Thermionic Mods

ErickPulido

Member
different clipping diodes check the stockade for examples or another pedal like the mahayana drive etc, different led colors what I do is I read other schematics to get ideas, just try and listen ;)
 

Chuck D. Bones

Well-known member
The mod you make depends on the sound you're trying to achieve. I don't mod pedals just because I like modding them, I do it to get a particular sound. If the Thermionic does what you want, then I'd leave it alone.
That being said, D2-D5 should be something other than 1N5817 IMHO. The other Friedman pedals use 1N4148.
Asymmetrical diodes can produce more even-order harmonics. The effect can be pretty subtle. If you do mess with the diodes, I'd suggest replacing D2 with a short and/or D5 with an LED. Maybe try sockets for those so you can experiment.
 
The mod you make depends on the sound you're trying to achieve. I don't mod pedals just because I like modding them, I do it to get a particular sound. If the Thermionic does what you want, then I'd leave it alone.
That being said, D2-D5 should be something other than 1N5817 IMHO. The other Friedman pedals use 1N4148.
Asymmetrical diodes can produce more even-order harmonics. The effect can be pretty subtle. If you do mess with the diodes, I'd suggest replacing D2 with a short and/or D5 with an LED. Maybe try sockets for those so you can experiment.
variety kind sir since I already have 3 of the same. IN4158 it is. Any other mods to make it sound different?
 

DGWVI

Active member
If you're not averse to overblown broken fuzz that pushes the opamp to it's limits, you could increase R5 to 1M, and decrease R4 to something like 2.7k (or lower). This will have a ton of gain, but will be gated

Or, instead, and in order to retain some control over the amount of saturation and clipping, you could decrease R9 to something like 2.2k. Again, that'll give you a fairly ratty, broken sound with the Gain pot up. Increasing C8 to 1u would be advisable, as it'd keep the frequency cutoff the same as the stock values
 

Chuck D. Bones

Well-known member
I would be careful if you alter R9 or R11. R10 and R12 each make a clean bleed for that stage. Notice how they are matched to R9 & R11. Hard to believe that you would want more gain from this circuit, but if you do, try it in small increments. All of the usual high-gain caveats apply. Keep the input wiring away from everything else. Make sure you have very good grounds between the jacks and case.

You could also retune the tone controls. You haven't told us what tone you're seeking, only that you want it different. So why not just build a different pedal? If you're after high-gain, consider the Valhalla, Malachite, Tyrian or Sanguine.
 
I would be careful if you alter R9 or R11. R10 and R12 each make a clean bleed for that stage. Notice how they are matched to R9 & R11. Hard to believe that you would want more gain from this circuit, but if you do, try it in small increments. All of the usual high-gain caveats apply. Keep the input wiring away from everything else. Make sure you have very good grounds between the jacks and case.

You could also retune the tone controls. You haven't told us what tone you're seeking, only that you want it different. So why not just build a different pedal? If you're after high-gain, consider the Valhalla, Malachite, Tyrian or Sanguine.
I’d like a harsher distortion this time around and a bit more bass on this next build.
 

Chuck D. Bones

Well-known member
I’d like a harsher distortion this time around and a bit more bass on this next build.
Try these:
For more bass, make these changes:
C3 = 22nF
C8 = 220nF
C11 = 100nF
C18 = 470nF

For harsher distortion make these changes:
R9 = 2.2K (increases 2nd stage gain 20dB)
C8 = 1uF, if using electrolytic then + lead to the left (2.2uF for more bass)
R10 = jumper (reduces 2nd stage clean bleed)
Crank the trimpot all the way up

The "more bass" changes extends the lower end down another octave.

There are more things we can do to increase the harshness; see if these are enough.
 

DGWVI

Active member
So, this thread got me wanting to take one of these to extremes, so I added a Little Debbie board to one of my last orders.
So far, the mods I've done, and am happy with are
  • C1 and C3 - 1u
  • C5 - 2.2u
  • C7 and C16 - 10u
  • R6 - 100k
  • R8 - 2.2k
  • R9 - omitted/ jumper
  • R12 - 1M
  • C8 and C9 - 100p
  • C10 - 1n
  • C12 - 27n (didn't mean to change this, just what I grabbed from the bin)
  • C13 - 4.7u
  • L1 and L2 - infrared LEDs
  • D1-D4 - 1N4740
  • L3 and L4 - white LEDs
  • Current op-amps are all MCP6002 that I happened to have on hand. NE5532 seemed to have a weird gating with the gain up

So, it basically picks up where the stock Thermionic family leaves off as far as amount of distortion. It doesn't have that jangly sparkle that my Therm DLX seems. Much more of a fuzzy sludge thing going on, but most chords still ring through with decent clarity. Definitely brings the heavy doomage. When I really dig in to my low A, it starts to do that saggy ducking thing that Rats like to do with the distortion cranked
The gain pot acts more like the sustain pot on a Big Muff type circuit, it's high gain no matter what setting. Feedback is easy to attain with it cranked. The noise floor doesn't seem to appreciably increase which is nice.

