mandocaster said:
Sounds as though the standby switch itself might be at fault....it sends the high voltage to the power tubes when turned on, and the sound would cut out if that voltage is interrupted.
Yeah, it could be the standby but this amp is really new. It normally takes a few years for the switch to go wonky.
I just dakked over to SchematicHeaven to refresh my memory of the circuit. From the symptoms described I'm not strong on it being a tube, even the rectifier.
Of course, you should ALWAYS swap in a new tube as the first test! It's quick and easy. Many times I've wasted troubleshooting only to find it was a tube all along. Still, I got a hunch it's something else.
My money's on a bad coupling cap! If a cap goes leaky it lets unwanted high voltage from the previous stage pass through with the signal to the next one. This voltage slowly screws up the action of the next stage.
Caps often will start up fine and then start to leak over the next hour or two. If it happens in the preamp the volume is ok but the tone will go sour.
If you look at the tubes when the amp is screwing up see if the outside plate in the tube has any orange or red glowing spot on it. This is a sign of the bias on the tube being overridden by the leaking cap and the tube is jammed full on and starting to cook itself. If you remove the shields from the preamp tubes you can see the same thing. I'm not talking about the normal heater glow from the top and bottom but the actual outside plate surface that you see through the glass. Remember there are two separate sections in the preamp tubes.
The rectifier doesn't use bias of course but if there's a much bigger than normal current draw its plates can start to glow as well.
If it were on my bench I would put a meter on the bias voltage/output tube grid feed splitter point and let the amp cook for an hour or two. If the voltage won't stay steady that's a sure sign.
The coupling caps are cheap and it wouldn't hurt to "shotgun" change 'em just in case, either. Why charge the player for hours of time checking each individual cap when $10 will eliminate the couplers as suspects in 10 minutes of labour?
The fact that snapping the standby resets the problem is also telling. It resets the charge on a failing cap, letting it start off ok again.
Finally, the re-issue DRs I've seen are not hand wired but rather printed circuit board, as I recall. If so, I'd flip the board over and touch up every solder joint. Printed circuit boards are notorious in tube amps for getting intermittent problems after a few years (like the few years after the warranty is over!). The constant heating and cooling from the tubes causes the solder joints to "flex" as they expand and contract, causing them to go flaky. The board was almost certainly originally soldered by a wave-soldering machine process. You could have a weak solder spot right from the factory that shows up in just a short time.
Now that I've listed the scary stuff, by all means try the tubes first! If it is a tube it's 90% likely to be a 6V6. Preamp tubes only handle signal and don't wear like output tubes. I've seen 50 year old and more 12AX7s still going strong. Often when output tubes get near the end of their life they start up ok and then crap out as they get good and warmed up, just as you describe.
Despite this problem, you chose perhaps the best sounding of all Leo Fender's designs!:rockon:
