Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Jamlid on May 28, 2022, 01:32:42 pm
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Hi folks,
I recently took my 5F2A chassis out of it's cabinet in order to put a handle on said cabinet. However, once I re-installed the cab, put everything back together and attempted to use the amp, it made a loud humming noise.
Weird, because the amp worked fine before and I was careful not to knock anything when disassembling/reassembling.
Anyway, I thought it best to treat it like a new build and took some voltages. Firstly I did this with just the 5Y3GT installed:
5Y3GT
Pin 2: AC voltage switching between 0 and 300 ish - 500VDC (This is v high no?)
Pin 8: Same as above - 500VDC (also high)
Pin 4: Same as above
Pin 6: Same as above
Then with no tubes installed:
5Y3GT
Pin 2: 357 VAC
Pin 8: 357 VAC
Pin 4: 5.3 VAC
Pin 6: 0.0002 VAC
So maybe, somehow, the pwr xfrmr has failed?? Could this have happened because of a faulty rectifier?
Any help is appreciated, thanks.
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Those are crazy voltage readings. I suspect a faulty meter or operator error. Check the meter battery.
Pull the 5Y3 and set aside. Set your meter to measure AC VOLTS. Connect one probe to pin 2 and connect the other probe to pin 8. Should read about 5VAC. Now connect one probe to pin 4 and the other probe to pin 6. Should read about 600VAC (depending on your PT specs). If these two measurements are OK the PT is probably OK too.
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Hmmm 🧐 … Ground loop through the amp handle? Are the tee-nuts (for the new handle) clamped all the way in (so they’re not both touching chassis)?
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Thanks both. The screws do not go all the way through the holes I drilled, and I was careful to avoid said hole lining up with the chassis so that's not the problem. Will take suggested readings when I get a few moments