Harmony H400 modification

[bbakehorn]

I've modified a Harmony H400 to include an isolation transformer as well as a bridge rectifier. It's still not working though, and I'm looking for some assistance. The 12AU6 tube is not glowing and I'm not sure if my circuit is completely correct. Also the speaker only hums a bit. I've included my oscilloscope test along with a schematic of the modifications. I made test points (e.g. TP0). I used a 100x probe so there may be some errors. I didn't patch the ground from the rectifier to the ground of the circuit because that didn't seem to make sense.

[Jazz P Bass]

Why did you add a rectifier?

The isolation transformer should be fine (120Vac to 120Vac) by itself.

[bbakehorn]

http://karaslittlebrother.tumblr.com/harmonyh400a
http://www.instructables.com/id/Isolation-transformer-upgrade-for-old-guitar-amps/?ALLSTEPS


It was my understanding that the 35W4 would cause the isolation transformer to overheat without the bridge rectifier. I couldn't find an exact schematic modification for the H400 though.

[g1]

That can be the case with a half-wave rectifier if the isolation xformer is not sufficiently over-rated for the job. What model of isolation xformer are you using? You could try it without the bridge and see if it overheats.
In any case, you will need a ground reference for the bridge.
Can you post the non-modified version of the schematic?
Edit: never mind, it's here (your first link is not working):
http://karaslittlebrother.tumblr.com/harmonyh400a

[bbakehorn]

Here's a link to the original schematic. If I do put the rectifier ground to chassis ground, what is the purpose of C5 & R3? I went with the Triad N68-X as specified in the link. I have a H400 not an H400a. There seems to be a significant difference. I can try it without the rectifier, and post the results.
http://www.prowessamplifiers.com/sch...rmony_h400.pdf

[Enzo]

You needed the isolation transformer yes, but that bridge screws everything up. It is the reason your 12AU6 is dark.

Look at the original, the 117vAC comes in through that 100 ohm resistor, through the heater of V3, then the heater of V2, through the primary of a small transformer, then back up and around to return to the power switch at bottom left. That is all AC current from your mains. Note that little trip through the small transformer primary. That transformer is there to power the 12AU6 heater. Transformers only work on AC. Your bridge made it DC.