5E7 Clone - Les miserables

[fdesalvo]

Hello, all.

I just completed this build. All voltages are within ~5% of the schematic. I'm getting a sharp buzz that is not effected by the volume controls and it persists with or without anything plugged into the inputs. A side note - the bass control introduces treble and the treble control introduces bass..is this some quirk? All wiring is per the fender schematic, save the filaments, which are twisted and routed into the corner of the chassis, well away from the other wires. My grounding scheme is a'la Hoffman, which has proven to be very quiet in times past.

Problem persists with v1/2 pulled. Puling PI resolves noise, with only a very slight hum in the background. extremely slight.

Clueless.

[TomCarlos]

Frank... was this a kit or something you built from scratch? Just curious because I was repairing a Gibson amp and failed to ground a lead on the output transformer secondary. That was causing a loud "squeal." So I don't know if a squeal is the same as a buzz. But check that out.

[fdesalvo]

Hey, Tom. Good evening.

This is a scratch build. The last scratch build I did had an an763 preamp mated to a cathode biased pair of 6l6s and she fired right up without issue, but this one has me chasing my tail.

I've taken the input jacks out of the equation and can tell you the ot is good to go. Thanks for swimming by my man.

[DRH1958]

It seems as though the problem lies in the tone controls since pulling V1,2 don't help but pulling V3, where the cathodyne PI is the second half, stops it. This indicates to me the problem lies between these two tubes which is where the tone controls are. Plus, the stack controls operate backward, bass increases treble,etc., so something amiss there. I would re-check that wiring as it's a bit of a different animal than a lot of tone controls.

Fender_bandmaster_5e7_schem.pdf

[fdesalvo]

Thank you, DRH.

My fear is that I'll have to yank the board. The presence control works as expected at least, haha. That's what I get for soldering for 6 hours straight. I need to take breaks.

Best,

[DRH1958]

Not exactly sure of how it's built but maybe a quick check is to see if you can identify that the wiper of the treble control goes directly to the grid if the stage preceding the cathodyne PI. Possibly on the wrong side of that 220k resistor or something? Also to be sure the treble control has a cap on tabs 1 and 3, between the wiper of the pot.
If this doesn't fix the buzz, maybe for the tone function fix, just re-label the controls if they work properly. No reason to re-wire all of it, just re-label them if the sweep of the pots do the correct control of tone. Possibly just got the labeling mixed up?

[fdesalvo]

HA!

It just occurred to me - to check the 100Rs on the heater string. Yep - I put 100Ks there. Will swap these out tonight and report back. Thanks for the advice so far.