Debugging a nice 60/120 hum in a reverb circuit

[mikepukmel]

Thanks!

[nickb]

Thanks Nick. When you write "move your wire" do you mean the shorting jumper? So, short the grid to the ground side of the bypass cap on that tube?

Correct

Ah, ok and second test, just turn off the tube. but this also keeps the supply out of the equation at that point in the circuit.

The point is to disconnect the tube but leave the supply in place. I don't really suspect that supply but never say never.

Re node D Yes, I checked a couple of times, think its OK. Will check again. I put 4 photos covering the amp guts viewed from the top, post #46, should be fairly high res, on the previous page. I didn't get shots of the cap board, though, will do that when I get home tonight. The photos show the extra cap and dropping resistor that I have removed while going through the suggested tests.

Todo:
- different routing of shorting jumper on reverb recovery grid.
- disconnect reverb recovery plate lead
- try a different rectifier tube

[ric]

Thanks Ric! I checked with the gurus on MEF, and this is what they suggested for soldering to the steel chassis, like fender and clones. With a big soldering iron, works great. It's really a flux, rather than a corrosion inhibitor. I think the name means that it is not acid core, like some plumbing solder fluxes.

Gotcha, the flux in a little tin. I actually have some in with my welding equipment box.

I was thinking of the stuff I've been using lately to rewire the house, a completely different thing using a similar name.

Ah, the joys of aging.

Hope you get the hum tracked down. You can always figure the first build is what it is and use imperfections as a reason/ excuse to build version 2.0...

[mikepukmel]

Correct



The point is to disconnect the tube but leave the supply in place. I don't really suspect that supply but never say never.

Progress!! I got home, dead tired as usual, but threw some water on my face and got enough steam to try some of the changes out (didn't want to wait all the way till the weekend).

1) Move ground short wire from input pin2-nearby ground, to input pin2 to ground side of cathode cap/resistor. Check connections, power up. Turn the reverb knob to 10, no hum at all. The only thing that increased was the 'johnson noise, a little white/pink noise. Turned off, drained filter caps. Triple checked, looks like this is "it".

So, the nasty signal is making its way into the tube through my nightmarish grounding wiring? I did something dreadfully wrong. Maybe several things.

Now the ground bar is wired to the lugs soldered to the chassis. So, what I don't understand is how this signal could get in that way.

[mikepukmel]

My all time favorite quote from the old TV show "Mash", maybe my all time favorite quote ever. By Colonel Potter:
"You're askin' a man who graduated 246 out of a class of 248 to do his BEST?"

[mikepukmel]

So, in the original amps (the ones that work ) there are 3 wires leading from the - side of the cathode cap/resistors to the brass plate. Then there is one black ground wire coming from the cap board, through the grommet and over to the brass plate. But its inbetween the rightmost cathode ground and the next one over.

There is one more ground wire coming from the cap board that is grounded soldered to the chassis, over by the power transformer. The main difference I can see in my ultra-humluxe is that instead of running a ground wire from the bus bar right down to the chassis, I ran a wire from the middle of the bus bar over to where the PT CT, reservoir caps (-) and bias are all 'grounded' to the chassis. Is the problem that this 'star' ground is badly constructed?

[Tom Phillips]

... Is the problem that this 'star' ground is badly constructed?...

Yes. You have used the star ground point you created with multiple solder lugs at the PT end of the chassis for too many connections. If you are going to follow the approach used in the old Fenders that point should include only the PT center tap and reservoir (First stage filter cap) connection. That is a very "dirty" & noisy circuit point and it is best not to connect other circuit grounds directly to that point. There is much to say about the merits and pitfalls of various grounding schemes. Sorry I don't have time to add more details about your situation at the moment. Maybe later. Meanwhile, there are lots of good existing MEF threads on the subject. Here is one suggested thread to read http://music-electronics-forum.com/showthread.php?t=33322

[mikepukmel]

Thanks for the info Tom, and thanks for the link. (understood regarding time, and this thread is getting pretty long ) I've been reading myself blue re grounding schemes. I got it pretty wrong. This morning, backed out most of the mistakes. Starting to understand more bits and pieces re Fender multiple ground points, what they put at each point. I'll get it right sooner or later!

