What reverb tank would work with this Univox U-1226 circuit?

[rshipp]

This is my first post here, so I hope I'm doing this right. I just finished repairing an old Univox U-1226 head (what an unusual design!), and it seems to be working great so far. I haven't pushed it hard yet, so I'm crossing my fingers for now. It was missing the reverb tank when I got it, and I have no idea which tank to order for it. This particular circuit seems to use a 6AN8 to drive the tank straight off one of the plates directly through a small capacitor without the usual transformer to match the impedance to the tank. It then uses half of a 12AX7 for the recovery stage (the other half being used by the tremolo circuit). I know that reverb tanks are rated by input and output impedance, but I don't know enough about electronics formulas and math to figure out what this circuit would require (this is my first major tube electronics project). I'm hoping someone will either know from experience what tank would work, or will be able to tell me what tank impedances I might need. I'm attaching (I hope) that part of the schematic.
Thanks for any help you can give me.

[Enzo]

Spring reverbs all work the same way. You drive the transducer at one end with a signal, then the other end picks up the vibrations and sends them to a basic gain stage for blending back into the amp. The OUTPUT end of reverb pans are all remarkable similar. The point where the pans differ is at the INPUT end.

Solid state drives usually use higher impedance input pans. The Fender type tube drive uses the transformer to drive a low impedance pan input. Your tube type from the plate would want to see a high impedance pan. Fewer amp makers use this method, but notable among those who did was Ampeg.

You would want a FB type reverb pan. As in 4FB2C1B, or 8FBxxxx, or 9FBxxxx.

If you have an EB pan, such as 4EB2C1B, you could try it. Might work, probably not as well.

The Fender transformer drive types use AB pans, as in 4AB2C1B. Those would not work.

[rshipp]

Thanks Enzo---that's the advice I was looking for!

[lowell]

I'm always curious about this too. Wouldn't you want a tank with an input z of around 67k-100k or higher if using a 12ax7? Here the 6AN8 datasheet lists the Ra at 170k for the pentode side and 4.7k for the triode side. http://tdsl.duncanamps.com/show.php?des=6AN8A

Cheers to Enzo for knowing the tanks of the top of his head!

[nickb]

Nice article here: The Valve Wizard

Also see thread http://music-electronics-forum.com/t16071/


Nic