Bassman Head Silverface Hum

[misterc57]

This amp looks like it has been completely gone over with perhaps every cap replaced and some resistors.

But it has two main issues. There is a hum at idle (I think it is at 120 hz). Volume at zero. Also any light finger tapping of any section of the chassis produces a thump into the speaker.

It had one dead output tube when I got it. I swapped in a pair of working 6L6. Voltages at the output tubes look good.

In other pics or videos of this model amp I notice that the brass (I think it is brass) front control plate (where all the pots and jacks connect) is soldered to the chassis. This one does not have that, instead there is a wire going from the control plate to the turret board at two points.

Looking for suggestions on where to start. Thank you, MC

[Leo_Gnardo]

There is a hum at idle (I think it is at 120 hz). Volume at zero. Also any light finger tapping of any section of the chassis produces a thump into the speaker.

There's some possibility these may be related. Maybe not. I'd pursue the thump source first, tapping around with a chopstick. It could even be a piece of wire, I've seen that happen. As you say, all the caps have been replaced. Looks like Mallory 150's, a favorite in my workshop. I would not expect a new one to make a racket, but don't rule it out.

I think Fender expected the clamping action of all those pot nuts would provide sufficient conduction between the brass ground bus plate & chassis. It wouldn't hurt to make sure by soldering that plate to chassis with a big iron, or at least running a lead from it to a ground terminal.

Hum might arise from a pre tube. Remove & replace, one at a time. I'm sure you've been thru the drill. If that brings no joy, we must look further.

All the power filter caps have been replaced, presumably not too long ago. Just because they're new, don't assume all are good. Or that the previous tech made all good solder joints. Look carefully with a bright light, poke & probe around, maybe you'll find a dry or cold connection.

Note that each filter cap stage has a parallel film capacitor, an idea borrowed from some high falutin' hi fi guru no doubt. It's a belt & suspenders method, in case you don't trust the filtering ability of your electrolytic capacitors at very high frequencies.

Even though your output tubes are new, they may be far enough out of match with each other that hum crops up in the output signal. Quick test would be to pull the drive tube & if you still have hum then check bias currents on your output tubes. I see this Bassman has dual biasing - 150 ohms from each cathode to ground, and negative voltage on the grids as well. One thing I don't see, and maybe it doesn't much matter anyway, amps with this bias method had a capacitor wired from one output tube cathode to the other. If my memory's correct, it was 5 uF 50V non-polar electrolytic. If you think this is a necessary part let me suggest upgrading it with a film cap, the kind that might be used in a speaker crossover. You might find these with higher voltage ratings, no harm to use one of those.

[52 Bill]

The output section has been partially changed from the original 1968 circuit to a sort of blackface version. The one thing that I noticed was that the two resistors that feed the bias voltage to the grids are of different values and the big cathode resistors are still in circuit. I would think that you would want to build it as one circuit or the other, not a sort of meld of the two.

[misterc57]

Thank you for the suggestions so far. I have a few things to try and check out.

I am not familiar with the differences or advantage of the blackface versus this amp's original circuit.

Seems that a lot of folks want to "blackface" their amps. I am not sure why. If I go that route, what would be the schematic to follow. I will see if I can find some articles on this subject.

I agree about not blending circuits and choosing one. Can you tell by examining the pictures, which circuit (original vs BF) would be the path of least modifications from what is currently there? Perhaps that would be a good starting point.

[g1]

Some of those solder joints to chassis look kind of blobby. Like where the two 100R resistors for the artificial center-tap (off the pilot lamp) connect to chassis.
Give those blobs a yank with some pliers to verify the connection is solid.

[Leo_Gnardo]

Thank you for the suggestions so far. I have a few things to try and check out.

I am not familiar with the differences or advantage of the blackface versus this amp's original circuit.

Seems that a lot of folks want to "blackface" their amps. I am not sure why. If I go that route, what would be the schematic to follow. I will see if I can find some articles on this subject.

I agree about not blending circuits and choosing one. Can you tell by examining the pictures, which circuit (original vs BF) would be the path of least modifications from what is currently there? Perhaps that would be a good starting point.

I'd start with eliminating those cement block cathode resistors, maybe substitute a pair of 1% 1 ohm resistors so you can conveniently measure bias current instead. Then you'll have to re scale the bias voltage selection, no biggie that's just one resistor from the bias pot to ground. Might have to try a couple values until you can select bias voltage from say -45 to -55V. Also use a matched pair of resistors at the bias distribution point. 52 Bill noted your amp has 2 different values, they look to me 100K and 220K. Fender typically uses 220K so go with that.

[misterc57]

Still processing all this info. I am not sure which schematic to use. Attached is the one I have found. Also below are the voltage readings I took at the output tubes, and both tubes were within 1% identical readings. Amp is connected to an isolation variac.

