Framus mid control?

[ChopSauce]

So, that (roughly)



would be the plan.

Any other suggestions/comments/criticisms...

[pdf64]

… to add to the confusion, Merlin Blencowe suggests (Fig. 14.6: Practial high-gain preamp circuit; p.258 of his "preamp book") that (almost) this tone control be placed in a similar configuration as suggested by Rob Robinette and advocates that the filter should not be loaded (by the following stage?) - as far as I understand things - which sound as the exact opposite of what the simulator suggests.
…

Can you clarify what’s confusing?
ie the default RL in TSC Framus sim looks to be 1M.

Also my Blencowe preamp book is the earlier edition, but are you sure you’re comparing like with like? Rob’s site contains plenty of guff. With regard to that suggestion, I suspect he is speculating, rather than writing from analysis / experience.

[ChopSauce]

Well, I think I agree with you entierely but what is the "RL" in the Framus control simulator: right load?

Merlin put it (the 1M) on the left/input side, on his book - if that is -and it's no verbatim copy but so close ...

According to the simulator, the Framus scoops the mid whatever load is before or after it - which might explain why Merlin has had no trouble with it on its multiple gain stages preamp - but it seems to only vanish on full tilt and manages for the higher treble output when placed after a 1k impedance - hence the cathode follower above, which is supposed to comply with the simulator.

[Colossal]

Chopsauce,

Have you scrapped the control? If not, try placing it after the coupling cap on the second stage and before the entrance to the cathodyne, not after the Tweed Vol and Tone controls.

[ChopSauce]

scrapped? Not sure what that means...

Well I already wired the cathode follower section as depicted above and I am ready to connect that to the rest of the amp.

The simulator shows that the response is fairly enhanced after a 1k impedance, vs something at least ten times larger - as should be a typical triode gain stage, if I'm not mistaking - so I went for the cathode follower!

Tomorrow I'll test the above section straight into an EL84 (but a cold clipper/pseudo-half PI before) power section.

AND ... yes: I meant tweed controls but the amp is no tweed at all, more something of an overdrive "skinny"/86Special kind of amp - but the tone stack. The whole thing needs to fit into an AC4 sized-chassis that I had left...

[Colossal]

scrapped? Not sure what that means...

American slang for have you 'thrown it into the scrap/junk pile'?

Well I already wired the cathode follower section as depicted above and I am ready to connect that to the rest of the amp.

Ok, so you have already changed it then.

The simulator shows that the response is fairly enhanced after a 1k impedance, vs something at least ten times larger - as should be a typical triode gain stage, if I'm not mistaking - so I went for the cathode follower!

When driven by a follower, the high end is preserved and response should be quite flat when the control is all the way up.

AND ... yes: I meant tweed controls but the amp is no tweed at all, more something of an overdrive "skinny"/86Special kind of amp - but the tone stack. The whole thing needs to fit into an AC4 sized-chassis that I had left...

Yes, Tweed amps are quite flat in the EQ and the Framus will scoop out that midrange, morphing it into something different.

[pdf64]

RL equating to the ‘load resistance’ makes sense to me. eg a 1M pot or grid leak.

[ChopSauce]

Of course: RL shows on the simulator ...

I tested the above, which turned to be somewhat weird and pretty useless in my situation. The Framus mid control works as expected - which means that there is a sudden boost of all frequencies when turning the pot rightmost. Also, the tweed control is too much of a loss with such an input impedance as is the output of the Framus ctrl.

As an alternative simple yet versatile tone stack, I'm going to test a Big Muff style tone control combined with a variable resistor to act as a presence control (1)

The values of components were tuned to scoop mids around 500Hz, like the Fender TMB controls - according to the simulator (2).

The curves were plotted for different R2 values corresponding to a 250k pot wired as a variable resistor in series with a 22k resistor, with a 50k increment.

bigMuff+pres.png

The lower R2 value, the more it scoops.

Note that the tone pot has been adjusted for each plot, to accomodate for a drift towards low frequencies, as the presence resistor (R2) value decreases.

I'll test that tone stack after a cathode follower and report accordingly, but at least that one should work, in theory.
_
(1) http://www.muzique.com/lab/tone3.htm

(2) https://www.guitarscience.net/tsc/bigmu ... pot=Linear

[danman]

I recently completed a Tweed ODS build that was posted here on the forum several years ago by a fellow member. I included the Framus mid control in the OD section of this amp and it does work fairly well. With the pot on 10, the gain loss is very minimal. When dialing down the pot, you do loose a tiny bit of volume but it does an effective job of shaping the mids. This is my first go with a Dumble type circuit but overall the build turned out very nice. This was the original post by nickfl in which he details the design and schematic.
https://ampgarage.com/forum/viewtopic.php?f=6&t=30492

Edit: I went the posted values for the potentiometer, resistor and caps but do plan on experimenting with the values a bit to see what the results might be.

[ChopSauce]

Thank you very much for the report!

It will probably be of interest to tweed "tweakers" that the Framus mid control works fairly well as suggested by Rob Robinette.

I will anyway go for a scaled "Big Muff with AMZ pres. ctrl", which gives "finer" control - as I can afford the loss of a triode as a cathode follower.
NB: a MOSFET follower would probably work as well for a fraction of the cost.