Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dbishopbliss on November 11, 2025, 10:20:08 am
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A friend has a Friedman Dirty Shirley that stopped changing gain between 4 and 10 a few weeks ago. It reaches one level of gain and then stays the same. Then just the other day it started make a squeal (listen to link below).
He has tried swapping the power tubes, preamp tubes (V1, V2), Phase Inverter and Rectifier and it still has the same behavior. His is an early version with no structure switch. The treble, bass and presence don't change the sound much. The mid knob impacts it the most.
Some people on the Gear Page have suggested using deoxit on the volume knob, but that doesn't seem quite right to me. I think that a bad pot would make things scratchy, not squeal like that.
Any thoughts?
https://drive.google.com/file/d/1nJ9VTW5eu3sBnz3dcdvseOXf-wqZE9Kc/view?usp=sharing (https://drive.google.com/file/d/1nJ9VTW5eu3sBnz3dcdvseOXf-wqZE9Kc/view?usp=sharing)
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Generally speaking, the first thing you should do with pretty much any amp that comes into the shop is to properly clean all the pots, jacks and tube sockets. It's surprising how many problems this can solve. This step really cannot be skipped. If a customer tells me they already did it, I do it anyway, as they may not know the correct procedure.
Don't just clean the volume knob, clean ALL the pots, JACKS and tube sockets. If there are open contact type switches that can be cleaned, do those too.
If the problem persists after proper cleaning, go from there.
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That's classic oscillation. If you have a schematic and photos that you can share, then I think you'll make more progress with responses than just audio of the problem.
I've seen oscillation be caused by something as simple as a loose nut on a pot to as crazy as grid wire proximity to a choke mounted close to the input on an aluminum chassis. Check hardware tightness and any other connections to ground/shield and lead dress. If it worked properly at one point, then I'm leaning towards loose hardware or contamination interrupting solid connections.
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Here's a schematic I found over at the amp garage. No idea how accurate it is but there is one person that said they followed it to make their own.
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That's classic oscillation. If you have a schematic and photos that you can share, then I think you'll make more progress with responses than just audio of the problem.
I've seen oscillation be caused by something as simple as a loose nut on a pot to as crazy as grid wire proximity to a choke mounted close to the input on an aluminum chassis. Check hardware tightness and any other connections to ground/shield and lead dress. If it worked properly at one point, then I'm leaning towards loose hardware or contamination interrupting solid connections.
Here are some gut shots.
(https://i.ibb.co/7xy87p8N/57-C635-FF-65-C3-4-BA1-98-A9-BE7-C51-CEB3-BF-1-201-a.jpg) (https://ibb.co/KcyRCsRz)
(https://i.ibb.co/jZHZzfNg/23117596-388-D-4-BF1-B33-D-D94-DC13903-F1-1-201-a.jpg) (https://ibb.co/tP3PzKG8)