5E3 power supply

[Randall]

I have a dead 5E3 clone in shop. The guy thinks it is a 1955, I get to tell him it is not a real one, lucky me.

Anyway, the 5000 ohm screen resistor is toast. It has old cardboard Mallory 16uF filter caps in it from 1964, they all test good out of circuit and are not showing signs of bulging or leaking, surprisingly. In wondering why that resistor baked, I would suspect the second filter cap, or one 6V6 gone bad. However, the tubes test good with no shorts, and so do the caps, so what else could it be? I want to convince him to replae the caps, but he is really hung up on keeping everything original because he thinks he has a collectable here.

http://www.ampwares.com/schematics/deluxe_5e3.pdf

Also, I don't have a 5000 ohm resistor on hand, would anyone have any qualms putting a 4700 in there? It is within 10% tolerance.

[Jon Snell]

The cause can only be the screen grid on the 6V6 or the capacitor.
They fit 4k7 Wire Wound or 3W MO now in the same place.
Check the location sticker for the serial number that includes the date code;

Otherwise it is worthless.

[Helmholtz]

Also, I don't have a 5000 ohm resistor on hand, would anyone have any qualms putting a 4700 in there? It is within 10% tolerance.

No problem at all.
(That resistor is no screen resistor, it's a power supply filter/dropping resistor even though it feeds the screen node.)

[Enzo]

A bad power tube is WAAAAAY more likely than a shorted cap to burn that resistor.

And never forget: it is JUST a guitar amp, so of course 4700 ohms is just fine.

Look on your 5E3 layout drawing, says all readings + or - 20%.

Good thing the guy doesn't have a classic '57 Chevy, he'd want the original dry-rotted bald tires on it.

[Randall]

The power tubes tested good, and I tap on them when I go thru the five different short tests. I also let it idle for about 45 minutes and monitored it. I hope he doesn't have an intermittant tube.

[g1]

An intermittent short in a power tube that only shows up with high voltage applied and maybe only when pushed hard. That is what I would suspect.
Have you tried the tap test with the power tubes in the amp delivering power to a load?

[Randall]

"An intermittent short in a power tube that only shows up with high voltage applied and maybe only when pushed hard. That is what I would suspect.
Have you tried the tap test with the power tubes in the amp delivering power to a load?"

After asking this, I did let it get warmed up while playing mp3 thru it, occasionaly tapping tubes. Sure enough one time a RCA 6V6GTA started to red plate briefly, and the volume dropped down a bit. I tapped it again on the opposite side and it returned to normal, and has been there since.

I can't think of a reason for a tube to red plate in a cathode biased amp other than the fault of the tube?

[The Dude]

..........I can't think of a reason for a tube to red plate in a cathode biased amp other than the fault of the tube?

Never say never, but yeah. It seems you found your problem.

[Dave H]

I can't think of a reason for a tube to red plate in a cathode biased amp other than the fault of the tube?

A leaky grid coupling cap could also do it.

[Randall]

I have swapped tubes from side to side to see if it follows. So far though, nothing.