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

[tubeswell]

After reading this article AMZ Presence Control again, I had an idea about adapting a previously 'successful' Big Muff tone stack I had for 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 there, 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 wondering 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

[Steve Conner]

Hi tubeswell

Hang a MOSFET cathode follower off the plate of the EF86 to get a low impedance drive for the tone stack, then all the theory in Orman's article should work. Stuff the FET into a copper pipe end cap and if anyone asks, it's a nuvistor.

Let me know how it works out- I'm looking for a better one-knob tone stack to replace the useless one in my new hybrid.

[riz]

I love the Big Muff, thanks for the link! Safe to say that's why the opamp version of the pi sounds the way it does? My opamp box is way more defined in the upper frequencies than the older transistor circuit, same box. Love 'em both.

[tubeswell]

Hang a MOSFET cathode follower off the plate of the EF86 to get a low impedance drive for the tone stack, then all the theory in Orman's article should work. Stuff the FET into a copper pipe end cap and if anyone asks, it's a nuvistor.

Thanks Steve

Just out of interest, I was contemplating a 6U8 in V1 with the triode part as a follower in the upcoming build (and then having the recovery either as a driver-cathodyne pair or a LTP). So you reckon that the 1M pot and 180k will work (to give a variable midscoop to mid hump control)?

[Steve Conner]

Well, if you build a scaled version of Orman's circuit, and drive it off a cathode follower, you should get the same frequency responses that he plotted. The question is mainly whether you like them.

Another issue is the large-signal behaviour of the CF when overdriven. It can supply more current on positive peaks than negative. You can get a weird effect where it is capable of charging the large bass capacitor in the tone stack much quicker than it can discharge it, and this is supposed to be partly responsible for the sound of Marshalls and other amps with CFs. Your circuit may or may not suffer from this, and I have no idea what it'll do to the tone of it.

To scale by a factor of X, multiply all resistances and inductances by X and divide capacitances by X. So if changing from a 250k pot to 1M, multiply all other resistors and pots by 4, and divide all capacitances by 4.

[tubeswell]

Thanks for that Steve. I understand about the 'inverse' scaling of caps vs resistors and I should've added that I 'scaled' the tone stack values out of the values in Merlin's 1st book (470k resistor and 500k pot values, and 1nF and 3n3 caps) as opposed to the AMZ values. However with the bass cap I went the other way and used a way bigger bass cut cap (4n7) because this gave a better 'balance' when driven by an unbuffered EF86. Although I see how this would change when you add a buffered stage to drive the tone stack (so that would become about 1n7 if scaling from Merlin's design for a mid scoop)

It wasn't so much the actual frequencies I was thinking about in my initial query, as the ability to change from a mid scoop into a mid hump. I made the assumption that if Jack Orman's version used 25k pot and 3k3 series resistance, that would equate to about 180k series resistance with a 1M pot. But I guess I should suck it and see.

[Steve Conner]

Well, from the article it seems that Orman's circuit can change from a scoop to a hump. So your scaled circuit should do the exact same.

The "Body" control just changes the time constant of one of the RCs. If the two time constants are the same, you get a flat response with the tone knob midway. If they overlap, you get a hump. If the low-pass is lower than the high-pass, you get a scoop. The centre frequency of the hump/scoop is (roughly, I think) the geometric mean of the high and low pass frequencies.

I ignored interaction between the two circuits through the track of the tone pot, but I don't think it is hugely important.

[tubeswell]

Many thanks for that Steve. You just greatly increased my understanding about how that tone stack works. :-)

[tubeswell]

FWIW the initial idea was pretty hopeless (don't try it at home), so I ended up using Merlin's 'Bone Ray' tone stack, which works exceedingly well.

[jrfrond]

Hi tubeswell

Hang a MOSFET cathode follower off the plate of the EF86 to get a low impedance drive for the tone stack, then all the theory in Orman's article should work. Stuff the FET into a copper pipe end cap and if anyone asks, it's a nuvistor.

Let me know how it works out- I'm looking for a better one-knob tone stack to replace the useless one in my new hybrid.

I've often thought about giving the BMP tone control a shot too, and I build a pentode preamp design as it is. I TOTALLY agree with using a buffer for low-Z drive, but why a MOSFET Steve, which would necessitate a low-voltage supply? I'd probably use 1/2- 12AX7 as a buffer and the other half as recovery, since it's a real lossy circuit design.

Just for shits and giggles, I'm thinking that one could experiment with this design in a negative feedback loop, albeit with modified values. Just throwing this out there on the table.....

[tubeswell]

Total as-built schematic (including voltages). You will see that I used a the 'AU7' side of a 12DW7 for the low impedance buffer.

The 'Bone Ray' tone stack is something Merlin Blencowe came up with (from adapting a Big Muff TS by adding a mid control) FWIW it works very well (I scaled the cap and resistor values so I could use 1M pots).

[jpfamps]

I like the aesthetic of a single tone control, and experimented with a BMP circuit, driven by a cathode follower, in a couple of amps.

It worked reasonably well, ie you could dial in a good sound quickly, however the useful range of the control using the circuit values I experimented with was between high noon and 3 o'clock, which I didn't think was great ergonomically.

I considered adding further controls to the BMP, but I felt this was moving away from the simplicity of the one knob tone idea, as well as adding more components.

Perhaps I should have been more persistent..........

I've now moved to a plate driven version of the Gibson GA70 bass/treble control (which were copied by Vox), which I think gives best bag for buck, and has a very component count.

[tubeswell]

I like the aesthetic of a single tone control, and experimented with a BMP circuit, driven by a cathode follower, in a couple of amps.

Me too. Less tone-sucking componentry is more.

It worked reasonably well, ie you could dial in a good sound quickly, however the useful range of the control using the circuit values I experimented with was between high noon and 3 o'clock

I think that depends on the total mix of impedances, and gain of stages, as well as size of OT etc. I usually end up tweaking each particular circuit to bring out whatever mojo is hidden away in there. (But I'm just a hobbyist, so I don't care if a uselessly fritter away hours working on one thing).