Hiwatt Classic 50 Crackling when cranked

[kdawg]

Hello... I've been working on a troublesome Classic 50 (DR504) that was converted to a Classic 100 (DR103) by adding two power tube EL34's. Unfortunately, they never upgraded the OT and it fried. PT also fried because of a power surge, so I replaced both with Mojo 100W rated.

I put in the adjustable bias circuit and that is working. Question is - they never changed the HT2 supply circuit to the DR103 circuit, so there are still 1K5 screen resistors and no 470R 10W. Is that essential to change over for the higher current draw?

There are two big issues right now, I will be checking them out soon:

1. Fairly loud hum (seems like a 60 Hz) that varies with Master volume.. forgot to check if channel input volume affects it also.

2. At running the amp at about 70% up, it starts making a frightning crackling and arcing type sound. I looked at tubes in the dark and didn't see arcing there, could it be PI V4 tube losing bias somehow? I will check that. Any other suggestions?

Thanks

-kdawg

DR103 PS: http://mhuss.com/Hiwatt/Schematics/DR_100wPS.pdf

DR504 PS: http://mhuss.com/Hiwatt/Schematics/DR_50wPS.pdf

DR 103 Output: http://mhuss.com/Hiwatt/Schematics/DR_100wOutput.pdf

DR 504 Output: http://mhuss.com/Hiwatt/Schematics/DR_50wOutput.pdf

[kdawg]

I solved the hum issue, broken wire on power filter cap to V2... I have to check on crackling now, could that be a cause of over or under biasing? We're talking extremely loud volumes here - I have to wear earplugs just to test it. All voltages look good on the preamp tubes.


Preamp: http://mhuss.com/Hiwatt/Schematics/DR_Pre4Input_v1a.pdf

Thanks

-kdawg