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Heathkit W-7a bias voltage fluctuation

Scott Burton

Scott Burton

New Member
Hello, I have myself a pair of Heathkit W-7a mono blocks. They had been re-capped before I purchased them. I noticed one sounded a bit distorted in the higher frequencies so I changed the tubes and it sounded weaker in higher frequencies than it’s counterpart.

I checked the bias of both amps with my bias probe. The one that sounds fine needed a slight balance adjustment and settled in at around 42mv. The other one would slowly climb to around 30mv then jump to 100+ then down to 10 and back up in both tube sockets. I let it warm up and it kept doing it.

I don’t have the schematic yet but figured some of you may know where to look to troubleshoot what may be causing this. I guess that also explains the sound issues I was having with this amp…
 
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A link to the manual: https://audio-circuit.dk/downloads/heathkit2/Heathkit-W7A-pwr-sm.pdf

See the instructions starting halfway page 36. When reading these instructions, I don't understand how/what you were measuring.

Heavy current fluctuations can be caused by oscillations or dirty/failing adjustable pots. But when looking at the schematic of this amplifier, intermittent contact of either of the two adjustable pots (one sets the balance, the other bias) would affect the current of both output tubes, not just of one of them.
 
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Thanks. It was happening to both output tubes. I am using a bias checking tool. The kind where you plug it into the tube socket and then the tube into it. Not the built in bias check terminals on the Heathkit. How would I know if it were an oscillation vs a bad pot?
 
if you clean or replace the pots and it stops, it was the pot

honestly thats a lot more likely than an oscillation problem anyway, but it wouldn't be a bad idea to clean the tube sockets and pins while you're in there.
 
Sorry, I missed the bias tool part in your first post.

What bias tool are you using? You measured 42 mV, 30 mV, etc.. How do the measurements translate to current? Is the resistance over which your bias tool measures 1 Ohm?

The manual advises 65 mA per EL34 (which is rather hot... amounting to a combined plate and screen grid dissipation of close to 32 Watt per EL34).
 
It is 1ohm. I missed the 65ma in that manual. Yeah that is pretty hot. :eek:

I just used Deoxit on the as voltage pot and it is still occurring. I will spray and work in the balance pot too and test. I have ordered an Alan Bradley 15k pot (based on the specs in the manual) to replace the voltage pot.

What’s odd is, I can hear a slight click right when it jumps to 100+. I think that might be the tube responding to the voltage jump. Might be irrelevant but I thought I would mention it. Thanks for all of your help.
 
are these things cathode biased ? I don't have a schematic handy. Not too unusual for cathode bias amps to be run on the hot side since the bias drifts downward as output goes up. Run it too low and it will develop crossover distortion before it gets to the point of clipping.
 
The schematic is on page 3 of the manual (see post #2). It's fixed bias, so not cathode bias.

I must say I never saw such hot bias before in a fixed bias amplifier (see post #5). It's running in class A alright.
 
Another possibility is bad tubes. What kind are they?

Then again if it's in both sockets the bias supply makes more sense.
 
EL504 said:
I must say I never saw such hot bias before in a fixed bias amplifier (see post #5). It's running in class A alright.
Click to expand...


probably spec racing. Crank the bias way up to try and minimize distortion. Beats the tubes to death but it sure looks good on a THD meter.
 
I have tried with 3 sets of matches tubes. All the same result.

I will replace that bias voltage pot (which will be a pain since it’s behind the filter cap block) I might also replace the balance pot too once I get them delivered. I will report back if that fixes the issue or not.
 
Finally received the pots and replaced them. It is still happening. I am going to try and trace back from the tube socket and see where it might be happening. :(
 
I had a W7 that had weird voltage fluctuations where the amp would suddenly draw a lot of current and PT started audibly humming with power tubes arcing...The cause ended up being a bad surgistor. I replaced it with cl-80 and it fixed the problem. I am to this day not sure about the mode of failure.
 
The original surgistor has a set of contacts that close, don't they? If so I can believe damaged contacts causing the voltage to jump as they make or break.
 
That’s a good place to look. Those are the original surgistors too. I will give that a check and see what I can find. Thanks.
 
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