[SOLVED] Soundfile - help with amp debug

[surfsup]

I recorded my amp. A short clip, 30 seconds long.

axHO with two gains and one mastervol

G1 adn G2 are both at max with MV=0 at start

as I turn up the MV, this is what you hear.

http://chicagocadcam.com/ChrisHahn/axHO.mp3

When all three are 50%, the amp is nearly silent. I would really appreciate help from teh experts (clip gets loud at end). there's a subtle high pitch oscillation always present and a hiss that gets unusable over about 65% max output. The hiss sounds loud in the clip, but the guitar is way louder.

[xtian]

Is there a problem? The AX84 HO is a high-gain amp, and if you have both gain knobs wide open, it's going to be noisy, no doubt.

[surfsup]

The hi pitch oscillation is always there. I'd like to at least eliminate that and reduce the hiss to about half. The hiss is too much at higher levels.

My ideas:
Add a cap at input jack to tame high freq
Change Ck at v1a
Change coupling cap
Redo grounds, star gnd PS and PT at a lug near PT

Would any, all, or none of these help?

[VacuumVoodoo]

High pitch oscillation could be microphonic tube in the preamp. Gain always results in noise/hiss.
I would be more concerned with the mains 60Hz hum, it sounds like it comes from the heaters. Is heater wiring balanced and grounded i.e. via CT or with resistors or a pot?

[surfsup]

You can See the 46v elevation circuit in the last three pics here, and the two 100Rs coming off the lamp, as well.

http://chicagocadcam.com/ChrisHahn/amp.html

I dont really hear much hum just mostly hissing, and the hipitched ive changed out v1 and v2 with 6 other tubes, same thing. Metal shields over them too. The hi pitch oscillation started after i ran shielded from the mv to the octal, i think.[/img]

[Randy Magee]

Did you ground the shield only on one end?

[surfsup]

Yes.

[Richie]

Did you ground the output jacks to the chassis, if not try that.

[surfsup]

In photo 1 and 3 you can see the green lead come off the output jack copper ground buss and run under the board and grounded at the second filtering cap...

Being new i dont have spare components, etc lying around. I was thinking of breaking the ground buss into three: one for the first RC, one for the power tube, one foe the preamp. Leave preamp grounded at input jack, run another lead for the power tube to a point near the PT on chassis, run the ground for the first RC to this same point with the PT ground.

Ah, maybe i'll just order up some stuff and try each thing anyway. Maybe i'll learn something...

[ampgeek]

Is it common to use both 100R reference resistors and a center tap ground reference (elevated or not) at the same time? I was under the impression that it was one or the other.

Should be pretty easy to test and might be worth a shot.

Good luck,
Dave O.

[VacuumVoodoo]

In photo 1 and 3 you can see the green lead come off the output jack copper ground buss and run under the board and grounded at the second filtering cap...

Wrong. It should be grounded at the first cap. Heater DC Offset should also be grounded there.
On V1 socket you have a yellow heater wire very close to cathode pin nr 3. Move it up and away from the pin. It's a hum back door.
As for the high pitched oscillation - without schematic to correlate with hardware photos I can't say anything.

Here is tested, verified and quiet grounding scheme for a SE amp.
One thing only needs correcting: the green/yellow wire from mains input socket is safety ground and MUST be connected to chassis by it's own dedicated screw close to to the socket. No, don't use socket or transformer mounting screw for this.

[surfsup]

Okay thank you for that schematic. I see what i was going to do was not quite the same, I will segment the preamp grounds into individual nodes as well, and run to the star. So I will have:

1) Input Jack chassis ground - only input jack
2) Mains chassis ground - only the mains
3) chassis ground near the PT (not a bolt) where all other segmented star nodes are grounded for each preamp triode, EL34, OT, PT, PS

Here is my amp's schematic (EL34 instead of EL84) in case you have an itch to take a closer look. Thanks so much.

[surfsup]

Okay I tried to follow your SE ground scheme: problem is on my build V2 preamp and V3 powertube are all off the same B2+, so taking the methodology, I used a lead from V1, one for V2/V3 and one for the PS (C1/C2). On first shot, I highlight the grounds with red/white arrows. I think the hum is a bit worse now but the static is better, could the heater wires need redoing (again)?:

[img2000]http://chicagocadcam.com/ChrisHahn/FULL.JPG[/img]
[img3008]http://chicagocadcam.com/ChrisHahn/SIDE.JPG[/img]

[surfsup]

FINALLY! Solved the humming issue. I the pics from previous post, I took the long purple lead from V1 (first preamp tube) and ran it instead to the input chassis ground so both were grounded there.

Way, way way way better! Also took out the V1 grid stopper. much quieter!

Still some hissing but now I can put G1/G2/MV all at 7-ish (maybe 7.5) and it produces very little hiss and almost no hum. Guitar is loud at this point. I will never put em all at 10 so I'm good with this build now. Time for a faceplate and cabinet.

[VacuumVoodoo]

I was going to write about V1/Input/ grounding when I looked at the photos but then I read you already got rid that ground loop. Good job!