Tracking down hum

Paul Lebow

Member
2019-12-05 5:44 pm
I have an early 60's Ampeg M15 guitar amp - a pet project. Replaced a blown OT but having hum problems that I can't trace. Since it is completely dependent on volume level, I thought it was in the preamp stage. Don't see any hum on the B+ voltages there or on the grids. There is about 20v p-p ripple on the 6L6 power tube B+ using HV scope probe, but none on the plate voltages for previous stage. I replaced that cap but ripple is still there.

Any systematic way to trace down the source?
 
I had a similar issue I could not solve in a Heathkit mono tube amp I rewired for guitar. Massive hum, volume control dependent. Checking the DC filter cap string with my DMM set to AC, fractional mV readings on all the plate voltage sources in the preamp tubes.

The only thing that would give a substantial reduction in this hum was when I replaced the heater supply of the 1st two 12ax7 gain stages with a battery! This amp has a single 6.3V filament winding running all tubes, including the rectifier tube.

I quit the idea of having a super gain lead metal amp and demoed out the 12ax7, leaving a single triode gain stage, followed by a volume control, through a couple of baxandall tone controls, into an ordinary pentode input 6BQ5 amp design. The hum is gone.

I dont know what I did so wrong! With the input shorted, when you turned up the volume the hum was like holding the tip of the guitar cord with your fingers - on any other amp. I also dont see how it correlated to the heater supply - how does anyone else make a hi-gain preamp without resorting to a floating DC heater supply? Yeah - floating - if one side of the heater was grounded, back to insane hum.

I only mention this here, now because it happened yesterday / today and it sounds so similar to what you're experiencing, maybe the cause is common. Hope someone can give some insight. Thanks.
 
Firstly and very important for your safety, remove the capacitor of death, C1. A 0.05uF dangerous component and fit a three core mains cable with an earth lead going to the chassis of the amplifier!


If the hum is mains frequency, (50 or 60HZ) then the problem is not main smoothing. It could be something as simple as a heater leak in one of the 6SL7 valves. If the hum increases with the volume control, suspect V2 for an issue, it may even be as simple as a poor ground. Connect pins 1 and 4 to ground to see if that is the problem, you may find V1 has a poor ground, maybe.
 

Paul Lebow

Member
2019-12-05 5:44 pm
Great suggestions. 20v p-p ripple on first 400v B+ cap is 120hz but I haven't looked at the hum output with the scope yet - sometimes hard to distinguish 60 hz from 120 hz just by listening. Is the 5% ripple OK on that first cap?
There are 3 6SL7's, preamp, vibrato, inverter. I swap back and forth, remove vibrato tube, same same. Hard to believe they all have same heater leak but... Will try grounding grids ( pins 1 and 4.
Thanks again, pulling my hair out :-(
 

Paul Lebow

Member
2019-12-05 5:44 pm
Its an old circuit still with death cap and no 3 wire ground, yet. Will get to that. I know there is some chassis AC current flowing depending on orientation of AC plug in wall, hum changes and can see slight spark touching chassis to true ground on one orientation.
 

Paul Lebow

Member
2019-12-05 5:44 pm
Well the valve gods are playing with me. Turned on the amp this morning to measure actual hum to speakers - except for some slight buzz, the hum is virtually gone at full volume, audibly and on scope. Still 20v 120Hz ripple on the 400v filter cap. I'm sure that once I mount the new OT on the chassis and button it up back in the cabinet, the hum will return :-(
 
Yes - Ripple of 5% seems appropriate from what I've read and goes to almost zero on the caps down the chain.
For some reason the hum has settled down on the Ampeg M15 - Also realized that Ampeg didn't use switching jacks back then so unused channel volume pot needs to be turned down to zero or else its an open antenna.
Put a 4000ohm Hammond OT despite the learning later that the original was most probably 6K - but it sounds so good through the 15" speaker and "accordion" input with volume at 10 that I'll probably keep it that way. Will check to see if 6L6's are stressed.