Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: 72Blazer on June 05, 2024, 10:11:41 am
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Background: I built this clone close some years ago. It’s stock with the exception of Sluckey’s trem-o-nator. All controls work as intended and the amp sounds good-again with a low noise floor when all knobs turned to zero. Been using it for a couple of years. I’ve been working on the board (cleaning) removing leaking DC voltage to help quiet the amp (still got work to do). Its got a very slight noise floor with all control knobs turned down/not bad.
New issue: When the reverb pot is turned up the noise floor increases and there is what I would call a constant scratchy oscillation and motor boating sound. The higher the is turned the more prevalent the noise is. This oscillation and motorboating sound goes away when the reverb is switched off using the foot switch. The motor boating cycles on and off-meaning it will be gone for roughly 4 or 5 seconds then return for a bit then go away again-but with no exact periodicity.
I have pulled every single tube in the amp (one at a time) and the problem is still present with the exception of when the PI tube is pulled of course. I swapped out the PI tube with another 12AT7 and the problem still exists. I don’t think it is a problem with V3 reverb driver or the V4 reverb recovery tubes. I have adjusted (moved around) the lead dress wires to the preamp tubes has had no effect either. I reflowed all the grounds to the main board. Running out of my low experience troubleshooting techniques.
Any suggestions?
Vr
J.
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One reason that pops up now and again is a bad filter cap on the reverb driver node. It's probably also your power tube screen node IE the one after the choke.
Dave
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V4A and V4B cathodes share a single 820Ω/25µF. Separate the cathodes and give each one it's own 1.5K/25µF. Does this help?
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Thanks. Will do and I'll report back later.
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I tried your separating the cathodes suggestion, but now I don't have reverb. Was I supposed to still have the cathodes jumpered? they are not jumpered now.
The oscillation/motorboating and excessive overall noise is GONE!
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Not jumpered.
Pin 3> 1.5K/25uF combo > Ground. Same for pin 8
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That's how I got it wired TVDT-but no reverb now.
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Either you didn't do this correctly or you accidentally messed up something else. Measure the resistance from pin 3 to chassis. Repeat for pin 8. Put the probe directly on the tube socket pin. Do you measure 1.5K for each pin?
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Sluckey: Yep it's on me. I did not not do a good tack solder job (1.5K resistor to the 50uf/50v cap) on the newly installed combo going to pin 3.
Measuring about 1.5k on each of the cathode pins and have great reverb now.
Noises gone.
Awesome fix and much appreciated!
Vr
J