Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: texwest on October 24, 2021, 11:13:51 am
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Hi all, I have a PR build that has been one of the quietest amps I have ever heard. When I first powered this amp up a few years ago, I thought it had a problem because I couldn't hear it hum at all. But now it has developed a reverb hum problem.
When I turn the reverb pot to 0 it goes away. Turn up the knob it gets louder. It's not an incredibly loud hum.
The volume of the hum is independent of the amp volume. It stays the same with amp on 0 or 10.
This amp also has a 1 meg dwell pot that does not affect the hum either.
I tried a different set of reverb cables and reverb tank. Tried new tubes in the reverb send and return positions. Still the same.
What should I be looking for at this point?
Thanks!
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Maybe try to reestablish a good connection between the return RCA jack and the chassis.
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Maybe try to reestablish a good connection between the return RCA jack and the chassis.
Interesting. I just checked all the voltages and they all look good. Used a different reverb tank and cables again. I used an alligator clip connected to the chassis and touched around all around the jacks and shielded cables with no difference. When I disconnect the input or output reverb cables one at a time the hum goes away.
But very interesting is that when you pull the reverb input cable the hum slowly fades away. I'm thinking that should be a clue?
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OOps sorry
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... when you pull the reverb input cable the hum slowly fades away...
I wonder if that slow fade may indicate that the HT B node cap is bad? ie the hum is coming from the driver stage down the reverb springs.
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OK I put on a couple rubber cloves and held a 22uf at the second node and it lessened the hum. Held it on the 1st node and it helped more. The 22uf at the 3rd and 4th node helped nothing which makes sense, since the reverb comes off the 2nd node.
Then held 22uf at 1st and 10uf at the 2ndd node and it helped a little more. But its still there a little bit of hum with the reverb pot at 10.
It's helping but not sure if I'm just filtering the B+ better with 44uf 32-22-22.
Its odd how I can turn the amp up to 10 and hear no hum as long as the reverb pot is down. If the first node and second node were bad you would think I would get general hum in the amp
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How certain are you this was not an issue you just missed before? I ask because i have a Princeton build with 40-20-20uf cap can and it has always exhibited this same behavior. As have two other fender made PR reissues. The hum is very faint but certainly noticeable and only comes in as the reverb control gets turned up.
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...I wonder if that slow fade may indicate that the HT B node cap is bad?...
Wild thought: the recovery stage has no grid-leak except via the tank pickup. Disconnect, open-grid, it will slowly charge-down toward tube cutoff.
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Its odd how I can turn the amp up to 10 and hear no hum as long as the reverb pot is down. If the first node and second node were bad you would think I would get general hum in the amp
With the balanced stages those nodes feed, noise caused by common mode interference such as HT ripple would tend to be greatly attenuated.
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How certain are you this was not an issue you just missed before? I ask because i have a Princeton build with 40-20-20uf cap can and it has always exhibited this same behavior. As have two other fender made PR reissues. The hum is very faint but certainly noticeable and only comes in as the reverb control gets turned up.
Haha, you might be onto something there. I was thinking about selling this amp and when I heard that hum I thought I couldn't sell it like that. I think I would have heard it before because in general this is an incredibly quiet amp. I always prided myself with how quiet this amp is. LOL Honestly I could live with this much hum, but would love to figure it out.
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OK so far I have changed out all the lock washers and flowed all the solder on the reverb jacks. Replaced the .02 and 500pf on the reverb input stage. Replaced the .02 on the reverb recovery. Switched out two different tubes in both the reverb send and return. Pulled out the chassis put it on top of another amp and used the other tank and cables.
The only thing that has lessened the hum is adding the two caps to the 1st and 2nd nodes.
So I'm thinking at this point the reverb input stage might be amplifying some B+ ripple and either the first two caps are going bad or it needs more filtering to not hum.
But what about the reverb transformer? Could that be the cause?
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But what about the reverb transformer? Could that be the cause?
I can’t envisage how :w2:
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Well Jordan's post got me thinking about what have I changed recently. I recently put a new production Accutronics tank in this amp. So I got out the old Accutronics tank that I stole out of an organ and plugged it in and low and behold not nearly as much hum!
The tank I was originally ABing with is a vintage Gibbs tank in a vintage fender and it hummed but just not as badly. I also figured out that the Gibbs tank never bothered me on that old fender because it has more background hum to begin with. The new Accutronics bugged me on this PR build because it is so quiet at idle.
I'm sorry to you guys and myself for leading us on this wild goose chase!!
OK so now I know that the new production Accutronics are a bit noisy and seem to be sensitive to Ripple in the B+