SEVERE GSG Hum Problem

[TomMcNally]

How about some new closeup pics of the back of the new RCA jacks
and a shot of where they connect to the board.

Have you checked to make sure nothing on the underside of the
board is touching the chassis?

It's got to be something simple we're missing here ...

... tom

[TomMcNally]

Another thought ... have you tried another source, with totally different
cables, etc? There is a cable or grounding problem somewhere, and
if you wired the RCA's and new RCA's correctly, the problem must
be outside the amp.

Have you checked all resistor values around the 12SL7 ? I see you
used some of those 5 band resistors, which are easy to mistake.
You might want to test them all with your meter.

... tom

at work playing DJ as usual on a Sunday afternoon
http://nj1015.com

[cddeluca]

that I didn't think of. With all these recent mods I didn't reconnect the signal in - I just plugged in the speakers and turned it on, hearing the same old hum and thinking nothing's changed. (I did ohm everything out at the driver in the last post - everything's spot on)

So I just reconnected the signal in (no drive, just the cable to the pre) and it's now MUCH quieter than it was initially. It still hums a bit (I still can't get over the total silence when the inputs are shorted) but these speakers (BMS 4592ND on Speakerlab horns) are about 110 dB at one watt so some hum isn't surprising.

The problem must have been induced on the wiring from the RCAs to the board barrier strip at the opposite side of the chassis. Maybe the mic cable was tuned just so or maybe relocating the wiring across the center of the board, as you do, did it. I don't know but i can live with this level of hum. It's audible from the listening position but very low volume.

BTW, the reference to the 12SL7 - typo I presume? Or have I been off planet this whole time?

Thanks to you and Shannon for all the help.

Best, Charlie

[TomMcNally]

Sorry - I have so many 12SL7's from building Darlings I typed that
by mistake.

Glad you have it under control, a few guys have reduced any residual
hum by adding a second choke in place of R17A/R17B ... it's a simple
mod.

Knowing that you have speakers that sensitive, you could even be
hearing AC hum magnetically coupled from the power transformer
to the output transformers.

Compared to my 300B amp, the get*set*go is really quiet, but I don't
have speakers like yours.

... tom

[Shannon Parks]

Email me some full res pix - I'll host'em. May catch something with those. Also, have you been able to test with any alternate tubes at V1 and V2? Which power transformer are you using?

Shannon

[cddeluca]

Hi Shannon

Yes, I tried a couple of different 6SL7s and 6AX5s; made no difference.

The power transformer is a new Edcor XPWR005-120.

Like I wrote earlier, it ain't bad now. There's no real reason why it should be any better - I really just re-did what was already there. The only things that are different than they were before this thread started are the CL-90 is no longer on the HV center tap and the outputs are grounded, neither of which should have really made a difference.

I'll send a couple of pics when I get the camera back. Thanks.

[EWBrown]

Do you have the GSG PC board single-point grounded to the chassis?

That's an easy one to overlook, and can cause a hum issue.

(been there, done that, more than once)

/ed B in NC

[cddeluca]

I have the PC board connected from J6 to the IEC socket ground lug. There doesn't seem to be any other continuity.

Thanks.

[Shannon Parks]

I'll send a couple of pics when I get the camera back. Thanks.

Cool - please do. I want to see that input wiring you were describing, and the terminal strip.

Shannon

[cddeluca]

Hi Shannon:

Yesterday I finally received a pair of 6C4C I'd ordered when I initially couldn't quiet the amp down.

This morning I replaced the WWII era (VT numbered) 6B4G (I tried two sets and they both were identical in terms of noise) that had been in there and the amp is now MUCH quieter - it's now the quietest amp I have - of course it's not black silent - given the speakers I have nothing could be - but it's now everything I could expect of it.

Thanks to you and Tom for all the effort in helping resolve this problem - I sincerely appreciate it.

[Shannon Parks]

Excellent news! Next time I come to New Orleans I'll have to drop by for a listen and maybe some crappie fishing on Lake Pontchartrain.

Shannon