Gretsch Mystery Amp Hum c.1960

[misterc57]

A new hum issue (they just keep happening). I am not sure of the model of this amp, closest I could find was a model 6161 but the schematic is slightly different.

When I first brought this amp to operating AC level it seemed to work fine. It had a 2 prong cord and I thought I would update to a 3 prong, well that is when hum started.

At first it was 60 Hz. I disconnected the mains ground from the chassis and that made no difference.

I had since updated all filter caps (22 uF, 10 uF, 10 uF), updated the 6973 output tube pair cathode cap to a 47 uF. I have all these caps and the mains ground going to the same ground connection.

Now the hum seems to be 180 hz to 240 hz, jumps from one to the other. If I touch the volume knob it seems to change sound. This is very odd; if I touch the two screws on the left side of the amp top, the hum changes sound, these screws hold the chassis in place and are currently hanging out of the cabinet, not connected to anything. If I move my hand near the chassis, hum sound changes.

The 3 input jacks are not shorting jacks.

The upper part of pin 8 of the rectifier broke off while soldering (must have been fatigued from something) so for now I jumped pin 8 to pin 1 and moved connections to pin 1.

The hum volume changes with the volume pot. Hum is present with no instrument connect. The hum is present with a guitar connected and present when playing.

The tremolo works but is turned off.

The model 6161 schematic shows two channels, I do not have that. My V2 has one half of the tube unused.

Scope readings taken at the speaker connections.

Thank you in advance for any guidance. MarkO

[misterc57]

Forgot to mention that if V1 is pulled there is no hum. I have tried a different 12AX7 in V1.

Also attaching the 6161 schematic.

Gretsch_6161.pdf

[Helmholtz]

Please never disconnect the safety ground from a chassis. And always connect the ground clip of the probe to circuit ground.

Your scope pictures show unstable (double) triggering. Double traces are never real, as you can't have different voltage values at the same time.
Get acquainted with the trigger options of you scope. Do not solely rely on the autoset mode.

Frequency is either 60Hz or 120Hz, nothing like 180Hz or 240Hz showing.

[Jon Snell]

Is the hum coming from V3?
Please do not use it without a safety ground connection, that is dangerous.

[Helmholtz]

Forgot to mention that if V1 is pulled there is no hum.

So the hum enters via V1 and the power amp (including the PI) is fine.

[g1]

With amp volume all the way down, does the hum go away?
How about if you short the input jack to ground?

[Enzo]

Agree on scope, when you see waveforms like that, turn the vernier knob on your sweep until ONE series of waveforms is displayed. It won't be 240Hz hum. Your input jacks are picking up the hum. Plug shorting plugs into the jacks and see if it quiets down. When you bring your hand near the unshorting jacks they pick up your presence

[Helmholtz]

It is not a time base/sweep but a trigger stability problem. Trigger level should be adjusted manually. It might help to select trigger source AC line.

[misterc57]

Thank you all for the warning on having the chassis grounded at all times. Done. I forgot to mention that I also removed the .01 death cap.

Volume pot at zero is no hum.

The are three input jacks. First two are labelled "Bass" and each has 148K to ground at the tips. The third jack is labelled "Treble" and it is OL to ground at the tip.

There is a grounded metal shield over the three input jacks.

I grounded all three tips, no change.

I then placed the tube shields over the 3 12AX7 tubes. This helped a lot.

Now there seems to be less hum and it varies unevenly with the VOL pot adjustment. At one point there was no hum when dialing the VOL pot back and forth but then it came back at different spots on the pot.

I am now questioning the VOL pot as a source of added hum at different points in the rotation. The pot was cleaned, but?

[g1]

Maybe check for corrosion/oxidization where pots mate with chassis.

[mozz]

Is there a cap on the V1 cathode?

[misterc57]

Maybe check for corrosion/oxidization where pots mate with chassis.

This helped. No corrosion but the inside chassis is painted black, I sanded down to bare metal and now the VOL pot is no longer acting funky or reacting to my touch. Hand waving interference is also gone. I did the same for the TONE pot.

Interesting on this amp, 1/2 of V1 and 1/2 of V2 are not used.

pin 3 of V1 is 4K to ground. There appears to be a cahtode cap buried in there but I am not 100% sure without removing a lot of obstacle components.

I wonder if this amp would benefit having a grid stopper at V1?

I know have a audible lower amplitude 60 cycle hum (according to my phone app) that kicks in around VOL 4 and gets louder as you dial up the pot.

I took a scope picture with VOL at 10. Hopefully I got thee scope right this time.

Issue is improving incrementally. Thank you.

[Helmholtz]

Scope pic shows HF oscillation around 70kHz.
That's not your hum.

The period of 60Hz is 16.7 ms (milliseconds). Scope time scale is in µs (microseconds), so the frequency displayed is 1000 times higher.

[mozz]

I asked if there was a cap on V1 due to sometimes it does help get rid of hum. Maybe there is one and it's open.

[misterc57]

I asked if there was a cap on V1 due to sometimes it does help get rid of hum. Maybe there is one and it's open.

Found my way to the original 25uf 50V cap. It read 47uF after being pulled. I replaced it with a new 47uF, same capacitance I used for the power tubes cathode cap.

I sanded away the paint for the 3 input jacks.

After the above it is sounding better but there is still an annoying 60 Hz hum but not as loud as it was previously, but loud enough to be noticed and annoying.

The amp is lacking gain. So I have to put the VOL at 10 to get to a bedroom level loudness, and at 10 the hum is the worse.

Interesting that the 12AX7 originally in V1 was a Ruby high gain, which did make the amp louder, however with that tube the hum is unbearable.

Below scope images show the hum, the lower amplitude uses a regular 12ax7 and the higher amplitude uses the HG tube.

Helmholtz, I think I got the scope correct this time!!!

I have two unused halves of 12ax7 available, would it be possible and worthwhile to add another gain stage or would that just make the hum louder? Or is the hum and gain two different issues?