OPT primary windings resistance

[Gareth Magennis]

Hi,

I have a malfunctioning 2 x EL34 valve amp that clips around 12 Watts.
HT and screen grid supply all there, 500v+, phase splitter signal does not
clip when output stage does.

OPT split primary windings measure 78 ohms and 90 ohms.
This seems very high to me but I'm no expert.

Seems kind of strange that both primaries would exhibit equally excessive
resistance rather than a partial short in one or both, but so far I have had
no joy from the manufacturer to confirm or deny these readings are off
normal scale.

Could these readings be normal? I don't want to have to buy another OPT to
find out.



Cheers,


Gareth.

 

[Phil Allison]

Gareth Magennis wrote:

I have a malfunctioning 2 x EL34 valve amp that clips around 12 Watts.
HT and screen grid supply all there, 500v+, phase splitter signal does not
clip when output stage does.

OPT split primary windings measure 78 ohms and 90 ohms.
This seems very high to me but I'm no expert.

Seems kind of strange that both primaries would exhibit equally excessive
resistance rather than a partial short in one or both, but so far I have had
no joy from the manufacturer to confirm or deny these readings are off
normal scale.

Could these readings be normal? I don't want to have to buy another OPT to
find out.

** Those readings are probably normal for the particular output tranny.

I assume you have tried new EL34s.

If you run the amp with no load (with a sine wave gen, on the bench) do the screen and cathode currents still rise with output level ?

Have you checked any screen and cathode resistors for correct value?

You do have a *damn good reason* for not mentioning the make and model - right ?


..... Phil

 

>"You do have a *damn good reason* for not mentioning the make and model - >right ? "

Now there's something we ought to take to town. Seriously.

Many years ago I came to SER, told the male and model, and got the fix. It was a Pioneer bigscreen and those people are fucking insane. the problem was in the shutdown and that print was a hundred bucks.

I get an answer back someting like "R 131".

Is was. It was a 100K and you know how those are. the old days, anything like 75K and up, might just open up on its own.

How did we,,,, oh yeah.

Peole, MAKE AND MODEL. GOD DAMN. COme in here "I have a circuit with two transistors" without mentioning whether it is in a high efficiency furnace, a Mercedes Benz,a sump pump, a microwave oven or a Teddy Ruxpin. It SDOES make a difference. If you knew why you would not be asking.

When you ask a question include all information you can gt, and get it. Supply vlotages and maybe waveforms.

 

[Mike]

On Thu, 19 Mar 2015 00:21:49 -0700, jurb6006 wrote:

When you ask a question include all information you can gt, and get it.
Supply vlotages and maybe waveforms.

Some questioners find typing so hard they keep the number of letters to a
minimum...

Mike.

 

[Gareth Magennis]

"Phil Allison" wrote in message
news:f1f97027-03db-47ac-887c-e926d0de6d73@googlegroups.com...

Gareth Magennis wrote:

I have a malfunctioning 2 x EL34 valve amp that clips around 12 Watts.
HT and screen grid supply all there, 500v+, phase splitter signal does
not
clip when output stage does.

OPT split primary windings measure 78 ohms and 90 ohms.
This seems very high to me but I'm no expert.

Seems kind of strange that both primaries would exhibit equally excessive
resistance rather than a partial short in one or both, but so far I have
had
no joy from the manufacturer to confirm or deny these readings are off
normal scale.

Could these readings be normal? I don't want to have to buy another OPT
to
find out.

** Those readings are probably normal for the particular output tranny.

I assume you have tried new EL34s.

If you run the amp with no load (with a sine wave gen, on the bench) do the
screen and cathode currents still rise with output level ?

Have you checked any screen and cathode resistors for correct value?

You do have a *damn good reason* for not mentioning the make and model -
right ?


..... Phil





The Amp is the Blackstar with the PIC.

The schematics I have do not include the OPT or output valves, I just
assumed they were standard.

This amp is rather unconventional in that the "phase splitter" is driven by
a pair of Mosfets. One is driven by the "PA In" signal, the other looks
like it might be via feedback from the 4 ohm OPT tap??

