Big Muff TS looking for Mid-scoop to mid-hump control?

[tubeswell]

After reading this article http://www.muzique.com/lab/tone3.htm again, I had an idea about adapting a previously 'successful' Big Muff tone stack I had in a EF86 pre-amp.

This Big Muff used values that I scaled from the Big Muff in Merlin's 1st book, so that I could use a 1M pot instead of a 500k pot for the tone control. The resulting tone stack gives a nice wide range.

But I also liked the characteristics of a 5F2A tone stack that I previously had in this circuit. The 5F2A tone stack after the EF86 tended to lose some highs and had a nice voice (more of a mid-high hump) for guitar, but I had to have it tuned about 2/3 to 3/4 of the way to the 'treble' side to get it to a point where I liked it.  But it still didn't have the chimey highs that I like, and I realise this is probably a lot to do with the highish output impedance of the EF86 (but I digress slightly).

Anyhow, when I put the Big Muff TS (variant) in the amp, it took away the mid-high hump that the 5F2A had and seemed to replace it with a nice mid scoop with a huge range, that produced a way better hi/low balance, but at the expense of gain. So I was pondering about this and seeing Jack Orman's article again, I am pondering about how to adapt his idea to my scaled values of the circuit in Merlin's book. Hence the attachment. Any comments, criticisms, suggestions etc welcomed. TIA

[Matty_V]

I used that amz tone stack in my latest build. My amp is a totally different beast than yours, but I really like the wide range of tones it provides. The mid scoop is great for cleans! I find that I'm almost on the "bass" end of the tone control so I may tinker with the component values a bit to make it more usable. I guess I don't have any advice for you. I'll be interested to see what you come up with and maybe adapt some of it to mine. Good luck!

[tubeswell]

Here is a complete schematic idea for comment/suggestions

A few specifics I wondered about.

1) not sure whether to use a CF stage or not after the EF86. The obvious reason to use one would be impedance buffer for tone stack. This would be different to a Route 66 approach (which doesn't use a CF stage - and has a different stack). Any comments?

2) If I do use a CF stage not sure whether it should be a 12AU7 with 22k Rk or a 12AX7 with 100k Rk. The 12AU7 drawn on the schematic is direct-coupled to the EF86 and a 12AU7 might steal a lot of current (from the EF86) but this could have a nice assymetry under overdrive conditions. (or not?) Any comments/ideas? (The EF86 voltage notes from an earlier build which ran the same gain stage setup are attached FYI)

3) Tone stack. Shown a couple of variants (both using 1M pots). Which would be better for a hump/scoop control? Any bets? Or would I be better off with a typical Fender TB tonestack or one of those brown fender tonestacks? or...? (Reason I am thinking hump/scoop control is to better cope with the higher gain I would get with the EF86 morph control in 'pentode' mode. whilst retaining treble-bass balance of a single tone control for when in 'triode' mode - hence the (modified) big-muff concept TS in the actual circuit).

4) Tone stack recovery stage. (Borrowed from a 6G3 driver). Would this stage really be necessary in this amp? The reason I put it in is because when the EF86 morph control is in 'triode' mode, the amp ought to behave more like a 6G3 (methinks) (or not?).

5) I think I'll go for an LTP here. This is again borrowed from the 6G3. Alternative would be to not have the TS recovery stage (in 4) above) and have a cathodyne stage here. (Gaining a spare 1/2 a stage). The NFB insertion point would have to change. Methinks the LTP would give better drive for the output stage given the 'lowish' HT voltages expected.

6) Not sure whether to use a 12AX7 or a 12AT7 in the LTP. I have tried 12AT7 as LTP tubes before and they do have a nice mojo. Any comments?

7) the LFO stages are borrowed from a 5G9 and work well with fixed-bias 6V6s at those voltages. If I went with one triode and a mosfet follower (as I have done in other maps) I could get another spare triode. (And if I went with a cathodyne PI, then I could also probably to a reverb - because I'd have a whole spare pre-amp tube.)

8) I have put the standby between the 2 reservoir caps. I can't see the harm in doing that. Can anyone else?

TIA

Pete