[Patrick Turner]

Hi Stephie,
Thanks for sending the schematics and the F response.
There is much food for thought.
The problem with the spice .asc file is that it won't open in anything else so I printed it so I could scan it, but RHS info didn't fit on page and there's no select-image and rotate function to get a better page fit so there's hours of mucking about to make a nice looking schematic.
My LTspiceIV program does not seem to have triodes.

""" triode_nh.txt into the Spice (under LTC/LTspiceIV/lib/sub folder and the triode_dd.asy into the sym directory to have the tube models available""" is a bit beyond my very dumb PC skills. I guess I can work out from what you have which are the a, g, k tube connections and then put in a triode of my own making and generally end up with something that can be put on the bench in front of me so it can be made.

I've never used gyrators. Reverse log pots are mostly unobtanium. They have a purpose in wien bridge oscillators, but need to be quasi log types, ie, between linear and log, to get evenly octave calibrations on a dial.

Today I compared a version of a Baxandal with a mid control I devised with the standard Fender with mid control. I ended up concluding my own mid control with pot and 2 caps attached to work on the R below bass pot to 0V wasn't any better than the Fender stack. The addition of CF buffer to drive the stack gave +3dB gain, but not a huge F response difference.
For the Fender stack, If the bass and treble are set at 10, ie both at max, and mid 10kpot at 1, ie, 10k shunted, then you get -9dB dip at about 320Hz and Q is more than 1 and slopes of dip are greater than -6dB/octave, to the circuit works to exploit phase shift some how, and probably no need for anything else. I'll prepare a set of graphs for my customer to show the B, M &T settings to get various profiles, perhaps that will make him happy and sound will be OK, but this amp has only 2 x 6V6 and is a combo, and maybe he has to change his speaker driver.

Your circuit shows nicely symmetrical peaks and troughs for the mid F. Having thought about it all, when would peaking the mids be ever used? The mid dipping is to make sound less harsh, but boost a sheen on top of everything.. Playing a melody would make upper notes too loud. Chords contain many F from bass to highs, and if the mids are toned down and highs boosted, they are supposed to sound better.

Anyway, I'll be able to ask my customer to bring his axe and we can try a few different settings, and see what he thinks. My concern is to get the best sound, even if it seems a bit irrational and counters the strict conservative approach I have taken for hi-fi amps.

I'll also put in a 2k2 pot in series with 820r GNFB resistor to allow reduction of NFB.

People have been confused with potentiometer properties and their type classification for a long time.

Duncan's tone stack program lists type A log, and B log pots and linear pots.
Afaik, Log A pots have 10% of total R between wiper and bottom at 50% rotation. Such pots suit volume controls, and the Fender tone stack has a "sort of" Log A for bass.

At 50% rotation, the Fender bass pot measured 260k total with 21k between wiper and bottom, so pot goes from 8% of total R to full R between 5 and 10 settings.
This is a type A pot.

The Fender has a treble pot which I measured at also 260k total but at 50%
rotation the wiper to bottom = 60k, so 23% of total. AFIAK, this is a type B log pot.

Linear have 1/2 total R at 50% rotation.

But in the world of pots, there are pots and pots, and type A are usually 10% log pots, and type B is a linear pot, so at most stores where pots are to be purchased you have only A and B type, ie, log and linear.

But then there are "type C audio taper" pots, and then there are all the vast array of pots which differ from what you'd want them to be because the manufacturer is a cheap production site somewhere in Asia and the idea of making a very good pot costing more than $0.10c to produce is quite outlandish, foreign, and way too difficult especially if it is a log pot, where a dual gang type might give +/- 3dB difference between channels at different level settings, and its impossible to compensate for pot errors with a fixed R strapped across one pot or both, somewhere, and hence there's a need for a channel balance control. The pots in the Fender Deluxe re-issue look like $0.08c types and because of vast numbers wanted by Fender, they are so cheap,
and Fender can insist on and enforce some quality control.

But elsewhere in say Jaycar or Dick Smith stores there are log pots which seem to have say 3 sudden kinks in their R increase, thus showing the process of depositing carbon is done in 3 squirts of C at 3 different rates, rather than a smoothly changing variation of the rate of C deposition. Seems like that to me. I ain't no expert on just how log pots are made.

However, many log pots sold in Oz for $5.00 are quite long lasting and usable if you buy 4 and choose the best one, and you seal up the openings with cardboard and tape and you cramp the rivets of lugs to avoid loose lugs.
Long nose pliers helped with a vice does this without over-tightening. Repeated soldering of lugs can soften track material and lead to loose lugs and intermittent operation. In other words, the $5.0 pot needs work to make it passable.

Alps "Black", 27mm square bodied pots seem to me to be very long lasting and quite precise enough without "flat" spots where say 20 degrees of turn doesn't alter the Vo level much.

Best of course are switched attenuators. I've never had troubles with DACT.
But the best of Alps carbon pots and DACT are far too expensive for Fender to include as standard items.

But, I did find that I could fit a standard 24mm dia old style linear 10k pot onto the Fender Deluxe PCB to make a mid control, and placed where I had removed one of the 2 "Normal" channel inputs. It is held to board with turn of Cu wire and then wire links to "Vibrato" channel bass pot some 200mm away. The nut which was on 6mm input jack suited the pot, and hole size was correct. Works fine, and give the owner the mid control he wants like in the old 1965 Twin Reverb amps.

Patrick Turner.

Patrick Turner