Fender ProTube Twin-Amp reverb feedback

[tot_Ou_tard]

Thanks for the replies guys! Extra thanks to oldguy for checkin' out my pics. I'm glad you like 'em.

So, I just tap on the tubes and listen for strange sounds, eh? I guess that sounds easy enough. Even for me.

In fact, I'll go give that a try right now.

A microphonic tube will be obvious, kind'a the dental hygenist hitting the exposed nerve.

[Mick]

A microphonic tube will be obvious, kind'a the dental hygenist hitting the exposed nerve.

Well, I removed all of the metal shields (vibration dampers?) from the preamp tubes and let the amp warm up. Then I tapped each tube several times, including the power tubes. All I heard was a tiny bit of noise that sounded like small vibrations coming from the springs in the reverb tank. And they all sounded the same. I did the test several times, with the reverb dial set higher each time. None of the tubes had any obvious difference. Oddly enough, this time I was able to turn the reverb dial up to 8 without getting the feedback. But if I turn it any higher the feedback starts right up.

Is it possible that I have the bias set too hot? Can that cause feedback? The bias setting instructions on Eurotubes.com says "Super's and Twin's have a higher plate voltage so with the JJ 6L6GC's we see them come out of crossover distortion at about 34mA but we typically like to bias them between 38 to 44mA and most players like them best at 38 to 40mA." I set the bias on this amp to 39mA.

What should I try next?

[Moshe]

What should I try next?[/QUOTE]


I would swap out the tubes one at a time until the feedback goes away.

[Mick]

I would swap out the tubes one at a time until the feedback goes away.

Okay, I'll try that...

But what if the problem is actually a combination of two or three bad tubes? This amp has a total of 12 tubes (seven 12AX7's, one 12AT7, and four 6L6GC's) and any of them might be microphonic, right? I could be swapping tubes in different combinations until hell freezes over!

When I was a kid they used to have vacuum tube testing machines at the store. I remember my dad taking tubes from our TV in for testing. Why don't they have something like that nowadays?

[markb]

Okay, I'll try that...

But what if the problem is actually a combination of two or three bad tubes? This amp has a total of 12 tubes (seven 12AX7's, one 12AT7, and four 6L6GC's) and any of them might be microphonic, right? I could be swapping tubes in different combinations until hell freezes over!

When I was a kid they used to have vacuum tube testing machines at the store. I remember my dad taking tubes from our TV in for testing. Why don't they have something like that nowadays?

So that people can charge you extra for doing the testing for you. But really, without the cynicism, a tube tester like that won't pick up a microphonic tube. For that you need an audio amplifier like, say, a guitar amp. Get two new 12AX7s and a new 12AT7 and swap each one with the new ones. If you swap one out and there's no difference you can say that's a good one. The problem won't be in the power section so you can save yourself the cost of the 6L6s and a bias adjustment straight away. And when you've found the bad one and thrown it away, you'll still have a spare or two for when the next one goes bad :

[Mick]

So that people can charge you extra for doing the testing for you. But really, without the cynicism, a tube tester like that won't pick up a microphonic tube. For that you need an audio amplifier like, say, a guitar amp. Get two new 12AX7s and a new 12AT7 and swap each one with the new ones. If you swap one out and there's no difference you can say that's a good one. The problem won't be in the power section so you can save yourself the cost of the 6L6s and a bias adjustment straight away. And when you've found the bad one and thrown it away, you'll still have a spare or two for when the next one goes bad :

I have two extra 12AX7's. I ordered two Ruby 12AX7A's from Guitar Center before I decided to replace all of the tubes with JJ's (Note: The Ruby tubes I got from GC are just JJ tubes with a Ruby label added). I don't have an extra new 12AT7, but I do have the original Groove Tube 12AT7 that came with the amp. I'll swap those in and see what happens. If that doesn't work, then I'll buy a new 12AT7.

[Mick]

Why does the amp only feedback when the reverb is turned up?

If it's a microphonic preamp tube causing the feedback, then shouldn't it feedback even when the reverb is turned off?

Why does the reverb setting have any effect on the feedback?

If there's no feedback when the reverb is turned off, doesn't that indicate that the problem must be somewhere in the reverb circuit?