Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: inapier on May 18, 2026, 07:07:51 am
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I recently completed an AC30 build using the revision 7 layout on the internet as a guide. I had built a few amps before like the 18 watt Marshall, a couple of tweed deluxe and a deluxe reverb but nothing with as many components as the AC30.
Everything worked perfectly when I powered it up for the first time except that when the trem is engaged, the trem is very strong even with the trimmer pot turned to 0 ohms. I installed a switched 500k pot to be able to switch from trem to vib and be able to adjust the intensity. The switch works exactly as it should and does indeed switch the trem function to vibrato but with the pot all the way off the trem and vib throb is quite strong and gets stronger as you turn it up and the last 1/4 sweep of the pot is unusable as the throb starts to induce some distortion artifacts.
I’ve triple checked my components and wiring layout against the diagram several times and don’t see where I’ve made a mistake. What would cause the trem and vib to be working strong with the pot all the way off?
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Here is the layout i used for the build. The amp sounds incredible and I’m glad i put in the work to build it. I just need to drag this thing across the finish line and figure out whatever problem is causing the trem to throb with the pot turned all the way off. I’m sure its something simple but I’ve stared at it for so long I cant see the forest for the trees.
https://turretboard.knucklehead.dk/wp-content/uploads/2012/03/ac306normalrev7.pdf
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The issue has been sorted out thanks to help from a person who knows these amps pretty well, user pdf64 on the gear page. My intention was to replace the 500k trimmer pot on the tag strip with a switched 500k pot mounted where the trem/vib switch is located on the front panel. This way I would be able to adjust the intensity and switch between trem and vib.
Problem was that with the pot all the way off the intensity of the trem was still strong, and couldn’t be adjusted any lower. Pdf64 pointed out that the 150k resistor listed as R73 connected to the 500k pot and then to ground would have to be bypassed for the intensity to be turned down to nothing. I switched the connection of the 500k pot to the grounded side of R73 and bingo. The intensity of the trem and vib is now adjustable from completely off to full on right there on the front panel.
This is a great mod for the AC30 because the intensity needed for the vib to sound good is different from the intensity needed for the trem to sound good. It was always a compromise with the tag strip mounted trimmer pot. This provides a much more functional channel in my opinion.