6L6 pin 1 : -31.5
6L6 pin 2:
6L6 pin 3: 412
6L6 pin 4: 413
6L6 pin 5: -31.5
6L6 pin 6: 412
6L6 pin 7:
6L6 pin 8:

More info...

The blobby solder joints to chassis are solid.

Bias distribution point resistors both measure 100K. We are talking about the two older style resistors at the top of the below photo, right?





Fender-Bassman-50-Schematic.pdf

[misterc57]

For measuring current what wattage should those 1% 1 ohm resistors be? I will need to order some.

[Helmholtz]

As the tubes already have cathode resistors, you can use the cathode voltages (pin 8) to determine idle currents. Just divide cathode voltage by the (measured) resistance value between cathode and ground. Both cathode voltages should be identical for least ripple hum.

If you use 1R resistors, they won't see more than 0.25W.

[misterc57]

Progress

Pulled the 12AT7 driver tube. No hum. Replaced V1, hum is gone. Playing the amp is okay now.

Measured the following current at pin 8 of the 6L6's.

6.79 VDC / 147.6 ohms = .046
6.66 VDC / 149.6 ohms = .045

Still have thumping noise when lightly tapping the chassis. It is now most apparent at V1 and components on the turret board near V1. Touching the wire going from the board to V1 pin 1, is the most sensitive.

[Leo_Gnardo]

Bias distribution point resistors both measure 100K. We are talking about the two older style resistors at the top of the below photo, right?

With the super close up photo, yes I can see they're both 100K. And you measured them besides. Extra points for that.

Should you wish to get a smidgen more hi frequency response, could change them to 220K. But, it is a BASS amp, and I'm sure plenty bright as-is. Nothing further needs to be done there.

Although we don't expect 1 ohm current sense resistors to have to withstand more than a mere fraction of a watt, it's my practice to use 3 watt resistors for this function. We've had discussions on this here in the past, hundreds of entries. Some folks install low wattage resistors and have reported they mysteriously go open-circuit. Unlike some, I don't consider the current sense resistors to have a dual purpose, that is, to also act as overcurrent fuses should an output tube short. FWIW here's what I use: 1 ohm 1% 3W WW resistors - Mouser has 'em $1.46 each, 71-RS02B1R000FE70

Of course you can skip those current sense resistors entirely and measure bias current through the handy "Voltage-on-each-OT-primary divided by OT-primary-resistance" method. A valuable lesson I learned here on MEF.

I see you found the hum culprit, what a relief!

[Helmholtz]

Should you wish to get a smidgen more hi frequency response, could change them to 220K

I can't think of a reason why the value of those resistors should influence treble response.
Rather lower values will roll off some bass and somewhat reduce available grid drive by loading down the PI.
Removing the 2nF grid-to-cathode caps, OTOH, might result in a little more treble. But they may be necessary for stability.

[misterc57]

Update

Not so much the heaters but every other wire connected to V1 makes awful noise when touched with a wood chop stick. Every component connected to those wires at the turret board also makes noise when tapped or touched with a wood chop stick. The noise is worse at the tube wires/pins then at the turret board.

I know the amusing solution is to remove the wooden chop stick from the amp.

I reflowed the 6 (non heater) wires at both V1 and at the turret board. No difference. I retention all V1 pins and sprayed them with cleaner. No difference. I am beginning to suspect the tube socket.

This was odd. At one point in my many power ON of the amp, the hum was back, got louder as the amp warmed up and then after full warm up faded away and disappeared.

V1 voltages seem good...

Pin : VDC
1 : 196
2 :
3 : 1.53
4:
5:
6 : 205
7:
8 : 1.55
9:

[Leo_Gnardo]

I can't think of a reason why the value of those resistors should influence treble response.
Rather lower values will roll off some bass and somewhat reduce available grid drive by loading down the PI.
Removing the 2nF grid-to-cathode caps, OTOH, might result in a little more treble. But they may be necessary for stability.

Right you are and thanks for the correction. A quick back-of-the-envelope calculation shows the corner frequency is subsonic, and it may actually be a good thing to select 100K resistors here, which would help roll off super low frequency "fluff".

When I encounter those 2 nF (0.002 uF disc caps) I try with, and without, looking on the 'scope for ultrasonic "chirps" while sweeping low frequencies played at higher volumes through a loudspeaker. The speaker is important - I seldom if ever see such ultrasonic artifacts when driving a resistor load. In any case, if I encounter this problem, the caps go back in circuit. Then I do a final test, playing an actual bass, and using JBL speakers so any hi frequency racket will be revealed to the ear. Also while monitoring the output on the 'scope. Honestly, I can't hear any difference in treble response with or without the 2 nF caps. Maybe bats and dogs can, but none of them ever brought me an amp to fix.

[Helmholtz]

Also while monitoring the output on the 'scope. Honestly, I can't hear any difference in treble response with or without the 2 nF caps.

Changes to frequency response within the feedback loop will be mitigated by the global NFB, which tends to equalize the response. That's until the power amp clips.