Vox AC30, vintage

N

N_Cook

Guest
Owner decided to power up on full 240V mains after being in an English
garage for many years. Worked perfectly before "storage" and well for a
few minutes and then arcing and sparking crackling noise so he switched off.
3 amp mains fuse ok but at half mains on a variac taking about 1 amp
with all valves removed. I'm assuming the excess current draw does not
disappear at 100% mains, I've not tried, but about 1.2 amp at 50% and
not distressing let alone blowing 3 amp at 100% before my powering,
curious? Unusual, for these days, winding pattern,? no magnetising buzz
heard at 50% mains or other any odd noises.
With variac "mains" of 34V ac, secondaries give, multiplying by 240/34,
465V, 6.3V, 5V, ie ballpark values . "HT" is ratioed 27 to 39 V ac wrt
ground, but 27+39 multiplies to 465V .
I've not desoldered the mains Tx primary or secondaries yet, but what
could be the failing in the Tx to give otherwise good scaled outputs.
There is a lot of tar leaked out of the Tx , but that was in antiquity
presumably and normal.
HT caps and o/p tx have been replaced at some time and a dummy load/sp
attenuator added for recording
 
"N_Cook" wrote in message news:noi5q2$gtc$1@dont-email.me...

Owner decided to power up on full 240V mains after being in an English
garage for many years. Worked perfectly before "storage" and well for a
few minutes and then arcing and sparking crackling noise so he switched off.
3 amp mains fuse ok but at half mains on a variac taking about 1 amp
with all valves removed. I'm assuming the excess current draw does not
disappear at 100% mains, I've not tried, but about 1.2 amp at 50% and
not distressing let alone blowing 3 amp at 100% before my powering,
curious? Unusual, for these days, winding pattern,? no magnetising buzz
heard at 50% mains or other any odd noises.
With variac "mains" of 34V ac, secondaries give, multiplying by 240/34,
465V, 6.3V, 5V, ie ballpark values . "HT" is ratioed 27 to 39 V ac wrt
ground, but 27+39 multiplies to 465V .
I've not desoldered the mains Tx primary or secondaries yet, but what
could be the failing in the Tx to give otherwise good scaled outputs.
There is a lot of tar leaked out of the Tx , but that was in antiquity
presumably and normal.
HT caps and o/p tx have been replaced at some time and a dummy load/sp
attenuator added for recording





Insects? Now carbonised?



Gareth.
 
On 11/08/2016 18:26, Gareth Magennis wrote:
"N_Cook" wrote in message news:noi5q2$gtc$1@dont-email.me...

Owner decided to power up on full 240V mains after being in an English
garage for many years. Worked perfectly before "storage" and well for a
few minutes and then arcing and sparking crackling noise so he switched
off.
3 amp mains fuse ok but at half mains on a variac taking about 1 amp
with all valves removed. I'm assuming the excess current draw does not
disappear at 100% mains, I've not tried, but about 1.2 amp at 50% and
not distressing let alone blowing 3 amp at 100% before my powering,
curious? Unusual, for these days, winding pattern,? no magnetising buzz
heard at 50% mains or other any odd noises.
With variac "mains" of 34V ac, secondaries give, multiplying by 240/34,
465V, 6.3V, 5V, ie ballpark values . "HT" is ratioed 27 to 39 V ac wrt
ground, but 27+39 multiplies to 465V .
I've not desoldered the mains Tx primary or secondaries yet, but what
could be the failing in the Tx to give otherwise good scaled outputs.
There is a lot of tar leaked out of the Tx , but that was in antiquity
presumably and normal.
HT caps and o/p tx have been replaced at some time and a dummy load/sp
attenuator added for recording





Insects? Now carbonised?



Gareth.
Click to expand...

No smoke or burning smell. Next thing to try is variac powering again
but on the 110V Tx setting, then IR thermometer to see where that
current is going
 
"N_Cook" wrote in message news:noigue$oeu$1@dont-email.me...

On 11/08/2016 18:26, Gareth Magennis wrote:
"N_Cook" wrote in message news:noi5q2$gtc$1@dont-email.me...

Owner decided to power up on full 240V mains after being in an English
garage for many years. Worked perfectly before "storage" and well for a
few minutes and then arcing and sparking crackling noise so he switched
off.
3 amp mains fuse ok but at half mains on a variac taking about 1 amp
with all valves removed. I'm assuming the excess current draw does not
disappear at 100% mains, I've not tried, but about 1.2 amp at 50% and
not distressing let alone blowing 3 amp at 100% before my powering,
curious? Unusual, for these days, winding pattern,? no magnetising buzz
heard at 50% mains or other any odd noises.
With variac "mains" of 34V ac, secondaries give, multiplying by 240/34,
465V, 6.3V, 5V, ie ballpark values . "HT" is ratioed 27 to 39 V ac wrt
ground, but 27+39 multiplies to 465V .
I've not desoldered the mains Tx primary or secondaries yet, but what
could be the failing in the Tx to give otherwise good scaled outputs.
There is a lot of tar leaked out of the Tx , but that was in antiquity
presumably and normal.
HT caps and o/p tx have been replaced at some time and a dummy load/sp
attenuator added for recording





Insects? Now carbonised?



Gareth.
Click to expand...

No smoke or burning smell. Next thing to try is variac powering again
but on the 110V Tx setting, then IR thermometer to see where that
current is going






It'll be going through the fried spider.


Gareth.
 
N_Cook wrote:

I've not desoldered the mains Tx primary or secondaries yet, but what
could be the failing in the Tx to give otherwise good scaled outputs.
Click to expand...

** Primary winding shorted turns will do that.

Take a good look at the voltage selector - maybe something is bad there.


