1970 Silverface Bassman - Hum in Bass Channel only

[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

[pompeiisneaks]

Have you tried swapping out V1? Chopsticked the crap outta the area? Do you have a scope to see where the hum is arriving?

~Phil

[Stevem]

To sus this out better what hum do you have , 60 HZ from the heater, or 120 HZ from the power supply.
Is the hum a lower pitch then your low E string?
Also how much AC ripple do you have on pins 1 and 6 of V1 as compared to V2?

If you place a short across either of V1s plate resistors you will kill the gain from that section of the tube and then maybe narrow down better where it's coming from.
Another simple test is to short pin 2 and 7 to ground.

Also try cleaning the switch contacts on both input jacks by running some 400 grit sand paper thru it or a strip of thin card board like a business card .

That tube socket may be conducting , as in its bad also!

[Saransk]

It is definitely 60 Hz

Well the tube socket is new
The "doghouse" caps are new
The amp is extremely quiet with tube V1 (Bass channel) pulled
New set of input jacks - all match the quiet Normal channel
Ground plate not "floating"
Voltages within 5% of schematic on V1 - both cathodes especially good.

"Chopsticked" the wiring - no change in hum - especially the grounds seemed to be really solid.
But a bit microphonic - tap a wire got a noise through the speaker.
But could a "Film" cap be microphonic.
Did get some loud noise when I checked the voltages at the tube socket - quiet on the B+ side of the plate resistors (new) but a loud noise on the tube side.
I think these is partially due to the lack of more coupling caps. - it becomes essentially a hum injection
Cannot find any "Smoking Gun" as to why there is loud hum in V1 (Bass) but none in V3 (Normal) through the output.
It matches the Normal circuitry, the heater wiring is twisted tight, raised, and is on the same pin(s) as every other preamp tube.
I've not grounded pin 7, but if that cuts the hum, now what?
And why does the hum go away when the volume control is turned all the way up - (to 10)

[Lauri]

And why does the hum go away when the volume control is turned all the way up - (to 10)

When you turn the volume control all the way up the hum from first half of V1 is just the right amplitude to cancel itself out when combined with the hum from second half of V1.

Try a different tube at V1. Put new cathode bypass caps if you haven't already.

[Stevem]

What level of D.C. Voltage if any do you read on the input grid of V1b on that bass channel?
It would not be the first time that a Fender oil board started to conduct D.C. Voltage!

When this takes place you can move all the tone stack components onto the channel pots themselves to bypass the board terminals.
This also makes for a better sounding amp since your shortening up the grid wires by a bunch!

[Lynxtrap]

Do check (again) all connections to ground, and also measure resistance between chassis and every component/solder joint to make sure you have no connection where you shouldn't. With the amp turned off.
Also double check that all resistors and filter caps are connected as they should. Fenders can sometimes be confusing with the series caps and placement of the dropping resistors.

[Stevem]

Try this even if just for kicks!
Put a separate ground on the last filter for the preamp section and land that new ground wire back at the PT filament ground , or crimp a ring lug on it and bolt it down to one of the PT mounting bolts.

Note that many Fender amps do not have enough stages of filtering , as anything more then 2 preamp gain states on one filter is asking for de- coupling issues!

[Lynxtrap]

...and a different idea to try if nothing else helps: Place a separate filter cap for V1 locally on the board, grounding with the cathode resistors/caps for the same tube.

[Firestorm]

New jacks? Make VERY sure that you've wired them correctly. Bad ground reference on the input grid will hum a lot. I stared at the inputs on a '54 Twin for days before realizing what I had screwed up.

[Bombacaototal]

I am wondering if this issue was resolved and what was the resolution since I am having a similar issue.

On my amp Channel 1 is extremely quiet even with volume at maximum. Channel 2 has a constant hum which is present at the same loudness regardless of where the volume pot is set (zero or maximum). I attach below a recording of the hum. To my ears it is a low frequency (60Hz or lower) and for reference I am in the UK (London).

If I disconnect both channels and leave only the power section the amp is very quiet too, and therefore it is fair to conclude that power tubes, rectifier, transformers, etc are all fine.

I have tried the following:
1) replace 12ax7
2) checked and re soldered every single connection of the pre amp paying extra attention to any grounds
3) chopstick poked everywhere without finding anomalies

Each channel is using independent supply caps from the same cap can (which shares a common ground) from CE Manufacturing and I am pretty sure this is not the source of the hum.

Voltages are fairly similar on both channels

My grounding scheme (similar to dumble) is as follows:
Long buss bar across both pre amps and input jacks with isolation washers, with the bar, jacks and pre amp supply cap can grounded at a star washer. Power Tubes and PI are grounded together on another point and PT and the large value supply caps (220uF) are grounded on another star washer.

I noticed Hofmann has 2 different cross bars for each pre amp and grounds them separately at the jacks. My jacks are isolated but I am wondering if I should chop the buss bar and create a second star washer point?

Any other ideas or suggestions

[Stevem]

On the second channel preamp tube do this check.
One at a time short pin 2 and pin 7 to ground and report back on the results.

[Bombacaototal]

On the second channel preamp tube do this check.
One at a time short pin 2 and pin 7 to ground and report back on the results.

Thanks Stevem.

Yesterday I shorted pin 7 (second triode at the end of the tonestack ) by adding an alligator clip to the pin and the other end to the side of the chassis and the channel went dead quiet. As soon as I unplugged the chassis end of the alligator clip the hum was even louder.

I did not try pin 2 but I can do it as soon as I am home.

[Stevem]

Yes, that result when you removed the ground end of the jumper is normal.
If grounding out pin 2 when you get home then kills the hum then there's a issue with that channels input jack wiring, or the number two input jacks built in shorting switch needs a cleaning .

With nothing plugged into any of the amps input jacks a normal good jack should read a short across them until you plug a cable in.

[Bombacaototal]

Thanks for the help Stevem. I have just tried and grounding Pin2 makes little difference on the hum unfortunately