Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: alange5 on July 21, 2018, 03:34:47 pm
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I recently bought a '63 Vibroverb reissue. I opened it up to adjust the bias (pot installed by previous owner) and found the plate and B+ voltages to be extremely high. I'm measuring in the neighborhood of 875-900V on the 6L6 plates.
Schematic of the power supply:
(https://preview.ibb.co/gKWSJd/schem.png) (https://ibb.co/fnwuyd)
I disconnected the PT, fed it 120V through the primary, and measured 330V across the secondaries (no visible center tap). I reconnected the PT and measured secondary voltage along the rectifier. Voltage looks good (ballpark 350) at the cathodes of CR1 and CR3, and at the anodes of CR2 and CR4. However, at the cathode junction of CR2 and CR4, voltage jumps way up to 870ish volts. That spot joins the two diodes, the first filter cap, the choke, and the B+ OT primary
Any ideas?
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I don’t know why folks feel is vital to measure plate voltage. Doing so has a tendency to send amps into oscillation, which I suspect is what’s happening here.
Do things straighten up if the phase splitter tube is removed?
Really, just use the HT / B+ Vdc, at idle the plate can only be a very few volts lower.
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With everything connected as usual, measure the AC voltage between the two PT red leads (CP11 and CP12). What have you?
Has this PT been replaced? Perhaps with a PT meant for an old Vibroverb that doesn't use a FWB?
I'd be careful. 800V will destroy a bunch of filter caps.
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> I recently bought a '63 Vibroverb reissue.
Did it work? (Unlikely to work right, or for long, at 900V.)
What ELSE did the previous owner do inside?
While hanging a long meter lead off the power tube plate will tend to induce oscillation, you also say you find 900V on the filter cap and OT CT point. That seems really unlikely.
Is your voltmeter trustworthy? Compare on known-happy amps? Change the meter battery?
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Did it work? (Unlikely to work right, or for long, at 900V.)
What ELSE did the previous owner do inside?
While hanging a long meter lead off the power tube plate will tend to induce oscillation, you also say you find 900V on the filter cap and OT CT point. That seems really unlikely.
Is your voltmeter trustworthy? Compare on known-happy amps? Change the meter battery?
Yes, it did (does) work. I've played it for about one hour total. No telling whether or not the high voltage was always present, or developed recently. Best I can tell, the only mod that was done was the bias pot. It looks like it was done by a tech about 10 years ago.
I measured with three meters - two digital, one analog, and used different meter leads. The readings are consistent. I get about 350V at the PT secondaries and diodes, and 800+ past the diodes on all three OT primaries, cap, and choke. The high voltage continues all the way down the rail. V1's plates show about 475V.
I'll investigate more tomorrow
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With everything connected as usual, measure the AC voltage between the two PT red leads (CP11 and CP12). What have you?
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With everything connected as usual, measure the AC voltage between the two PT red leads (CP11 and CP12). What have you?
430V AC
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Something very strange going on. 430 x 1.414 = 608. This is the theoretical maximum dc voltage you can get from that PT.
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...measured 330V across the secondaries...
430V AC
What changed?
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I have no idea. Must have been oscillation or something else affecting the reading. I was reading crazy voltages with the chassis pulled from the amp but still connected to the reverb tank and speakers. With the chassis moved to the bench and hooked to a dummy load, everything looks normal. I now have 405V on the plates with a pair of RCA 6L6GC's.
Sorry for the goose chase but thanks for the help.