Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: ltkrepp on July 25, 2026, 08:29:37 am

Title: Help with 1987 build
Post by: ltkrepp on July 25, 2026, 08:29:37 am

Hi all,
I'm hoping someone will see or help point me in a direction to solve an issue I'm having with a Marshall 1987 build. All voltages are correct until I plug a 12ax into V1. Once a tube is in that spot, voltage drops to roughly 29v on V1. Assuming the tube is running away drawing a load of current. I've checked the cathode resistors (820 and 2.7K ohm) and they're good. V1 cahode bias cap (330uf) is in the right direction and is good. The 10k and 8.2k dropping resistors all read as they should.  All the grid connections and connections to the cliff jack appear correct. Tube socket pins, etc all look good. Tubes in all other positions V2-V5 do not have this issue. I've subbed other 12ax7's into the slot known to be good and they all exhibit the problem. Attached is a photo of the V! preamp section. Any help/suggestions/ideas would be greatly appreciated.

Title: Re: Help with 1987 build
Post by: shooter on July 25, 2026, 09:06:11 am

socket wired wrong?


ohm socket pins (tube side) back to the parts they should be soldered to, ohms near zero?

Title: Re: Help with 1987 build
Post by: ltkrepp on July 25, 2026, 10:08:15 am

Yes. Traced and put to a meter every socket pin connection to its destination on the board. All went to the correct places onthe board and had continuity.

Title: Re: Help with 1987 build
Post by: sluckey on July 25, 2026, 10:15:37 am

Measure the voltages on V1 pins 1, 2, 3, 6, 7, and 8. What have you?