What I have found today is that the problem is the bias is too negative,
i.e. the bias pot will not take it low enough.
Normally this would mean huge amounts of crossover distortion, but in this
amp it doesn't. Instead, increasing negative bias means the output level
just drops and drops.
Not seen that before, and am scratching my head slightly.

I can't figure out why the bias voltage is so high, I need to talk to the
tech at Blackstar, but he has been on holiday.
The original repair I did was to replace both Mosfets, 4 2W resistors and
all the 1uF 450v caps in that part of the circuit, though the bias
adjustment circuit seems to be independent of anything here.
The PCB around the Mosfets was burnt by the overheating resistors. I've
had this same problem before on a different Blackstar - the recommendation
is to drill holes in the PCB between the Mosfet legs. Maybe something
similiar here to the Marshall conductive PCB problem?


Something's not right though. Though I suspect the OPT is good now, thanks
Phil.

(Adding more resistance into the bias circuit to reduce it brought the power
up substantially, so I suspect this just needs sorting)



Cheers,



Gareth.

 

[Phil Allison]

Gareth Magennis wrote:

I have a malfunctioning 2 x EL34 valve amp that clips around 12 Watts.
HT and screen grid supply all there, 500v+, phase splitter signal does
not
clip when output stage does.

OPT split primary windings measure 78 ohms and 90 ohms.
This seems very high to me but I'm no expert.

Seems kind of strange that both primaries would exhibit equally excessive
resistance rather than a partial short in one or both, but so far I have
had
no joy from the manufacturer to confirm or deny these readings are off
normal scale.

Could these readings be normal? I don't want to have to buy another OPT
to
find out.



** Those readings are probably normal for the particular output tranny.

I assume you have tried new EL34s.

If you run the amp with no load (with a sine wave gen, on the bench) do the
screen and cathode currents still rise with output level ?

Have you checked any screen and cathode resistors for correct value?

You do have a *damn good reason* for not mentioning the make and model -
right ?


The Amp is the Blackstar with the PIC.

The schematics I have do not include the OPT or output valves, I just
assumed they were standard.

This amp is rather unconventional in that the "phase splitter" is driven by
a pair of Mosfets. One is driven by the "PA In" signal, the other looks
like it might be via feedback from the 4 ohm OPT tap??

What I have found today is that the problem is the bias is too negative,
i.e. the bias pot will not take it low enough.
Normally this would mean huge amounts of crossover distortion, but in this
amp it doesn't. Instead, increasing negative bias means the output level
just drops and drops.
Not seen that before, and am scratching my head slightly.

I can't figure out why the bias voltage is so high, I need to talk to the
tech at Blackstar, but he has been on holiday.
The original repair I did was to replace both Mosfets, 4 2W resistors and
all the 1uF 450v caps in that part of the circuit, though the bias
adjustment circuit seems to be independent of anything here.
The PCB around the Mosfets was burnt by the overheating resistors. I've
had this same problem before on a different Blackstar - the recommendation
is to drill holes in the PCB between the Mosfet legs. Maybe something
similiar here to the Marshall conductive PCB problem?

** So the problem with low power was there when you handed back the amp ?

Something's not right though.

** For sure - have you doubled checked those new 2W resistors for CORRECT value?

Though I suspect the OPT is good now, thanks Phil.

** There was never any chance it was the culprit.


.... Phil

(Adding more resistance into the bias circuit to reduce it brought the power
up substantially, so I suspect this just needs sorting)



Cheers,



Gareth.

 

[Gareth Magennis]

"Phil Allison" wrote in message
news:39d933a6-f17f-4540-a885-306f46d7de9b@googlegroups.com...

Gareth Magennis wrote:

I have a malfunctioning 2 x EL34 valve amp that clips around 12 Watts.
HT and screen grid supply all there, 500v+, phase splitter signal does
not
clip when output stage does.

OPT split primary windings measure 78 ohms and 90 ohms.
This seems very high to me but I'm no expert.

Seems kind of strange that both primaries would exhibit equally
excessive
resistance rather than a partial short in one or both, but so far I have
had
no joy from the manufacturer to confirm or deny these readings are off
normal scale.

