Princeton Reverb (Blackface) problem

[MWJB]

"Do you think the can may still be bad, even with a low mearured leakage? I will check with the scope once I get past this problem." leakage & ripple hum are not the only signs of shot filter caps, mushy response from the amp usually sets in before anything else.

Be sure to keep track of plate voltage and plate current, a Princeton PT is only usually rated for 70ma, you don't really want more than 30mA per tube, or 22-24mA if using Russian 6V6s.

[tubeswell]

Sorry, you lost me on HT (if PT= power Tranny)

Sorry: High Tension = High Voltage (secondary winding) - the one on your PT that supplies the B+ (via the filter). If the rectifier shorts, it will most likely dump that high-voltage AC onto your filter cap(s), which will cause them to majorly malfunction, ending up shorted, with the result that the HT AC voltage is now directly shorted to ground, which is not good for the PT. The diodes stop this from happening, because they rectify the VAC coming off your HT winding before it hits the rectifier tube. The rectifier tube otherwise puts out a rectified voltage with the normal voltage drop.

Do you think the can may still be bad, even with a low measured leakage? I will check with the scope once I get past this problem.

What MWJB said. Plus if the electrolytics are over 10 years old, then you may as well chuck in new ones (for the price of grief that could otherwise cause you).

(R.G. didn't do a post in this thread (yet) - his suggestion comes from an article in an on-line mag somewhere, which you should be able to find if you google 'immortal amplifier mod')

[Iceviper]

Thanks for the speedy replies and great info guys, at this point I can't wait to get home and look at the beast. I pretty much am tone deaf when it comes to music, I just look at the amp as a piece of equipment that I like to work on. My final test is running a freq generator through the spectrum in and measuring distortion on the appropriate load. I'll probably change the can just on the advice that it'll make it sound better when running a guitar through it. I'll post with results after I work on the unit tonight.

[Iceviper]

One last thought I had at work would be to slide the SSR back into the rectifier socket. With all of the tubes out and the B+ and negative grid bias supply wires disconnected, fire(hopefully not a bad choise of words) up the amp and see if I develop B+. If so, I believe that will point to the rectifier tube being bad. If I still waste the fuse, I'm missing something. Any thoughts appreciated.

[tubeswell]

One last thought I had at work would be to slide the SSR back into the rectifier socket. With all of the tubes out and the B+ and negative grid bias supply wires disconnected, fire(hopefully not a bad choise of words) up the amp and see if I develop B+. If so, I believe that will point to the rectifier tube being bad. If I still waste the fuse, I'm missing something. Any thoughts appreciated.

Yes the SS rectifier (which amounts to the same thing as the protection diodes when the rectifier tubes is shorted) will at least stop AC from being dumped onto the filter caps. If there is still no B+ with the SS rectifier, then either the filter caps must be shorted, or the HT winding may be fried. (Do you still get high VAC off each end of the HT winding without a rectifier installed?)

[Iceviper]

The voltages with the rectifier tube out are at spec (650 CT, 6.3 & 5). The filter caps are out of the circuit since I disconnectedthe B+ wire from the rectifier socket, along with the negative grid supply wire. I'm on the way home, I'll post when I find something. Thanks for your help.

[Iceviper]

Got it. The amp still popped the fuse with the SSR. I pulled SSR and started checking with the meter again when I noticed 8 ohms to ground on either side of the filament. Further checking showed the fillament windings of the NEW transformer were the culprit. After removing the transformer and taking off the housing I found the flux band that Hammond attached to this transformer was the cause and that when I mounted the unit using their supplied hardware I had inadvertantly shorted the case to ground (Hammond did not supply isolators for the mounting screw shafts like the paper units on the OEM transformer, only plastic shoulder isolators). Long story short, I used heat shrink on the mounting screws that pass through the transformer getting the desired isolation. B+ is up to 360VDC sans filter can, 475VDC with can with about 1VAC of 120HZ ripple(these measurements were made without any other tubes installed). The negative grid voltage is at -36VDC. I'm happy, so now its off to find a new quad can. Thanks for the help everyone, you guys made it easy for me. Hope to talk soon.