Excessive Hum

[Bob M.]

I have built an amp for my friend. It sounds great but has a little too much hum with nothing plugged in, volume control off. It's 2x 6L6, cathode biased, 1 preamp tube (2 gain stages), one long-tailed pair phase inverter, very Fender-esque, similar to the Norm Channel of a BF Bandmaster.

I've ground referenced the heater circuit w/ 2x 100 ohm resistors, I've used shielded grid wire in the preamp and star grounded the amp. Power tubes are matched. The heater circuit reads 3.13 VAC both sides exactly. The filtering is: B+, 47uf@450Vdc, screen, 22uf@450Vdc, PI, 22uf@ 450Vdc and Preamp, 22uf@ 350Vdc. The layout is linear from left to right and very compact using alot of component leads as wiring aka Matchless. It sounds very good but has too much hum. What methodologies can I use to track this hum down and eliminate it?

Thanks in adv.,

Bob M.

[Bruce / Mission Amps]

If the outout transformer is closer then say, 3 inches to the power tranny, undo it and rotate it 45 degrees and see if that helps.

[MWJB]

You don't mention any voltages in the amp, how much current are the 6L6s drawing?

Can you describe the hum, 60Hz/120Hz?

[Bob M.]

Hi,

Thanks for the replies.

I will try the output transformer re-orientation/re-positioning.

The B+ voltage is 430 Vdc (screen 422Vdc, PI 400Vdc, preamp 340Vdc). The matched 6L6s are pulling about 30mA each. I could size the power tube cathode resistor for a little less current but I like the sound better with just a little more current thru the tubes.

Of course, it's a low steady hum but I'm not sure if it's 120 Hz or 60 Hz. How can I differentiate? It's present regardless of any preamp control setting or volume control setting. It's not very loud but as this amp has all new components, including transformers, I thought I might be able to be rid of it.

Thanks again,

Bob M.

[MWJB]

I would think that you could go up on the plate current a little (so that's not likely to be a cause), especially with a solid body guitar, you should be OK with 40-50mA per tube. You'll lose at least 30v via the cathode resistor, so as far as the tubes are concerned your plate voltage is really under 400v.

Low steady hum, irrespective of controls sounds like it could be filament related. Pull your pream tube, does it still hum? Check routing of the heater secondaries from the pilot light to 1st 6L6, avoid running them back over the PT. Check for grid wires that run too close up to the heater wires, ...long shot but it may help to have the heater wire from 6L6 pin 2 run to the other 6L6 pin 2, likewise for the 6L6 pin 7 heaters (polarity doesn't matter for preamps).

You said before that you used 2x100ohms for a virtual centre tap for the heaters (I assume that they have a good solid ground connection). If you have a 6.3VAC CT that you're not using on the PT, and after biasing to taste you have 35-40vdc at the cathode resistor, you could try connecting the CT to the hot side of the cathode resistor to dc bias your heaters? You would do this instead of the 2x100ohm virtual CT.

[Bob M.]

First, I tried an identical, brand new output transformer with long (15") leads and positioned it every which way - no improvement at all.

The heater wiring is solid, well twisted pairs, away from every other wire. I could lift it/bias it above ground by 40~50 Vdc or so. I might try that.

I did clip in another 60uf@500Vdc cap across all four filter caps, one at a time. It made no difference whatsoever to the hum amount at any position. Then I tried putting in another stage of filtering prior to B+ so I had a 47uf@450 cap at the rectifier output, then a 150 ohm/7 watt resistor, then at B+ I connected a 22uf@450 cap. This extra filtering, like a PI filter, works sometimes but not this time. The hum was still the same. Awhile back I got some caps that were labelled backwards (pos and neg reversed). I'm wondering if there is a proof positive test to make sure that one of my filter caps (47uf@450Vdc) although wired correctly may in fact be reversed in polarity. It seems odd but now I'm reaching out for more arcane solutions.

