Fender Bassman 50 with Master Volume

[Cliff Schecht]

Are all of the (selenium?) rectifiers KNOWN to be good?

[xtian]

You've got the tone control leads for the normal channel kind of wrapped under and around all the B+ leads coming from the doghouse. You do not want to do that. The tone leads should be as far away from those noisy leads as possible.

OK. I'll rewire that now.

I also see that you've got higher than spec plate voltages, but you've set your negative bias voltage at spec. That might hum. You need to adjust the bias for the actual conditions, not to the schematic.

Would you expect the noise to change as you change the bias control? It does not.

[xtian]

Are all of the (selenium?) rectifiers KNOWN to be good?

Do you mean the diodes? I don't know. I think those are original (1974).

I have enough UF4007 diodes to replace them. Is this the right component? (OK, found other threads about 1N4007 vs. UF4007…so I'm going to put them in.)

[xtian]

Crap. Moved the bundle of tone stack wires, and replaced all the recto diodes. No freakin' difference. Same voltages (fine, I think) and same noise.

[Firestorm]

Hmmm. Well, since the Bass Channel was the one humming to begin with, you should probably verify that all grounds are good (jacks, pots, etc.) Since the hum is most pronounced somewhere in the middle of the volume pot's travel, you might want to try chopsticking the lead from the volume pot wiper and the leads between the pots; '60s lead dress has them all up against the chassis.

Not sure if it's related, but you have your screen supply cap grounded with preamp filters. It really belongs with the main filter ground.

[xtian]

Not sure if it's related, but you have your screen supply cap grounded with preamp filters. It really belongs with the main filter ground.

Help me with this. I thought the only screens in this amp are in the pentodes. All that power amp stuff is grounded on the end towards the bias pot. And all the preamp stuff is grounded near the input jacks of the respective two channels.

What are you looking at?

And thanks for the help. I'm following all the leads I can get.

EDIT: Oh, wow. I see what you're talking about, now that I'm looking at the power filter caps. There is no separate ground for the screens/PI B+ and the B+ for the pre amp section. Is this usually wired differently than I have it?

[xtian]

I can't find any modern looking examples of filter boards that are wired with more than two ground wires exiting the board. But I found one old one, attached, that clearly has more ground wires exiting from a third hole in the chassis.

[Firestorm]

On Fender Reverb amps, they grounded the screen filter with the preamps because they also used the screen supply to power the reverb transformer. It isn't really the right way. The screen supply has high ground currents which should be kept away from preamp supplies. Just disconnect the ground wire between the screen filter and the preamp filters and run an insulated wire from the screen cap cathode over to the ground on the main filter caps.

But as I said, it sounds like the Bass Channel hum predates the latest work, so I'd be looking for ground problems in that channel, along with lead dress issues.

[xtian]

There are two components to the noise I'm hearing. The first is simple 60Hz hum (and I mean EXACTLY 60 cycles, as detected by my iPhone app). This, I can work on with lead dress and grounding schemes, I think.

The second component is more overtly bad, because it's a buzz, probably at the same frequency (sounds like a squared off wave, like clipped hum). I made good progress on stomping this out by replacing the plain wire from pin 7 on V2 to the board, which was running overhead, with shielded cable.

There's still trouble, because as I wave my hand around the chassis, it is interfering with some magnetic field that is inducing the buzz. If I hold my hand just so, I can almost completely cancel the buzz!

A different problem is that when I open the volume on the Bass channel, it's just too damn loud, as evidenced by the WHOOSH of white noise. Really sounds like it needs to be tamed. Options?

[Firestorm]

I probably would have left the PI filter grounded with the preamps, but the PI will tend to phase cancel ground current hum, so OK.

There's still trouble, because as I wave my hand around the chassis, it is interfering with some magnetic field that is inducing the buzz. If I hold my hand just so, I can almost completely cancel the buzz!

That suggests you have a sensitive lead(s) that should be rerouted or shielded. Probably a grid wire. You should be able to isolate the stage it's coming from by bypassing the plate resistor of each stage. Tone cap leads are especially suspect. Sometimes I use shielded quad cable to replace all the tone stack leads. YMMV.

A different problem is that when I open the volume on the Bass channel, it's just too damn loud, as evidenced by the WHOOSH of white noise. Really sounds like it needs to be tamed. Options?

Most often, white noise will be a tube or a resistor. The 220K in front of the treble pot could be noisy. Actually, you might want to lose that and just use a 250K pot. Also check the pot itself (50K?); in that tone stack, the treble pot is almost like a second volume control; it it's noisy, you'll hear it. If you reused any 100K plate resistors from the previous circuit, change 'em. And try rolling some tubes to find a quiet one. If the Bass Channel is too gainy, you've got an easy fix: change the cathode resistor on the third stage until you find one that does what you want. It's 1K5 now: try 2K7, 3K3, 4K7.

[Structo]

Guys, shouldn't the heater hum balance pot have the two heater wires to the outside lugs and the center lug should go to ground?

He said he has the HT center tap on the middle lug.

[xtian]

Guys, shouldn't the heater hum balance pot have the two heater wires to the outside lugs and the center lug should go to ground?

That's the way it is wired.

He said he has the HT center tap on the middle lug.

My HT center tap (red/yellow) is grounded. My PT does not appear to have a heater center tap.

[Gibsonman63]

There's still trouble, because as I wave my hand around the chassis, it is interfering with some magnetic field that is inducing the buzz. If I hold my hand just so, I can almost completely cancel the buzz!

It sounds like you have the amp chassis on your bench out of the cabinet. Do you have a flourescent workbench lamp turned on or a CRT monitor close to it? I have been guilty of this one myself. You may try sliding it back into the cabinet. There should be an aluminum shield stapled to the top inside of the cabinet.

[xtian]

My voltage at the power tube plates is 450vdc, and the recommended value is 420--mine is about 7% higher. So should I raise the recommended -44vdc bias voltage to -41?

@Firestorm: I replaced the third stage cathode resistor with 4K7, and this took a lot of the buzz out. Still plenty of gain available.

My heater wiring is suspect. See where it doubles back to get to the balance pot? That's the place, if I hover a metal tool in that area, that it's easiest to silence the buzz--directly halfway between the V1 and V2 tubes. Then, floating my tool down along the path of the heater wiring, I hear node, antinode, node, antinode, as I pass each tube.

Anyway, time to put it back in the head cab and play it a little.

[Gibsonman63]

You may check you heater wiring. It should have the same polarity from power tube to power tube. Fender didn't pay attention to this, so you have a 50% chance that it is wired correctly. That will knock down a bit more hum, but this would affect both channels and not just the one.

Balance pot on the heaters?