondary windings and in practice I can achieve a
wide range of AC voltages by selecting the connections between the tapped
secondaries. When transformers with high voltage secondaries are almost
unavailable new, this arrangement is often used by experimenters with tube
equipment of modest current requirements. The heater supply will be from a
separate 6.3V heater transformer in the same box

Have a look through a few component catalogues - you might be able to do
better than 2 transformers back to back.

Some suppliers sell transformers with 2 individual primaries (Radiospares
certainly used to) you put the 2 primaries in series for 230V & parallel for
110V.

With the 2 primaries in series; you have a 230V autotransformer tapped at
50% - just ignore the secondaries, unless you can think of a use for them.

 

[Ian Field]

"Phil Allison" <phil_a@tpg.com.au> wrote in message
news:b53g2vF8lu2U1@mid.individual.net...

"John Fields"

There's another certainly less expensive alternative, and that's to
use a capacitor in series with the primary of the dipper's power
transformer in order to use its reactance to drop ~ half the mains
voltage across itself - with very little loss - and put the rest
across the primary of the transformer.

If you're willing to measure, and post, the current into the various
dippers when operating at their rated input voltages, I'll be happy to
show you how to figure out the value of the capacitor.


** The load current for those dippers is gonna be nothing like a sine
wave.

Firstly, there is half wave rectifier in the DC supply - so the iron core
of each AC transformer will be off set creating a highly asymmetrical
current wave.

Then there is the issue of 50Hz operation in lieu of 60Hz - driving the
cores hard into saturation. Normally this produces a spiky current wave
peaking around each zero crossing.

The combination of the above currents will look just horrible on a scope.

Putting a series cap in the primary circuit will not behave predictably.

The simplest solution is a resistor in series with each heater winding, to
bring the voltage close to 6.3.

I have the feeling there's something dangerously wrong with JF's idea - I
probably tried it myself as a teenager many years ago, all I can remember is
not to try that again!

Thinking about it - its probably one of those hidden gotchas. If you
calculate Xc for normal operating current; it won't get the heaters going
from cold resistance - if you increase the capacitance enough to get the
heaters started; it goes into runnaway as soon as the heaters start to heat
up.

 

[Ian Field]

"Peter Howard" <rover110@bigpond.com> wrote in message
news:ksi301$gd1$1@dont-email.me...

John Fields wrote:
On Fri, 19 Jul 2013 13:11:26 +1000, "Peter Howard" <user@bigpond.com
wrote:

This is a question about a piece of vacuum tube test equipment
commonly used by amateur radio
operators but as all the ham radio newsgroups I've looked at seem
rather inactive I thought I'd ask it here. I live in a 240Vac mains
country and I have several USA spec GDO's. They are tube types, a
Millen, a Lafayette and a
Heathkit and are of course meant for 110Vac mains which is why I've
only just
got around to doing something to make them usable on 240Vac. A step
down transformer is not the answer. The one I tried put 8.8v on the
6.3V tube heater. Maybe because the transformer is marked 230V-110V
to suit the nominal 230V Australian mains but in practice the local
juice is anywhere between 240V and 250V.
Each GDO is very similar with a small power transformer and a single
half wave rectifier diode with two electrolytic caps and a resistor
for filtering.. I
replaced electrolytics where necessary and also a defunct selenium
stack rectifier (shows how old the Millen is) and brought them all
up on a borrowed variac to the point where the heaters had 6.3v on
them. The B+ line on all is around 130Vdc, hardly surprising when
two are designed around the same 6AF4 triode and the other a very
similar 9002 type.

I have it in mind to power these dippers from a step down power
supply consisting of two identical back to back transformers in a
box. They have multiple taps on their secondary windings and in
practice I can achieve a wide range of AC voltages by selecting the
connections between the tapped secondaries. When transformers with
high voltage secondaries are almost unavailable new, this
arrangement is often used by experimenters with tube equipment of
modest current requirements. The heater supply will be from a
separate 6.3V heater transformer in the same box

I have two choices. One is to remove the transformers from the
dippers, leave the rectifier and filter caps in place and bring in
the heater supply and a higher voltage AC feed for the rectifier
through a multiwire connecting cable. The connecting cable with plug
and socket are suitably rated for the voltages and currents involved.
The other is to do the rectifying and smoothing in the transformer
box and make it a DC feed direct to the B+ line in each dipper.

