Old Sano 60 Cycle Hum

[BungleSim]

I'm working on an old Sano amp for the guitar shop I work at. Schematic and photo of the preamp attached. The amp works and sounds pretty good but the owner wanted me to modify the preamp a bit to give the two channels some distinction from each other. Originally V1 shared its cathodes with the 2K/25uF values. I separated them to make one of the inputs higher gain by giving that stage (V1A) a 2.2K and 0.68uF cathode resistor and cap (circled in red). Initially I had the RC near the input (top-left in the photo) but then I relocated it to its current position.

Now the HI input has an obnoxious 60 cycle hum that increases when you turn its volume up, even when nothing is plugged in and the input is grounded. The hum is 1Vp-p at maximum. The LO side also has a slight 60 cycle hum, but it stays very low (0.1Vp-p) and when you listen to the amp it's a hum that one would expect from an amp from the '60s.

Any pointers on how to efficiently troubleshoot this? I've swapped the preamp tubes (2X 6SN7, 6SL7 PI, pair of 6L6s, and a tube rectifier that I am unsure of at the moment) to no avail. The hum goes away (or matches the LO side) when I remove the 2.2K/0.68uF and return V1 to a shared cathode configuration.

As I type this I am realizing that I do not know if the heaters have 100-ohm resistors to ground. I need to double-check that. Any other tips would be greatly appreciated!

[Leo_Gnardo]

I do not know if the heaters have 100-ohm resistors to ground. I need to double-check that. Any other tips would be greatly appreciated!

Well yeh, I'd see what's up with filament grounding arrangement, first. Some hum relief may be found by floating the filaments on a low positive voltage - some self biased amps refer filament center tap or resistor-balanced virtual center tap to output tube cathodes 15-25V more or less depending on amp. Ampeg was just down the road a piece from Sano - in the 60's they floated the filament CT or VCT typically on an uncharged 0.1 uF film cap. Sometimes that helps. Cheap, fast & worth a try.

One of the drawbacks of having smaller preamp cathode caps is you're no longer effectively bypassing hum frequencies. True that you shave off a lot of low frequencies to voice a channel as specifically bright or mid-rangey but you lose the hum suppressing advantage of having a larger cap. You find out fast many pre tubes hum, but keep trying, you may find one that doesn't. But do get the filament grounding sorted first, you may be pleasantly surprised and need go no further.

[Stevem]

Yes, that bussed grounding layout must go!
You should also Plunck the chassis down on a sheet of foil just to confirm that it's not radiated 60 or 50 hz drifting around.

[Phil_S]

I'm always ready to acknowledge there are others here who know more than me. My gut reaction is this. The hum wasn't present when the amp came to you. You separated V1 cathodes, made separate RC filters, and moved the ground point. The first thing I'd do is put it all on the original ground point.

You want to say a ground loop will be 120Hz? I don't think it's a ground loop. I think you did something that's allowing 60Hz heater signal to bleed over to the audio circuit. This is a lead dress problem. FWIW (not much), that's my 2¢.

Good call from Steve on putting a cover over the chassis. If you don't have anything that works, just glue some kitchen foil to cardboard. I keep one of those handy and I've been amazed at how it will cure a non-problem. If the idea spooks you, remember, you can orient it with the foil side out and it works just as well.

[SilverFox]

There was a recent thread here discussing the hum reduction benefits of large caps on the cathodes of preamp tubes. Even values as high as 250uf. The gist of the link was, the larger cap offers a low impedance path to ground at 60 hz. After determining if the amp becomes quieter when returned to it's original state, I would change the .68 value to perhaps a 25uf and change the resistor instead or alternatively or both, change the second preamp stage to alter the voicing.

I also wonder if the input grids should be modified to the typical input scheme of a 22k to 68k input resistor with a 1M to ground. But I don't know enough about the 6sn7 to offer this as anything more then a suggestion.

Are there any other Sano amps that could serve as an example schematic?

silverfox.

[Malcolm Irving]

In the photo, the new resistor lead and cap look very close to a heater wire/terminal. Might just be a perspective effect of the camera, but it's a thought.

[Phil_S]

In the photo, the new resistor lead and cap look very close to a heater wire/terminal. Might just be a perspective effect of the camera, but it's a thought.

I've been thinking about this, too and am glad someone else commented on it. I keep coming back to no hum when you got it, hums after modification. Therefore, it is the modification that's the problem.

There are two aspects to the mod.
1) Separated cathodes.
2) Grounded separated cathodes at different locations on the buss. One runs close to the likely source of the 60Hz.

If this was mine, I'd reroute the one that crosses over the heater pin. Specifically, I'd go a bit sideways across the socket and ground it in the same spot as the other cathode. You may have to extend a lead on the resistor.

[Smokebreak]

Nothing to do with the issue but to keep one of the cathodes stock, you'd need to use 4K/12u cathode.
For the mod I'd bump the Rp to 220K, K 1K5/1U, 22n coupler. Maybe split the 2nd stage too

[Structo]

Wow, that is a heck of a ground bus!

I agree, if it hums after mods, then mods probably made it hum.

I don't care for the way those old Sprague can caps are just laying there on the circuit.