Understanding the ENGL gain trick and how to do it

[screamersusa]

I could use a little clarification of the ENGL od trick.
They go from a standard 12ax7 100k/1.5k-22uf combo to another with a 330k/3.3k-1 or 10uf combo with a 1nf snubber cap across the 330k.
I take it they're lowering the voltage to the second stage and raising its gain so it clips faster?
Can a variation of that be utilized in a 24v valvecaster type circuit if one were to drive the second 12au7 stage from a BS170? (if not I'll use that Marshall tranny to build an ENGL OD at 300v but I hate HV)
What I like about it is the added compression allowing legato notes while playing heavy riffs.
I like the VH4's tone and the Savage120 playability. Cant find both in one preamp except the GNX3K custom patches.
It would be cool applied to a fet circuit as well.

I suck at math, which is ironic because I used numbers to move pictures for 20+ years .

[DrNomis]

If it's any consolation,I'm not the greatest at maths either,anyway,basically,what Engl are doing when they use a 330k/3k3 resistor combination in a 12AX7 gain stage,instead of the usual 100k/1k5 combo,is that they are reducing what's called the "saturation current",this is the maximum current that can flow from the cathode to the anode(plate) when the tube stage is fully on,this makes the DC load-line of the stage less steep,and the stage will theoreticaly produce more voltage-gain,so yes,it will be more easier to overdrive till it clips....

The total resistance of the anode,and cathode resistors determine the saturation current as found by this ohm's law formula:

I =E/R

I= Current in miliamps.
E=Volts.
R=Resistance in Ohms.


When the 12AX7 Tube stage is running,the current that flows through both the cathode resistor and the anode resistor causes a voltage-drop across the cathode resistor,this raises the potential of the cathode,so that it is more positive than the grid,effectively making the grid more negative than the cathode and thus biasing the tube stage.....


I'd imagine that the snubber cap across the 330k resistor is to tame the high end harmonics....


Note that tubes,like the 12AX7 behave more non-linearly when fed lower supply voltages.....

[screamersusa]

Awesome reply. I get it, thanks a zillion.

Last Night by accident, I discovered the Groove Tubes Trio uses the same trick in the scream channel except at 200v instead of Engl's 400v. And it has the clean type(s) I'm looking for (A weakness in the Engl)
I grabbed that schematic real quick since you can't touch one for under a grand used. Thats what I'll use that marshall tranny and old gibson chassis for. When I get daring

Since My major project is the FET based stomp preamp, which fet would be best to apply this trick to. I.E. which would clip more like the 12ax7 in that type configuration.
At the moment I'm looking to mix mfp102's and BS170's. The Mfp102 sounds more bell like to me than the J201.

[DrNomis]

Awesome reply. I get it, thanks a zillion.

Last Night by accident, I discovered the Groove Tubes Trio uses the same trick in the scream channel except at 200v instead of Engl's 400v. And it has the clean type(s) I'm looking for (A weakness in the Engl)
I grabbed that schematic real quick since you can't touch one for under a grand used. Thats what I'll use that marshall tranny and old gibson chassis for. When I get daring

Since My major project is the FET based stomp preamp, which fet would be best to apply this trick to. I.E. which would clip more like the 12ax7 in that type configuration.
At the moment I'm looking to mix mfp102's and BS170's. The Mfp102 sounds more bell like to me than the J201.

You'd probably have to compare the load lines of a 12AX7 Tube, and the Fets that you're planning to use, Fets tend to have characteristic graphs that look a bit like Pentodes, to my eyes, but I've read articles saying that they produce distortion harmonics similar to 12AX7 tubes, which are twin-triode tubes.....

The best way to find out for sure is to experiment with different devices till you get the results you're after....

[mjones99]

Aside from DMOSfets, tubes are much easier to bias up than solid state devices. Tubes also are much easier to set up the overdrive voicing and gain structure. putting a 330K/3K3, not only adjusts the gain of the tube, it will be voiced differently as a large Rk puts it closer to cutoff and makes a raspy tone, where as a lower Rk gets you into saturation, giving a smoother warmer tone. If you look at the schematics of classic tube amps and look at the values of the Rk, Ra, and coupling and bypass caps you can see how those components are used to create the sound of the amplifier.

[RnFR]

Awesome reply. I get it, thanks a zillion.

Last Night by accident, I discovered the Groove Tubes Trio uses the same trick in the scream channel except at 200v instead of Engl's 400v. And it has the clean type(s) I'm looking for (A weakness in the Engl)
I grabbed that schematic real quick since you can't touch one for under a grand used. Thats what I'll use that marshall tranny and old gibson chassis for. When I get daring

Since My major project is the FET based stomp preamp, which fet would be best to apply this trick to. I.E. which would clip more like the 12ax7 in that type configuration.
At the moment I'm looking to mix mfp102's and BS170's. The Mfp102 sounds more bell like to me than the J201.

remember the difference between JFETS and MOSFETs. you are talking about a couple different kinds of devices there.

[mjones99]

Awesome reply. I get it, thanks a zillion.
Since My major project is the FET based stomp preamp, which fet would be best to apply this trick to. I.E. which would clip more like the 12ax7 in that type configuration.
At the moment I'm looking to mix mfp102's and BS170's. The Mfp102 sounds more bell like to me than the J201.

The problem is that FETs dont bias exactly like tubes do, and they dont behave the same at all. On a tube you can adjust both the Ra, and Rk to voice the stage, you really cant do that with an FET. You can get closer with a depletion mode device than enhancement. The BS170 has to be biased with voltage divider or drain feedback, the jfet can be biased in self bias like a tube, but it only sounds best (or at all) at a certain bias. The tube can be biased all over the place in contrast and not only will it work it will sound good, but different at each bias point, this cant be done with jfets/mosfets I dont think. Maybe a DMOS like lnd150 or DN2530 or one of the other supertex dmos but i havent worked with those enough to know, I understand that you can put an LND150 right in the tube socket in the appropriate pins and it will bias up and work right but I have never used any. I have 5 LND150 and 10 DN2530 that I got from supertex samples but havent had chance to try them out yet. I think if you are a student supertex will send you several samples, I know they will if you have even a small company.