Prof Tweed

[Kipper4]

http://runoffgroove.com/professor.html


I just built it last night.
Had a hard time biasing the fets when it got late. You all know how that is.
Got an early start this evening and rebiased the sucker.

Mods
10nf in the feedback loop
2u2 for source bypass caps
1nf caps in parallel with q1 and q2 drain trimmers
used all j201

Sounds great just what I expected and wanted. Nice fenderish drive and sweet at lower drive (vol) settings too.
thanks to everyone that was involved in the design and anyone who posted their mods., I read up last night in bed.
I've learnt a fair bit about biasing fets ith my recent experiments.
The stock tone control is ok.

However id like to try a differant tone stack
I've had a brainwave that id like to axe the stock prof tweed tone control and put output of the tweed into an active tonestack
like the one used in the BossFA1
schematic here

http://www.freeinfosociety.com/electronics/schemview.php?id=2323

it uses half of an op amp.
My question is.
Is it doable?
Will the tweed output just be too much for the op amptone control. or can i just put a resistor to limit the tweed output before the op amp?
Will I encounter impedance or phase problems by doing this?
would I be better of leaving  out the last Q3 stage and going straight to the op amp tonestack or will this totally anhialate the tweeds character?

thanks for reading
Any help is welcome
Rich

[samhay]

I quite like the Princeton-style tone stack in the Tweed, but I hear that most people don't. Is it the interactivity with the gain pot that you don't like or the simply the frequency response.

In any case, the active tone stack you linked needs to be driven with a low impedance, so will probably only work well if driven with an op-amp. Seeing as you would need 1 op-amp anyway, then 2 isn't such a problem if you use a dual. It is active, so you may need to drop some signal before feeding it, but that should be easy enough to do by adding a series resistor before the gain pot.

[Kipper4]

Cheers Sam
I'll probably use a dual and make 1st a buffer anyway I'll give it a go

[samhay]

Upon a moment's reflection, and seeing as you probably have gain to spare, why not go for a 2-knob passive tone stack with a similar frequency response to the one you posted. Perhaps some thing like this: (from here: http://amps.zugster.net/articles/tone-stacks) wired before the volume pot (you will need to up the value of the 22n cap too).

[Kipper4]

Cheers buddy
I'll breadoard that one and see how it goes.

[Kipper4]

Yep Thanks Sam that works ok for me. nicely interactive too.

[Kipper4]

Quick question.
Why put the tonestack before the vol pot?
why not after or indeed after Q2?
or just before the output pot?

[Kipper4]

Quick question#2

why not use the 18 watt tonestack seen about 1/3 down the page here

http://amps.zugster.net/articles/tone-stacks#TweedPrinceton

It seems to me the volume would not be so affected by this one. put me right if i'm way off.
thanks
Rich

[PRR]

> Quick question

Quick answer: we are talking $2 of parts at most. Hay-wire it up and listen.

I'm quite sure this is how the 5F6A was "designed". Leo had a drawer of parts and a hot iron, and futzed.

[Kipper4]

Thanks Paul
I'm sure Leo was emminantly more qaulified than I.
I guess it was more of a design question. rules and regs. "Expected" "best" place or type.
I can futz all day and i likely will one night after work.
I'm not done yet, Like a tortoise. I go slowly and hope some of it sinks in.
Short term memory is not a strong point after my mispent youth.

[samhay]

>Why put the tonestack before the vol pot?
Because it probably works best when driven with a low-impedance source with the output then fed to a high(er)-impedance load (like a 1M volume pot).
You can cetainly try moving it around the various gain stages, but keep the above point in mind.

>why not use the 18 watt tonestack seen about 1/3 down the page here
I was playing with something similar recently. It might work quite nicely. Also remember that these are designed to work with valves and as you are playing with FETs, you can scale the resistance down (and capacitance proportinally up) to keep the noise down and/or to accomodate pots that you have in your parts bin.

[Kipper4]

Ahhh my old friend impedance.
Cheers Sam. I'll do some more.
Probably a good time to look into calculating impedance.

[PRR]

> I'm sure Leo was emminantly more qaulified than I.

How? Quicker on the draw with the iron?

We know he didn't play guitar.

> I guess it was more of a design question. rules and regs.

As I read the eveolution of Leo's amplifiers, his grasp of electronics was sketchy. He fixed radios. He knew roughly what value parts to expect to find, and how connected, could fake-it when part numbers were rubbed-off or parts disconnected, and extended that workbench experience to new builds.

[Kipper4]

his grasp of electronics was sketchy. He fixed radios. He knew roughly what value parts to expect to find, and how connected, could fake-it when part numbers were rubbed-off or parts disconnected, and extended that workbench experience to new builds.

You learn something everyday.
Cheers Paul I never knew that. my grasp is still a little sketchy too but i'm experimenting more as time goes on and am starting to futz my own designs also.
I just enjoy learning and making stuff. Its been my lifes work and I love it.
Thanks to forums like this and the good people who are willing to share their knowledge and for their patience.
Rich

[Kipper4]

I guess i could drop a jfet voltage follower before the tonestack to sort out the impedance issue and put it anywhere in the circuit then.
is my conclusion after reading this

http://www.muzique.com/lab/imp.htm

I have seen this done in some high gain jfet amp emulations that i have made recently.
Food for thought leads to food for thought.

[Kipper4]

Yep
The one that Sam suggested sounds best.
The 18watter was terrible.
Thanks guys.
edit
I'm gonna scale it down to 2x 25k pots with
100nf and 2.2nf and a 10k and1k

[samhay]

Cool, sounds like you are making progress.
Are you going to leave the tonestack between the 1st and 2nd stage?

[Kipper4]

Sure am. I can get some great tones out of it. I might try a 47nf before the stack instead of the 100nf I have in there now.
Which by the way probably didn't help the other stack I tried with a voltage follower after. Q3.
It's all good fun.

[Kipper4]

[/quote]

I scaled down the values but its just about the same stack

input> 22k> A50k bass>Gnd
input> 1nf , B50k treble. 47nf,2k2>Gnd

tonestack output (treble lug 2)>A1M vol pot

fet biasing by ear.
Sounds great. Thanks Sam and PPR

[Kipper4]

Demo here

https://youtu.be/4h-MlA9Npd8