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One tube reverb circuit

12/7/2000 10:52 PM
Dave B.
One tube reverb circuit
I've noticed alot of decent sounding amps use a single dual triode (12AX7)for their reverb circuit.  
(Bedrock, Hoffman, Boogie, and others)  
I'm sure they didn't go out and have custom transformers made for this purpose, Right?  
Does everyone who builds these circuits use the standard fender reverb tranny? How does this affect tube life? Obviously the Fender reverb tranny is made for 2 12AT7 triodes in parallel. And most of the one tube reverb circuits I've seen use a single 12AX7 triode for the drive circuit.  
Any insight in this matter would be appriciated.  
12/8/2000 4:25 AM

I think it's been discussed but a reissue reverb OT is a better match for a single 12AX7 triode then the parallel 12AT7 sections.  
12/8/2000 6:06 AM
Dave B.

That's good to know, I'll try those one tube circuits with confidence.  
By the way your wire went in the mail today.  
12/8/2000 12:13 PM

Just bear in mind that these 022921 reissue reverb driver OT /I got one from Sovtek recently/ come with a paper that indicates 14000/4 Ohm primary to secondary impedance which doesn't look to me like a good impedance match for a single 12AX7 section but at least you can try and say what is the result.
12/9/2000 3:28 AM
frank paxton
12AX7 vs 12AT7 Reverb Tube
Bruce: Are you saying that you can use a single triode stage out of a 12AX7 (rather than both stages in a 12AT7) to drive the transformer and then use the remaining 12AX7 stage as a recovery amp ? This sounds good ! Is there a schematic somewhere I can look at or is it just a straight substitution ?  
/frank paxton
12/9/2000 7:22 AM

Although I've done it number of times, there is no schematic.  
You have to tweak the circuit a little bit.  
It is not quite as fat as a normal Fender reverb but I don't know anyone, who actually plays out, that uses a Fender amp with the reverb up past about 2.5 to 3, so in a new circuit, after a little tone and gain tweaking, a 12AX7 works OK after all.  
If you look at the specs of a 12AX7 and then the specs of the 12AT7 driven 8 ohm reverb tank, think 24K-30K (my actual measurements) into an 8 ohm tank load here not 4 ohms, I think you'll see that a standard reissue Fender reverb OT and stock reverb tank runs fine with one section of a 12AX7.  
Reverb is one of the easiest of all circuits to get working and sound decent so this is not brain surgery.  
12/10/2000 4:17 AM

Bruce said:  
>>Although I've done it number of times, there is no schematic.<<  
Here's the next best thing to a schematic, from Doug Hoffman's site.

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