Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Yeatzee on March 09, 2026, 07:41:59 pm
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Getting Deja Vu with my own personal GA40 I had a similar issue with and never got around to solving it. Have an early GA20 in, matches this schematic:
https://www.prowessamplifiers.com/schematics/Gibson/ga20.pdf
The amp was serviced somewhat already before it came to me, with replaced electrolytics and a 3 prong. The rest was largely untouched. Dimed the amp was barely bedroom volume. I measured the output wattage and it was putting out ~0.6w (2ishvac into 8ohm load) clean. Yikes! Here's what I've tried and done so far:
New rectifier, no change.
New power tubes, no change.
Replaced all of the waxy coupling caps with Mallory 150's, lowered the bias resistor to 150R from 220R, dropped the screen resistor from 10k to 2.2k. This got me up to a whopping 2.7vac clean into an 8ohm load for .9w. Really rocking! Same for both channels.
Here are some reference voltages with all of the tubes in the amp:
Plate to Cathode - 262vdc
Plate Current - 36mA
6v6 Plate Voltage - 275vdc
Screen Voltage - 255vdc
5Y3 Output B+ - 285vdc
Bias is hovering around 90% plate dissipation. Screens were crazy low (210vdc) before I reduced the resistor from a 10k to a 2.2k. I've not run into an amp that ran the screens so low compared to the plates so I decided to just experiment and get it closer, still a big drop with the 2.2k. Obviously that wasn't the fix either way, more of an aside than anything.
So where to next? I've been at it for hours so taking a step away, and hoping you guys can inject some new ideas as to what I'm missing! Amp otherwise sounds great, just low output. Worst case OT. Also don't have a spare 6SL7 phase inverter so may need to order one to test if that could be it.
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Adding some more voltage readings for reference. Seems quite low, but I have nothing to go off of. 1meg resistors dropping the voltage a ton for the preamps, but as designed?
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Welp, decided to test the OT primaries and secondaries. Primaries seemed normal about 212ohm each side to CT, plate to plate 423ohm. Secondaries, injecting 3.4 volts in via my variac to the 8ohm tap I got a voltage drop of about 1 volt so it's pulling current. DANGIT.
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What's the AC voltage on the primary when you inject 3VAC on the secondary?
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Altering the design of the amp by changing component values is not a troubleshooting technique.
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Altering the design of the amp by changing component values is not a troubleshooting technique.
I've not run into an amp that ran the screens so low compared to the plates so I decided to just experiment and get it closer, still a big drop with the 2.2k. Obviously that wasn't the fix either way, more of an aside than anything
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What's the AC voltage on the primary when you inject 3VAC on the secondary?
100v on the primary = 2.6v on the secondary. I've been told a decent drop on the secondary from input voltage indicates a misbehaving OT secondary. Is that not the case?
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http://www.geofex.com/ampdbug/outtrans.htm (http://www.geofex.com/ampdbug/outtrans.htm)
Here's a guide that explains a few more test methods for output transformer secondaries
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Almost 9w clean with a 5e3 OT subbed in.