HELP!! Newbie has Valve Jr Power Loss

[Auherre756]

My beloved VJ head has suddenly lost as least 50% of its output power. At first I thought it would be as simple as changing out the preamp or power tube. No such luck. I attempted to test each component in the circuit for shorts (Note: I'm no electronics wiz, but I can perform some very basic testing, repairs, mods, etc., with lots of instruction) Alas, I couldn't find anything obviously wrong or broken. The voltage across the PT (red wires) is 280V. Not sure if this is is an issue. What else should I be checking?

 

Any advice or ideas would be most appreciated. Thanks.

[cGil]

If you happen to have unplugged the volume pot for some reason, and then plugged it back in, check the connector. If a pin got bent while plugging it back in, that'd do it. A bad triode in your 12ax7 can do it, too, so definitely try another tube. If don't have one, get some (plural). It's no fun chasing gremlins if the tube doesn't work.

 

Gil...

[Auherre756]

If you happen to have unplugged the volume pot for some reason' date=' and then plugged it back in, check the connector. If a pin got bent while plugging it back in, that'd do it. A bad triode in your 12ax7 can do it, too, so definitely try another tube. If don't have one, get some (plural). It's no fun chasing gremlins if the tube doesn't work.

 

Gil...[/quote']

 

Thanks for the suggestions, Gil. Interestingly, the first thing I tried was replacing the preamp tube with a known 'good' one. No change. After a lot of connector and terminal shaking, component tapping (the ol' wooden chopstick routine), anode/cathode voltage testing, etc., I removed and reseated the EL84 power tube...again. And, what do you know? My output power is back to normal. I'm curious as to why things didn't improve the previous times I pulled (and replaced) the EL84. Maybe this time around scraped the terminals clean of oxidation.

 

Thanks again.

 

Rich

[cGil]

Here's a trick I use a lot. First, I buff the tube pins a little with some 0000 steel wool. Then I squirt a little ProGold into a small container (like the cap from the spray can). It only takes enough to dip the pins of the tube into. Then I use the tube's pins to treat the pin receivers in the tube socket. Whatever you do, DON'T SPRAY THE SOCKET! You don't want dust sticking to it and creating a path for arcing between the pins.

 

If the sockets are old and crusty, I'll use a Zero-Residue cleaner to spray it with, and a toothbrush to knock any ancient crud loose. The inside of the nastiest pin receiver can sometimes be prepped with just a quick poke of a round needle file. Doing this may require retentioning the pin receiver afterwards with another poke outside of, but next to the receiver. I use a smooth steel poker for that job. After all that, I wrap up with the tube pin-dip transfer method described above to apply ProGold metal treatment to the pin receivers. Keeps 'em from getting corroded and messing with the signal transfer.

 

I use this same method on 1/4" phone jacks if I don't want to get messy with the overspray blasting all over the inside of the amp. Besides, you only need to treat the actual plug contact points inside the jack, anyway. The rest is wasted spray anyway.

 

Gil...

[Auherre756]

Gil...Good ideas that I'll make note of. Thanks again for your help.

 

Rich

[StarGeezers]

CGil, my Stock VJr had some of what looked like to be beige glue inside one of the pin sockets , scraped it out as best as possible , but still requires some tube wiggling from time to time ... Grrrr!!!!

[cGil]

Beige glue? Like the stuff that's all over the v1 & v2 pcboards got on the socket, too? Yuk! That's a new one.

 

Gil...

[StarGeezers]

worse, it seems very resistant to cleaning...eventual scraping produced brass base metal ,still scritchy scratchy sometimes, requiring a wiggle ...Grrrr!!!!!