1965 Bandmaster hum

[Bombacaototal]

I recently purchased a 1965 AB673 Bandmaster. The amp had been previously served which included a full recap (supply and cathode), plates on stack changed to 1W MF (I think) and some of the PI resistors updated. The bias electrolytic cap had not been changed but I replaced it by an F&T.

The issue I am having with the amp is a 60HZ hum, which as fas a signal to noise is negligible but one not playing it is evident.

I have replaced power tubes to TAD 6L6WGC and biased the amp. I have checked and replaced all pre amp tubes and these are fine, not the issue.

Hum happens with both volumes and vibrato controls set to zero. So I think it's probably between the stack recovery and output.

Is this potentially a ground loop? Or could it be a cold solder, although if it was the latter I'm not sure I'd have signal

[pdf64]

Is the heater circuit balanced with respect to the chassis?

Starting at V1, what valves need pulling to stop the hum?

[Stevem]

Let’s narrow things down more here.

Open up the dog house and unsolder / lift up the down stream end of the 1K resistor, the end that does not connect to the choke.

This will unpower all the preamp tubes and take them out of the equation.

If your heater center tap off of the PT ( green yellow wire ) is making a good ground connection then there are only a few other possibilities for your 60 hz hum condition.
With your ohm meter check the continuity from each leg of the heater winding to ground to confirm this.

1) one of the new filters is bad.

2) the output tubes are not matched close enough.

3) the output tubes are biased too hot and at the edge of the plates starting to glow, or are glowing.
What is the procedure your using to check your output tube bias?
4) the bias filter is bad,

5) if this hum takes place even when you first turn the amp on and while it’s in standby, then the power transformers magnetic flux is coupling into the OT ( unlikely with them being so far apart) or the power transformers coil core is loose, or the transformer itself is not screwed down good enough.

[mojotom]

I have noticed on some already serviced amps that a new cap job can really help hum reduction. I had one 64 Vibroverb I worked on with some issues and I kept the first two serie filter caps as they looked new and fine but once everything was done I still had a bit of hum and changing those two really finalized the amp.

Those old filters caps ran for 40+ years but new ones can loose efficiency (less filtering, les tight bass, more hum) in about 10-15 years depending on use. It means a serviced may need servicing like an old original one and sometimes there will be some work to be done or undone.

Others hum sources on those Fender could be conductive board and leaking/damaged signal capacitor.

[Bombacaototal]

Attached the audio of the hum, click at the end is when I turn the standby

I pulled out V1 (1st stack), V2 (2nd stack), V3 (trem) with no difference. Only when V4 (PI) is out the hum is gone

Tried 2 different power tubes and hum is still there

It is biased per schematic, -48V between the 220K resistors at the PI

Thanks for the input Steve, I have replaced the bias filter cap and it made no difference. Bias and power tubes seem not to be the cause as well. The hum only happens when I disingage the standby

I guess only two options left, bad filter caps or heaters. Or coupling cap leaking DC

[Stevem]

So let me get this straight, so only when the amp is in standby your hearing this hum, and at that point is the hum being heard from the speaker , or off the power transformer/ chassis?

[Bombacaototal]

So let me get this straight, so only when the amp is in standby your hearing this hum, and at that point is the hum being heard from the speaker , or off the power transformer/ chassis?

I only hear hum when the circuit is live (IE standby not engaged). With the standby there is no hum. And I can hear it through the speakers

If you go through my recording you will hear first the circuit live, then the pop when I engage the standby and then there is no hum

[Bombacaototal]

Didn't have time to work on the amp furthermore today, but was going through the pics and I can't find the heater CT. I think it's usually by the pilot light. Probably explains the hum. Will open it tomorrow

[sluckey]

Didn't have time to work on the amp furthermore today, but was going through the pics and I can't find the heater CT. I think it's usually by the pilot light. Probably explains the hum. Will open it tomorrow

Your heater CT is yellow/green and it's soldered to the chassis at the same point as your HT CT, which is yellow/red.

[Bombacaototal]

Didn't have time to work on the amp furthermore today, but was going through the pics and I can't find the heater CT. I think it's usually by the pilot light. Probably explains the hum. Will open it tomorrow

Your heater CT is yellow/green and it's soldered to the chassis at the same point as your HT CT, which is yellow/red.

Many thanks Steve. Your are right, and I just went back to the schematic of the amp (attached) and it doesn't use the 100ohm balancing resistors

[Stevem]

Just because you have the bias set at -48 volts does not mean that the output tubes are matched close enough to not have one be idling too hot and creating a hum.

[Bombacaototal]

Just because you have the bias set at -48 volts does not mean that the output tubes are matched close enough to not have one be idling too hot and creating a hum.

Thanks Steve, I tried 2 different pairs of matched 6L6WGC tubes by TAD. They were pulled from my SSS and were not too far in range. The Bandmaster humed with both different pairs. These tubes have cathode straight to ground. Should I measure plates only to double check?

[Stevem]

Ok here’s a test that takes all of 30 seconds.
With the amp in play mode remove the the 12AT7 next to the output tubes.
if the hum goes away then we need to narrow down what’s going on in the preamp section even more.

To do this one channel at a time locate the two 220K mixer resistors that feed the 12AT7 input grid cap.

Next unsolder each resistor one at a time and if in doing that the hum goes away I will then bet that on the input side of that resistor you have a DC voltage that should not be there.

This voltage will be from a leaky coupling cap.
If the vibrato channel is passing the hum then the .1 cap off of the plate is bad, if the normal channel shows DC voltage/ hum then the .047 cap off of that tubes plate is bad.

[Stevem]

Ok here’s a test that takes all of 30 seconds.
With the amp in play mode remove the the 12AT7 next to the output tubes.
if the hum goes away then we need to narrow down what’s going on in the preamp section even more.

To do this one channel at a time locate the two 220K mixer resistors that feed the 12AT7 input grid cap.

Next unsolder each resistor one at a time and if in doing that the hum goes away I will then bet that on the input side of that resistor you have a DC voltage that should not be there.

This voltage will be from a leaky coupling cap.
If the vibrato channel is passing the hum then the .1 cap off of the plate is bad, if the normal channel shows DC voltage/ hum then the .047 cap off of that tubes plate is bad.

With what I am taking about here I am assuming that you really have a 120 hz hum and not a 60 hz hum.

[Matthews Guitars]

I have seen old Fenders where a ground connection had simply come loose from its solder connection to the chassis. But it wasn't obvious until I moved the ground wire and found that it wasn't connecting to the chassis anymore. So take a minute to physically check all grounds. One just might not be a ground anymore.