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12/22/1998 1:27 AM
B. R.
I have one 9-pin preamp tube socket left on one of my homebrew amps. Im interested in adding reverb, and I have seen some schematics (Gibson, I think) using a single 12AU7 as a driver and recovery tube. Any opinions or comments on this approach? I do have a single 12ax7 stage before the PI that I can use as a mixer. I am not looking for a real deep reverb sound, just a touch to give the tone a little space.  
Bill R.
12/22/1998 3:00 AM

Fender usually uses a paralleled 12AT7 to drive the reverb tank, and 1/2 of a 12AX7 for recovery. Check the pittman schems for Deluxes, princetons etc. Matchless used this approach in the Chieftan reverb as well. If you can, I would use this setup.  
Vp=410v through the transformer primary  
impedance resistor= 1M  
There are some gibsons that use a single 12AX7 as the driver and recovery tube. Check the "Hank Amplifier" and others in the pittman book. I am not a big gibson reverb fan though, I tend to think the fender sounds much better.  
12/22/1998 3:46 AM

Well, you've got two triodes in a 12AU7 and Fender effectively uses "two" triodes to drive their reverbs, it's just that one "triode" is the twp halves of a 12AT7 for greater power capacity (5w combined). The reverb recovery tube is usually a 12AX7/7025 which has a greater amplification factor which is usually plumbed in for greater stage gain. One-half of a 12AU7 has a plate dissipation of 2.75w which may be a little anemic and an amplification factor of 17-20. Other possible options are to use a 6FQ7/6CG7 which can dissipate 4 w in one section or a triode/beam power tube such as a 6GW8. As I've been considering freeing up part of a tube socket in one of my projects, I've been considering converting to the 6GW8 to drive/recover. You could also consider the 12BH7 which has the same heater current draw and basing as the 12AU7 (3.5w per section). All of the above would give you more drive to the spring but, unfortunately, note offer as much potential voltage gain as may be necessary. Just checked the Gibson schematics in the Tube Amp Book and most use either dissimilar triodes such as a 12AXy/12AU7 mix or a triode pentode such as a 7199 - the Hawk uses a 12AX7 and you might be able to copy this circuit.  
12/22/1998 5:22 AM

Checkout the tech notes on the 6BM8 at Svetlana's . Eric Barbour has a reverb driver schematic that looks interesting...  
12/22/1998 6:04 AM
Jack Orman

"Checkout the tech notes on the 6BM8 at Svetlana's . Eric Barbour has a reverb driver schematic that looks interesting..."
I was going to suggest this tube as well. You could use the pentode section to drive the springs transformer and use the triode section as a recovery amp. I don't remember the Barbour schematic but it would be worth checking out.  
regards, Jack
12/22/1998 9:23 AM

Have a look at varous Mesa Boogie schematics. Some of their amps (Mark III, Caliber 50, Studio 22) use 1 9 pin tube (12AT7) for reverb drive and recovery.  
12/26/1998 9:03 PM
B. R.

Thanks for all the replys. I found the Mesa Boogie circuits, and will probably trythat first. Thanks, Bill R.
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