Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: kevin_taylor on December 03, 2025, 03:44:29 pm
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Hello,
My name is Kevin and I’ve been soaking up a lot of info that everyone is generously sharing on this forum for several months now. Trying to wrap my head around this deep hobby. I’ve built a a few amps over the last couple years including the Hoffman 5F2A, which went pretty well. Most recently, I built a Vibro Champ based on the AA764 layout and schematic in a Fender Champion 30 chassis and cabinet that I got for free on Craigslist (some circuit board traces were burned up around the power supply section). For the power transformer, I used the Hammond 290AX with secondary taps for 275v and 325v.
I’d like to get some help with which voltage to go with. My understanding is that the higher voltage might give more headroom and the lower voltage earlier breakup which I would prefer. Maybe that answers my own question.
Changes I made to the circuit are:
Upgraded the 470 ohm 1 watt cathode resistor to 2 watts as noted on the AB764 schematic
Added 470 ohm 2 watt resistor to screen of 6v6 based on Hoffman 5F2A
Added 1.5k ohm 1/2 watt resistor to grid of 6v6 based on Hoffman 5F2A
Impedance selector switch
Another question I have is that the screen voltage I measured is slightly above the plate voltage even with the 470 ohm resistors on the screen. My understanding, as well as the voltage listed on the schematic is that it should be slighter later than the plate. Is this an issue I should be worried about?
Here are my voltage measurements from the two different secondary taps. If anything stands out as off or if more information is needed, please let me know:
325 volt secondary
Power supply
Node 1: 380v
Node 2: 372v
Node 3: 347v
5Y3
Pin 2: 380v
Pin 4: 340v
Pin 6: 347v
Pin 8: 380
6v6
Pin 2: 3.3v
Pin 3: 360v
Pin 4: 370v
Pin 7: 3.3v
Pin 8: 21.3v
Cathode resistor: 466 ohm
Plate dissipation: 14.6 watts 104.3%
Plate current: 43.2
275 volt secondary
Power supply
Node 1: 335v
Node 2: 327v
Node 3: 307v
5Y3
Pin 2: 327v
Pin 4: 295v
Pin 6: 295v
Pin 8: 332v
6v6
Pin 2: 3.3v
Pin 3: 317v
Pin 4: 329v
Pin 7: 3.3v
Pin 8: 18.53v
Cathode resistor: 466 ohm
Plate dissipation: 10.5 watts 76%
Plate current: 36.5
275 volt secondary with 2 470 ohm cathode resistor in parallel (231 ohm measured)
Power supply
Node 1: 325
Node 2: 307
Node 3: 287
5Y3
Pin 2: 327v
Pin 4: 295v
Pin 6: 295v
Pin 8: 332v
6v6
Pin 2: 3.3
Pin 3: 285
Pin 4: 305
Pin 7: 3.3
Pin 8: 12.93
Cathode resistor: 231 ohm
Plate dissipation: 14.4 watts 102.9%
Plate current: 52.9
I didn’t have any other 2 watt resistors besides 470 and 220 ohms to experiment with.
I am leaning towards the 275 volt taps and trying a 270 or 330 ohm Cathode resistor after ordering them to bring the dissipation down to 95% or so. Is there anything else I should be looking at? Based on the advice here, I will revisit my 5F2A to make sure that is on the right track as well, though it uses a different transformer with slightly different voltages.
Another issue with my build is the Champion 30 chassis angles downward towards the back of the cabinet so I had to adjust the layout to avoid mounting components too close to the speaker including the output transformer inside the chassis. So far everything seems to be working as expected.
Thanks for any help you can offer.
I’m including photos before I am told to add them. :)
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Additional photos
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+ faceplate with model name correction.