"Fiddy" 50-watt build log

[xtian]

Marshall 1987. This is the second build, both detailed in this thread.

[Cameron]

Finished the build. Voltages look good; a bit lower than my last build. Plates are at 402v and bias is set to 38ma (about 60%). Sounds great, but has too much hum when you turn up the volume much. Chopsticking didn't reveal anything.

Gonna rest my brain for a while.

Try moving the grounds for the first tube ...to the input jack ground.

[xtian]

Well, ground-loop hum is getting me down. I had a small success early when I moved the star ground from it's own post at the lower left to the tag strip where the two center-taps are grounded to the chassis. That improved the hum on the master volume to within tolerance.

But I still have a lot of hum on the volume control end of the two channels, confirmed by grounding the input of the PI, and observing that the hum is introduced pre-PI. (I also swapped in new preamp tubes to eliminate a faulty 12ax7.)

I then took about a dozen steps, and nothing helped in the least. I moved ground leads around for the three multisection caps, the ground buss for the controls, moved the star ground to the input jacks and lifted the center-tap tag strip (you can see a nylon nut there now), moved the ground for the bias voltage around, chopsticked wires each time…NO change in hum from the pre-amp sections.

Oh, one more thing. In earlier builds this year, I had been patting myself on the back for placing the PT and OT by using the headphone trick. And for this build, I did the same, choosing a slightly off-axis position that reduced hum in the headphones. But now, I loosened the nuts, and while the amp was running, I rotated the PT and OT into the position that had caused the worst hum in the headphones--NO CHANGE in hum! So my confidence in the headphone trick, or rather, how hum is introduced into the circuit by the PT, is shaken.

Since I've eliminated easy solutions, I think my next step will be to remove my fancy parallel/cascaded input switching system and simplify the input wiring.

Any suggestions are welcome.

[Colossal]

Since I've eliminated easy solutions, I think my next step will be to remove my fancy parallel/cascaded input switching system and simplify the input wiring.

Any suggestions are welcome.

Before you do that, you might consider separating the the bias circuit ground off the common preamp ground buss and over to the common single star ground point on its own. Also, move the black ground wire for the middle cap can off of the buss wire above the pots and straight over to the common star ground point.

[xtian]

you might consider separating the the bias circuit ground off the common preamp ground buss and over to the common single star ground point on its own.

This was actually my thought, too! Aw, OK. I'll give you full credit. Yes! This is a major improvement!!

Also, move the black ground wire for the middle cap can off of the buss wire above the pots and straight over to the common star ground point.

Did this next. No change.

Much happier overall, though. Thanks!

[xtian]

Pair o' Fiddys.

[Reeltarded]

Good deal! Crank it up and make a clip!

[xtian]

Got the front panel done. Faceplate arrives in a week. Lookin' cookin' !!

[Structo]

Is that Plexiglas?

Clear or blue?

[xtian]

White, frosted plexiglass. Had to slap my head hard last night, because I realized I cut the opening the size of the chassis, not the size of the faceplate! D'oh!

[Reeltarded]

You and me.

Measure twice, cut once. Then, measure the right thing and cut again.

[Structo]

Gawd I hate when I do that!

[xtian]

Right?!? Well, gonna solve that by cutting my first router template.

[xtian]

There, that's better. Much nicer with the MDF router template.

No, I didn't mismeasure again…that's my 1/2 scale faceplate.

[xtian]

Finished Fiddy BKB glamour shots.

Took it off to consign at the local guitar shop.