Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: BlackCrowe604 on December 03, 2023, 02:31:11 am
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Hey Everyone,
Looking over a vintage, slightly-modified blackface DR for a friend. It's ben recapped in the past with ICs in the doghouse, and coupling caps etc throughout the amp. Lead-dress isn't amazing by any means (maybe that ties into the problem I'm having?).
The issue is, the reverb when turned up a little past half way starts to squeal.
I've done the usual things, rolled tubes etc, no change. But the strange thing is, it works fine when the chassis is out of the cabinet on the bench and away from the speaker (no squeal even all the way up).
When I have the chassis on the bench, and I move my hand closer to or between the output tubes, the squeal shows up. It's like it's acting like an antenna?
The 6V6s are vintage Hammond brand from a hammond organ. Screen grids were added at some point to this amp. One was cooked. I replaced it, re-installed the old tubes and cooked my replacement haha.
So, while I wait to get a new set of 6V6 and install another screen grid resistor, I was wondering if anyone here may have experienced something like this before?
My thoughts are: replace any coupling caps in reverb driver & recovery circuits, and possibly try shielding the tank better? Or replacing the RCA cables as well? (they look like generic cheap plastic-molded ones like the ones you get from radioshack etc).
Someone else has suggested to me that I may have a grounding issue. Any thoughts on this or where to check?
Thanks everyone, I appreciate any insight on this one.
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Before doing anything relying on chance , post good pictures of the wiring.
"Lead-dress isn't amazing by any means (maybe that ties into the problem I'm having?)."
You could be right
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Maybe the V4 cathode bypass cap is bad.
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Is the pan in a bag?
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Is the pan in a bag?
We are to early to have some answer, he is sleeping :laugh:
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Here's some t shooting info from member Chip "Fresh Start".
From this thread https://el34world.com/Forum/index.php?topic=9681.0
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Hey Everyone,
Looking over a vintage, slightly-modified blackface DR for a friend. It's ben recapped in the past with ICs in the doghouse, and coupling caps etc throughout the amp. Lead-dress isn't amazing by any means (maybe that ties into the problem I'm having?).
The issue is, the reverb when turned up a little past half way starts to squeal.
I've done the usual things, rolled tubes etc, no change. But the strange thing is, it works fine when the chassis is out of the cabinet on the bench and away from the speaker (no squeal even all the way up).
When I have the chassis on the bench, and I move my hand closer to or between the output tubes, the squeal shows up. It's like it's acting like an antenna?
The 6V6s are vintage Hammond brand from a hammond organ. Screen grids were added at some point to this amp. One was cooked. I replaced it, re-installed the old tubes and cooked my replacement haha.
So, while I wait to get a new set of 6V6 and install another screen grid resistor, I was wondering if anyone here may have experienced something like this before?
My thoughts are: replace any coupling caps in reverb driver & recovery circuits, and possibly try shielding the tank better? Or replacing the RCA cables as well? (they look like generic cheap plastic-molded ones like the ones you get from radioshack etc).
Someone else has suggested to me that I may have a grounding issue. Any thoughts on this or where to check?
Thanks everyone, I appreciate any insight on this one.
I wouldn't go shotgunning parts in a 67 without direction. It sounds like you have a few problems. One is affecting the output section and cooking power tubes. Fix that first. Then focus on the reverb.
If I'm following you, one of the existing tubes is taking out screen resistors. I want to make sure that's what you mean because it could read like your amp killed a replacement set of tubes.
There's a lot of debate about the power handling is a screen resistor with some advocating 3W+. What did you have in yours by chance?
Now for the reverb. It's unlikely coupling caps are the problem. There's a 500pf at the input to the driver and a .0033uF at the plate of the recovery. Both of these are keeping the low frequency from getting out of hand and blocking DC. You're experiencing high frequency squeals, not blowing triodes and scratchy pots.
You might be referring to the cathode bypass caps. In which case that is a likely culprit. The bypass cap and resistor in the recovery stage is shared with the final triode stage much like the 2nd triode of the input stages of the two channels. It saves cost when the caps are healthy but causes problems when they're not. There could be positive feedback between the two triodes if the cap isn't strong. Replace just that cap and find out.
To future proof it you can add a small tagboard and give each triode their own cathode resistor and capacitor. You'll need to sort out the resistor value to keep the load line constant, but this gives you the flexibility of being able to shape the low end of the final preamp triode and even the reverb (though the couplers don't leave much to work with) by tweaking the value of the cap.