..... Phil
 
jurb...@gmail.com wrote:

Valve rectifiers or solid state ?
Click to expand...

** It's almost certainly one of these, with a GZ34.

http://bmamps.com/Schematics/vox/Vox%20AC30,%2036%20(1960%20JMI)%20Schematic.pdf

The OP mentioned a 5V heater winding.


.... Phil
 
Tried on 115V selector setting at about 20% (UK) mains and current draw
went up to about what it was with 50% of UK mains on 245V setting.
IR thermo showed the Tx was warming up nicely even at 20% of 240V main,
current too low on variac ammeter to measure but .2 to .4 A ac.
I wonder how hot this Tx got , presumably in hours of use, stinking the
place out with tar smell (noticeable at just 10 deg C over ambient),
before it was stored away?

Anyway trying a bit of the tar on DVM-R, squashing the probes together
over a bit of it, perhaps about 0.2mm thick, could get a few megohm.
Trying on a megger over a 1mm or 2, easily got 500M to 1G. So has anyone
ever tried removing one of these Tx and baking in a low oven , outdoors
presumably, to drain liquid tar out, then replace it with what ?.
I'll disconnect all the wires and see if there is a pair of points I can
measure with a Megger, if any of the team think its worth trying in an
oven or other ideas?
Conductive resistor path inside would explain the linear characteristic
of the excess current , but primary , secondary? With Tx removed I can
check inter winding leakage and to frame,screen etc
With Tx removed, would it be worth removing the outer cooked paper and
underlying cloth to get a closer look at the windings, perhaps tar is
just leeched from the cloth originally (for anti-damp purposes and not
holding windings in place? Where to research Tx construction of the 1960s?
The owner said that while it was working properly last week and the
start of the arcing/sparking noise , before switching off,there was a
puff of what he called steam rather than smoke, before switching off.
Perhaps related to damp or fried insect and nothing to do with the tar
resistor problem. I've not had anywhere near full HT on this amp.

Valve rectifier, not SS, BTW
 
On Friday, August 12, 2016 at 9:30:10 AM UTC-4, N_Cook wrote:
If paper burns at 451 deg F, what temp for near to charring, the voltage
& current ratings are just about readable with the right light on the
Tx, as black print on now almost black grade of brown paper, colour
maybe partly due to tar vapour of course
Click to expand...

http://www.keypolymer.com/potting-encapsulating/ is a source for new potting compounds.

451F is the *FLASHPOINT* (on average) of paper. NOT the burning temperature which can be much higher or lower depending on external conditions. What this means is that a piece of paper will catch fire in the presence of sufficient oxygen at 451F. Char temperatures are much lower, starting as low as 200F depending on the type of paper and external factors.

For instance, wood, from which paper is made, burns from between 900F and 1,200F, and can be much higher if oxygen is forced into the fire. Paper is mostly cellulose without the volatiles, so burns at a lower temperature, again unless oxygen is added.

Peter Wieck
Melrose Park, PA
 
If paper burns at 451 deg F, what temp for near to charring, the voltage
& current ratings are just about readable with the right light on the
Tx, as black print on now almost black grade of brown paper, colour
maybe partly due to tar vapour of course
 
On Friday, August 12, 2016 at 2:21:47 PM UTC-4, N_Cook wrote:
Hang on a bit , a quote on submarine cable manufacture
"Two layers of coal tar-impregnated jute over the armor wires complete
the cable structure."
Click to expand...

Not much burns underwater.
Jute resists seawater better than most other natural fibers.
Coal tar, while vicious stuff (and one of the ingredients for Greek Fire), also does rather well under water, unlike asphalt.

Would this be the specification used by the Great Eastern? That is about the correct vintage...

Now, without reference to the NET, what is significant about the Great Eastern? There are masny things, but exclude cable-laying for this answer.

Peter Wieck
Melrose Park, PA
 
Hang on a bit , a quote on submarine cable manufacture
"Two layers of coal tar-impregnated jute over the armor wires complete
the cable structure."
 
The Tx came out easy enough, as did the outer layer of paper, after
warming with hot air, so cannot have been that cooked.
Frame and wires to each other showed nothing to 30M on DVM-R or to 20G
on megger.
Enamel of outer 6.6V,6A winding looks fine, but blacker paper under that
layer. No sign of any tar, looks as though it is beeswax? , looks like
whatever the yellowish wax that was covered on those notoriously leaky
waxed paper caps, its that which has carbonised to black tar-like
material probably, and flowed out of the Tx. No cloth seen in lower
layer either, seems paper and wax. Over about 2mm of ome of the wax/tar
on the inside of the removed paper, got a Megger reading of 5G.
I'm assuming a distributed "carbon resistor " over a large area (to
distribute .5KW of heat) rather than shorted turns as at no point has
there been any of that nasty magnetisation buzz of shorted turns.
How to determine if the "resistor" is on the primary, nearest core so
hottest, or the HT secondary winding?, unlikely the high amp windings.
Nothing to loose placing the Tx on a tray in a low oven, but which
orientation, as in normal use or the other way up and hope enough wax
flows out. It would be nice to take some before and after ohms reading
of something , if anyone has any ideas.? Leaves the possibility of near
enough solid carbon at the main hotspot, that will not budge at all.
You'd think after decades at this game you'd have seen all possible
fault scenarios.
 
I just thought , take R and L readings with a RLC meter, for the
sections of primary (assuming same gauge of wire through the length) and
2 sections of HT secondary and they should be proportionate, R to L of
each section. Then retake readings after heating.
BTW I rechecked the Tx ,after removal, on variac
 
You must log in or register to reply here.

Welcome to EDABoard.com

Sponsor

Back
Top