Issue all of a sudden - power gone

[klauswilliams]

Hi all - hoping someone can help me out here.

I built a homebrew - started off as a 1987, ended up adding the 2:1 layout, then a bunch of extra switches: bright cap, V1 gain, V2 gain, tone stack. NOS pre's, JJ output. All has been perfect for a couple years. Then, over the course of a week, the tone started to come and go - between normal and a tone sorta like when the tubes aren't hot yet, or when you power down and keep playing for those few seconds. I noticed it (or maybe it was coincidence) when I switched between the 1987 and 2204 sides. But now, it's all that thin bright weak tone, no more normal tone at all. I swapped each pre out 1x1, same deal. Had to be tubes right?? Tried swapping with another I know is good, nada. Tried all 3 inputs, switch settings, nothing made any difference.

Well after that didn't show anything, I took voltage readings and everything is pretty much normal, with the possible exception of V1, pins 1 and 3. Before, they were roughly 160v and .95v. Now, they read 195v and 1.75v. All else is normal as you would expect. Are these a clue? I'm stumped. Here are a few pix:

http://i912.photobucket.com/albums/ac32 ... CN0374.jpg" onclick="window.open(this.href);return false;
http://i912.photobucket.com/albums/ac32 ... CN0372.jpg" onclick="window.open(this.href);return false;
http://i912.photobucket.com/albums/ac32 ... CN0371.jpg" onclick="window.open(this.href);return false;

Any ideas?

Chris

[neikeel]

I am afraid those symptoms sound quite a lot like a dead or dying OT. Have you had any mishaps or been running big mismatches?

A component failure is most likely with your story, not a wiring fault.

[klauswilliams]

Man say it ain't so.... No mismatches really - amp has a pre-master and PPIMV, but otherwise I wired 16ohm tap from OT straight to speaker jack - no impedance selector. Cab is 16ohm 4x12. No use of attenuators either. OT is a M****n, was brand new to this amp. I agree this does sound like component failure though.

[Thiez]

You have a schematic of your amp? So we can see your mods etc. It's hard to judge by the pics.

[klauswilliams]

Unfortunately no schematic. But it's essentially SDM's 2in1 layout found here:
http://home.comcast.net/~jbjdav26/2in1version2.GIF" onclick="window.open(this.href);return false;
http://home.comcast.net/~jbjdav26/1959% ... 20grnd.jpg" onclick="window.open(this.href);return false;

On top of that, I added these things:
- 3in1 tone stack switch
- switch for stage 1 Rk and Ck
- switch for stage 2 Rk and Ck
- Resonance control
- zener diodes to drop voltage just after mains filter cap (issue here?)

Numerous other component value changes but no circuit changes other than above. Unless something in the circuit has been slowly stressing a component over 2+ years, I would guess the circuit is fine. Hope so anyway.

[Thiez]

You can solder of the OT and measure it to make sure it's ok or not. It;s hard to make it completly sure, but your can start with measuring the resistance. Most of the time a failure in transformer is caused by shortened windings.

[klauswilliams]

I unhooked the OT primaries and secondaries and took measurements. OT is a M****n.

Primaries as follows:
brown + white: 45ohms
white + red: 90ohms
brown + red: 50ohms

Those seem awfully high, obviously.

On the secondaries I was only able to test between gray (16ohm tap) and orange, being that the yellow and green were both taped off individually and not used (no impedance selector). But that measurement was 0.5ohms, seemingly OK.

Thinking back, M*****n gave me the secondary measurements via a phone conversation, gray=16, yellow=4 and green=8. I wonder if they were mixed up there and I've unknowingly been using the incorrect tap. Long shot maybe.

[neikeel]

All those static tests sound ok.

So are Chris's colour codes.

[klauswilliams]

Well that's good. I was assuming it would read much lower. Have to start checking stages now.

[klauswilliams]

Going back to voltage readings, the ones different from before are all on V1 - pin 1 is now 195v (was 148ish), pin 3 is 1.75v (was 0.85), pin 6 is 223v (was 185ish), and pin 8 is 2.3v (was 1.7). Not huge changes, but something. Does this point to preamp power supply or something in that chain? I isolated most of my switching and none of them seem to be the issue. I guess another way to describe the tone now is a cocked open wah, but with reduced overall volume. Real nasally and thin.

[neikeel]

I suspect that somewhere in preamp something has gone awry, voltages are high in V1 because maybe V2 is not drawing any juice, check your plate feeds and the tone stack grounding. Also some people have had the tone stack mica cap go south.

[klauswilliams]

Got it sorted out, unfortunately after swapping out about 15 preamp components to try to isolate the problem. Turns out the mid pot ground wire had a cold solder joint where it connected to the ground buss. Because of switching stuff around a bit after the amp was built, I ended up having to use a short jumper wire from the pot lug to ground wire. Irritating since it seemed to measure OK with the meter when testing. Oh well..good side is that I finally drew up a schematic for my amp and of course learned a bit in the troubleshooting process. Thanks for the pointers all.

[neikeel]

I suspect that somewhere in preamp something has gone awry........................the tone stack grounding.