I'm trying to choose between these alternatives and both seem
equally good. Can anyone think of a reason to prefer one method over
the other?

PH

---
They both seem excessive.

There's another certainly less expensive alternative, and that's to
use a capacitor in series with the primary of the dipper's power
transformer in order to use its reactance to drop ~ half the mains
voltage across itself - with very little loss - and put the rest
across the primary of the transformer.

If you're willing to measure, and post, the current into the various
dippers when operating at their rated input voltages, I'll be happy to
show you how to figure out the value of the capacitor.

I've read a lot of interesting replies with gratitude but your idea of
using a series capacitor sounds the most promising. When I come to think
of it, I have seen LEDs powered direct from mains with a capacitor and a
half wave rectifier.

BTDT! - a series dropper capacitor is a *REALLY* bad idea - see Phil's
post - he's actually bench tested it and posted a couple of reasons not to
do it.

The LED examples you've seen are an entirely different can of worms - you
still need a small series resistance to protect the LED from turn on surge,
the capacitor passes spikes completely unattenuated.

If you're driving anything with tubes (& heaters) calculating Xc for normal
operating current; wont get the cold (low resistance) heaters to start
glowing - if you increase the capacitance to overcome that hurdle; it'll
runnaway when the heaters star glowing.

 

[John Fields]

On Mon, 22 Jul 2013 11:54:55 +1000, "Peter Howard"
<rover110@bigpond.com> wrote:

John Fields wrote:
On Fri, 19 Jul 2013 13:11:26 +1000, "Peter Howard" <user@bigpond.com
wrote:

This is a question about a piece of vacuum tube test equipment
commonly used by amateur radio
operators but as all the ham radio newsgroups I've looked at seem
rather inactive I thought I'd ask it here. I live in a 240Vac mains
country and I have several USA spec GDO's. They are tube types, a
Millen, a Lafayette and a
Heathkit and are of course meant for 110Vac mains which is why I've
only just
got around to doing something to make them usable on 240Vac. A step
down transformer is not the answer. The one I tried put 8.8v on the
6.3V tube heater. Maybe because the transformer is marked 230V-110V
to suit the nominal 230V Australian mains but in practice the local
juice is anywhere between 240V and 250V.
Each GDO is very similar with a small power transformer and a single
half wave rectifier diode with two electrolytic caps and a resistor
for filtering.. I
replaced electrolytics where necessary and also a defunct selenium
stack rectifier (shows how old the Millen is) and brought them all
up on a borrowed variac to the point where the heaters had 6.3v on
them. The B+ line on all is around 130Vdc, hardly surprising when
two are designed around the same 6AF4 triode and the other a very
similar 9002 type.

I have it in mind to power these dippers from a step down power
supply consisting of two identical back to back transformers in a
box. They have multiple taps on their secondary windings and in
practice I can achieve a wide range of AC voltages by selecting the
connections between the tapped secondaries. When transformers with
high voltage secondaries are almost unavailable new, this
arrangement is often used by experimenters with tube equipment of
modest current requirements. The heater supply will be from a
separate 6.3V heater transformer in the same box

I have two choices. One is to remove the transformers from the
dippers, leave the rectifier and filter caps in place and bring in
the heater supply and a higher voltage AC feed for the rectifier
through a multiwire connecting cable. The connecting cable with plug
and socket are suitably rated for the voltages and currents involved.
The other is to do the rectifying and smoothing in the transformer
box and make it a DC feed direct to the B+ line in each dipper.

I'm trying to choose between these alternatives and both seem
equally good. Can anyone think of a reason to prefer one method over
the other?

PH

---
They both seem excessive.

