Marc Bolans fuzz? HH IC100S as a pedal.

[anotherjim]

I don't usually post designs I haven't made, but I started converting a service schematic for a possible pedal and might as well put it out there.
This old HH solid-state amp had the most raucous fuzz distortion provided by a switch and "Sustain" knob. Not to everybody's taste, I should add, but with humbuckers and its low input impedance, it can be smooth and thick.
And the distortion device is none other than a 741 opamp clipping its heart out with some band-limiting filters around it. With +/- 16v power rails too!

The amp in question looks like this...


The only service schematic...


Taking Ch1 alone as far as the tone makeup amp I drew this up...

I left out the "Presence" control as I don't remember it being helpful with the distortion in use.

[Mark Hammer]

The 741 is fine for use when the gain aimed for doesn't exceed the maximum voltage swing.  Your drawing shows the 741 powered by +9V (the original factory schematic doesn't) and enabled for a max gain of near 700x.   That will most definitely exceed the voltage swing of the chip, even if the supply voltage was higher.

The venerable MXR Distortion+ uses a 741, with a 9V supply and has a max gain of about 213x.  Most builders/modders will find that it is difficult to achieve a "clean" sound from the circuit, even in the complete absence of clipping diodes.  I guess HH found they could simply exploit that.

[anotherjim]

Mark, I scaled the gain down -  see notes on schematic and compare to the original. My 47k feedback R8 is 150k in the original. It's meant to clip the opamp - that's the effect. The original max gain exceeds 2000, in theory. I expect it's working more like a comparator then.

Also, the factory input amp also has 47k feedback changed on mine to 15k (R9).

The one I'm concerned about is the final stage. Flat, the tone loss is -20dB (1/10th) but with full bass or treble, insertion loss is only -3db when it might cause more clipping. Of course, there is the volume control before that.

You don't need the clipper amp to go clean, you switch it out.

[iainpunk]

a big thing to keep the output stage from clipping is the 22k/4k7 (R11 / R15) voltage divider, taking out a lot of the amplitude.
this said, why not replace the Vb buffer and output stage with a cheaper (dual / FET based?) opamp?

i really, really approve of not using diodes, both on an emotional as a technical sense.
if the clipping amplitude is higher, but with the same relative clipping, it takes longer for the slew rate to catch up. if its then attenuated, the slope is less steep than it would be with the limited amplitude from clipping diodes. this softens the tone.

cheers

[anotherjim]

If a lightly loaded 741 can get within 1v of the supply rail, then the original has a total 30v swing. at 9v this becomes 7v, the 1v rail limit becomes a bigger hit.  So a more authentic gain reduction is by 4, not 3. I did ruminate over this but it will need to be a hot guitar to clip the first noticeably and the clipper has adjustment. I did own one once and remember low settings of the Sustain knob weren't that good - but I was playing bass through mine.

Out of the clipper, notice 22k into 100n, starting at 72Hz, then the 22k also divides when it gets to a 4k7.

I've also considered different opamps in just the same places, although I was wondering about a NE5532 -  please see note lower left on the schematic.

[ElectricDruid]

You've changed a few things around the amp where the presence control used to be too, I see. Is that deliberate?

R24/C17 (your schem) were 6K8/4u7, are now 10K/1u.

C18 (your schem) was 6u8 (I think? not clear) is now 100n.

Is this right / what you intended?

Thanks for this, btw. It looks fierce! As you said, basically a comparator fuzz.

[Rob Strand]

FWIW, there used to be HH Amplifier schematics on the web.

The circuits from the late 70s and early 80 had a tube sound section.  That section was never drawn out in full on any of the schematics - it only appeared as a block.   I've never seen it drawn up although I was guessing it was a CMOS opamp or CMOS chip.

[anotherjim]

H-H must have had cheap surplus 6u8 caps. They are overkill. Some places don't stock 6u8, so 4u7.
The last amp shouldn't have much impact, so I changed some things. The Stage/Studio "master volume" switch is fixed to studio (looks funny in the HH scheme - that's a screened cable to the switch).

The Presence is deliberately left out. I remember it being quite harsh sounding. It was a very hissy amp as you might expect. 
However, this is also the amp that Wilko Johnson started his career with, so maybe the presence boost was part of his attacking cleanish sound?

One thing unclear about the presence pot noted as A/Log. Would that be reverse taper or Audio taper different from the standard log?

The IC100 predates the VSMusician range with the secret valve sound circuit. I've never seen a schematic of that. The VS range were highly successful in the UK. Along with Carlsbro, HH nearly wiped the likes of Marshall out when punk/new-wave happened but then Stadium rock/metal happened and players wanted high gain stacks.
As for Mr Bolan, the IC100 carried on as preamp as it doesn't mind being without a speaker and it has a handy DI output jack.

[anotherjim]

I've got the basic first 2 stages on breadboard - connected in series for distortion always.

The Good...
The distortion is solid and dense.
Does not fart out on decays.  Maintains almost as long as the string vibrates.
No huge hiss. Quieter than a MBP. Not surprising with that 22k/100n 70Hz low pass on the output. I do wonder about those filter values, especially the cap. It absolutely roars with a smaller cap.

The Bad...
Does not clean up well from the guitar vol knob. Any noise on the input yields nasty IMD.
Absolutely no dynamics.

[anotherjim]

I've changed the output filter to a variable traveller low pass. Much more useful and can still go as dark as the original.
And the 1k gain pot can usefully be larger like 10k. Not much difference 0 to 1k.