Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Tiny_Daddy on August 10, 2011, 06:19:52 pm
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I am re-building an ancient VOX AC100. It has had plenty of previous repairs, good bad and ugly. Question is, were the output tube grid resistors really 47K as I see on the schematic? Seems to me this would make for muddy sound. Maybe they should be 4.7K?
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It look really a 47 K . Strange desing with 100 ohms on the plate.
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On a Gerald Weber book Tube Amp Talk page 243, he said about this amp " The four EL34 tubes where in fixed bias with the bias voltage set at 35 volts with Zener diodes. These amps run very hot at 35 volts bias and I often change the bias to 42 volts , especilly with EL34 build today......
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Someone has removed the bias zener and added a bias adjust pot. Also the output transformer is 25% larger than stock with no brand or other markings, just a plain gray end bell. Unknown impedance so I will get a replacement from Heyboer.
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I built myself a clone of an AC100 with the guts of a Sovtek Mig 100. I added a second, higher gain channel with an EF86 and an overall cut control, and will be adding power scaling to it. I got sidetracked on a Bogen CHB100 project that is almost done, so after that it will be back to the AC100 project. Anyway, I used the 47k grid resistors just like the schematics, and it sounds just fine for guitar and decent for bass as is. I made mine switchable for cathode bias or fixed bias, using slightly larger cathode resistors in cathode bias mode, but will be going to something larger still to get it to run cooler. It is pretty brutal on the tubes otherwise. I used a Heyboer choke and OT and they are very nice btw. There were actually a few different OT's that were used in these stock. Check Jim Elyea's book for pics.
These AC100s, and AC50's are absolutely killer amps that most people have no idea about. They sound like they are in the same family as an AC30 though a bit cleaner, but don't have that often overly bright zing that an AC30 can have with some guitars, and also take to humbuckers better than an AC30. With my second channel, my clone will get more gain than any AC30 and still has that super nice clean sound these amps do so well.
Good luck with yours!
Greg