Princeton Reverb: Hum, Low B+, dead PT ...

[Bluefinger]

Hi,

I have a problem with my PR build. When I first fired it up, there was nothing but a relatively loud hum coming from the speakers but no sound from the instrument. None of the controls had an effect on the sound. I thought I'd take a chence and take quick voltage readings (which turned out to be a bad idea). The PT was wired correctly but I had only 250 VAC on pins 4 and 6 of the rectifyer and 97 VDC on pins 8 and 2. There were also a couple of volts AC on the preamp plates. As soon as I was done taking the readings the PT went up in smoke ...

I checked the wiring and everything looked good ... any ideas?

thanks

[MWJB]

Inadvertant grounding of the heraters via the pilot light terminals touching against the chassis?

Next time power up without the tubes installed, it'll help isolate issues should they arise.

Any pics of the build in case there is something obvious we can eliminate?

[Bluefinger]

the heaters have a virtual center tap and meassure 50 ohms to ground on both sides. Sounds ok to me and nothing seems to touch the chassis ... center taps and screen are grounded, 220 and 240 volts wires are isolated off, 230 is connected to the on/off switch, heater wires are looking fine, bias is taken from the rectifyer, rectifyer is wired correctly ... I can't seem to find a mistake. You think that there could be a problem with the PT itself? I have a feeling that I have overlooked something tho ...

I'll post pics this evening ...

[MWJB]

What make was the PT? What's your nominal wall voltage, out of interest (probably not related to your issue)?

[Bluefinger]

That's the PT: 125P1B (=020873/022772) MAINS for: FenderŪ Champ A - mains transformers
waal voltage is 230 Volts

[MWJB]

That's a good quality PT, I'd be surprised if it was suspect.

If your wall voltage is 230VAC, it's good practice to hook up the next highest primary tap e.g. 240VAC...if your B+ & heater voltages then come up low (not that I have seen that on a TAD/Mojo 125P1B PT wired for 240VAC), you can always switch to the next lowest tap.

[Bluefinger]

I don't think that there's something wrong with it either ... I think I'll recheck the wiring again with a fresh brain ...

[Bluefinger]

Ok ... The UPS man brought a new tranny yesterday so I wired it up exactly like the old one and fired up the amp with only the rectifyer plugged in to trace down the error. Guess what ... no incorrect voltages. And with all tubes in place the amp ran perfectly with all voltages within specs and it sounds great. I doublechecked everything again, compared it with the pics I took of the old wiring ... nothing. I'm a bit confused now. Did I really have that one in a million faulty tranny? Guess I'll never know ... those TAD trannies are extremely reliable usually ... hmmm