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Reverb tranny impedance???

2/18/2004 6:37 PM
Chuck Reverb tranny impedance???
How can I tell what the best plate load impedance would be for a reverb circuit. (or any other circuit for that matter)  
If I wanted to try a parallel 12AU7 or a triode connected EL84 for example. I'm sure the answer is in the tube spec sheets, but I can't understand them well enough yet.  
Thanks for any help.  
2/19/2004 7:41 PM
Ray Ivers

Assuming a reverb tank with 10 ohms input impedance, the design criteria for a reverb drive stage would be very similar to any single-ended output stage driving an 8 ohm speaker.  
For a given triode at a given plate voltage, using the tube-manual ballpark plate loading as a starting reference point, increasing the load impedance decreases both output and distortion, and vice versa (within limits). The 'best' load impedance is the one that achieves the results you want (maximum power, minimum distortion, somewhere in between, etc.).  
Most of the reverb drive stages I've seen use an output of 500 mW to 1W. A parallel 12AU7 or EL84 triode stage (along with many other tube types) will be able to do this, using the standard 10K/8 ohm Fender-type reverb transformer.  
This unit, while easily available and widely used, is not exactly the beefiest 1W SE transformer the world has ever known. I realize we don't need to supply the drive transducer with a squeaky-clean hi-fi signal, but it would be nice to not have it hard-clipped or running deep into transformer core saturation, either.  
If you intend to determine the best tube type and drive circuit/transformer for the tank you've chosen, that's great - let me know and we can crunch some numbers. OTOH, if you're going to go with the Fender unit there are plenty of existing circuits out there that can be slightly tweaked for your tube choices.  
2/19/2004 8:46 PM

Proper impedance matching is expected to provide the best performance, but recently when building a circuit to drive a reverb tank removed from a solid-state Conn organ we decided to take our chances and just use the proven Fender circuit.  
That way if it didn't work, or if the unproven short tank with *folded line* spring was not very toneful, we could just use a regular Fender replacement tank and the outcome would be a known quantity.  
This reverb tank for solid-state drive has input impedance of 1475ohms, so I thought I would just treat it like the load on a small power amp where different amps have the ability to drive different impedance loads, and the lower impedance the load the more difficult it can be for an amp to drive unless it is CAPABLE of handling such a low impedance load.  
It sounded good in spite of the drastic mismatch, and compares well to a regular low-impedance-input tank on the same amp.  
So my feelings now are that the Fender transformer-based drive allows a tube to drive about any reasonable load (at least as low as 8 ohms). It just naturally continues to put out about the same signal voltage at 8ohms as it does into 1000ohms since with the step-down transformer the output is low-impedance capable and loads as low as 8ohms do not *load the output down* like would happen with a higher-impedance-output drive circuit.  
My simplistic feeling was that a circuit *optimized* for the solid-state tank would not drive a typical Fender tank. OTOH, using the Fender-style transformer it can easily drive the low-impedance tank (no surprise) or just about anything having that much impedance or more.  
I think the advantage of matching impedances is more important with higher power tubes and where more significant currents are involved. The reverb drive application is enough like a small-signal voltage amplifier with step-down that it can perform just as well there as it can as a voltage to current converter.  
2/20/2004 1:24 AM
Thanks guys. I didn't figure the impedance match was ultra sensitive. But I didn't want to be grossley out either. I have an old Stancor 10k/8ohm tranny from an old TV set that was running a single 6V6. It looks a damn site better than the stock Fender unit. Which I believe is 25k/8ohms.  
I have a reverb circuit that has never worked right. I bought the parts new from Torres. In a stock BF type arrangement it distorted badly at any level. I checked and rechecked the circuit. No trouble there. All voltages and current readings good. Subbed in another tank, same problem. Swapped tubes, same problem. Nothing left but the tranny.  
I think I'm going to try an EL84. I have alot of em and it'll look cool in there.  
As for using the load specs from the tube data. I can do that. No brain needed. But I've never seen the load specs for a 12AU7 triode or even a triode connected EL84. Not that those specs don't exist, I just couldn't find them.  
Thanks again.  
2/22/2004 4:37 AM
Dave B

Check out the Svetlana tube site. Eric Barbour did some circuits for the 6BM8 tube. One of those circuits was for a reverb driver that used a 10k transformer. This tube is cool because you get a power pentode for a reverb driver and a triode for reverb recovery all in one 9 pin tube.  
2/22/2004 4:57 PM
Thanks Dave. I will check that out. I've heard good things about the 6BM8 in reverb circuits. I may not use it on this amp though. I already have all the triodes in place and accounted for. So I have 1 9pin socket in place to drive the tank. I really don't need to free up a triode for anything.  
How easy is it to get 6BM8s? And for how long?  
2/22/2004 9:08 PM
Dave B

Svetlana (Now Sovtek) still makes them.  

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