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Troubleshooting bias of old diy amp with Acrosound TO-300 transformer

I am trying to restore an old diy amp which uses an Acrosound TO-300 transformer.

When I check the bias of the two EL-34 tubes, one reads 34v, while the one is 10v. In fact, the tube with the higher bias gets much warmer then the other.

All this while I have the amp on variac at around 80vac. There is a bias potentiometer between the tubes, but twisting it doesn't seem to change much anyway.

The amp was supposedly repaired professionally before I acquired it and some caps have been replaced, while others have been kept.

What should I start investigate first?



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I'm back on this amp...
While having it on with a variac around 80v, I was turning its Bias pot left and right to see if anything changed. It would affect a little bit the voltage of the right EL34 tube. Like from 36v it would go down to 35v, but that's pretty much it, still the other EL34 (left one), would instead test 9v or so...
Old amps have aged parts - perhaps check all resistor values, and all coupling caps for leakage (very important), and when EL34's are removed that all supply rails are as expected when variac increases and the rectifier starts up and preamps get up to voltage.

Is it fixed bias or cathode bias (my guess, especially if it is similar to a Williamson). Pot wipers can go intermittent. Do you have any other EL34 to compare with?
Diagnosis is the art of logical elimination. You start at one end of a system and work your way through it until you find the fault.

On an amp you don't know, the first thing is to draw up a schematic. If you don't have that you are flying blind.

If you have a schematic, you should have been able to work out nominal values for each test point eg anode voltage, grid voltage, cathode voltage. This gives you a steer on what to expect as you test.

Once you have the schematic, start measuring key metrics and recording the values at test points on the schematic. If you don't do this, you have no information to make a diagnosis with.

A typical approach is to pull ALL the tubes and measure the entire amp "open circuit" ie with no tubes in. You then start (usually at the finals) putting the tubes in place and checking static voltages. Work your way backwards through the amp to find the issues.

Write it all down...
Drawing a schematic is probably easier than you think. You know most of the circuit elements from the start: power transformer, rectifier tube, ps caps, output transformer, output tube, etc. Start by drawing in all these components and labeling them. For the circuit you can guess that it is either Williamson or a variation of it or Mullard 5-20 or a variation (plus a preamp tube). So keep those schematics in mind and you should recognize it right away as one or the other. That lets you fill in most of the schematic. You apparently have a VR tube so we can assume that the screen grid is stabilized by it. That's another element to draw in. Use a pencil, and a big piece of paper. Re-draw and erase as you go along. Keep drawing and labeling and before you know it you will have the schematic.
I would start by immediately pulling the output tubes, especially if one is red plating.
Then check your bias voltage on the socket pins, should be pin 5 I think.
Your bias voltage should be the same on either socket.
If it isn't you have a problem. Unless you have an adjustment pot for each tube, but this is rather rare.
Fix it before continuing.
Then turn your adjustment pot to both extremes, to find your bias range.
You want to be sure your desired bias voltage is within that range.
You then adjust your bias voltage to the most - voltage you can get.
It should now be safe to put your tubes back in.
They definitely should not red plate now.
Then try to adjust your voltage down to your desired level.


2012-05-27 7:34 pm
I would start by immediately pulling the output tubes, especially if one is red plating.
Then check your bias voltage on the socket pins, should be pin 5 I think.
Your bias voltage should be the same on either socket.
The first you should check is the el34 to make sure it is good.
Unless the amp has fixed bias , then there will be no voltage on pin5 unless the coupling caps are leaking. If it were fixed bias the P5 voltage would be a negative number. From the value given +35vdc indicates cathode bias. The pot could be a balance pot for the bias voltage which was used in older equipment like Heathkit w5m, Knight kb85 , and others. I suspect that the pot may be defective or the contacts are corroded. The value of the pot is around 200 ohms but should be marked on the back of its body. Measuring the contacts to see if it matches the value listed and while you rotate the adjustment knob should tell you if it is performing correctly. If it is not the pot then you could check the tubes, or maybe the bias circuitry.