marshall reissue hum balance

[rockstah]

what does this do?

looking at a schematic and the hum balance circuit we see two 100 ohm resistors and one 100 ohm pot that connect to the heaters-

http://www.drtube.com/schematics/marsha ... -60-02.pdf

i thought hum balance was a way to compensate for tubes that arent matched closely.
can someone explain whats going on since this hum balance seems to connect to the heaters?

the amp blew the v5 tube and took out these two resistors and the pot. sure we could just replace the pot and the two resistors but I'm trying to figure out what happened so i don't change the parts out and fire it up and they just blow again!

bad tube?
bad ot?
bad speaker cable?
why wouldnt the amp blow a fuse before this happened?

Thanks,


Mark

p.s. also on the board we see w2 and w3.

[rockstah]

Larry? SDM? Dan? Andy? anyone?....

[novosibir]

The hum balance is to minimize background hum (humbucking principle), caused by inproper twisted filaments inside the cathode pipes of the preamp tubes.

When a output tube is going south, sometimes the HV from pin 3 (plate) is arcing to pin 2 (heater) and is looking for the easiest way to ground...

... and that's through this hum balance circuitry - where the huge current of the HV is toasting these wimpy parts.

Check your output socket for any burning path between pin 2 & 3 - if so, clean it (acetone), shape it and insulate it after properly cleaning (ladies nail paint) - and of course swap in new resistors, the trim pot and healthy output tubes.

Larry

[rockstah]

thank you Larry!

[Brad H]

The purpose of the hum balance is to adjust your heater voltage on each side of your tube sockets. 6.3v heater voltage should be an even split of 3.15 on pins 2&7 of power amp tubes, and pins 4/5 & 9 of preamp tubes. I have a 2203x that I was re-biasing and rolling preamp tubes through for better tone, and noticed Marshall had set the hum balance unevenly from the factory. It was 3.9vac on one side of the tubes, and 2.6 on the other. They do this because they only put one to two twists in the heater wires, and the cathode wires are very close to the heater wires, which can cause hum. I set it right in the middle so I have 3.2 on each side. Older Marshall’s didn’t use this pot, so they were already even. My opinion...this is a garbage way of controlling hum instead of taking the time to put more twists in the heater wires, and ensuring the lead dress is neat. THAT will eliminate hum. When I build amps, I put a minimum of three tight twists between each tube socket, and never lay cathode wires across heater wires. Never have I gotten hum from heater voltage.