Noisy first and/or second stage - 60Hz hum

[mlp-mx6]

Hey folks - I have built a Kxmxt Express, the "asteroid" preamp into a 6V6 Express power amp. I have a TERRIBLE problem with 60Hz hum in the 1st or 2nd stage. I have isolated it to there because if I ground the input to the 2nd stage (that is, zero the volume control) the noise goes away completely. Though not exact, I did copy the Francesca approach to wiring the sockets, with the heater wires against the chassis. I cannot seem to locate the source, or do anything about it.

Has anyone else encountered this issue? Any suggestions?

Thanks,
Michael

[lastwinj]

have you tried DC heaters?

one thing you can try is a put a switch on the heaters, let the amp warm up, make sure the noise is coming thru, then swithc the heaters off. if this gets rid of it, problem is in the heaters.


possible bad filter cap. what ground system are you using?

germ

[rfgordon]

For 60 Hz noise, here are a few things to try.

1. make sure the same wire line feeds the same heater pins. I always use a green & black twisted pair, so I can keep that straight. If you switch one tube's heater wiring, it won't be humbucking.

2. To test to see if lines of shielded wire will help, cut small (1/2" wide) strips of foil. With the amp OFF and unplugged!!, wrap the foil around a wire and alligator clip lead the foil to ground. (The foil by itself will add noise, so don't forget to ground it.) Turn on the amp and try it.

3. Another thing on heater wires, twist them pretty tightly.

4. Make sure heater CT is grounded. If you don't have a heater CT on your PT, use two 100 ohm resistors to ground for an artificial center tap. I once built and amp and forgot this step--made awful noise till I remembered the resistors!

5. Review your overall grounding scheme. double and triple check for ground loops.

[mlp-mx6]

Gents, thanks for the replies.

I did keep the wires aligned by the pins (used orange and white to make sure).

The wires were not twisted tightly - they were wrapped around each other a few times like the Francesca pictures. This is the first time I used this approach. I am much more used to twisting and floating the heater wires like a Fender. (it is also the first time I have had this problem. Coincidence?)

Heater CT is grounded.

Ground scheme is rechecked (and simple). I used a ground buss and EVERYTHING is grounded to it. All input jacks are isolated from the chassis. The buss is only grounded to the chassis near the input. Again - if I ground the input to the 2nd triode it is VERY quiet, which makes me suspect the 1st stage.

I am considering DC heaters. I can rig up a 12V heater DC circuit for the 1st tube pretty easily using a Radio Shack xfmr that I already have.

I can also clip in a different filter cap to see if that is the issue. However, I would expect that to sound like 120Hz, not 60Hz. Th caps I have in are fresh Atoms. (Probably the last big Atoms I buy, but they should be good.)

Again, thanks.
Michael

[UR12]

If you are sure it is the first stage then there is a couple of things to check. If you are sure it is 60 hz that you are hearing and not 120hz then you should be able to rule out your filter caps. It's either your heater wiring or you are picking up 60hz through one of your grid wires. Do you have a shielded wire from your input jack to the grid of v1?. If you are running a nonshielded wire from the vol control to the second grid of v1 you might want to try and move it around or replace it with a shielded wire also. You say that your input jack is isolated, you might want to ground it to the chassis to see if that helps. If you have pics that might help.

[mlp-mx6]

Dana,

I did use a shielded wire to the first stage. I'll try to move the 2nd-stage wire. Maybe I'll use shielded for that too. I ran it under the board similar to Francesca, so I can't move it very much.

(Just tried it, no help)

Thanks again!
Michael

[UR12]

If you can ground it at the vol control and it gets quiet then it's definately something in that area. Do you have a bottom plate (shield) on the amp?

[novosibir]

Michael, simply try another input tube - maybe then your prob is solved!

I've experienced, that i.e. the reissue TungSol 12AX7's sometimes do have huge hum issues - sometimes permanently - sometimes the hum rises up after 5-10 min warm up of the tube.

The TS's do have wimpy aligned filaments inside the cathode pipe. Hopefully the current batches are better in this respect, because it's otherwise the best sounding tube from current production IMO.

Larry

[rhinson]

Michael, simply try another input tube - maybe then your prob is solved!

I've experienced, that i.e. the reissue TungSol 12AX7's sometimes do have huge hum issues - sometimes permanently - sometimes the hum rises up after 5-10 min warm up of the tube.

The TS's do have wimpy aligned filaments inside the cathode pipe. Hopefully the current batches are better in this respect, because it's otherwise the best sounding tube from current production IMO.

Larry

+1 on that----i've had similar issues with these and their other 12ax7 (EH version) as well. i've had to send back a couple in each batch of 10-20pcs i've ordered for a good long while now. i don't recall having that problem with the old sovtek 12ax7wa's or wb's-----but they obviosly don't sound nearly as good as these newer offerings. rh

[mlp-mx6]

Folks, thanks!

I swapped the EH tube for an OLD RCA 12AX7 (pulled from a 70's Vibro Champ) and it is resolved!

I hate that I bothered you all with this when what should have been my first approach was not tried. I *assumed* that a brand new tube would be cool. Plus - I had used that tube in a Dumble circuit with no issues. Guess the Wreck circuit is MUCH more sensitive. It is certainly higher gain.

Now to crank it and hear the TONE!

Many thanks,
Michael

[novosibir]

I hate that I bothered you all with this when what should have been my first approach was not tried.

No need to be embarrassed, because 'this' is it, for what forums are.

Larry

[Allynmey]

We all get nailed by that every now and then!