First question on my build

[Chuck H]

Photo below. The red "resistor" symbol is where you want to locate the other 220k resistor as per Dave H's suggestion.

[Chuck H]

I see that you stated that the amp is biased pretty cold. Do you have a suggestion for voltages that I can try to see what happens?

If you're going to play bass through it I think the bias level you have is probably OK. If you ever want to crank it for overdrive and play some Randy Z type bass stuff then it might suffer from a little ugly crossover distortion. No suggestions on current at this time. See below...

It's somewhat fortunate that your bias supply is a simple one. It has over a volt of ripple which would be VERY loud hum. The overall power amp circuit is designed to mitigate this ripple hum with the opposing phase of the output tubes. The output of either side of the push/pull power amp needs to have pretty good balance for this to work. Since adjustment of the bias supply causes inconsistent hum I suspect you have a problem with output tube imbalance which fails to cancel this ripple hum.

Have you measured plate, screen and grid bias voltage at every output tube socket? (the thread is getting long so I can't remember if we did this) Since you have the hum reduced with the bias adjustment right now we can't count on an intermittent problem presenting itself, but I think I'm on the right track here.

It might help if you had a steady signal (100-ish mVAC) to drive the input of the amp while taking voltage measurements. It would allow us to discover what the current draw of each output tube is when conducting signal. This could end up being a critical and necessary test.

I suggest replacing that bias pot and touching up ALL the solder joints between the bias circuit and the power tube grids with fresh flux or a little flux core solder. Just wrap your head around "any joint that get's bias voltage to the grids" and it'll be easy to follow the circuit.

FWIW I use to burn pots now and then when I was green and using a big Radio Shack soldering pen. Modern pots are sort of tender and you should get in and out fast to avoid fouling the contacts between the lugs and the resistance strip inside.

[J Luth]

Thanks Chuck that's where I thought you were saying to put it.

I'll give it a try tomorrow. The wife will kill me if I do it tonight. Well that is if we have electricity tomorrow. the light are blinking right now . We have another Nor'easter hitting us right now only this is not as bad as the last one we got last Friday. A lot of people lost power we only lost it for a total of about 2 hours some people are still without power. That one was rain and high winds, this one is heavy wet snow and a bit windy but not as bad as the 93MPH winds of the last one.

[J Luth]

Back in post #9 I listed a lot of voltage readings in the amp. The only thing not listed was the voltage at the 1R resistor to ground on the power tubes.
The amp is only used to play bass only and currently I am playing mainly with blues bands, with a little bit of soul and R&B thrown in. Hell I use mostly flat wound string with these guys when using my P bass style guitars No Randy Z FUZZ. Well I do use rounds also on my Jazz Bass style.

On the Bias cap whats in there is a 47uf 160V would large value cap be better like 100uf or something?

[Chuck H]

On the Bias cap whats in there is a 47uf 160V would large value cap be better like 100uf or something?

Eventually, 100uf, yes. But not right now. Even a 47uf there gives 70mV of ripple. We may end up using that for ripple noise for trouble shooting I know it's counter intuitive WRT modern medicine, but we're not going to treat symptoms before solving the problems. The symptoms are telling us what we need to know right now. In fact, you mentioned paralleling another cap into the bias circuit temporarily. Take it out please. It could partly mask what we're looking for.

[J Luth]

Well so far the power is still on.
Will remove the other cap. 220K resistor is installed the way you showed. I'll see what happens tomorrow. I'll recheck the voltage's on the power tubes.

[Dave H]

On the issue of the voltage divider to the PI. Dave is correct that there's a voltage division in the signal via the plate loads at the stage feeding the PI. But with your arrangement if you just tack a resistor to ground you are connecting HV to ground because the division happens before the coupling cap (which blocks DC). It's a high enough resistance that it won't damage anything right now, but it's not ideal so we should change it. You can't simply move the voltage divider to the other side of the coupling cap because the PI bias must remain isolated from 0V via the coupling cap. The first solution would be then to sandwich the voltage divider between two coupling caps. But we don't need to do that. Dave recognized that there would be a voltage division because the HV supply has a very low AC impedance. So... You don't need to take the second 220k resistor to ground. You can take it back to the HV node for the same signal division and avoid shunting HV to ground here.

