The Amp Garage

Stevem wrote: ↑Tue Oct 13, 2026 11:54 am Try this.

Leave all the tubes in and unsolder the down stream end of r37 which is the 1K 1 watt after the choke .

Do you still have a hum from the speaker?

Stevem, with r37 disconnected as you suggested the hum stopped.

Stevem wrote: ↑Tue Oct 13, 2026 11:54 am Try this.

Leave all the tubes in and unsolder the down stream end of r37 which is the 1K 1 watt after the choke .

Do you still have a hum from the speaker?

I disconnected the down stream end of r37 and the hum from the speaker stopped.

Replaced both diodes and the problem stayed the same. Measuring with a scope: With both V6 and V7 installed there is a sawtooth waveform of 7v p-p at pin 7 of each tube, which gradually amplifies to 35v p-p as the tubes warm up. Measuring V6 pin 2, with no input signal to the amp, the signal gradual...

Thanks Stevem,

I am trying to find the specs for those diodes but haven't found them. They are labeled D-1 404. I will try a 1N5408 when I can get a couple.

Phil

Thanks Stevem, I must say up front that I am new to tubes so please forgive ignorant questions and statements. This is the schematic I am working from : https://www.prowessamplifiers.com/schematics/Gibson/Ga_20rvt.html V5a (12AX7 normally 12AU7) is the preamp to the power tubes V6 and V7 (6BQ5). Wit...

Stevem: Thank you for these very helpful suggestions: Stevem: Try the amp with just the output tubes in to see if it hums, if not add the preamp tubes back in one at a time from the outputs on back to the first one. Palan: I remove all the tubes except for the output tubes and the hum is there but v...

R.G. Thank you for these great test point suggestions. Here are the results: RG: You could use your voltmeter set to AC to measure the AC ripple voltage on each bypass capacitor. Palan: Measurement of AC voltage at each bypass capacitor. 1st bypass cap, input of L1: 8.2V with a scope: 20v p-p at 120...

doubts*

Hello, I had a 120hz hum in a Gibson GA20RVT amp and replaced a few capacitors, which were not original. That corrected the problem. While looking at another issue the chassis slipped and fell a few inches and the circuit side landed on the back of the speaker magnet. Now it has a worse 120hz hum. I...