Eliminating ground loops in a new build amp

[Bob M.]

I've built a 2 channel amp, somewhat similar to a Fender combo amp. Channel One is quiet as can be. Channel Two hums more that slightly, moderately, but it's useable unless you play the amp above 7 or 8 on the volume control. I have reverb in both channels. I think I have a ground loop in Channel Two. Today, I'm going over everything in channel two first, checking for wiring errors. I have my cathodes of the Channel Two preamp tube (12AX7a) grounded together with the ground lug of the Channel Two volume control and the Middle control and from there proceeding to a central star grounding point. I need some troubleshooting tips or methods to find/isolate this ground loop in Channel Two. When I play thru Channel One, the amp sound excellent and very quite. Channel Two is just hummy. Any suggestions are appreciated.

Thanks in adv.
Bob M.

[Greg Robinson]

Have a read of Merlin "The Valve Wizard" Blencowe's chapter on grounding. If you haven't read his books yet, I can highly recommend them.

[pdf64]

Michael at Modulus amps has devised some nice layouts that incorporate the good practices that Merlin documents.
In short, the negative terminal of each HT filter cap becomes the star point for the local circuits it supplies. Each star has a wire linking it to the next. The only connection to the chassis is at / close by, the input socket.
See if this helps

[Helmholtz]

Did you exclude other possible reasons of hum? Is it 60Hz or 120Hz?

A schematic and pictures could help.

Do you have a scope?

[Bob M.]

Thanks for the replies. Yes, I've read the Dr. Valve article (re-read it yesterday). My star grounding scheme is pretty much like as you've stated and, yes, I have a scope (several). This is probably my 30th or so build. I'm been using mostly my own designs on gigs/tours since the early 90s. Almost all have been very reliable, quiet and great sounding (to my ears). I usually do an original circuit amp for friends/clients about 3 or 4 times a year, less since the pandemic.

I checked my wiring very carefully for errors in the past few days. Voltages are what might be expected from the circuit at hand. The amp sounds great but Channel Two is hummy. Yesterday, I moved some wires around (I'm using an unconventional layout), re-directed some ground wires and shielded a grid wire that had a medium length run. I still have the hum but now... When the volume control of Channel Two is on 4 or 5, the amp is dead quiet now but the hum returns as I increase the volume up to 8. However, as I decrease the volume from 4 to zero the hum grows louder again and is about the same at zero (no volume) as it is at 8 on the volume control. So, I have hum at both extremes of the volume control setting but no hum in it's middle range. This is a new one to me and I thought I'd seen many/most types of anomalies,or malfunctions over the years. The hum seems to be isolated in the 3rd gain stage, a stage after the first two preamp stages, but just prior to the phase inverter stage, similar to a Fender combo amp from the 60s.

The tube that is associated with this stage is a 12AX7a and one half is used in this 3rd gain stage and one half is used in the reverb recovery stage, just like a Fender combo amp. The reverb circuit works as expected, a little hissy when the reverb control is advanced but nothing out of the ordinary for an amp using this reverb circuit. I am summing the two preamps together so I have reverb in both channels but I'm doing this in a way I've used forever in two channel amps with no problems or after effects. I've removed and re-measured most of the parts associated with this 3rd stage. I'm pretty convinced (but not absolutely) that it's a grounding and/or layout problem. Any ideas you may have I'm open to trying.
Thanks,
Bob M.

[Helmholtz]

Please post a scope pic of hum at speaker output including scope settings (input sensitivity + time base).

(Still hoping for a schematic and guts' pics).

[SoulFetish]

Can you post a schematic and/or layout drawing? or a photo? what are you using as a summing node to mix the two channels?

[catalin gramada]

Run a hefty wire from output to input. Bolt this wire at the input jack to chassis (and only there !). Solder the supply filter caps along to this wire in a order exactly as B+ supply the stages are. Tie each stage ground to its relative ground supply cap. Done !
Here a pic of one bus ground as I did it in one of my project, to get an ideea. You may mount this wire on isolated standoffs, as I did, or to any isolated points from chassis. No other contact to chassis but only the end of the ground wire bus.