Blues Junior power transformer smoking - can it be rewound ?

[chazpope]

H, bought the amp from ebay this month - plays nicely - but as soon as it gets anywhere near middle volume - smoke / smell comes out of power transformer. If played 'not so loud' there is no smoke. It is a 'green board amp' and not sure if this ads up but I was told by prev owner that the amp is 5 years old.

Now looks like the UK spec transformer is about $160 so I now am not so happy.. I found the US verison for $40 ish - but the voltage wont fit

I have no valve experience but I have wound my own transformers in the past, I wonder if anyone has done it for this one? I mean how many turns, what wire, is it possible to keep the primary and heater coils etc, or should I just measure the voltage and hope to reproduce it.

also considering that the amp still plays beautifully - should I be looking for any other problem (bias too hot?) that may be causing this ?

[Jazz P Bass]

First of all, you have to determine what is pulling current to make the transformer "smoke".
If this is not done correctly, a new transformer will also smoke.
As this appears to be a fire waiting to happen , I would kindly suggest taking it to a good tech.

[melvin]

Are you sure it's the power transformer smoking? It seems like the volume might not matter too much as far as the power transformer is concerned. If your tubes are going into runaway, however, it can cause all kinds of things to smoke, including traces on the circuit board.

[Chuck H]

Right. +++1 on that observation. If the PT is inclined to fail then any playing level should cause the problem. It's a class A amp so it draws the same current no matter what it's doing. Maybe the OT? Does the tube glow red? Not the little orange filament glow but big red patches on the metal plate. I'm guessing the bias circuit and/or filter caps and possibly the power tube are bye bye in this amp.

Melvin, who is that in your avatar?

[g1]

Chuck: I believe Melvin's avatar is Clara Rockmore, "The world's greatest theremin player"
Clara Rockmore Pictures - Free Clara Rockmore Photos

[melvin]

g-one! it is indeed Ms. Rockmore doing her thing...

[chazpope]

Are you sure it's the power transformer smoking? .

Yes I am sure, I had the back open and could see it coming from there, on and off as volume goes up/down, this happened before the PT itself had the time to become even warm. I will have a look at the valves tonight but I would have noticed if they were glowing red or something.

Is there any way to 'check the bias' on this amp ? I will proceed to check the dc readings next but I do not have a generator to do consistent AC checks

[Jazz P Bass]

Bias is checked at TP30
-10.7Vdc.

It's a class A amp so it draws the same current no matter what it's doing.

I do believe this amp is Class AB.

[jrfrond]

I've never been able to get the specs for the BJ PT, but I suspect that it has a very low VA rating. I have seen these cook in more than one amp, for absolutely no reason whatsoever.

Many mfrs. are now getting away with using less copper and core iron in their tube amp power transformers by switching from the old full-wave rectifier design to a bridge, where they can "steal" about 30% extra VAC for the B+ supply, but it also lowers the VA rating of the transformer. For those who are not familiar with VA rating, it is simply the product of the voltage and current handling abilities of the transformer.

The BJ is Class AB. Don't even THINK about trying cathode-bias Class A on these, unless you beef-up the PT.

[melvin]

In any event, if it turns out to not be the power transformer's fault, but excessive current draw from something, the transformer's insulation is most likely compromised and you will have to pull it apart to assess the damage (re-insulating may be possible). And I'd be willing to bet, if you need to, you could find a beefier replacement from the states that has export taps on the primary.

[Chuck H]

+1 on AB. I was thinking of the Valve Junior. My bad. Still, If it HAD been a VJ my assesment would have been correct

Surprising that the PT could be that sensitive to the difference between idle and full power. If it can be compromised before it even gets hot that tells me there is a short in there due to poor insulation or grossly underrated wire.

[chazpope]

melvin, not sure what you mean - if the laquer is cooked no insulation would fix it. looks like all us sites sell the 49967 model PT and that is us voltage only, not keen on the expensive MM ones. I will keep looking though, also I will check the bias tonight - I read this thing re biasing valves individually but could not grasp how to achieve the 'correct' bias after installing mods, it talks of 2.xV rather than 10.xV and it was not clear where to gauge that, so first I plan to just increase the 22K resistor and see what happens. also I will make sure it is set to 240 rather than 230v mains. plse give me more clues on how to check if valves 'work correctly'

[melvin]

chuck H, I am also surprised, which jfrond's experience illuminating. It seems to me that too much biasing could push the 'underrated' transformer over the edge, as well as "no reason whatsoever", which seems even more likely.

As far as the re-insulating goes, I am only referring to the limited damage of cooking the insulation on the wires leaving the transformer, not the coating on the windings themselves. There have been cases where the former is damaged but not the latter.

Also, I would not trust the bias check to just the voltage measurement at the grids, but instead, a measurement of the current flowing through each branch of the primary of the OT.

[chazpope]

Fixed !

Problem is gone - it was not the PT smoking! I had installed a shunt just under it to reduce home volume and that's what was smoking, yes I know I am a plonker

While it was open I measured some interesting voltages:
mains 244V, B+ 347V, C- 10.9V
across OT CP2-CP1 3.45V over 95.5 ohm while the others side CP2-CP3 3.2V over 94.2 ohm - notice quite a diffference there, also it means over 12W of standby power.

amp was set for 230 V - so I switched it to 240 and B+ became 330V

should I try to ballance out the difference of current in the valves by installing twin bias pots, and should I bias for less standby power anyway ? I play mostly distorted and dont want to 'clean up' the amp sound

[ThermionicScott]

Problem is gone - it was not the PT smoking! I had installed a shunt just under it to reduce home volume and that's what was smoking, yes I know I am a plonker

Nice, good to know it wasn't more serious!

While it was open I measured some interesting voltages:
mains 244V, B+ 347V, C- 10.9V
across OT CP2-CP1 3.45V over 95.5 ohm while the others side CP2-CP3 3.2V over 94.2 ohm - notice quite a diffference there, also it means over 12W of standby power.

It's normal for one side of the primary to read higher than the other -- the "outer" wrap will need a little more wire to get the same number of turns, so the resistance is a little higher.

amp was set for 230 V - so I switched it to 240 and B+ became 330V

Only one volt off from the schematic. What's your idle dissipation now? I bet it's under 12W.

- Scott