Well, I've never built a reverb...

[dwinstonwood]

But, I kinda like the Surf thing.

Another project idea is percolating. I'd like to attempt a Fender AB763-type reverb. But, I also want to build a single (Normal) channel 6G6-B. Just to get the thought processes going I've been trying to model that unusual tonestack in LTSpice. I can't figure out how to wire an LTSpice pot into a variable resistor, but this seems to work. I also guessed on how to make the 350K/70K tap pot. So, I'm not sure if that's right, either.

Any thoughts are welcome.

[PRR]

I did not know LTspice had grown a potentiometer?

How would you make a real variable resistor out of a real pot? Basically what you did there. My SPICE would cry about the dangling end; as in real life it is reasonable to tie that to wiper.

What drives this network?? Looks like an AX7 Plate, which is not a perfect voltage source but a saggy thing. Maybe you can hide that parameter in the VS? I like to make it explicit. In much Fender work, assume a stage Zout is around 39k.

The simple pot models do not have taps. What you did there will NOT show what happens when wiper is turned below the tap. And that's really the only distinction of this network: a serious top-cut. Remember that a 350k pot can be sliced as 350,000 1Ω resistors, or 35 10k resistors, or five 70k resistors. Now tap-in the mid network, and tap-out six "possible wiper positions". This is not quite right because it misses the VOL pot loading but that's minor. You have to place multiple probes (instead of sweeping multiple wiper points).

On the real Audio-taper pot the "2" point is really the "50% turn". So you might split the bottom 70k and see what you get at "1".

[dwinstonwood]

Thanks PRR, I built the pot models (represented by the symbols) using Kuehnel's Circuit Simulation book; the same for the voltage source. I'm pretty sure it has the Zout written into the model, if I remember correctly. There's a lot of Spice code not visible in my schematic (.lib pot_V1.txt, for example), all from following Kuehnel's instructions.

I'll keep messing with it. But, it's really just to get me started brainstorming about the amp as a whole, and where I want to go.

[dwinstonwood]

Thanks again PRR for your input. I fiddled with LTSpice some more and came up with this.

As I understand it, the tapped treble pot was a Hi-Fi gadget that gave a sort of Bass boost at lower volumes - sort of like a Loudness switch.

I set the Bass and Treble steps to 100% resistance, 50% (which would not be 12 O'clock on the knob), and wide open.

My results show that with the pots down all the way I get what amounts to a Bass boost, i.e., a low volume Loudness switch turned on.

At 50% and wide open there is the traditional Fender Mid scoop.

Of course, everything I did in my modeling could be all wrong!

[brewdude]

I was under the impression that it was the PI that was more important to the blonde/brown fenders then the tone stack… admittedly, my experience is limited.

[dwinstonwood]

That's interesting. I wasn't aware of that. The Tweed has a 470 coming off the cathodes, and a 10K tail; the 6G6-B has an 820 and a 6.8k; and the AA864 has a 470 and a 22k tail. I honestly have no idea how all that affects the operation.

But, now I'm going to read these two pages:
http://www.valvewizard.co.uk/acltp.html
https://www.aikenamps.com/index.php/designing-long-tail-pairs-the-load-line-approach

Thanks for the push.

[brewdude]

And, I think, AB763's use a 12AT7 PI.

[66Strat]

That's interesting. I wasn't aware of that. The Tweed has a 470 coming off the cathodes, and a 10K tail; the 6G6-B has an 820 and a 6.8k; and the AA864 has a 470 and a 22k tail. I honestly have no idea how all that affects the operation.

But, now I'm going to read these two pages:
http://www.valvewizard.co.uk/acltp.html
https://www.aikenamps.com/index.php/designing-long-tail-pairs-the-load-line-approach

Thanks for the push.

You may find this calculator helpful.
https://www.ampbooks.com/mobile/amplifier-calculators/long-tailed-pair/