Could these readings be normal? I don't want to have to buy another OPT
to
find out.



** Those readings are probably normal for the particular output tranny.

I assume you have tried new EL34s.

If you run the amp with no load (with a sine wave gen, on the bench) do
the
screen and cathode currents still rise with output level ?

Have you checked any screen and cathode resistors for correct value?

You do have a *damn good reason* for not mentioning the make and model -
right ?


The Amp is the Blackstar with the PIC.

The schematics I have do not include the OPT or output valves, I just
assumed they were standard.

This amp is rather unconventional in that the "phase splitter" is driven
by
a pair of Mosfets. One is driven by the "PA In" signal, the other looks
like it might be via feedback from the 4 ohm OPT tap??

What I have found today is that the problem is the bias is too negative,
i.e. the bias pot will not take it low enough.
Normally this would mean huge amounts of crossover distortion, but in this
amp it doesn't. Instead, increasing negative bias means the output level
just drops and drops.
Not seen that before, and am scratching my head slightly.

I can't figure out why the bias voltage is so high, I need to talk to the
tech at Blackstar, but he has been on holiday.
The original repair I did was to replace both Mosfets, 4 2W resistors and
all the 1uF 450v caps in that part of the circuit, though the bias
adjustment circuit seems to be independent of anything here.
The PCB around the Mosfets was burnt by the overheating resistors. I've
had this same problem before on a different Blackstar - the recommendation
is to drill holes in the PCB between the Mosfet legs. Maybe something
similiar here to the Marshall conductive PCB problem?

** So the problem with low power was there when you handed back the amp ?






I didn't hand back the amp because when I came to bias it it was only
outputting 12 Watts, but the signal was clean, no crossover distortion, so
never suspected bias as the culprit.

I kept trying to contact the techs at Blackstar to try and get them to
measure one of their OPT primaries and compare it to mine, but they never
got back to me.
Should have taken a few seconds to determine whether mine was good or not.
But their main man was on holiday.



Cheers,


Gareth.

 

[Gareth Magennis]

"Gareth Magennis" wrote in message
news:sIQOw.1148269$4b6.1040281@fx44.am4...



"Phil Allison" wrote in message
news:39d933a6-f17f-4540-a885-306f46d7de9b@googlegroups.com...

Gareth Magennis wrote:

I have a malfunctioning 2 x EL34 valve amp that clips around 12 Watts.
HT and screen grid supply all there, 500v+, phase splitter signal does
not
clip when output stage does.

OPT split primary windings measure 78 ohms and 90 ohms.
This seems very high to me but I'm no expert.

Seems kind of strange that both primaries would exhibit equally
excessive
resistance rather than a partial short in one or both, but so far I have
had
no joy from the manufacturer to confirm or deny these readings are off
normal scale.

Could these readings be normal? I don't want to have to buy another OPT
to
find out.



** Those readings are probably normal for the particular output tranny.

I assume you have tried new EL34s.

If you run the amp with no load (with a sine wave gen, on the bench) do
the
screen and cathode currents still rise with output level ?

Have you checked any screen and cathode resistors for correct value?

You do have a *damn good reason* for not mentioning the make and model -
right ?


The Amp is the Blackstar with the PIC.

The schematics I have do not include the OPT or output valves, I just
assumed they were standard.

This amp is rather unconventional in that the "phase splitter" is driven
by
a pair of Mosfets. One is driven by the "PA In" signal, the other looks
like it might be via feedback from the 4 ohm OPT tap??

What I have found today is that the problem is the bias is too negative,
i.e. the bias pot will not take it low enough.
Normally this would mean huge amounts of crossover distortion, but in this
amp it doesn't. Instead, increasing negative bias means the output level
just drops and drops.
Not seen that before, and am scratching my head slightly.

I can't figure out why the bias voltage is so high, I need to talk to the
tech at Blackstar, but he has been on holiday.
The original repair I did was to replace both Mosfets, 4 2W resistors and
all the 1uF 450v caps in that part of the circuit, though the bias
adjustment circuit seems to be independent of anything here.
The PCB around the Mosfets was burnt by the overheating resistors. I've
had this same problem before on a different Blackstar - the recommendation
is to drill holes in the PCB between the Mosfet legs. Maybe something
similiar here to the Marshall conductive PCB problem?

