Improving the Dr. Boogey

[valdiorn]

I've been reading this thread talking about various mods you can do to improve the Dr. Boogey. One thing struck me as odd and that's the third gain stage. Looking at the Boogey schems there is a source resistor of 3.9k at the 3rd JFET, but in the original Mesa schematic it's actually 39k.

They go on about how that stage doesn't give much gain and it's only for asymmetrical clipping and could potentially be replaced with diode clippers for less noise. I can understand that in the original Mesa schem the gain is small, about 100k/39k = 2.56 if I'm not mistaken (I'm flunking "Electronics 2" at college ), but in the boogey with a 3.9k resistor the gain looks like it could be considerable...?

So, why is the resistor 3.9k and not 39k? what does it change and what would happen if I replaced it with a 39k like original mesa schems??

[puretube]

i don`t trust those "original" schematix (blindly!) ... 

[anchovie]

i don`t trust those "original" schematix (blindly!) ... 

Maybe, but in this case the Mesa does have a 39K resistor - it's the "cold clipper" stage from the Soldano SLO-100 that has been ripped off by various other amp manufacturers hoping to appeal to the modern headbanger.

I suspect that in the Dr Boogey it was changed to a 3.9K to make it possible to bias the stage at 4.5v. The best way to approximate the extreme asymmetrical clipping that is produced in a SLO/Mesa/Framus/Peavey/Whatever by the stage would be to use a single diode (anode to ground) as the intention is to slice off the positive half of the waveform. A germanium or Schottky would chop off even more than a silicon, though whether the difference is audible I don't know.

Modern high-gainers seem to be moving away from using this stage though as it does add quite a bit of compression, so there are a recent designs that take more of a "JCM800 modded to hell" approach. I'm planning to JFET-ise a Krank Revolution preamp at some point to see how that comes out.

[MohiZ]

I suspect that in the Dr Boogey it was changed to a 3.9K to make it possible to bias the stage at 4.5v. The best way to approximate the extreme asymmetrical clipping that is produced in a SLO/Mesa/Framus/Peavey/Whatever by the stage would be to use a single diode (anode to ground) as the intention is to slice off the positive half of the waveform. A germanium or Schottky would chop off even more than a silicon, though whether the difference is audible I don't know.

Why wouldn't it be possible to bias the stage at 4.5V with a 39k source resistor? It would just need a bigger drain resistor as well, say 220k. A jfet with its gain and source both at the same voltage (for instance, no emitter resistor and gate pulled down to ground as it currently is in the Dr. Boogey) naturally clips the positive side of the (input) signal.

[anchovie]

Why wouldn't it be possible to bias the stage at 4.5V with a 39k source resistor?

No idea. I only suspected, I have no proof. I'd personally go with a diode anyway.