Not Happy New Amp Day

[duhvoodooman***]

E-w-w-w-w-w, that big filter cap and around it definitely isn't looking good! Actually, the cap may just be singed--looks like it was the resistor below it that went up in smoke.

Calling Tung, calling Tung.....come in, Tung!

[Katastrophe]

Ewwwwwww. Someone call that amph a doctah!

I hope it's a cheap fix, Robert.

[bcdon***]

I'm not an amph doctor, but it's just a cap and a few resistors. The real issue is what caused the blow out.. perhaps you didn't seat the tubes properly.

[tunghaichuan***]

I'm not an amph doctor, but it's just a cap and a few resistors. The real issue is what caused the blow out.. perhaps you didn't seat the tubes properly.

Correct. That node is the B+ supply for the EL84 screens. The one good thing is that the resistor blew on top of the board, i.e., not on the trace side. The carbon can cause problems later on if it gets on the PCB's traces.

It is a fairly simple circuit, so any good tech should be able to fix it.

My guess based on the photos is that either one or both of the EL84's screen grids were damaged in shipping; they are not the most robust of tubes. But it is hard to say for sure as I don't have the amp in front of me. Your tech should probably check all the tubes to make sure they are good.

You will want to have the tech check R24 and R25 to make sure they aren't toasted as well. Those could be increased in size. right now they are 100 ohm. Anywhere from 470 ohms to 1.5K ohms would provide more protection for the EL84 screens.

Finally, I've heard bad things about the Illinois caps (marked "ic"). Depending on the age of the amp, you may consider having those replaced when the amp is serviced.

Sorry about your misfortunes with the amp, but hopefully it should be relatively painless to have it fixed.

[kiteman]

Finally, I've heard bad things about the Illinois caps (marked "ic"). Depending on the age of the amp, you may consider having those replaced when the amp is serviced.

Sorry about your misfortunes with the amp, but hopefully it should be relatively painless to have it fixed.

You heard right tung. I got a terrific book on tube amps and the author despises ICs as they're pure garbage (in his words) and says replace all of 'em and as a result the amp will sound 10x better.

I hope all the best for you Robert.

EDIT: now that the posts are updated I had thought that Robert switched the tubes.

[duhvoodooman***]

I got a terrific book on tube amps and the author despises ICs as they're pure garbage (in his words) and says replace all of 'em and as a result the amp will sound 10x better.

While he doesn't badmouth them, it's interesting to see that Bill Machrone had replaced the IC filter cap in the first filter stage of his Pro Junior in that photo I posted earlier in this thread (repeated below). He states that the two Xicon caps he substituted "....have lower ESR (equivalent series resistance) and better ripple rejection." But he left the IC caps in the 2nd and 3rd filter stages, so he must not consider them "pure garbage".

BTW, his Pro Junior is a newer version (2006) and had a 47 uf IC cap in that first filter stage (Robert's has two 22 uf IC's there). Machrone replaced it with two Xicon 47 uf's to cut the voltage ripple in half.

[duhvoodooman***]

Here's a photo off Bill Machrone's site that shows how that section of the board should look. Looks to me like R32, the resistor below the 22 uf filter cap, is what let go:

I'm not an amph doctor, but it's just a cap and a few resistors. The real issue is what caused the blow out.. perhaps you didn't seat the tubes properly.

Yeah, exactly. And pray that it didn't take out the power transformer with it....

[Eric***]

Here's a photo off Bill Machrone's site that shows how that section of the board should look. Looks to me like R32, the resistor below the 22 uf filter cap, is what let go:

Is that picture a modded board? It looks like there are a couple of wires -- one yellow and one blue -- that aren't in Robert's photo. Maybe those had something to do with it?

Don't know, just my uneducated observation...

[duhvoodooman***]

Is that picture a modded board? It looks like there are a couple of wires -- one yellow and one blue -- that aren't in Robert's photo. Maybe those had something to do with it?

Good point! Those wires are part of one of BillM's mods for his Pro Jr. I was just trying to show what the unsinged components in that area of the board were....

BTW, here's a link to a Pro Jr. schematic:

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

Looks like R32 is a 2.7 Kohm 2W power resistor that steps the B+ voltage down from 319 VDC to 294 VDC.