Silverface Bassman - Hum in Bass Channel only - Stumped

[Saransk]

Resurrecting a 1970 Silverface Bassman
New caps, new plate resistors, new Classic Tone Power Transformer, etc.
Power amp redone as an AA165/AA864 - Preamp AB165
Everything works well except.... Hum in the Bass Channel - doesn't change with plugging guitar in or increasing volume.
Hum Disappears if V1 is pulled, and I have to turn the Normal Volume up to past 8 to get any real noise/hum.
With V1 in, hum doesn't change with volume until Volume is turned up all the way then it goes away.

Checked grounds - okay. resistance matches both channels. swapped tubes around - no change.
Preamp grounds attached to the brass grounding shield - Transformer (HV/Heater) & power grounds to one-point.
I "moved" the Cathode Resistor/Capacitor for V1 2nd half from the standard location, with its long wire, to the space near the V1 1st half which shortens the wire from pin 8.
At this point, considering both channels are wired the same, I have no idea why the Normal channel has no hum and the Bass Channel hums.  I thought it might be the volume control not grounding and the grid (pin 7) is floating - but the ground seems okay.

Not sure where to go next - don't want to have to do the whole "star" ground, especially since one channel is fine and the other isn't.
Problem is in only one channel
Any ideas really welcome

[Willabe]

The input jacks might not be getting a good ground. 

Might be loose, might be corroded. Try taking the bass channel jacks out, then clean the jack ground sleeve, jack star washer, and the area where they get bolted in and make contact with the brass plate/chassis. Then reinstall them.

Also check solder connections, 68K and 1M R's ground.

And make sure the jack switch is closing/working on the bass channel 2 jack. If that switch stays open, could be your problem. And/or, if the switch contacts are corroded that could cause a problem, slide a strip of sand paper between the contacts to clean the contacts.   

Better now?   

[mresistor]

I would use emery cloth to clean the contacts.. 

[sluckey]

Power amp redone as an AA165/AA864 - Preamp AB165

Recheck your mod work.

[Saransk]

Brand new switchcraft jacks -(Fender® Input Jack Replacement Kit for two channel amplifiers from Studio Sound Electronics)
Both channels wired the same - grounded through brass grounding piece. 
Jack#1 - 1 Meg connected to A - Ground & Switch and B - Tip/Out
              68K connected from B - Tip/Out and wire to Pin 2 V1
Jack#2 - Switch Connection connected to Out on Jack #1
              68K connected from Tip/Out to wire to Pin 2 V1 (Connected to Jack #1 68K)
Like found here:http://el34world.com/charts/CommonHookups.htm

As for the Output section - no hum or other issues - The only hum is found in the Bass preamp - V1

[John]

Hum Disappears if V1 is pulled, and I have to turn the Normal Volume up to past 8 to get any real noise/hum.With V1 in, hum doesn't change with volume until Volume is turned up all the way then it goes away.

Try clipping in a 470k resistor from grid to ground, using clip leads. See if that makes a difference.

[Saransk]

I suspect I should check both grids with the first check on pin 7.
If the Hum goes away when clipped to pin 7 - bad Volume control?
If Hum goes away clipped to pin 2 - something not right in the input wiring?

Also - I noticed in one photo there is a 220K resistor from the "deep" capacitor's connection to the "Deep" switch to the Ground connection of V1 pin 3

[mresistor]

Is it 60hz hum or 120hz hum?  If 60hz it would relate to filaments..  maybe a bad tube socket or something letting filament voltage leak or be induced to the other elements of the tube.. if 120hz hum then that's a different story.. Possibly a ground loop? You can go to Youtube and listen to the difference between 60hz and 120hz tones..

[Saransk]

It is definitely 60 Hz
But I can't figure out why only one channel
And when I turn the Volume for the BASS channel past 8 - the hum goes away.
The Treble/Bass controls don't affect the hum.
When the Volume control is at minimum resistance, the grid resistance at pin 7 is around 500K  - the Bass & Treble Controls plus the 8.2K resistor at the end.  This become the equivalent of a 470K load on the 2nd triode grid.  When the Volume Control is at the max resistance the grid Resistance is about 1/3 or 167K
the problem is everything in the Bass Channel measures just like the Normal Channel, except the Bass Channel hums.
The only thing I can think of is the 4 grounds in the circuit - the two cathode grounds, the tone stack ground, and the volume ground.
While they read as good grounds at this point I'm kind of guessing.

[PRR]

> 4 grounds in the circuit - the two cathode grounds, the tone stack ground, and the volume ground.

Why??

These should all be the same wire. Not scattered points on the chassis.

[sluckey]

Power amp redone as an AA165/AA864 - Preamp AB165

Recheck your mod work.

You modified the preamp to be an AB165 preamp. I'm still thinking you have a wiring error.

[Saransk]

There are 4 separate grounding points in the layout.
1 - V1 Cathode (pin 3) - grounds at the far right eyelet
2 - V1 Cathode (pin 8) - grounds at the same eyelet as V2 (pin 8) - I have tried moving this to shorten the cathode lead and grounding it with V1 (pin 3)
3 - Volume Control  - grounds to itself and to the ground shield through its case
4 - The Tone stack - grounds the 8.2K resistor to the Bass control case and to the ground shield through the case.
Fender never used the classic "star" grounding set up.  While the Power Transformer winding "grounds" were separate, and the ground from the first Capacitor Node were grounded together, all the rest of the grounding was done using the full chassis ground plane - that big brass shield.  There were at least 5 ground wires attached to my amp - no counting the controls and jacks.

Most, if not all, Silverface Bassman used the same eyelet board - Fender just rerouted the power amp feedback, the Phase Inverter connections, and the "mixing" or the two preamp stages.  If you look at the AA371/AB165/AA165 layouts you'll see that the eyelets are in the same place. - That's why it is easy to "Blackface" a Bassman power amp and get rid of the weird feedback loops, the Bias circuit, and PI design.  As for the AB165 preamp it was just the same as the AA371. And there are only 10 components

[PRR]

Are you saying that grounding to various eyelets and pot bodies is better than how Fender did it?

[derekiseric]

Not sure if you tried this already, but on my Deluxe Reverb I took the ground wires fender had used on the brass plate away and used the Hoffman pot ground with bus wire and that cleared up some strange noises I had when it was stock.