Thanks Chuck, looks like I missed that it was DC coupled. The 220k originally connected to the tube plate of the other channel which is close to the same potential. I should have added a cap in series with the 220k to ground but as you say with the high resistance values it doesn't really matter. Even if you connect it to the B+ node there is still DC potential difference but it's less than when it's connected to ground.

[Chuck H]

Yeah, I considered all that, but Jim had indicated that it reduced gain "not in a bad way". So I left the division as it was. To get to actual stock gain level I think the shunt in the divider should be closer to 330k, maybe even 470k depending on other things yet unmentioned. But that can happen with later tuning. We still need to fix this thing

[J Luth]

Chuck,

Looks like it will be next week sometime for more trouble shooting.
I have to do some cleanup here after the storm last night and a gig this weekend. A few limbs down and a few more that are broken and hanging over the deck and cars.

Also I need to place an order for some parts unless I can get them local. There are a couple of places local I can get parts from but I may not be able to get to them. I found a possible bad .1uf cap and I don't have any left. Going to get a replacement bias pot also.

[Chuck H]

Bummer. Hope there's not much expensive damage. I have projects ongoing for a year or more sometimes. Life is what happens to you while making other plans.

[J Luth]

No real damage just a bit of a mess. I have most of it done now, not doing any more until after Saturday's gig.
My projects never end. When we bought this house, our third, I made a 10 year plan. Well it's been over 15 years and the list doesn't look any shorter. Just feels like I am going in circles.

In post #92 you wrote "It might help if you had a steady signal (100-ish mVAC) to drive the input of the amp while taking voltage measurements." Are you looking to power the amp at a steady 100v AC rather than whatever is coming out of the wall, which does vary quite a bit here especially when the farm/ice cream stand just up the street is open. If so, I do have a very old variac that works, it only had a 2 prong outlet and no meter. I recently put on a 3 wire line cord and mounted a box to the side with a grounded outlet. Still have to test it to see if the smoke escapes when I turn it on with a load on it.

[Chuck H]

That's mVAC (millivolts) of "signal" to put into the input of the amp while measuring things like current on the output tubes when conducting that signal. I was thinking the hum could be an imbalance in the push/pull operation of the amp, and it still may be, but you futzing with the bias and hearing the hum come and go is probably a better place to start.

[J Luth]

DOH! That's what I get for not reading your post 3 times, my dyslexic eye's and brain never saw the m in mVAC. Although the type of dyslexia I have is not real bad it is a PITB. That's why I was never able to and still can't sight read sheet music or charts.

[Chuck H]

Well that's a much better reason than why I can't read sheet music

I went from french horn in junior high school straight to rock guitar and never read sheet music again I was a good enough player at 20 that a guy asked me if I wanted to audition for the guitar parts in a San Francisco presentation of God Spell, but I didn't get it because I wasn't proficient at sight reading. I can pigeon through the notes if I brush up a little and I get timing structures and note time designations ok, but I'm far from where I could be if I hadn't dropped it.

[J Luth]

You would think by now I would learn my lesson and type out a line rather than copy and paste from a post, but no. Then I would have seen it. It's actually a bit easier for me to copy and paste but sometimes it backfires.

Cool on the offer for an audition you got. Even though you state you couldn't sight reading the offer still says a lot.

Now I sit here waiting for another storm to hit only this one is going to be all snow, possibly 12 - 18" or more. Need to send Mother Nature a message to let her know that it's March not February. Were not supposed to get this type of snow storm in March, it an unwritten rule. March is usually more rain than snow.