Hoffman Stout TMB Troubleshooting Assistance

[coltojt]

Greetings all,

I recently completed my 2nd amplifier build, the Hoffman Stout TMB. Unfortunately, I've reached a wall in my troubleshooting and am needing input from the community at this point. I've made only two changes to the original layout: Fender-style pilot lamp and Pure Tone Output jacks. Wiring the output jacks with only a positive tip and ground sleeve terminal presented some confusion -they are mono jacks so perhaps I should have used stereo? The Switchcraft L12a output jacks have three terminals versus the Pure Tone output jacks.

The amp is producing sound although way too distorted at normal volume levels. I've checked the layout and that all of the voltages and resistors are in the correct ranges according to the schematic (highlight method). Using a dim bulb tester, I've found what I believe is a problem area in the power amp section -bulb is dim until I plug in the power tubes. However, I'm seeing correct voltages in the power tube section without the dim bulb tester. I'm very uncomfortable running it at this time without the dim bulb tester to further check voltages as both the power tubes and rectifier become unusually hot after just a minute of powering on.

What I've visibly observed in the preamp section is that the tubes are not producing normal glow or heat, while the power tube and rectifier sections appear to be drawing excess current. Please let me know if there are any glaring issues in my wiring from the images, tips in the right direction would be greatly appreciated.

[tubeswell]

The light bulb dimmer will rob some primary voltage, so the amp won't sound normal with it plugged in.


What voltages do you have:
1) on the power supply rail (High Tension voltages) at each filter cap node (including the B+)?
2) on each plate and cathode throughout the amp
3) on the output tube screens

[coltojt]

I failed to mention, I tested the output without the light bulb dimmer and was hearing the distortion at normal volume level. I will follow-up with the voltages this evening!

[tubeswell]

Using a dim bulb tester, ... -bulb is dim until I plug in the power tubes. both the power tubes and rectifier become unusually hot after just a minute of powering on.

Check all connections around the output tubes  - you may not have the grid leak resistors connected to ground or to the output tube grids

[coltojt]

I apologize for the untimely response, been a moment since I could take a deeper look at the amp. Yesterday, I went through the entire amp again, tested all resistors, checked all voltages on the schematic. I could not find a single issue with the wiring, resistor or capacitor values, or voltages. The voltages are perfect until I put the power tubes in, then voltages drop by half across the board (including preamp section) and the dim bulb goes bright. I'm definitely suspecting the cathode resistor or cathode bypass cap section. With the power tubes in place I'm only drawing 3.5V at the cathode. It's the only section I've tested that is off from expected measurements. I'm just unsure what this indicates, I'm thinking of replacing the cathode resistor even though it's resistance is technically spot-on.

[coltojt]

I'm a dum dum, it was the dim bulb limiter...

[dmp]

What did you use for the chassis? Nice looking build    Edit: nevermind, it is the stout chassis sold here and at mojotone