Please help with one knob tone control

[martin manning]

As a point of interest, this type of tone control appeared in the 1964 Supro Tunderbolt amp... long before the Big Muff. The attached schematic shows it, with some resistor and cap values you could try.

[Markusv]

Thanks Martin
Yes- nothing is new, is it??

Diagrammatics

I will have a good look at the DTS

With my limited understanding of Tone stacks it seems the one RC attached to the Tone pot is a high pass and the opposite RC leg is a High pass filter?
So basically the pot dials either one or the other in?

Re the -3db roll-off I really know nothing about the theory behind it and will have to investigate first.
For instance I have no real understanding what the term means, why it is desirable and how to apply it.
Math idiot here.

Thanks anyway
I will look and learn

Markus V

[diagrammatiks]

Well your coupling capacitor just sets a frequency roll off point for the next part of the stage.

you don't really have to calculate it but generally .022 is brighter then .047 etc.

You can use the calculators to get the best numbers for making sure your tone stack and your coupling cap frequency work well together.

I hate math as well. thankfully the online calculators at pentodepress do everything for you.

[Markusv]

Diagrammatics
(or anyone reading and willing to help)

I will be increasing the coupling cap tomorrow to see if I can get a little more bass response out of the set-up. The amp really has a great tonal range now but it may be lacking the warmth it had before

Another thing...
I think I am missing the gain it had before this Big Muff intervention. (the tone stack definitely introduced some loss)
So I wonder if it would help to change the parallel V1 stage to a single triode gain-stage and a cathode follower?

Any opinions before I get going?

Markus V

[tubeswell]

Diagrammatics
I think I am missing the gain it had before this Big Muff intervention. (the tone stack definitely introduced some loss)
So I wonder if it would help to change the parallel V1 stage to a single triode gain-stage and a cathode follower?

Any opinions before I get going?

Markus V

Hi Marcus

A number of ideas strike me.

You can configure the Big Muff for mid-scoop (more lossy but better high and low freq 'balance') or mid-hump (slightly more gain in the mids but not as good-a-highs).

Re: the input stage(s), you can reconfigure the parallel input stage to either;

1) a DC-coupled pair with a CF driving the tone stack (will give better bandwidth but maybe not that much more gain; or

2) a cascode stage in front of the tone stack, (a bit more gain than 1 but not as-good-a-bandwidth) and you may need to change the type of tube to a 12AU7 for best results in this regard; or

3) two cascaded stages in front of the tone stack (the most gain pre the tone-stack but you may have to muck around a bit with the inter-stage coupling and cathode bypass/boost frequencies to get the best effect through the tone stack); or

4) a stage in front of the tone stack and an (extra) recovery stage afterwards (more like the supro, or even a BF input stage/TS albeit with a Big Muff instead of a TB tonestack) - easier to control the recovery stage's cathode bypass/boost frequencies to achieve a more 'unfettered' sound through the tone stack

JM2CW and food for thought

[diagrammatiks]

if you have 2 stages to play around with...

the stage - tone stack - stage will give you the most drive to the pi.

A parallel triode has much lower output impedance then a single triode and more gain then a triode+cathode follower...

so, while the cathode follower but suffer from less overall insertion loss I don't know if that will make up for the difference in gain between the parallel triode and the dc coupled cathode follower.

[Markusv]

Hi folks

Thanks again

I have decided to go with Triode Gain Stage> TS>Triode Gain Stage >PI

After all:
I have only one dual triode to play with
I like the tone stack and all I want to do is recover the gain lost through its use
I also doubt a CF will do much to add gain. In fact I know it wont
I will no longer have the parallel stage so there is a lot of gain lost in the process

Make sense?

I think the main challenge is to select the size of coupling cap between this recovery stage and PI and size of grid to ground resistor if any is needed

Markus

[surfsup]

What about a mosfet CF to drive the TS? Dont need any room or a hole.

[Structo]

A CF stage won't add gain but it will keep the Tone Stack from losing gain.

[surfsup]

I am thinking about testing a MOSFET for this myself and mocked up a schemo to show how it would be done. There's been a comment that it could be "ice picky" because the MOSFET would not compress the signal like a normal gain stage from a tube would. But I would think in an application like this the MOSFET could be set up to "even out" the TS loss like you said by tweaking the value of the resistor on the Source to ground. (100k in the schemo below). For heat sinking, I don't think its necessary:

Power = V*I

So if there's 280V B+ (from OPs schemo) and lets say the first stage triode is drawing 1mA across the 100k plate resistor, so you would have 180V on the grid of the MOSFET.

P = (280-180)*(180/100k) = 0.18Watts

0.18watt * 63degC/watt = 11 degrees C above ambient temp

I am also wondering how this would work if one were to employ a MOSFET CF before a tone stack with a switch to cut out the tonestack, whether the vol differnce would still be a problem.

[img:890:560]http://chicagocadcam.com/ChrisHahn/sche ... lower2.jpg[/img]

[tubeswell]

I am thinking about testing a MOSFET for this myself and mocked up a schemo to show how it would be done. There's been a comment that it could be "ice picky" because the MOSFET would not compress the signal like a normal gain stage from a tube would. But I would think in an application like this the MOSFET could be set up to "even out" the TS loss like you said by tweaking the value of the resistor on the Source to ground. (100k in the schemo below). For heat sinking, I don't think its necessary:

Power = V*I

So if there's 280V B+ (from OPs schemo) and lets say the first stage triode is drawing 1mA across the 100k plate resistor, so you would have 180V on the grid of the MOSFET.

P = (280-180)*(180/100k) = 0.18Watts

0.18watt * 63degC/watt = 11 degrees C above ambient temp

I am also wondering how this would work if one were to employ a MOSFET CF before a tone stack with a switch to cut out the tonestack, whether the vol differnce would still be a problem.

[img:890:560]http://chicagocadcam.com/ChrisHahn/sche ... lower2.jpg[/img]

You can use an IRF820 for that as it 'works' as a straight sub for a triode gain stage. But you may need a 12V zener between the gate and the source to ensure that the gate-source voltage doesn't get to the point where is destroys the insulation between the two, and you also probably ought to have a 100-200R resistor in series with the gate and the previous stage's plate for stability.

[surfsup]

tubeswell,

But you may need a 12V zener between the gate and the source to ensure that the gate-source voltage doesn't get to the point where is destroys the insulation between the two

Don't the MOSFETs have an internal zener built in to them already?

[Markusv]

OK so here is a bit of an update

I kepth the Bigg muf tone stack and I discovered that if I tweak some values around the 1M tone knob I am getting:
More drive to the PI > The original setup cut too much mids and I was getting a very responsive tone stack but not in the range that I wanted. The amp was a bit thin and the lack of mids seemed to suck its ability to drive the PI and output tubes
The tone knob is a little less responsive now but the range is very useful all round

Values:
.022 coupling cap
On the Bass side: .01 /1 meg
On the treble side: .005 /1meg

I am getting a similar tonal range to my tremolo channel- which I like very much but there seems to be a bit more highs and towards a bass setting it sounds almost jazzy

Markus V

Hope this makes sense

[doctorfixit]

After trying all the other one-knob schemes, I put in a big muff , tweaked the values, and finally got rid of the honking mid hump that plagued all the others. Baxandalls were too lossy.