There's another certainly less expensive alternative, and that's to
use a capacitor in series with the primary of the dipper's power
transformer in order to use its reactance to drop ~ half the mains
voltage across itself - with very little loss - and put the rest
across the primary of the transformer.

If you're willing to measure, and post, the current into the various
dippers when operating at their rated input voltages, I'll be happy to
show you how to figure out the value of the capacitor.

I've read a lot of interesting replies with gratitude but your idea of using
a series capacitor sounds the most promising. When I come to think of it, I
have seen LEDs powered direct from mains with a capacitor and a half wave
rectifier.
So, here are some figures I took.
Millen GDO. 115Vac transformer primary current 39mA
Heathkit GDO 115Vac transformer primary current 44mA
Do those sound about right? The figures were taken with the only AC ammeter
I have, an analogue multimeter inherited from the Australian Post Master
General's telephone branch. It was allegedly a good instrument in its day.

In both cases 115Vac from the variac was the voltage that gave me heater
voltage of around 6.3V. I suppose I have a bit of latitude when ancient tube
manufacturers data sheets quote a heater supply tolerance of +/- 10 percent.
So, whats the procedure for getting the capacitor value? And of course, what
sort of capacitor?

---
What I did was ass-ume that the transformer's load would reflect back
to the input as resistive, allowing a reactance to be used to drop the
input voltage to the transformer with no loss.

My bad.

I have an EICO 710 which draws about 120 mA from 120V 60Hz mains, so
it looks like:

E 120V
Z = --- = -------- ~ 1000 ohms
I 0.120A

Then, since:

1 1
C = ---------- = -------------------- ~ 2.7ľF,
2pi f Xc 6.28 * 60Hz * 1000

I found 3ľF worth of 400V polyester caps, wired them in series with
the GDO and started cranking the variac toward 240V while watching the
output of a current transformer on a scope to get a picture of the
current through the cap.

I didn't get very far before the dipper's transformer started buzzing
furiously and the current through the cap jumped abruptly to about
150mA and wouldn't fall back unless I cranked back on the variac to
way farther than when the current jumped.

The dreaded saturation, but I never realized that it was hysteretic.
What a wonderful find!

After that I read Phil's posts and figured I was doomed, but decided
to press on anyway.

So, what I did was: (View with a fixed-pitch font)


.. 0-120VAC
..120AC>--+ / 0-240VAC
.. | / / +-------+ +----+
.. V<-O-+ +----+-----|CURRENT|-[CAP]-+RMS |--+-------+
.. A | | | | XFMR | |AMPS| | |
.. R | | +--+--+ +-+---+-+ +----+ | |
.. I P||S | RMS | | | | |
.. A R||E |VOLTS| +-+---+-+ +--+--+ [GDO]
.. C I||C +--+--+ | SCOPE | | RMS | |
.. | | | | +-------+ |VOLTS| |
.. | | | | +--+--+ |
.. | | | | | |
..NEUT>---+----+ +----+-----------------------------+-------+

My strategy was totally empirical in that I started with a small cap,
cranked the variac up to 240V, and if the voltage across the GDO was
lower than 120V, turned down the variac, increased the value of the
cap and then cranked the variac back up again.

I have a capacitor decade box, so that made the exercise fun.

In the end, I wound up with 240V into the cap, 112mA through it, and
120V across the GDO.

I tuned it to 100MHz and let it sit for hours, and it's still there,
mas o menos, since it's not rockbound.

Conclusion: A series cap works for an EICO 710, YMMV.
---

The other Lafayette GDO is already operating from AC delivered by cable from
my back to back transformers in a box using the diippers own internal
rectifier and filtering. Works fine. The transformers inna box don't even
get warm. Not surprising considering the modest current requirments of a
little triode.
The Millen though is built like a tank. While I was able to renew the filter
caps and rectifier, removing the power transformer will require some
disassembly to get at it. On the plus side, the transformer windings passed
the megger test okay so it seems a good candidate for the capacitor trick.

---
Why would you have to remove the power transformer?

--
JF