When I pull the preamp tube, the circuit is quiet, like off, no sound at all. Maybe I'll try floating the input jack or use a plastic one (I hate that though). Any ideas from anyone out there?

Thanks,

Bob M.

[R.G.]

When I pull the preamp tube, the circuit is quiet, like off, no sound at all. Maybe I'll try floating the input jack or use a plastic one (I hate that though). Any ideas from anyone out there?

That sounds like Mother Nature is trying to tell you that it's got something to do with the preamp tube, not the power amp, output transformer, etc.

Divide and conquer.

Plug in the preamp tube. Short the second grid to ground. If it hums, the problem is after the first grid and before the grid of the next tube.

If not, short the first grid. Hum? If yes, it's to do with the first triode's plate/cathode circuit. If not, it's on the first grid.

Is your input jack sleeve connected to the chassis? Or isolated and grounded right at the first preamp tube?

The right way to do this is to isolate the input jack from the chassis, and put a 0.001uF ceramic cap from the input jack ground to the chassis to trap RF. The input jack itself runs on a coax wire down to the first preamp tube, where it connects to grid and ground.

[Bob M.]

Excellent food for thought, I'll try these suggestion, R.G.

Thanks,

Bob M.

[Bob M.]

Short the second grid to ground. If it hums, the problem is after the first grid and before the grid of the next tube.

I did short the second grid to ground and the hum continued. What is between the first grid and the grid of the next tube is the tone stack, a standard Fender one but w/ 2x .022 and a 68K slope resistor. I can't see any problem with it. I have a shielded grid wire running to the grid of the second stage preamp tube. I also changed the new 12AX7A to another brand new one, just in case. Maybe I'll defeat the tone stack and just run a volume control to see if that stops the hum or couple the 1st stage output (plate) into the PI and see if the hum stops. I tried floating the input jack and that made no difference in the hum level or quality.

This is a baffling problem. I've built many, many amps in the past and this is a first of sorts, at least with this hum issue.

Bob M.

[R.G.]

I did short the second grid to ground and the hum continued.
Bob M.

If we're speaking of the same thing, then any hum cannot be coming from the second grid, or anything before it.

I suppose it is possible that the second cathode is connected to the heater winding by a solder thread or some such. But grounding the second grid is pretty much certain to stop any hum that's coming in through the grid.

To be sure we're in sync, the "second grid" we're discussing is on one triode of the first preamp tube that you pulled to kill the hum, right?

[Bob M.]

To be sure we're in sync, the "second grid" we're discussing is on one triode of the first preamp tube that you pulled to kill the hum, right?



We are in sync and that is absolutely correct.

Somthing strange happened. I pulled one end of the coupling cap that connects the preamp to the PI (V1, pin 6 to V2, pin 2). Then, the high voltage connection that supplies the 100K and 82K plate load PI resistors started sparking and hissing. I resoldered this connection and will re-test.

I don't think it's possible to have anything inadvertantly connected to the heater circuit but I will get out the magnifier and triple-check everything to make sure.

[Bob M.]

The Hum Solved!

Thanks to R.G. Keen's insightful troubleshooting help it became obvious that the hum was somewhere in the second gain stage - essentially somewhere in or around the tone stack or pots. I ran some experiments to confirm this from all other possibilities and had a closer look at my work in and around the tone stack and pots. I had used a thick solid copper wire as a ground buss bar, soldered to the rear of the 4 pots - Volume,Treble, Bass and Middle and then attached a ground wire from this going to the star ground point. I removed this buss bar and reconnected the grounds - left lug of the middle pot to the left side of the Volume pot and in turn connected this to the cathode cap and resistor for the 2nd stage, V1-pin8, which already had ground wire to the star ground point. Now the amp was very quiet, hum completely gone! I had a classic ground loop situation, having two ground paths for the 2nd gain stage.

Again, thanks, R.G.!