Hum from 20th Anniversary KT88 Shiva

[duncan]

Hey gang

Before I get the whole “take it to a tech, amps are dangerous” spiel, I want to start by saying that there aren’t any techs in my area that are working on amps, and none will work on this amp. Plus I’m comfortable working on electronics and I do basic amp repair on the side.

I’m starting this thread to see if this a common issue or if someone has fixed this before.

My amp is making a 60hz hum without any input. Hum exists with volumes down and with both nothing in the effects loop and with the effects loop jumpers.

I’ve installed brand new power tubes and biased them properly. Hum existed with old tubes and new tubes. Have rolled preamp tubes with known to be good tubes. Hum goes away when V1 is removed and comes back when v1 is replaced and warmed up. Hum also goes away when phase inverter is removed and comes back when replaced and warmed up.

Hum does not increase in volume at the same rate with an increase in amp volume.

Jorg at Bogner basically said take it to a tech. Figured I’d reach out here.

Video: https://youtu.be/TCqFuwNan5E

 

[Ventura]

Disrupting the signal chain by removing V1 or the Phase Inverter puts the problem on this side of the powetubes.

Bad filter cap?
The fact it stays the same volume up or down leads me to believe it's a bad cap.

Have you inspected the undercarriage of the chassis?? Check for any peculiarities? Follow your nose too - sniffing the board can lead you to burt-out or overly heated plastics/compounds that are blown or about to blow.

I'd flip that baby upside and have a look under the hood being mindful for discoloration of similar spec'd components, any bulging caps, yellowish goo around a soldered component, and the burnt smell.

Keep us posted.