6SN7 Push Pull Reverb Driver

[W5FH]

If anyone has the time to give me a little advice -
I incorporated a 6SN7 as a long tailed pair, per Merlin's write-up, driving a 2 watt MPS 22.5K (push-pull pri) to 8 ohm transformer. The tail R is 6.8K, cathode R is 910 ohms, with 820K grid bias resistors. Non inv grid has 0.1 ufd to cathode. Feeding with a 0.002 ufd cap to inverting grid from wiper of 500K dwell pot. The dwell pot high side has the following stages driving it: Input stage 6SL7 triode, tone stack, 6SL7 triode stage, 0.002 ufd cap. Just ordinary LTP set up except use of transformer. B+ is 332 vdc.
The 6SN7 stage is really drive hungry. I did not realize that as this is my first time to utilize it. At idle I measured 83.1 vdc on cathode and 73.3 vdc supplying the grid resistors (from junction of tail and cathode R's). I am measuring 10.8 mA of idling current flow to stage. Can someone advise me if I have the correct cathode and tail R's? I do have up to 415 vdc to power this stage. But I thought I would ask first. During initial testing I had an 8 ohm small speaker connected to transformer (instead of pan). I was using my generator with 1 khz sine wave and it took nearly 9 volts drive to get a reasonably loud signal out of speaker.
Thanks all.

[Stevem]

Can you post up a schematic of this push pull reverb driver circuit?

[W5FH]

Thanks reply Stevem-
Here is a schematic that I started from. Note these changes :
1. I used push pull transformer primary as plate loads
2. I used 820K grid R's
3. I did not use feedback components, just have a 6.8K tail R to ground
4. The 0.1 ufd cap on non-inv grid goes to ground, not to cathode as in my first post.
5. It is driven through 0.002 ufd cap off wiper of 500K dwell pot.

6SN7_LTP_PI.jpg

[Helmholtz]

Measurements look reasonable. Are you using an 8 Ohm input tank?
I'd estimate the differential gain to roughly 12, i.e. from input grid to full primary.
The OT steps down by 53 to the 8 Ohm secondary, so total gain from input to output will be only 12/53 = 0.23.
Means that a 9Vrms input signal should produce an output of 2Vrms or 0.5W into an 8 Ohm speaker.
Acc. to Accutronics the 8 Ohm input tank needs a nominal current of 28mA. This should be achieved with a 1Vrms input signal.
There's not much you can do to significantly increase gain with this circuit.
E.g. an ECC81 would give only 25% more gain.
The typical Fender SE reverb drive circuit with 2 parallel ECC81 triodes might have a 4 times larger gain.

[martin manning]

I incorporated a 6SN7 as a long tailed pair, per Merlin's write-up, driving a 2 watt ...

Can you link to Merlin's write-up so I can see what you are attempting?

[Helmholtz]

I found the circuit in Merlin's latest book, using an ECC81 with a small dual primary mains power transformer used as OT driving a 600 Ohm tank.
I think the OP should post a pencil drawing of his version.

[W5FH]

Thank you both for nice replies-
I will draw my particular circuit out and post. Here is some info on the amp it is used in. I built an octal version of the John Mayer "Wonderland Reverb" based on info from The Tone Geek under the "Fairfield County Signature" circuitry. As I am still developing my understanding, I thought the 6SN7 would be a good choice for a push-pull reverb driver. Last night I took the amp to a friend's studio and he test drove and evaluated it. The amp and reverb sounded very nice, and reverb was of a slightly different character. Previously I built a 12AX7 version of the Wonderland that made a very nice sounding amp.

[martin manning]

Helmholtz, which book is that? I'm still confused as to what the circuit is.

W5FH, I was thinking you are using the LTP outputs as the two phases driving a p-p output transformer, with the B+ supplied at the CT primary (same as Helmholtz, I believe), but I don't think that is correct based on the drawing above. If you have a link to the Fairfield schematic that would help too. It's often difficult to find a particular post on TAG via search.

[Helmholtz]

Helmholtz, which book is that? I'm still confused as to what the circuit is.

Speaking of Merlin's book "Designing Valve Guitar Ampliiers" (2025), page 175.
The OP's drawing is wrong. Missing CT connection and wrong pin numbers.

[martin manning]

W5FH, ok, I see your other thread with the Fairfield schematic.

[martin manning]

Helmholtz, which book is that? I'm still confused as to what the circuit is.

Speaking of Merlin's book "Designing Valve Guitar Ampliiers" (2025), page 175.
The OP's drawing is wrong. Missing CT connection and wrong pin numbers.

Thanks I don't have that one... I guess I need to order a copy.

[martin manning]

Is the circuit like the one at the bottom of this page?

https://valvewizard.co.uk/reverbdriver.html

[Helmholtz]

Is the circuit like the one at the bottom of this page?
https://valvewizard.co.uk/reverbdriver.html

That would be a transformerless SRPP driver.
The circuit from Merlin's book is an LTP used as a self-splitting PP amp loaded with a PP OT connected between the plates and CT to B+.

[didit]

IMG_2170.jpeg

[Helmholtz]

That's what I not wanted to do as the book is copyright protected.
Did you get Merlin's permission?
Otherwise better delete the post.