Rectifier oddness

[drewl]

So I'm working on an Ampeg B25, I know the two 5AR4 rectifier tubes I have are flaky, but they work if I power the amp up with the standby off, and the voltages can rise slowly.
If I power it up and then turn the standby off, the amp blows a fuse.
Is it the sudden slamming of the load that a compromised rectifier tube can't take?

[Raybob]

So I'm working on an Ampeg B25, I know the two 5AR4 rectifier tubes I have are flaky, but they work if I power the amp up with the standby off, and the voltages can rise slowly.
If I power it up and then turn the standby off, the amp blows a fuse.
Is it the sudden slamming of the load that a compromised rectifier tube can't take?

How old are the filter caps?

[Enzo]

What fuse is in your amp?


And in my head, it seems more likely old caps are making too much of a charging spike, rather than the rectifier itself. I could be wrong.

[drewl]

Well the caps are "only" 40yrs old..........
Mains fuse is supposed to be a 3A slo blo, and it popped a 5A slo blo I put in for testing.

[Enzo]

I'd disconnect the caps and clip something into their place for testing.

[drewl]

Yeah, I tried that with an intermittant JJ5AR4, and it still blew the fuse once.
replaced the filters since they're old, and I'm gonna put a fresh rectifier in when it gets here.

I think both these rectifiers were in my Korg AC30, which is known for killing them, I removed the standby in that amp since, and I wonder if they'll work in the AC30 if they can come up to voltage slowly without getting hit by the circuit load.

I never like rectifier tubes, they always seem to flake out on me.

[R.G.]

Hmmm. This sounds like it might be one of several things, including the caps issue noted.

It might be interesting to try the rectifier tube current clamp I genned up: Tube Amp Current Clamp It can set the available peak current to anything you like. The current through it never gets bigger than you set with the current sense resistor.

[drewl]

I'll have to try that RG, but it was the bad rectifier tubes I had.
Put one in my AC30 and it did the same thing.

[R.G.]

Yeah. I was just thinking it might be a way to soft-cushion the rectifier tubes - keep them going a bit longer, or protect new ones. Just speculation.

[1ampman]

My Hi/Fi audio buddies don't have good luck with JJ 5AR4/GZ34's. They don't hold up....

[drewl]

Yup, this JJ was basically new and is failing.
The other is a Sino, which came with my '93 AC30, so I'm guessing it's the original.