Reverb send / return 5F6A / JTM45 (correction)

[tompagan123]

I corrected an error in the  circuit topology from my original posting.

Greetings.  This is my first post.  There has been a lot of discussion on this topic but haven't seen my particular question posed.  I'm adding reverb to a circuit that's essentially a 56FA ("Western Electric" circuit).  Actually, it more like I'm taking a Princeton reverb like circuit and chassis, removing the vibrato, and modifying the preamp to have a bassman/JTM45 like topology.


Proposed Circuit Topology

Input pre (1/2 12AX7) --> Cathode follower driver (1/2 12AX7) -(rev send)-> DC Cathode Follower --> TMB Tone Stack --> (rev return) --> PI driver --> fix bias Cathodyne --> power amp.

The reverb would be very similar to the AB763:      (rev send) -->reverb driver (parallel 12AT7) --> reverb transformer -->  tank-- added recovery amp (1/2 12AX7) --> (rev return) 

So essentially what  I'm suggesting here is that I can steal my reverb send from the grid of the cathode follower through a coupling cap.  The signal has been amplified twice by this point and is affected by the volume control.   I realize that there will be a small amount of grid current flowing on the cathode follower (based on merlin's observations) which adds a soft clipping effect. The reverb send would mirror this which I can't see as a bad thing since it's part of the sound of the amp.     
Based on my limited knowledge, it would seem that tapping this signal should have little effect on the cathode follower itself, unless the reverb driver (parallel 12AT7s) were to go  into grid limiting . If this happened, it (might?)  interfere with / affect the dry signal.  Or would that only be the case if the reverb driver went into blocking distortion.    Any thoughts would be greatly appreciated because I'm on shaky ground theory wise.   

I realize that there are other ways to skin this cat (ouch), but this approach complements an existing blackface based design.  If this is difficult to follow I could send a diagram..      Much thanks in advance!

[tubenit]

There may be useful information to you here:  http://el34world.com/Forum/index.php?topic=7957.0

From your post, I can NOT tell if you are using an LTPI phase invertor or concertina since you said it is Bassman like and PR like?

With respect, Tubenit

[tompagan123]

Hi Tubenit.   The schematic is interesting.   It seems like you're  using a 100K resistor in a similar vein to the 3.3M || 10pf used in the blackface design.     To your question, my amp uses a concertina, so the RV2 side of the 100K would be connecting to the grid of a normal gain stage that drives the concertina, not to the concertina directly.   ** That shouldn't make much difference in operation compared to the LTP in your design, should it?

I had thought about using a 1.5M (without 22pf cap) which is what I currently use in place of the 3.3M || 10pf.   I wasn't sure how small I could go on that resistor without introducing positive feedback in the reverb circuit.    That's kind of why I had considered tapping off the cathode follower - for the additional isolation.
 
The other advantage your approach is the single tube reverb.   I had been planning on using the standard Fender parallel 12AT7 reverb driver just 'because'...  If I can use a single tube reverb driver then I don't need to lose the vibrato function.  I'm not looking for surf tones here. Just some built in ambience so I can keep my rig pedal free. 

Much appreciated!

Tom

[tubenit]

This could be an interesting build.  Not saying you should do this, just offering some ideas to consider.  I probably would add another filter cap on the B+ rail.   

I decided to not use the original PR reverb for two reasons:  1)  this allows a 100k mixing resistor since the reverb pot has such a high value & 2)  this one verbs more mid tones, IMO & I think it sounds warmer in tone.  The 100k is not going to attenuate as severely as a 3.3M does.

Note that I lowered the coupling caps after the PI to .047.   And it may sound "better" having a .01 cap after V1a instead of the .02?

With respect, Tubenit

[terminalgs]

It sounds like you want a Reverend Kingsnake or Hellhound.  you can track down a schematic with google pretty quick.

[tompagan123]

Good suggestions Tubenit.   I can see how the 1M reverb pot allows for the smaller 100K mix resistor.  That's slick.   And mid frequency prominence makes total sense.  That was also one of the reasons that I had considered sending from the CF grid - it's pre tone stack which means  pre mid scoop. I could also use a big grid stopper to minimize the possibility of grid current limiting by the reverb driver, which might also naturally roll off high frequencies.  Then roll off the lows by using a small value bypass cap.   

But your topology with the send/receive across the 100K and 1M reverb pots is elegant.    A couple notes

1) My build has a driver tube before the PI.   You're showing the 100K going directly to the PI.   I assume that would make no difference..

2) I've never tried a single tube reverb driver.  I was discouraged from doing it by merlin's article on valvewizard. Is the single 12Ax7 able to provide sufficient drive via the transformer  to properly excite the 8 ohm tank?   Just want to make sure I'm not missing any hidden assumptions.

Thanks again!

Tom

[tubenit]

Is the single 12Ax7 able to provide sufficient drive via the transformer  to properly excite the 8 ohm tank?

Please read the ARCHIVES thread on the one tube reverb.  I did my best to describe my experience with that design.  It is MORE than adequate for me & had waaaayyy more reverb then I personally use.

I will say the Dumblish cloners use the 1 tube reverb also and I've read many positive reviews.  It may or may not be sufficient to your tone tastes.  You will have to decide whether to try it or not?

You can put the insertion point prior to the driver tube if you wish to.  It's a matter of personal taste, IMO. Some of this stuff you have to experiment with on your own to see if you like it.  What works excellent for me may not be what you want at all? 

Also please note that the PR Bassman (schematic) is NOT a one tube reverb.

Tubenit

[tompagan123]

Tubenit,

  I do see now that the PR_Bassman you posted has a parallel reverb driver.  I missed that.  But to your point, I think I'll prototype the single tube reverb for this build because this amp is definitely not designed for surf, and I only use a tad or reverb.  I'll update when I have this guy running.

Thanks again for sharing the circuits!

Tom

[tubenit]

OK, in light of using the one tube reverb ................ then I would personally parallel V1.  You get 30% more gain and no increase in floor noise.  You can use a 5751 there which is a very musically sweet tube, IMO.

With respect, Tubenit

[tompagan123]

Interesting idea.   I could also add a switchable treble boost using a combo of full range partial cathode bypass in normal mode in combination a switchable small cap for treble boost.  Kind of like a built in dallas rangemaster.

Thanks again Tubenit!