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

[Gaz]

Is it possible to simulate the control with the Tonestack Calculator? It seems like your schem is slightly different... Let me know, because I'd really like to see how it works.

[andrew]

Hi Tubeswell, have you tried this tonestack? I used it on a 6sj7 build and liked it a lot. I am building another 6sj7 amp and would like to know how it compares to your design. https://tubeamparchive.com/download/file.php?id=9835 Thanks.

[tubeswell]

Hi Andrew. Thanks for your post. Looks interesting. Got any soundbytes? TIA

Pete

[andrew]

I'm sorry Tubeswell I do not have any clips. The treble control produces a thicker, louder,more distorted /complex sound as it is turned up. With the treble down it gets tame. The bass control just adds bass.

[tubeswell]

These were the sounds I got with the EF86 and a 5F2A tone stack (in an SE amp with a 6L6G pushing a celestion G10

Tremolo

http://www.nzguitars.com/forum/download ... hp?id=8232


Blues track

http://www.nzguitars.com/forum/download ... hp?id=8258



Then I switched to a big muff tonestack (same tube and speaker)

Trem

http://www.nzguitars.com/forum/download ... hp?id=8343


Blues

http://www.nzguitars.com/forum/download ... hp?id=8346


Zep

http://www.nzguitars.com/forum/download ... hp?id=8347


Me doin' James Burton

http://www.nzguitars.com/forum/download ... hp?id=8344

[andrew]

Thanks for posting the clips Tubeswell. It is difficult to really hear them on my little MacBook but they all sound nice and Fendery.

[10thTx]

Tubeswell,

I like your amp tone with that tone stack! James Burton tune was very nice! Tasteful playing too.

Thanks for sharing the sound clips. with respect, 10thtx

[M Fowler]

Pete, the amp sounded great liked your clips.

Mark

[tubeswell]

Yeah so much did I enjoy the EF86 concept with Merlin's triode/pentode morph control in that SE amp, I have commenced a project to do the same in a PP gigging amp.

[M Fowler]

I have done the parallel 12ax7 in several builds and should have just went with the EF86.

Your coming along nicely with cabinet built, I usually leave that to last which is the wrong way to do things.

Mark

[Structo]

The big muff stack was much fuller, sounds great!

[shane]

how about this one from Merlin?

Cheers
Shane

[tubeswell]

So further to these helpful comments, 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 one 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?

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.)

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

TIA

Pete

[Ian444]

I can't really help you much with ideas, but it is interesting that I found this thread, today I was playing with a test amp and did a circuit similar to this one except with 6P1P outputs (small 12W tubes). It is the first preamp I've ever played through with a pentode in it (ECF80/6BL8). I really like it, nice clean tones, and plenty of gain if you want it, with a smooth transition into distortion. With octal outputs it would be a lot of fun, so I can understand why you are heading in the direction you are. I don't know how the pentode in the ECF80 compares to the EF86 (check the datasheets), but it also has a triode section in it too, and a 9-pin base, might save you a half 12AU7 or something. The pentodes sure have scope for tweaking and interesting experiments. I skipped the pentode chapter in Merlin's book, now I have some bed-time reading, thanks for mentioning that, didn't know it was there even.

PS: that cct I linked to didn't work straight up for me, I used a 820K to pin3 and a 180K to pin6 of the 6BL8, just in case anyone tries it...