Princeton Reverb (Blackface) problem

[Iceviper]

I am presently repairing a Princeton Reverb for a friend. He had replaced the 5U4GB with a SSR years back. It worked for about an hour then smoked. He shut it off and stuck it in the corner for about 10 years. I found the power transformer cooked and one of the 6V6GT's shorted. I asked him to get me a new transformer, finals and retifier (I asked for the 5U4GB). He was sent a 5u3C, which he was told was a direct replacement. I tried to fire up the amp after parts replacement (quad can checked O.K. and was reformed)and found it blowing the 1 amp line fuse after about 20 seconds. I couldn't find any obvious problems and fired it up again with just the rectifier in place. The B+ would not come up and the fuse popped again. Mt tube tester freaks out when I tried to check the rectifier tube with 5U4GB settings. I can't find any info on the 5u3C. Any thought, help or info would be greatly appreciated.

[ThermionicScott]

TDSL is where I go for tube info: TDSL Tube data [5C3S (5Ц3С)]

By "freak out" does the tester peg the needle to the right side of the scale and twitch a little? That's what mine does with a healthy rectifier.

Sounds like there's a wiring problem yet to be solved -- does the fuse pop with no tubes inserted? (Also, you're sure they're "slow-blow" fuses? )

- Scott

[Iceviper]

The meter pegs to the right, and twitchs a lot. The tube tester (Hickok 6000A) also groans like its loading down. I've checked the wiring three times and isolated the rectifier (B+ wire removed, feed to vibrato circuit removed.) I am wondering if this tube is in fact a replacement for the 5U4GB. Any thoughts would be appreciated.

Joe

[ThermionicScott]

Yeah, they're pretty much the same spec -- that long filament sure looks cool though!

I would try it without any tubes plugged in, and if the fuse doesn't blow this time, check the voltages on the rectifier and output tube sockets.

- Scott

[JoeM]

Do you have a 5U4GB to check it against?

[Iceviper]

Sorry, forgot to reply to one question, fuse does not blow when powered sans tubes. Fuse is an MDL 1.

[Iceviper]

Unfortunatly not. My spare tube supply is low, I haven't worked on amps in about 15 years and just dusted off my test equipment last month. I'm going to ask my friend to get an actual 5U4GT. I can't think of anything else at this time.

[Iceviper]

Thanks, I'll try that tomorrow night after work.

[tubeswell]

filter cap shorted to ground maybe?

[Prairie Dawg]

This may be useful to you.

Winged "C" (SED) 5U4-G Black Plate

It still might be a bad tube, or a bad socket-I just went through that with a Mesa Boogie that was here for a few days.

[Iceviper]

I checked the quad can (checked good), and reformed it (leakage measured 2ua). At this point I have isolated the rectifier circuit from the rest of the amp and am still experiencing the problem.

[Iceviper]

Thanks, I'm going to look at that socket when I get home. Considering the heat that this unit experienced with the SSR, carbonising may very well be the problem. I'll let you guys know what I find. Thanks to all for the insights and help, much appreciated.

[MWJB]

Then it's the rectifier, if the fuse doesn't blow with it removed.

[tubeswell]

If the rectifier has shorted, that will also most likely cause the cap can to short (to ground) (due to it ending up conducting AC), which in turn will short the HT to ground, causing the PT to fry.

This is why R.G.'s suggestion of putting 1 x 1N4007 in series with each side of the HT winding and the rectifier tube anode (with the banded ends pointing toward the anodes) is a good idea. Prevents the caps from seeing AC if the rectifier tube shorts.

So even though you reformed the cap can, it might be a good idea to replace that as well - especially if its been sitting in a cupboard for 10 years (and if nothing still works, then replace the PT as well).

[Iceviper]

Sorry, you lost me on HT (if PT= power Tranny). I replaced TR1 with a new Hammond unit. I don't see the post on adding the blocking diodes, but thanks for that info. At this point, with the rest of the amp disconnected from the supply and it still blowing the fuse, I'm going to give the socket a good inspection, my thinking is that it must be that or the rectifier tube. 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. Thanks for the idea's and comment. You guys are very helpful.

I've been working in the electronics field (and with tube equipment) since the late 70's an I'm still amazed at how little I know and how much I can learn.