relationship of cathode voltage to noise?

[peesinstew]

Don't forget to add a filter cap to ground from the junction of the voltage divider. It will help keep AC off of your center tap.

[Firestorm]

Don't forget to add a filter cap to ground from the junction of the voltage divider. It will help keep AC off of your center tap.

Or tap it off Vs or a later node (per Kevin O'Connor). already well-filtered that way.

[skyboltone]

I got a 250 Ohm 2W pot from digikey and installed it.

For some reason it seems to dial in/out more buzz than hum. When I have it dialed in to the quietest setting, I think I'm getting more hum than I was getting without the pot. Luckily, I didn't have much hum to begin with. I do have it mounted in a hole that is right next to the PT, so maybe that has something to do with it. The hole was already there, so I used it out of convenience.

The stethoscope trick is in order Stew. Find out where the 60 cycle hum is entering the circuit. It could be OT primaries in close proximity to the PT core for all you know. Go stage by stage till you find out. If it's in every stage then the B+ to the center of your adjustable center tap will help. By the way, my experience with the buzz and the pot was the same. Amazing innit? Such temperamental little suckers.

Dan H

[peesinstew]

The hum goes away when the Volume is down, so I'm pretty sure it's the heaters injecting hum into V1.
With V1 removed, the amp is very quiet other than a very slight hiss that is only controlled by the presence. I can also dial in a buzz with the balance pot, but this is controlled by the presence too.

Haha, this is starting to sound like a bad horror movie.
The presence controls us all!!

[Firestorm]

Bear with me. I modified two of those 135-watt ultralinear Fender monstrosities. One was a bass amp that came out totally hum-free despite having an extra filament transformer to power a quad of KT88s. The other was a guitar amp that had a hum I could never completely kill (and I tried nearly EVERYTHING). Identical trannies in both, similar topology. Maddening.
Kevin O'Connor mentions, but doesn't explain anywhere I can find, a way to kill hum by finding the point where the hum gets injected and deliberately injecting more hum, out of phase with the first (like by using shielded wire, but connecting the shield to a source of out of phase hum).
Anybody know anything about this?

[Ron Worley]

It's topics (and responses) like in this post that make me realize that I have SOOOO much more to learn about tube amps!!

The best part of this group is that you can learn without being treated as the dumb $hit that I really am....

Ron

[briane]

Kevin O'Connor mentions, but doesn't explain anywhere I can find, a way to kill hum by finding the point where the hum gets injected and deliberately injecting more hum, out of phase with the first (like by using shielded wire, but connecting the shield to a source of out of phase hum).
Anybody know anything about this?

I've done this. Its called parasitic cancellation, and thats how it works. Highly not recommended, cause its a tone sucker, and a workaround for the real problem.

Attach one end of a wire to the out of phase grid associated with the pre-amp tube in question (leave other components connected). shrink wrap the other end, it attaches to nothing.

Take that wire and wrap it around the input wire to the other grid (which is out of phase with this new wires signal), along the length of run to that grid. It ussually takes a minimum of 2-4 inches of wrapping in length to make a difference. you can wrap tight or loose (ie each wraps spacing), each sound different. tight wraps suck more volume out.

This is a hacky fix, ussually used to address poor lead dress (thats how fender used it), and works by phase cancelation on the first input grid. pretty simple stuff really, but can fix problems that otherwise have no forseeable solution other than a total rebuild.