The only thing I could not "fix" was Fender fed the reverb driver tube with the dirtiest node in the power supply, but tied the cathode cap negative side and cathode resistor to the second preamp cathode and grounded those at the same point on the chassis. I ordered a turret staking tool, will add one more turret and break those two ground points. ...

[mikepukmel]

Hopefully bringing some closure to this rather long thread, that someone might get some use out of it at some point.

- Took apart the failed star ground.
- Put the power transformer center tap, and negative lead coming back from the 2 reservoir caps onto one lug.
- Found that I had mistakenly put 2 wires running from the ground bar to the failed star ground. They were separated by quite a bit of distance on the bar. Removed both wires from the star lugs, removed one wire completely. Ran a ground wire from about 2/3rds of the way down the ground bar to the chassis. Would have been about where Fender would have ran a wire from the cathode cap/resistor to the brass plate.
- Removed the 2 wires leading from the power tube cathode resistors to the star lugs. Shortened the wires, and soldered to the chassis right between the power tubes.
- Ran a wire from the first filter cap (the one right off the downstream side of the choke) to where the power tube cathode wires were soldered to the chassis.
- Ran a wire from the negative lead of the second filter cap to the ground end of the negative feedback resistor that grouds the phase inverter, then ran a wire from there to the chassis about where Fender would have put one.
- Found 3 or 4 tube socket wires that were too long (still after 3 re-wirings!!!!), shortened, and resoldered.
- Cut the end off the 'ground bar' that connected it to the phase inverter resistor that goes to ground.
- Ran a wire from the last filter cap, the D node, to the ground bar about where it is grounded to the chassis (about where Fender would have run a bus wire to the chassis).

Checked connections, powered up. Much less hum in the reverb! If I put the reverb on 10, its there, and noticeable, but at 3 or 4 (way more reverb than I usually play with), not noticeable. Amp does seem quieter overall as well.

As mentioned elsewhere, what I can't fix:
- The reverb driver tube is fed from the dirty end of the power supply, first filter cap node. But the reverb driver tube's bypass cap and resistor are grounded to the same point where the second preamp tube is grounded. This can't be good. I ordered a turret staking tool, will try to put one more turret in and move the reverb driver's bypass to a separate wire and run that over to where the first filter cap negative lead is attached to the chassis.
- Maybe last thing to do, move the ground for the bias board away from the "dirty signal" ground lug.

Thanks everyone for all the input, suggestions and help.

[mikepukmel]

Latest update. Tried several used old stock 12AT7 tubes in reverb driver spot. One was very quiet. With:
- fiddling with the grounding
- two new caps (off the plate of V2)
- replace 12AT7

Turn the reverb knob to 10, and almost as quiet as at reverb at 1. Just a little more hiss, but no hum.

These new tubes are pretty stinky. This is the 3rd tube replaced in this amp, and its not 'done' yet. Two EH 12AX7's severely microphonic. If I tapped my forefinger on the top of the cabinet, LIGHTLY you'd hear an audible sound in the speaker. Vendor replaced those.
12AT7 died, started crackling badly.

[Chuck H]

Are you saying the new at7 died now?

[mikepukmel]

Are you saying the new at7 died now?

Hi Chuck, nope, that baby is running smooth as a gravy sandwich.

[Chuck H]

I love a gravy sandwich!

[Leo_Gnardo]

I love a gravy sandwich!

I like Lumpy Gravy. With a schmear of Suzy Creamcheese.

[Chuck H]

I like Lumpy Gravy. With a schmear of Suzy Creamcheese.

May the spirit of Sheik Yerbouti smile upon you good sir