Tube Amp Archive

Ok so the preamp issue is fixed. Now I am trying to bias the hybrid output stage. In "Class AB" mode, all four output tubes are run in fixed bias, easy peasy. But in "Dual Class" mode one tube in each pair is switched entirely to cathode bias, with no negative voltage on the grid, while the other tu...

Woohoo! Well I figured out the issue in the preamp, the 100k cathode resistor (R36) on the cathode follower stage before the tone controls was open circuit. I found this almost by sheer chance as I was probing the circuit from the area I found that 1) I had just a few mV on the cathode of the follow...

Cathode biased outputs ride with their grids at a different DC level than fixed bias tubes. Is it possible that the capacitors separating the sets of grids are leaking? Thank you for the response R.G. I am actually using your "quick and less-dirty" oscillator as my test signal! I will look into thi...

Hi all. My friend brought a 2000 VHT Pitbull Hundred/CL with EL34's to me with some kind of volume fluctuation that I have not been able to reproduce. I did, however, find that it was red-plating a bit. The amp has a "dual class" mode via a switch on the back where one tube on each side of the power...

Thanks for the reply @maxkracht. I reflowed all high-mass component solder joints on the power board on Amp A. I am hesitant to remove that big resistor since it’s in there pretty good. I reflowed its connections and it measures fine when hot. I narrowed the sizzle down to the molex connecter for th...

Hi all, started my tube amp journey this year fixing amps for my friends. It’s been a great learning experience and I’m on a winning streak, but my latest two patients have me stumped. Two Peavey VTM-120 of many my friend bought from someone hoarding them have the same issue. He said he was using ea...