Hum In Amp

[plexi50]

I have a slight hum that increases with volume. There is no hum at volume CCW. This is series wire heaters. The wires are laying horizontal to the tubes. Should i raise them up like an old fender bassman to get the wires away from the tubes cathode? I think i got that right

[jjasilli]

What frequency is the hum?  60Hz?  120Hz?

Have you tried swapping tubes?  Loose connections; bad ground including loose nut on inoput jack; redplating?; unbalanced power tube bias?

[plexi50]

Bias is dead on for a change. 530 VDC on the plates @ -32.7 ma. Per tube. Low hum. I forget the diff between 120 & 60Hz

All preamp grounding is where it should be and away from the PT end of the chassis

I have not installed the choke yet so this may be the problem. I noticed when i used a bigger resistor value for the choke the hum was less apparent

Could running the heater wires above the chassis be a better alternative than having them horizontal as most marshall amps are?

I quess it's a matter of trial and error / I hate heater wiring

[PRR]

> when i used a bigger resistor value for the choke the hum was less apparent

Then it may be coming in the B+, not from the heaters.

Without "some" information about the amp, it's hard to suggest anything useful.

[LooseChange]

120hz is very close to the Bb (E string 6th fret) on you Guitar.

[plexi50]

Ok it's 120 Hz. I just finished installing a 5 H choke and now my screen and plate voltages are much closer than they were with the resistor

I think i messed up on one of the power supply nodes need to add another 50uf/500V cap tommorrow

The amp is like the Steve Luckey 2204/Plexi schematic

[PRR]

> I forget the diff between 120 & 60Hz

Because of the way the ear hears: not much.

60 and 120 are an octave. You know that two notes an octave apart sound a lot alike. That in a melody or chord, you can generally use the same note-letter an octave up or down, and it's still the same music, just a bit different.

In the output stage, pure 60Hz will usually be a low hum, stray "120Hz" is usually contaminated with overtones and sounds like "buzz". However the B+ filtering to preamps knocks-down the overtones more than the 120Hz. And typical guitar speakers don't do 60Hz well, nor can your ear hear it well, so the impression may be any stray overtones rather than the 60Hz fundamental.

(We suspect this is in the preamp because the volume control affects it, and it goes away when volume is at zero. The problem is *probably* in the stages before the volume control.)

Good commercial designs don't have much excess B+ filtering. You need all those nodes and roughly those values. Steve may have over-killed a bit, or not; sounds like he put that choke in there for a reason. It is surely possible to go through the math and invent a chokeless filter with acceptable voltage-loss and ample buzz-filtering... but why think that hard when a known-good plan is handy?