New Build 120hz Hum

[mumford]

I’m having hum issues with an amp I designed. I think it's probably a tube problem, but wanted to see if anyone saw any likely issues I had missed.
It features a 6SF5 common cathode stage followed by a DC coupled cathode follower into a FMV tone stack, followed by a 6SL7 that is acting both as another common cathode stage and a cathodyne PI. The power section is a standard 5e3. It sounds great, but hums like crazy (it’s 120hz).
Using an audio probe, I determined that it appears on the plate of V3 (a 6SL7). (There is no hum on the grid or cathode of V3.) I tried moving the ground point for V3's cathode to a number of different places and it made no difference.
Before I determined where it started (other than knowing it was in the V3/PI stage because it was not affected by the volume control and disappeared when I pulled the tube), I tried moving a lot of grounds around. I had originally just used 2 star points, one for the preamp and one for the power section. I modified the grounding a bit, but it made no difference at all. My current grounding scheme is:
Star 1: near the input jack:
▸ All non power section grounds are on a PC board plane except as described below. A single wire for that board goes from the V1 cathode to star 1.
▸ input jack
▸ 6SF5 and 6V6 shells.
Star 2: by the PT:
▸ All of the filter cap negative leads have their own lines to star 2.
▸ 6v6 cathodes go to the negative end of the reservoir cap.
▸ PI cathode goes to the negative end of its filter cap
▸ CT goes to the negative end of the reservoir cap.
▸ Speaker jack ground
▸ 6.3v CT
▸ 5v artificial CT (I am using the 5v as a temporary heater supply for the 6SF5s–I have a separate transformer I need to install to replace that setup).

Based on some things I’ve seen, it sounds like this may be a common problem with 6SL7s. Anyone have any thoughts on how to address it? I’ve got a supposedly low hum 6SL7 on the way. I’ve seen a couple of things on negative referencing the heaters, but am not really sure what this involves. I am nervous that if I rectify the heaters, I’ll exceed the current I have available (I’m at 1.5A now, and the PT is rated for 1.65). I’m not sure I need to deal with the heaters for this issue anyway, because it’s 120hz rather than 60hz.

It’s not my first build, but it’s my first design. Any thoughts on whether the tube seems a likely culprit, or obvious problems with my grounding scheme?

Here's the schematic:
[img973]http://img705.imageshack.us/img705/7766/schematic.jpg[/img]

[M Fowler]

The only thing I can see is to ground pin 1 of the 6SF5.

I determined that it appears on the plate of V3

This is actually V2b correct?


Mark

[mumford]

The only thing I can see is to ground pin 1 of the 6SF5.

I determined that it appears on the plate of V3

This is actually V2b correct?


Mark

Pin 1 is grounded.

V3 refers to the 3rd preamp tube, right? The 6sf5s are single triodes. What I'm calling v3 is one side of the 6sl7--the other half is the PI.

[passfan]

What about lowering the value of the coupling cap between v3 and v4. If it's coming off the power supply could this high a value be letting it through to the plate of v3 ? Just taking shots here, I know.

Just another thought...you don't have a fluorescent light on your work bench do you ?

[Andy Le Blanc]

If its coming thru the heaters you can try a hum pot with the cathode bias
as voltage source.

New stock 6sl7 are dicey, have you tried swapping around tubes?

If its consistent for that stage, you might have to add one more section of
filter just for it.

[mumford]

If its coming thru the heaters you can try a hum pot with the cathode bias
as voltage source.

New stock 6sl7 are dicey, have you tried swapping around tubes?

If its consistent for that stage, you might have to add one more section of
filter just for it.

Thanks. I'm pretty sure it's not the heaters, because it's 120hz (i.e., rectified). This is an old 6SL7, it may be bad. I ordered a new one from the tube store, hopefully it will kill it.

Another filter, huh? I'll def. consider it!

[Mikka]

Hi,

There is DC voltage and so probably DC current on the grid of the second triode of your 6SL7.
Your design is wrong on this point, for me !
Disconnect The 2M2 resistor from the 470k resistor and try your amp.

[Andy Le Blanc]

That's a method to provide a fixed bias and not incorrect.
Is not uncommon... if you do replace it use a cathode resistor between the
cathode and 47K
and use the 470k between the grid and junction of the 47k and cathode resistor.

[mumford]

I thought it seemed weird myself, but I got it from the merlin book, so I figured it was ok.

So I resolved 90% of my hum tonight by DC referencing the heaters via the power tube cathodes. Woohoo!

[RJ Guitars]

I have been solving hum issues as well in a single ended design here lately. I've got most of mine out too but I think I want to try this DC ref of the heaters. Do you a picture or scematic of how you did that?

thanks,

rj

[soma_hero]

Connect the heater CT to the cathode of the power tube.

[RJ Guitars]

no fancy footwork & no coupling stuff, just the heater center tap to the cathode point on the power tube?

rj

[Phil_S]

That's about it. If you have a pair of 100z resistors (recommended if no CT to create artificial CT) those are what actually get connected to the cathode. If you have a CT, connect CT directly the the cathode. Nothing fancy.

[ampgeek]

I am about to assemble my first single ended build and am following this with interest. Plenty of PP's under my belt and I haven't yet had to deal with humming issues.

My layout includes an elevated heater center tap which appears to have solved (for the most part) the OP's original issue.

But...doesn't the technique only reduce 60 hz hum from the tube's filiment supply? I do understand the theory behind how that works.

Seeing that it help reduced 120 hz hum has me believeing that I may not truly understand the technique and/or theory.

TIA,
Dave O.

[mumford]

Yep, I just connected the heater CT to the power tube cathode connection on my preamp board.