Thunderchief Mod question

[awilson40]

What do ypu think would be the results, sound wise, of removing one gain stage in the
Thunderchief. I would like mine to have a less agressive gain. I tried replacing the first fet
with a mpf102, and while it did reduce the gain, I didnt like the sound quality.

If I wanted tho try this can anyone suggest where to place a simple jumper to jump around
one stage that I can easily reverse if I dont like it ?
Thanks

[awilson40]

BTW...
what would be the effect of switching the 2nd fet with the MPF102 vs the first ??
Perhaps that would yeild a different gain.

I'm looking for more of a 70's crunch than the high gain.

[Mark Hammer]

I found mine a little "aggressive" for my tastes too.  Decent sound, just not the one I want all the time.

I don't pretend to understand discrete FET circuits all that well, but I'm picking up some things here and there.  One is that the caps to ground from the source pin alter the gain, or at least how much gain is applied across the entire frequency spectrum.  The 680nf (.68uf) caps provide a role in goosing the gain of their respective stages and especially for the mids and treble in those stages.  Another is that the gain is also dictated by the source resistor to ground in those same stages.  Consequently, the "aggression" in the circuit could probably be tamed a bit by:

a) swapping FETs for lower gain devices such as you have already done
b) completely removing one or both of the .68uf caps
c) increasing the source resistor in places where it is low-ish (here the candidate is the 2nd FET stage where a 1k resistor is used; 1k5 might help out)
d) sticking a small-ish value resistor between the source caps and ground, that will retain some of the intended tonal balance of the original but without the constant shrill overdrive (e.g., 220-470R from cap to ground)

Not ALL of these have to be done, and which ones OUGHT to be done would depend on what it is you want to preserve about the circuit.  For instance, if you wanted it to have a very hot output to overdrive something else, you don't want to reduce the gain too much, but may want to tame the treble a bit.

Hope that helps.

[petemoore]

a) swapping FETs for lower gain devices such as you have already done
b) completely removing one or both of the .68uf caps
c) increasing the source resistor in places where it is low-ish (here the candidate is the 2nd FET stage where a 1k resistor is used; 1k5 might help out)...bulky, but maybe a 2k2 or 5k pot and limiting resistor...
  d) sticking a small-ish value resistor between the source caps and ground, that will retain some of the intended tonal balance of the original but without the constant shrill overdrive (e.g., 220-470R from cap to ground)
  Yet another possible placement for a control pot wired as Variable resistor [1k ?]...if just to test, then replacable with a fixed resistor @ chosen value...
  Another place you might like a pot is between stages, say after teh first stage, splice a 'volume control' as gain limiter to the successive stages.

[awilson40]

Thanks Guys.
Do you think I should replace the 1K source resistors in both
the 2nd and the 4th stage to say a 1.5k ??  This seems to
be a common value for other Fet pedals ??

Kepp the suggestions coming, thanks again

[awilson40]

Well,
I replaced the 2nd source resistor with a 1k5
and increased the value of both .68uf caps
the first stage is a 10uf the 2nd us a 4.5uf (its what I had handy)

This did improve it, but not quiet enough.
Should I go larger on the source resistor

What would happen if I raised the value of the
source resistor on the final stage ??

Thanks
for the help

[petemoore]

Should I go larger on the source resistor
  I figured out: "why not?"
  Make that thing big, with long leg, enough lead left above board so a parallel resistor can easily be 'tagged on', a pot can be clipped on it, all the lower to lower-ish values can be sampled, then one or two fixed resistors which = ''optimimum resistance" in your mind can be fixed there.

[awilson40]

Thats a good idea, thanks
what are your thoughts on the source resistor on the final stage.
Would it really affect distortion, overall gain ??
Thanks again

[Jay Doyle]

With a cap bypassing the source resistor, increasing the source resistance will do two things, neither of them will reduce the gain however. First it will change the bias of the stage by adjusting the current through the JFET, second, it will alter the frequency where the gain goes to max.

The cap is there to stabilize the voltage at the source and reduce/eliminate the feedback that occurs when the source voltage varies with the gate voltage. That cap appears as a short to ground for all audio frequencies above the knee of the Rsouce/Csource. Increasing the resistance at the source will change the knee but for all frequencies above the knee, the gain will be the maximum the stage can give because the cap will still look like a short to ground. In effect, increasing the source resistor will INCREASE treble or more accurately, it will increase the gain of the treble frequencies.

Adding a resistor in series with the cap would be a better idea, or taking the cap out altogether. Another mod for that circuit is to lower the 100k feedback resistor that goes from the output of Q4 to the source of Q3.

This is an example of why I don't like to use trims as the drain resistor AND bias trim. By having a set value drain resistor you can tailor the gain easier. BUT we have to work with what we are given.

My suggestions:

1 - remove the second stage's source bypass cap.
2 - reduce the value of the 100k feedback resistor, try 68k or 50k.
3 - lower the value of the 220k gate resistor on the stage of Q4, this will load down the third stage's drain and reduce the signal fed to Q4
4 - remove the 470pF cap in parallel with the 470k resistor after the Gain pot to reduce the treble.

Hope this helps,

Jay Doyle

[awilson40]

Lots of great info
Thanks

[Jay Doyle]

Now that I think about it, adjusting the source resistance will also change the available headroom.

Increasing the resistance will lower the headroom available and therefore INCREASE the distortion.

[awilson40]

OOps...
So should I try a smaller resistor at the source of the 2nd FET ??
Do your other suggestions still look like the right direction to go ??
Thanks again

[uan]

i wonder if we can use the real 12ax tube in thunderchief to replace one of the JFET?

[swt]

take a look at the plexizer thread. I think that if you want crunch, and less distortion, the way to go is adding a cathode follower (buffer) and the marshall tonestack, after the second gainstage. This will load some gain, and also add the crunch you need. I think a good idea to load it even more is to scale down the value of the pots x10. use 25k instead of 250k, 100k intead of 1M, etc. The master after the TStack should be chosen by ear between 100k and 1meg, as the slope resistor too. Let me know the final result after this.

[swt]

i forgot to mention that the brown sound of early marshalls don't have the bypass cap at cathode two. remove that for less gain and better sound.

[awilson40]

Maybe the guys at OLC will put together a Plexizer kit
What do you guys think

[Jay Doyle]

OOps...
So should I try a smaller resistor at the source of the 2nd FET ??
Do your other suggestions still look like the right direction to go ??
Thanks again

First off, skip the source resistor change. Because of the trims on the drain also the DC bias trim you will have to rebias after you change the source resistor, which will change the headroom calculation, which will... You have too many things interacting, in my opinion, better to leave the DC bias be and try to find other places to lower the drive.

Adding resistors in series with the caps would be my first choice. Try half the value of the source resistor.

Then try lowering the value of the 220k gate resistor.

Altering the feedback resistor will also alter the 'feel' of the effect, so be sure you like it.

[Gary]

What do ypu think would be the results, sound wise, of removing one gain stage in the
Thunderchief?

Jay's suggestions are OK.
I like Mark's ideas even better.

Or you could make it as simple as removing the third stage.
Or you could reduce the value of the feedback resistor, like the older Marshalls did.
Or you could build the Eighteen instead because it has less gain.
Or you could hang loose while we polish the replacement for the Thunderchief.

[awilson40]

I actually ended up leaving the feed back Res the same,
setting the 1st and 2nd stage source resistors at 1k
ist stage source cap at 10uf and 2nd stage at 4.5uf

Then I went through a carefull biasing procedure using head phones to
really listen and ended up with a really good , smooth sound.

I will be watching for a TC replacement and might build the 18 also
just for comparison.