Bassman AA864 hum trouble

[sliberty]

I've built a one channel version (Bass channel only) of the AA864 Fender Bassman. I am in the debugging phase now, and have a great deal of hum. The hum seems to have both 60hZ and 120hZ components to it, so it is not clear as to the cause.

I've eliminated the deep switch for now, and although my plan is to eventually have a mid pot and a treble boost switch, neither is in the circuit at this time. Also, since there is one channel only, I have the second 220K mixing resistor going to ground.

In my troubleshooting, what I have done is to pull the small tubes from back to front, and the hum goes away regardless of which tube I pull. This seems to tell me that V1 is where the hum is because if the hum were in any other position, pulling tubes in front of that position should have no impact. To further confirm this result, when I lifted the .1uF coupling cap that connects V1b's plate to the 220K resistor that ultimately leads to V2b's grid (or just grounded at the plate side of that cap), the noise goes away totally. Frankly the amp is quite silent at that point.

So, first dumb question. Does this definitely mean that the hum is entering in or around V1 and the tone stack?

Note, another troubleshooting step I took was to leave that .1uF cap connected, but to ground the plate of V1a. This made very little difference in the hum, leading me to believe that V1a is not contributing much hum, and neither are the input jacks as they sit in front of V1a.

So second dumb question - am I correct that this further narrows the hunt to the tone stack and V1b?

Assuming that the problem is definitely in that area, I am wondering if my problem could be related to placement of my bias pot or of the OT with respect to the preamp circuit? My bias pot is fairly close to the treble pot. On the other hand, Fender's layout put the bias pot near the tone stack of the normal channel, so maybe that is totally safe???? As for the OT, it is under the middle of my preamp board, which puts it somewhat close to the cathode components (caps and resistors) of V1. Could that be a problem?

I'll post some photos soon. As I have been trouble shooting, the flying leads are starting to look a little worse for wear, but I will replace wires later after the amp is working properly.

Any and all help is welcome - this is very frustrating as you can imagine.

bassman_aa864_schem - bass channel only.pdf

[loudthud]

Most problems of this nature are caused by improper grounding. The filter cap marked 20@50V (sic) should have it's ground separated from the other caps. It should be grounded near the preamp.

[sliberty]

Thanks for the reply.

I have the caps grounded as follows:

Pair of 80uF caps (main filter) grounded at the same place as the various power supply grounds.
Second filter (20uF on the other side of the choke) is grounded near the power tubes.
Third filter (20uF connected to the PI) is grounded near V3 (the PI tube).
Fourth filter (20uF connected to V2) is grounded near V2.

V1 doesn't have it's own filter - it is essentially shared with V2. So I would expect that if the filter grounding was an issue, it would be introducing symptoms into both V1 and V2, but based upon my testing, it seems like the hum is exclusively in V1 -- unless my assessment is wrong.

As for the cap you referred to as 20@50V -- did you mean the bias filter cap (25uF)? This cap is grounded right to the bias pot back, and there is good continuity to the chassis. Just wondering - why would you ground the bias filter near the preamp? The bias supply is really related to the power tubes, so I would have thought if I were going to ground near anything specific, it would be the power tubes. Let me know as I may learn something new from this.

[g1]

Not the bias cap, but the one you are calling the fourth filter. It is incorrectly labelled as 50V on your schematic.
As you said, it is grounded near V2.
Have you tried various tubes in the V1 position?

[Enzo]

I will believe you have localized it to V1, sure. Now which half? Does turning the volume control to zero kill the noise or at least turn it way down? Or does it not affect it? Alternatively we could just ground the grid of V1b, but really, just turning the know sounds easier.

The volume and tone controls have ground conections, so where are they grounded? Did you run a wire to ground or did you just solder the ground legs to the back of the pot and hope the mounting hut makes a good ground?

Hum aside, is the amp strong and good sounding?

I doubt it has anything to do with your hum, but if you had the 220k mixing resistor from the "other channel" just sitting there< (And it has been removed from the schematic, right?) I wouldn;t have grounded it, I;'d just remove it. Look at the schematic, from the output of the channel yo0u made there is a 220k to the PI. If ther is another 220k to ground from there, that forms a voltage divider that cuts your signal level in half. Not only that it also completes a voltage divider with the 100k plate resistor. I think it will knock down the B+ for that triode about 20%.

[mac dillard]

Sure seems that you have a lot of ground points. I would look to have three ground points at the most. One for the mains ground, one for the power amp grounds and one for the pre amp grounds (right at the input jack)

[sliberty]

Thanks all for the posts. I appreciate your time in trying to help here. Keep it coming.

My bad, I forgot that the schem I posted had that type about the last filter cap. Well, as mentioned it is grounded near V2.

I also forgot that the schem is missing the 220K resistor to ground that would have been a mixing resistor for the second channel. As for signal loss, I figured I would assess that later and see how I preferred it. I'd been given the advise that if you want the bass channel to sound the way to does when the Normal channel is in place, you need that voltage divider -- but that remains to be seen. I've tried it with and without this resistor - still hums.

I have tried different V1 tubes with no benefit.

Grounds for the pots are soldered to the back of the pot. I do understand that this only works if there is good contact with the chassis, but based upon my meter readings, there is.

The volume control does not eliminate the big hum. So, that seems to indicate that it is not in the tone stack, right? It has to be in V1b itself?

I'll have to play the amp again and get back to you regarding how strong and good it sounds. I completed the amp 2 weeks ago and then got busy at work, so I just don't remember.

Mac - your suggestion about reducing the number of grounds would only eliminate one ground - the PI I suppose would move to be near the power tubes, the mains would be separated from the rest of the power supply grounds, and I would relocate the preamp ground to the input jack rather than near V2. Given that the symptom seems to be localized to V1b, I don't see how the PI change would matter, but I will try moving the preamp ground to the input jack and see if there is any impact.

[sliberty]

Prologue.

I redid the grounding completely on the amp. A friend suggested the following grounding scheme:

Place a lug close to the input of the amp. Connect all of the following to that lug:
All input jack grounds
All preamp grounds
All tone stack / volume pot grounds
Filter cap that is supplying the preamp
*
Place a lug close to the A/C wire entry point into the chassis. This is usually diagonally opposite from the input jacks. Connect all of the following to that lug:
HV center tap
Heaters faux center tap* (shrink wrap and do not use the real CT if it exists)
Bias circuit ground
Main filter caps

Place another lug close to the A/C wire entry point into the chassis. This should be separate from the lug above. Connect the following to that lug:
A/C ground wire
*
Place a lug in between the power tubes. Connect all of the following to that lug:
Power tube cathodes
Filter cap supplying the power tube screens
*
Connect all of the following to the ground lug on the output jack:
Filter cap supplying the PI plates
Presence pot ground


The amp is nice and quiet now. Obviously the old grounding scheme was creating ground loops.

Thanks for all of the suggestions.
Steve