** So the problem with low power was there when you handed back the amp ?






I didn't hand back the amp because when I came to bias it it was only
outputting 12 Watts, but the signal was clean, no crossover distortion, so
never suspected bias as the culprit.

I kept trying to contact the techs at Blackstar to try and get them to
measure one of their OPT primaries and compare it to mine, but they never
got back to me.
Should have taken a few seconds to determine whether mine was good or not.
But their main man was on holiday.



Cheers,


Gareth.





Blimey O'Reilly, finally sorted out the problem.
Turns out the phase splitter needs to be a 12AU7 and I hadn't noticed the
feint marking until I double checked today, and it wasn't on the schematic
anywhere.
So I'd plugged in the original valves at random, so the 12AU7 was not in the
correct position, then subbed in 3 brand new 12AX7 to find the same
problems.


Can't quite believe the difference between the 2 valves. If anything I
would expect less gain from the 12AU7, but obviously that is not part of the
equation here.

12AU7, you get 60 Watts from the 2 E34L's. Put in a 12AX7 and it clips at
10 Watts.

Blimey again, how does that work?



Gareth.

 

[Phil Allison]

Gareth Magennis wrote:

Blimey O'Reilly, finally sorted out the problem.
Turns out the phase splitter needs to be a 12AU7 and I hadn't noticed the
feint marking until I double checked today, and it wasn't on the schematic
anywhere.
So I'd plugged in the original valves at random, so the 12AU7 was not in the
correct position, then subbed in 3 brand new 12AX7 to find the same
problems.


Can't quite believe the difference between the 2 valves. If anything I
would expect less gain from the 12AU7, but obviously that is not part of the
equation here.

12AU7, you get 60 Watts from the 2 E34L's. Put in a 12AX7 and it clips at
10 Watts.

Blimey again, how does that work?

** 12AU7s have a much higher plate current rating than 12AX7s, 11mA as compared to 2mA. The low voltage gain also means the grid bias is quite different.

So, an 'X7 fitted where a 'U7 should be can be way off in its DC conditions - hence clipping the AC signal very badly in one polarity as you found.

Its nuts that so many amp makers do not label valve positions or supply valve line up charts with each amp.

Many owners think they can play technician and wind up making horrible mistakes with octal output and rectifier valves because of this - as well as installing valves out of key sometimes.


.... Phil

 

[Gareth Magennis]

"Phil Allison" wrote in message
news:8cecd4a6-a76a-486a-a36c-3e5d368c4a16@googlegroups.com...

Gareth Magennis wrote:

Blimey O'Reilly, finally sorted out the problem.
Turns out the phase splitter needs to be a 12AU7 and I hadn't noticed the
feint marking until I double checked today, and it wasn't on the schematic
anywhere.
So I'd plugged in the original valves at random, so the 12AU7 was not in
the
correct position, then subbed in 3 brand new 12AX7 to find the same
problems.


Can't quite believe the difference between the 2 valves. If anything I
would expect less gain from the 12AU7, but obviously that is not part of
the
equation here.

12AU7, you get 60 Watts from the 2 E34L's. Put in a 12AX7 and it clips
at
10 Watts.

Blimey again, how does that work?

** 12AU7s have a much higher plate current rating than 12AX7s, 11mA as
compared to 2mA. The low voltage gain also means the grid bias is quite
different.

So, an 'X7 fitted where a 'U7 should be can be way off in its DC
conditions - hence clipping the AC signal very badly in one polarity as you
found.

Its nuts that so many amp makers do not label valve positions or supply
valve line up charts with each amp.

Many owners think they can play technician and wind up making horrible
mistakes with octal output and rectifier valves because of this - as well as
installing valves out of key sometimes.


.... Phil




Thanks.
What is rather annoying is the number of times I phoned and emailed the
company describing in detail the symptoms and voltage readings etc. and not
once got a reply.

I can't believe they were sat there thinking "hee hee, he hasn't sussed the
12AU7 yet .....".


Gareth.