The SkreddZog build report

[jmusser]

I asked quite awhile back for someone to design a Hertzog Emulator, and the Skredd man came through for me. http://skreddypedals.com/circuits/SkreddZog.gif.  I used the Professor Tweed from runoffgroove http://runoffgroove.com/professor.html for the front end. First of all let me say that this circuit alone is a winner. I like it very well when used with the MPF102s throughout. It's very clean with a slight crunch, and reminds me a lot of early Beatles stuff. Having said that, the MPF102s do not work for the emulator, because they're way too bright. I subbed J201s, and they darkened it where it needs to be. After that, you have the phase inverter section. I screwed up on the transistor selection initially, and used 2N4401s, which I believe are NTE159, when I really needed NTE199s types to sub for the BC549C. I used some C1345s that I pulled out of an old 8 track deck. The NTE199s have a weird pinout of ECB referenced from the flat side. I used fixed 10K resistors going into the two Ruby amps, and 10uf caps across pins 1 & 8. After that, are two small Radio Shack isolation transformers (273-1374) which I believe together approximate a center tapped transformer. As this circuits sits right now, it needs a little more work to saturate the coils, and get things more watery, but man, does it have a heavy fuzz. It even fuzzes on the bridge pick up. It's getting close to the Hertzog sound, but is not quite there. Skredd shows a saturation pot for the transformer, that I will probably try to see what it does. The bias on the J201s is a real bear to dial in with the 100k trimmers, and it seems like if you just breath on them, they screw up the bias. Also, the tone control on the Professor Tweed does nothing at all. If you do a search under Hertzog Emulator, you will find the original thread for this. I believe it was sometime back in March.

[jmusser]

I'm just bumping this to see if there might be some suggestions on transformer saturation in this circuit. Right now, I feel this thing is headed in the right direction, but this part is lacking to get true Hertzog type emulation.

[KerryF]

I found the little secret to that circuit!  .  He copied those two LM386 circuits which can be found here: http://www.ciphersbyritter.com/NOISE/SPEC386.HTM .  I actually used one to drive my reverb tank to test to make sure it was working correctly.  Although that circuit does look pretty sweet  .

[tcobretti]

This is a little off topic, but have you guys seen this?  I think it is a JFET emulator of the Hertzog.  It was posted many moons ago.

[jmusser]

I don't remember it being exactly like that with the transformer, but I believe that may be the secret to getting the coil to saturate. I tried going through a 10K resistor from 9V to the primary, and that will have to be reduced, or eliminated altogether as shown in that drawing. I didn't know what effect it would have on that little tiny coil wire, and I didn't want to risk smoking it. Each wire is about like 4 strands of human hair on that Radio Shack isolation transformer. I'm doing three 12 hour shifts this weekend, but I will try to tack on a 1K there instead of a 10K before I leave for work tomorrow, and if no cigar, go to straight 9V. This is a wicked fuzz btw! It has a little up octave snarl to it.

[Skreddy]

Yeah; it's supposed to be a 10k pot in series with a 1k resistor.  I wouldn't go less than 1k in any case, as that would likely burn up your transformer and also it would have a similar effect to your signal as grounding it (only you're "grounding" the audio to +9v instead of 0v).

The theory that RG Keen mentioned about the saturating the coils with some DC offset was to simulate the effect of a saturated magnetic core without actually saturating a magnetic core.  I don't recall the exact language he used.  But it would compress the signal in one direction (the direction of the DC offset), so that's why my design uses two coils, phase-inverted, both saturated; to compress in both directions at the same time.

I'm getting really excited about this and would love to hear it.  I don't have the parts and the time at the moment to build this thing, but I'm thinking it's something I should do soon. 

Sound clips would be much appreciated!    Thank you for your time and effort!!

[jmusser]

I didn't have a 10K dual gang pot, and I wanted both transformers to saturate equally. I'll go with progressively smaller fixed resistor values, until I get to 1K, and see what tone I get. Once I get that part of the tone in the neighborhood, then I'll add gain pots to the Ruby chips. Then, I'll reduce the 220uf cap value in different increments too. This circuit is not very loud the way it is right now. It has a good thick fuzz and like I said, a snarl from some up octave that's in the mix. It has great sustain. It has some early Iomi tones to it, and some tones like Neil Young gets on "Keep On Rockin' in the free World". That's not a bad thing to be sure, but it's lacking the watery saturation.