Express Hum (60 or 120 cycle?) Help needed

[jlatrace]

Ive been trying to troubleshoot this on my own but have run out of ideas. It seems like either a 60 cyle or 120 cycle hum. Let me run down what I have figured out so far.

Amp is otherwise running great, but has excessive hum. Hum is there with volume on zero but at lower levels. Volume control does increases the hum. Hum disappears when I pull the PI. Hum also disapears when I pull V2. Sounds like it is somewhere between v2 and input.

I am going to attach pics for the group to review.

Before I rip out the heaters I wanted to see what he groups thoughts are.
Thanks,
Les

[jlatrace]

More pics

[Allynmey]

You have a wrong value cap in V1B. It should be .002uF not .022uF on the plate. That will let more lower freq. noise through. I don't see where your filaments might be bleeding into you signal though.

[jlatrace]

Could be, I'll try popping back in the .0022 uf coupling cap but I'm not hopeful because even when I cut the bass to "0" the hum is still there, but only from 2 oclock to full volume.

Sounds like maybe ground loop hum but I've checked and rechecjked my grounds. My scheme is to run the preamp and PI grounds to a wire buss that terminates with the input jack. Then run the power filters, bias supply, cathode grounds & PT grounds to a PT bolt. The AC earth ground is grounded near the AC entry point. The speaker jack is grounded at the jack and I have run a wire from the jack to the power tube cathode.

I'm still puzzled and need some input. Anybody else with a troubleshooting idea?

[smzinno]

jla - I believe your speaker jack should be isolated from the chassis if you have a separate wire going back to the main ground as you mentioned.

[jlatrace]

Thanks Steve. I've tried it both ways and it really didn't change anything. I left it because I'm not comfortable relying on the speaker jack as the only ground connection for the OT/speaker.

[Phil_S]

Thanks Steve. I've tried it both ways and it really didn't change anything. I left it because I'm not comfortable relying on the speaker jack as the only ground connection for the OT/speaker.

IMO, this is faulty logic. Redundant grounds = ground loop. Start by isolating the jack if there is a wire to ground. This may be one of several ground errors. It's hard to tell.

Before you go rewiring heaters, it's easy enough to determine 60 vs 120Hz. 60 is about low A/B and 120 is about an octave above that. (A=55, 110, 220, 440). Use your ear and play your guitar with it if you are stuck.

If 60Hz, then I would be thinking heater bleed. If 120, then it could be a B+ filter problem.

I am not fond of the way you ran the heater wire. I'm not saying it's wrong, as there is more than one way to do this. I've built a number of hum free amps and I run the heaters up in the air like this. (Note, the amp is about 95% complete in these pictures.) My work could use improvement, I'm sure, but I think it is a passable example. Heaters in the air and signal wires on the floor have always worked well for me.

[jlatrace]

Thanks for the advice. I have built many amps with no heater noise and very quiet, but none with the gain of the TW. Luckily I have these amps to A/B the noise floor with which helps me troubleshoot.

I moved the speaker ground as you suggested but no change was noticed. I then installed a DC biased heater and lifted the heaters to 40 volts. It was good to learn to do this and it did lower the noise floor a little, but my problem remains.

I have started to think the issue is in the pre-amp section, not the power section, since it varies with the volume & the bass control (with no guitar cord plugged in) and it stops when I pull v2 & V1.

I am focused on to the tone control section. This chassis has a long run from V1b to the bass controls. Those wires are super sensitive. Whenever I touch a chop stick or even get close to these wires the hum intensifies.

I'll keep you posted.
Les

[jlatrace]

Well I ran coax to the tone control just to see if it stopped the hum, no luck. But it did suck the tone from the amp, sharp and brittle describes the sound. I put it back with twisted wire from the board to the tone control, improved tone greatly.

The hum is really not too noticable until I get past 2 oclock. I really don't use the amp at levels higher than 2 oclock, so I guess I will live with it.

Next I plan to build another one and hopefully tackle the noise on the 2nd build.

Thanks for the help.
Les

[skyboltone]

How are the OT and PT oriented? Cores 90' to each other?

[jlatrace]

Good thought!

I did try to orient them 90' but I'm not sure I have it correct because I'm not sure how to compare the core of an upright OT (like a newer Marshall) to a laydown PT (like a blackface Bassman 50).

I will research and check this this weekend

[Firestorm]

With upright OTs, just aim the laminations perpendicular to each other because this will orient the end bells (the noisy sides) 90 degrees to each other, too. If one tranny is a Z Mount (laydown), almost any orientation works (Marshall did it both ways). If you have room, it's sometimes useful to unbolt one transformer and move it around, especially by rotation, while the amp is running and you can hear the effect.

The only way you're in trouble is if the transformers are both Z Mount unless you can get them far apart (and that can open the door to other issues).

[jlatrace]

Thanks. My PT is a laydown and my OT is a Z mount. I understand almost any way works, but, is it worth trying to move just to see what happens?

Now they are oriented as follows:
PT-End bells up and down (laydown).
OT- End bells on a Z-mount are towards front (control panelside ) and back (speaker jack side) of the amp.

The chassis is a classic JMP style chassis with the PT and the OT side by side.

[Firestorm]

I think a Z Mount IS a "laydown." I assume you mean that one is a laydown/z-mount/drop through transformer, and the other is an x-mount or stand-up transformer.

So your choices are to mount the OT with the laminations aimed at the PT or the end bells aimed at the PT. Like I said, Marshall did it both ways. I think Fender typically aimed the end bells of the OT at the PT.

[jlatrace]

Thanks for the answer and sorry for my confusion. You are right, the PT is a Z-mount and the OT is an X-mount.

Well since I have followed the way I did my JTM 45's (and they are wisper quiet) and it's already built, I'll leave it the way it is for now. It's too much trouble to pull and reroute the primary and secondary wires at this point.