Modded 2203 low sens makes no sense

[booj]

Schematics Photo Gallery - Photo 4 of 4 by Bob - MySpace Photos

It's a hummer! But not in the right sense. The high gain input, standard fair, works great. VERY LITTLE residual hum. The modded low sense input creates an almost intolerable robust 60 Hertz hum only when the guitar is plugged in, even when the guitar is turned down. When I added the 47 ohm resistor, the hum was reduced to an almost tolerable level. I don't know how that happened either.
Basically the grounding sysem is the high sens input jack is a grounded switchcraft, with everything tied to ground in sequence until it gets to the second stage where everything is tied to the 6.8k ground. Everything else from the Cathode follower to the Driver is grounded via the middle control pot. After that it's grounded by a buss bar in order from the Screen supply to the AC mains ground. That buss is grounded to the P.T. mount.
This is a new build. Just finished it tonight. No other issues. All comments welcome. The link is to my myspace page which is open to the public. Feel free to take a gander.

[loudthud]

So the 47 ohm is grounded at the 6.8K? If not, where?

[booj]

Yes

[loudthud]

It's unclear to me where the HV CT is grounded and if there is a continuous ground buss from the input jack back to the transformer bolt. If the current from the HV CT to the first filter cap has to go through the chassis to the input jack and back to the cap, that's where the problem lies. Check this thread: http://music-electronics-forum.com/t16328/ especially the schematic in post #8. The ground buss for the preamp dead-ends at the phase inverter and the power amp gets all it's grounds from the transformer bolt or other point near the transformer.

[booj]

[quoteIt's unclear to me where the HV CT is grounded and if there is a continuous ground buss from the input jack back to the transformer bolt. If the current from the HV CT to the first filter cap has to go through the chassis to the input jack and back to the cap, that's where the problem lies.][/quote]

Thankyou, Loud Thud, for your reply! I called the HV CT the P.T. C.T. I believe it would be the same thing. It is grounded on the buss bar (a 3" length of copper wire connected to the P.T. tranny bolt), right after the AC mains ground. So the current does not go through the input jack ground. It's a combination chassis/buss groonding system, with the last stop for the preamp section and the driver being the mid contol can, with everything else being connected to the buss. The ground contunues from the mid pot can through the chassis to the buss.

[booj]

For clarification, here's the scat.

http://viewmorepics.myspace.com/inde...ageID=53543410

[loudthud]

The new picture 5 is too small to read the text. I see three grounds, Green Delta, Black Delta and Red Delta. It seems like the hum is getting into the signal path between the Green Delta and Black Delta sections. Is there an actual chassis connection to the chassis at the Mid pot? Have you tried connecting that section to the input jack?

[booj]

Ha! It figures. It took 45 minutes to get that one posted. I'll try again., but yes, the ground goes to the chassis via the mid pot case for the black delta ground. Actually its just a colored ground symbol via Fender diagrams. I'm going to try a different ground point for the jack. Maybe use the J1 ground, since J1 is not in operation when using J2 ...duh.

[booj]

Another try. This is photobucket

http://s235.photobucket.com/albums/e...dette12509.jpg

[booj]

I have a new angle on the problem. I've finally scoped the output and when it just starts to clip (and it clips evenly) that there's some power supply hash on the peaks. Also at lower levels, when the pre is brought up and the master is maxed, that about midrange on the pre there's that old familiar click-like sound when you know your getting high freq. oscillation. And another item is the 470kII500pf combo which is tucked between the chassis and the pre pot seems to be the source of the hum, since putting my finger close to it makes the hum louder with exactly the same tone.
So it's not a grounding issue, but more of a placement + power supply problem.
With that unless somebody's got someting I'll drop this thread in a few days while dI work out the bugs.
Thanks again, Loud Thud for your feedback!

[loudthud]

It's normal to see power supply ripple on the peaks of the output at clipping. Would you say the hum is more 60Hz or 120Hz? Does it fade out when you put the amp in standby mode or cut off immediately?

[booj]

Well, I guess the problem was not a ground loop after all, but a layout thing, although it still makes me wonder why that little section between the first and second stage would be so vulnerable to hum pick-up. Anyway I just put a 22k resistor across the 470k/500pf combo. The low sense works quite well now. However, the high sense input sounds different naturally, since the circuit has been changed. It has more bass and actually seems to have less sensetivity.

Low Sens fix picture by myspaceblogphotobucket - Photobucket