Tube noob needs help

[Belva]

I recently got a Precision Electronics pa head with 2 7868 power tubes,2 6EU7 preamp tubes and a 6C4 rect. It's rated @ 35 watts & sounds great with tons of "testicular fortitude". Until I put a new lite bulb it the thing. Then I got nothing. No sound until I turned the master gain & the volume all the way up. Then all I got was distorted noise & not much of it. Pull the bulb & all is well again. I don't have a schematic for this amp. My only source is Antique Electronic Supply and that's a crap shoot. Any ideas? I can live without a bulb, but it would be nice to make this thing right & be able to see when the thing is on. Thanks

[StringKing7]

i think u broked it

[Belva]

Fortunately I didn't run it long enough to kill it. And it was that way when I got it. No bulb, but 6.5 volts across the contacts. Corrosion in the bulb socket? Light is bright when in place. I used a standard Fender GE44 bulb. No idea if that is the right bulb for it. But @ 50 cents it was worth a try.

[jimytheassassin]

This might help you

http://www.diyaudio.com/forums/showthread.php?threadid=113271 You'll have to register to see the schematic.

I have a suspicion, this fella had the same P.A., and he posted a modified schematic in the first post.. YMMV.. good luck.

 

BTW, the 6c4 is a triode, not a rectifier, Your rectifier is comprised of 4 diodes according to the schematic

[mike42]

Just a guess. Your pilot bulb is running off the tube heater circuit, and somethings messed up in the wiring, or your using a much higher wattage bulb than was intended, and pulling the whole circuit down. Try a much smaller wattage bulb.

[Belva]

I'll have to see if I can get a lower watt bulb. Can you get lower than a GE44?

I was told the GE44 is standard Fender issue.

[mike42]

Well I had an additional edit in there - I watched it load! -and it's gone. I am too old for this stuff.

 

My thought was, if this is a pretty vintage amp, and running 6 volt tubes I suspect it is, that the transformer may just be running out of steam. The fact that the bulb was missing may be a tip off that the prob was there before. Anyway, you might consider just using a 115V pilot assembly, and coming straight off the switch. There are a bunch of 115V self contained pilot units out there.

 

You could also have something else going on that's pulling the 6V down, but you'll have to get in there and mess around to figure that out. You might try measuring the 6V output with and without a bulb to see what you get, but if the power supply is that stressed you may be wise to not add anything else to it. Might also check your line cord, switch, etc., to be sure that the primary side of the transformer is getting good volts. Or ...my entire diagnosis could be wrong.

 

edit: try again - one more last, last thought - any possibility that putting a bulb in the socket is causing a high resist short across the socket itself?

[Belva]

I took this in for service. Turns out the bulb is wired into one of the preamp tubes. And that tube is on it's way out. So I'm having the tech do a basic tune up & oil change. New tubes & a cap job aint a bad idea for an old amp & no maintenance history. Also Precision Electronics is still in bidness. I plan on getting some numbers from it & seeing if they can dig up a schem.