Reverb mods for '70's Super Reverb

[Steve A.]

One of my friends likes to use a 12AX7 to drive the reverb transformer in his SF Super Reverb (with the push-pull master volume control)- to crash the springs more to get a Ronnie Earl sound.

Any thoughts on this? I didn't think that a 12AX7 would be "happy" there without some modifications to the circuit. And that it might be better to stay with the 12AT7 and just modify the circuit.

Thanks!

Steve Ahola

[Enzo]

The proof is in the pudding. They may not be happy there, but do they work? And what if the tube only last ten years instead of 20? AFter all, 6V6s should'nt be happy in just about anything Leo Fender ever stuck them in.

If the issue is plate dissipation, yes the 12AT7 can handle 2.5 watts and the 12AX7 only 1.2 by the book. But just how much dissipation does the reverb driver require in the first place? If it is only half a watt say, then neither tube is stressed. And that is forgetting for the moment that the amp parallels the two halves of the tube for higher dissipation. I have no idea what the reverb driver power situation might be.

[Ray Ivers]

Steve,

Here's some TubeCAD numbers to back up Enzo's post...

Assuming the reverb transformer has a 100R primary resistance, each half of a 12AX7 will idle at about 400mW with a shared 2.2K Rk, with the usual overvoltage situation of most Fender reverb circuits (either > 390V or > 430V across the tube). The 12AT7 will idle at about 650mW per side.

A 12BZ7 might be the ideal tube for your friend - it has the same voltage gain as a 12AX7, but has half the plate resistance and will drive the transformer/springs harder, plus it has twice the plate dissipation. It's a tall tube like a 12BH7, though, and won't fit under the stock tube cover. If he really likes the 12BZ7 he may want to rebias it, but it will work as just a plug 'n play.

Ray

[Steve A.]

Thanks for the replies! I guess I should have added that a friend of his had taken the 12AX7 out so its not in there right now. I told him I'd check here to get some expert opinions on it. I thought it might be a mismatch in terms of impedance as well as power dissipation, but if you two think its fine who am I to argue?

Steve Ahola

[Enzo]

Well, it is a reverb driver, and not something you listen to directly. So if the impedances don't match up, so what. If the plates are not overtaxed, and it sounds OK, that's all there is to it. It's not like you'd hear any crossover distortion of something. Yes, I know there's no crossover in SE stages. But if there WERE, you WOULDN'T.

[sbluestubes]

Has he removed the .0022 uf going to ground from the reverb return jack ? That will go a long way toward the blackface verb sound.