Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: evancorylevine on November 20, 2017, 10:01:46 pm
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My Weber 5F1 started making really gross distortion last night. It only happens at high volumes. I thought it was a mechanical buzzing maybe, but I don't think so after checking today.
Time for me to take some measurements? Where can I find a voltage chart?
e!
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I've built a 5F1 before. I didn't like it dimed. Was an unpleasant distortion.
I recently built a 5F2A, the Tweed Princeton which is a 5F1 with a one knob tone control. I don't like this one dimed either. But, it's cleans are very sweet! Especially when played with my 335 style guitar fitted with Alnico2 low-ish output humbuckers.
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Time for me to take some measurements? Where can I find a voltage chart?
Lotta 5F1 info here - halfway down are some voltages: https://robrobinette.com/How_Amps_Work.htm
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My Weber 5F1 started making really gross distortion last night. It only happens at high volumes.
So, it sounded OK at high volume before last night?
I'm not aware of a voltage chart but it's a simple amp that should be easy to fix. Change tubes first. If no joy, then measure voltages at nodes A, B, C, and every pin of V1 and V2. Post your readings here. There may be a clue in your voltage readings.
TIP... Once the amp is working properly, print a schematic. Then check all voltages and write them on the schematic. Put the schematic in an envelop and keep it with the amp. Good idea to do this for all your amps if you intend to work on them yourself.
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A loose resistor lead or something... maybe a loss of negative feedback? Could increase the gain...
As others mention it might not be expected to sound good at super-high volume, but if it sounded good before you're probably looking for one component which would make a significant change if loose (assuming you didn't drop it or something which would cause many components to come loose).
This is a simple amp and most loose components would just stop it from working. To me NFB is the best candidate for an error which would change the sound but not break it completely.
Microphonic tubes definitely cause unpleasant noises at high volume but I think you would have noticed and mentioned these types of noise. It would pretty obviously be "rattling" or "ringing" in my experience.
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I replaced the tubes to little to no effect.
The sound is basically a really obnoxious crackling around 11 oclock on the volume. It happens at higher volumes, but mostly as the level crosses that threshold during decay. It also doesn't happen for the first minute the amp is making sound.
Any clues? Should I take those measurements now?
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(edited some of sluckey's original here): Change tubes first. If no joy, then measure voltages at nodes A, B, C, and every pin of V1 and V2. Post your readings here. There may be a clue in your voltage readings.
TIP... Once the amp is working properly, print a schematic. Then check all voltages and write them on the schematic. Put the schematic in an envelop and keep it with the amp. Good idea to do this for all your amps if you intend to work on them yourself.
This ^^^
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Champ Tweed 5F1 layout with voltages
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I don't think I have ever played through my Tweed 5F1 at full volume, cranked some yes, it sounds best at lower to mid volume to me.
Could be cold solder joints your hearing.
Mark
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I don't think I have ever played through my Tweed 5F1 at full volume
plug it into a Marshall 4X12 with a nice Gibson guitar, then smile :icon_biggrin:
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Oh I have built many mini 5F1 or 5F2a head versions in which I run them through 212 or 412 and smile just not my practice amp Tweed 5f1 combo in my office sporting 8" speaker.
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Here are my voltages! What should I do?
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Looks like it is running a little hot. You could change the cathode resistor to say a 750 ohm to start and lower it from there untill you get to around 12-14 watts dissipation. Also do you have a Sovtek 5Y3 in there? They are known to drop less voltage than a real 5Y3. I run all my Champs/VibroChamps with real USA made 5Y3GT rectifiers they are still inexpensive and plentiful. However many people still run their Champs on higher voltages and higher dissipation and it doesn't seem to bother them. Your voltages aren't really that high so it could be something like a bad capacitor or bad connection. What speaker are you using? Try the amp with a different speaker. Make sure your speaker jack and wiring are good, no cold solder joints. Chop stick the amp when it is on, with one hand in yer pocket.
24.5 / 470 = .052 ma
386 - 24.5 = 361.5 * .052 = 18.77 watts
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I removed the cathode bypass cap and it fixed it!
...should I replace it?
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Won't hurt. I'd replace with a 25uf 50volt cap.
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Alright! I think this 25V one failed. Thank you all so much! I'm getting better at this! I'm learning! Thank you!