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I've finished wiring up another Plexi, but am having a heck of a time debugging this one.

It fires up, but is super noisy (hiss) and I'm getting no real output. Both channels behave similarly. All ground points are about .5 ohms. The jacks appear to be working properly. I checked the under-the-board jumpers for continuity and have it. They aren't grounding out, either.

The voltages on V1-V5 are all within spec.

Here's a symptom that I can't classify and I'm hoping one of you can suggest something. When I tap on the turret board with a stick it gets amplified. Thump, thump through the speaker. Thumping the tubes likewise. Not good to be sure. The whole shebang is microphonic. In the process, the signal from guitar is getting dumped somewhere--though I've checked each component and turret to make sure that stuff that is not supposed to be going to ground isn't.

The heaters on the rectifier are behaving oddly. Without the tube installed the 6.3 VAC lead from the PT (white) is only giving my 5V at pin 5. When I put the tube in, the voltage at the heaters continues to climb way beyond what I would expect. The rectifier I pulled from my working Plexi V6 so I'm confident that it works properly.

I've tried different tubes in each of the other positions, except the rectifier.

Any suggestions? I'm stumped at this point.

Lee

Rectifier voltage Seems odd to me. Is the rectifier heater connected to White 6.3V - Yellow 0V AC?

What about the other PT secondary voltages?

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Stephen
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Yes. White is connected to pin 5 and yellow to 4. With no tube in the rect socket, white is measuring 5Vac and the yellow about .5V. I've got the yellow/white tied off and have not measured it. The greens to V1-V5 heaters are measuring 3.45vac.

With the tube in, the voltage on the rectifier heaters measures both ac and dc (ac 85VAC) and 350VDC. I did not think this was possible, but that's the reading I'm getting.

If your V1-V5 voltages are within spec then your either not reading the voltages on the recto correctly or your DMM is not working (check your battery). If V1-V5 have ok voltages your recto is working regardless of what your DMM is saying.

The constant buss/hum/hiss sounds like a missing ground connection. Do you have a wire going from the buss bar to the preamp ground point?

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http://pdfelectronics.com

BTW filament voltages arte measured across the filaments NOT to from pin to ground! ie: pin to pin.
Stew

That was my thought as well, though the Plexi V6 sitting next to it does not behave this way with when measured similarly.


Definitely. The ground buss is connected to the chassis along with the input jack.