Wouldn't mind tweaking the bass section for more and louder low end. It has plenty, but my Cream Pie still kicks it's ass in that regard
 

Chuck D. Bones

Well-known member
C1 - Stock value rolls off at 22Hz, you moved it down to 0.5Hz.
C3 - Tight control used to range from 152Hz to 3.4KHz high-pass, now it's 1.5Hz to 34Hz.
C5 - Stock values rolls off at 72Hz, with the change to R8 you moved it down to 33Hz.
C7 - Stock value rolls off at 154Hz, you moved it down to 0.7Hz
C16 - Depends on next pedal in the chain.
R6 - 6dB increase in 4th stage gain, basically acts like the gain trimmer in the Thermionic cranked all the way up.
R8 - 20dB increase in 2nd stage gain.
R9 - Reduced 2nd stage clean bleed by 6db from stock. This was a good move after reducing R8 by 10x.
R12 - Increased 3rd stage gain by 13dB.
C8 & C9 - Restores high-freq roll-off after changing R6 & R12. C8 doesn't do much once the diodes start conducting.
C13 - Stock value makes a 15dB hump at 68Hz. With the C13 change, it's a 29dB bump at 15Hz. There's actually less boost from 50Hz on up with this mod. If I may make a suggestion, try reducing C12 to 15nF and make C13 470nF. You'll have a 25dB peak at 57Hz and more bass than stock from 20Hz on up. IC4.2 is actually a bandpass filter. The ratio of C13 / C12 sets the gain, the product of C12 * C13 sets the peak freq.
D1-D4 - This change should make very little difference. Why zeners?
L1 & L2 - slightly lower Vf, a little more compression in the 2nd stage.
L3 & L4 - higher Vf raises volume by at least 6dB and fattens the sound.
NE5532 has pretty high bias current which may interfere with operation as IC2 & IC3. Other stages should be fine. MCP6002 is rated for 7V max. It might be ok at 9V or it might die at any time. What's wrong with TL072?
 
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DGWVI

Active member
If I may make a suggestion, try reducing C12 to 15nF and make C13 470nF. You'll have a 25dB peak at 57Hz and more bass than stock from 20Hz on up. IC4.2 is actually a bandpass filter. The ratio of C13 / C12 sets the gain, the product of C12 + C13 sets the peak freq.
Sweet, I'll give this a try

D1-D4 - This change should make very little difference. Why zeners?
Just what I happened to grab. Been using them in place of 1N4148 in some hard clipper circuits, so they were on top of the pile

NE5532 has pretty high bias current which may interfere with operation as IC2 & IC3. Other stages should be fine. MCP6002 is rated for 7V max. It might be ok at 9V or it might die at any time. What's wrong with TL072?
The 6002s seem to be fine in this and a Sweet Honey I built recently. Probably try some other chips. Nothing wrong with TL072, other than I don't currently have any on hand. Current stock of dual op-amps I have are NE5532, MCP6002, TLC27M2, CA3240, CA3260, LM358, and MC1458
 

Chuck D. Bones

Well-known member
Wired that way, the zeners just act like regular silicon diodes. We've seen that done in other pedals, like the Disarray, probably for the same reason.
Replacing one of the 4 diodes with a jumper or LED would make asymmetric clipping, which could be interesting.

Semiconductors usually have no problem running a little bit outside the maximum ratings, but there is no guarantee they will meet spec or work at all.
 

DGWVI

Active member
Wired that way, the zeners just act like regular silicon diodes. We've seen that done in other pedals, like the Disarray, probably for the same reason.
Replacing one of the 4 diodes with a jumper or LED would make asymmetric clipping, which could be interesting.

Semiconductors usually have no problem running a little bit outside the maximum ratings, but there is no guarantee they will meet spec or work at all.
I like the 1n4740 instead of silicon for a little extra volume. As hard clippers, they sit in between red LEDs and 1n4148 with Vf of 1.2. probably could've just gone with 2 red LEDs for that stage, but I'm happy with it now.

I'm not even sure why I have or where I got the MCP6002s. But I've got a couple dozen of em, so may as well try em out. Also have a few ICL7621. Really not sure why I have so many CMOS op-amps

Just finished putting in the 15n and 470n you recommended for C12 & 13. Quite a bit more upfront low-end, but still kinda lacking compared to some of my other pedals. Really digging it, though. Way filthier than stock.
 

DGWVI

Active member
Other than completely rewiring the bass boost circuit, not sure what else we can do.
Oh, I think it's pretty great now. Thanks for all the pointers. If I get to feeling especially saucy for more low end, I've got a bunch of 1-band eq boards on hand
 
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