Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: limit6 on May 24, 2013, 03:27:59 pm
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So I've got my 73 Twin on the bench because it's been making a "wind blowing" and scratchy type sound even when no source is plugged in. All pots are scratchy even after a deoxit cleaning. Filter caps changed, bias cap too. Grid and screen resistors changed with metal films. Plate resistors changed with CC's. Swap power tubes for ones that are good, same thing. Change preamp tubes and the problem isn't as pronounced, but still there. I tap with a drumstick on leads from the blue caps and on the ceramics in the tonestack of both channels and hear a scratchy sound. Tap on pins 1 and 6 of the first few tubes and hear a scratchy sound. Hit the joints on the board to make sure they weren't cold. Any ideas?
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Which knobs change the sound?
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Yeah, 90% sure it's the 100K plate resistors. The ones arrayed in a "V" shape. They measure fine, they look fine, but when they are that old, they induce or create thermal noise and you get sea-rat noise. Doug sells 3 watt metal film ones. Just replace them all (6 qty) and you will be STUNNED at how much noise goes away. By far the cheapest & most dramatically beneficial thing you can do to an old Fender.
Wait...you say you already replaced the plate R's? Apologies, that sailed past me. I've done this to half a dozen amps and the rat noise just disappeared....even on a Champ it made a big difference. Still worth trying the metal film ones, though I imagine new CC ones would fix most of the issue.
SOMETHING is making thermal noise, some ugly resistor (or two or three) somewhere.
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PRR, it does it on all knobs. If all are set to zero on both channels and I only turn up the Master, the sound is present. Turn up both channels and it's a bit more prevalent, especially when turning up the vibrato intensity knob.
I'll put in metal films on the plate resistors and see if that does the trick. I'll report back. Thanks for the help.
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Ok, swapped in new 1/2 watt 100k carbon film resistors and the problem didn't go away. :BangHead: I'd have used metal film resistors but I only have an abundance of 1/4 watts lying around, which I assume wouldn't be suitable for an amplifier preamp circuit. Fresh tubes throughout would probably be a good idea, but I don't wanna shell out the cash because I'm selling this amp (not because of stated issues, simply to fund another purchase). I'm gonna throw it up for $699 on craigslist, but would feel unethical selling this amp if I knew that there was a problem with it besides needing new tubes. I'll probably convert it to adjustable bias too before selling. Does the price seem fair? For some reason people seem more than willing to pay low prices for an amp in as-is condition rather than spend $100-200 more for an amp that's fully serviced and ready to roll. :dontknow:
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> zero on both channels and I only turn up the Master, the sound is present.
Then it is between the channel pots and the master pot. Look at the schematic and figure which tubes that is.