FET->Tube equivalencies in ROG amp emulators

[Mark Hammer]

Looking over the schematics for the assorted ROG amp emulators and their actual tube amp counterparts, what one sees is essentially a FET where a tube ought to be.  Among amp-o-philes and techs, there are frequent recommendations to change preamp tube X for tube Y (e.g., change a 12AX7 for a 12AT7 or 12AY7, or vice versa).  The tubes are rated at different gains, and one assumes that the degree of push at each stage, and whatever degree of distortion is produced or not produced at each stage helps to form the eventual sound.

In the case of the various Runoffgroove amp sims, generally the same FET (a J201) tends to be recommended in each instance at each "station".  I'm just wondering if there is any means of doing with FETs what many do with tubes.  That is, swapping one FET for another of somewhat less (or more) gain at a given station/position.  If there is, is this a question of hand-selecting a FET according to some measurable spec, a question of tuning a FET in some manner via component value choices, or a matter of simply selecting a different part number?

Just wondering, partly because the Thunderchief I made sounds pretty decent but it doesn't seem to be capable of anything BUT over the top screaming high gain.  Something that would render it a little more open to sonic alternatives would be nice, and if sticking a different FET as the first or second device could accomplish that I'd like it.

[B Tremblay]

Our basic rule of thumb has been that if a bit less gain is needed, swap out the J201 for an MPF102 or 2N5457.  A side effect is that the overall tone will become brighter.

However, those three types were usually only ones we'd try while developing FET-based amp emulators.

We didn't choose those types according to datasheet specs; they're simply three of the more common FETs in the realm of DIY-fx.  It's not very scientific to be "easter-egging,"  but doing so would sometimes produce pleasing results.

I'd suggest trying an MPF102 (or 2N5457) as Q1 or Q2 - or both.  They should temper the blitzkreig a bit.

[R.G.]

It's worth while remembering a couple of maxims.

(1) JFETs do not distort or sound like tubes particularly
and
(2) the pre- and post-clipping EQ is as important if not more important than the actual clipping that goes on.

(1) is (one of) the reason(s) we still have tubes today - neither JFETs nor any other easily done simulation does sound like a tube. If we were pushing the envelope, the JFET characteristic is much more like a pentode than the 12A?7 triode that's universally what the JFET is supposed to be emulating.

(2) is the reason that JFET-for-tube emulators are somewhat pleasing - the various rolloffs and EQs of a tube amp are captured if the circuit is done well, and this captures some of the voicing of the amp in spite of the fact that other attributes like gain per stage, clipping points and clipping qualities do not necessarily match the tube the JFET nominally subs for.

Given (2), then yes, changing the gain of any particular JFET might be good, as that would change the effective clipping point and perhaps clipping quality of the following stage.

However, it's simpler to change a JFET stage's gain by changing the unbypassed source resistance for the stage than it is to deal with the changes in bias point of subbing in unlike JFET types.

By way of analysis then, you can separate the active devices from the passive devices in an amp emulation. The passive devices largely capture the voicing of the amp in the sense that the formants of a musical instrument carry the "character" and tone quality of the instrument. The active devices carry the clipping behaviour. The two together are the characteristic sound of the amp.

In that vein, Stephen Moeller's AC30 sim is a step closer in emulation than simple JFET-for-tube versions. He did the same as the ROG emulators in that the passive component "voicing" was reproduced, but he went a step further and subbed in an opamp simulator of the actual measured clipping behaviour of each tube in the signal chain. This produced a very pleasantly surprising emulator, much closer to the parent amp than any others I've heard.

So yes, mess with the JFETs. You can change gain easily by inserting unbypassed source resistance, and probably should do it that way instead of swapping JFETs at random. The J201 is a high gain device, so it's easy to turn the gain down.

[javacody]

RG, can you share with us what an opamp setup to emulate a tube would look like (schematic wise)?

[bwanasonic]

So yes, mess with the JFETs. You can change gain easily by inserting unbypassed source resistance, and probably should do it that way instead of swapping JFETs at random. The J201 is a high gain device, so it's easy to turn the gain down.

Pardon my ignorance, but is this "unbypassed source resistance"? (in red) :



Would this work in a useful way as drawn? What range of pot/trim would be a good starting point?

Kerry M

[javacody]

I think you'd change R3 to alter the gain.

[bwanasonic]

Also, found this in another thread:



Thanks Dragonfly
Kerry M

[Eddie]

My thunderchief has too much gain too.Most distortion comes from the second stage. I "measured" this by audio probe. I could leave Q3 and Q4 out.
I will try the mpf 102 in Q1.
I would happy to see a better thunderchief version (v1.3???).
Don`t get me wrong the thunderchief is the best pedal I built!

Does someone know the part values for the pritchard amp tube simulators? Does someone has access to the original patents?
http://www.pritchardamps.com/pritchardamps/tech_talk/db_magazine.pdf

Good read!

Thanks!

Eddie

[Ge_Whiz]

I'm too lazy to seek out J201s, so I've built three ROG emulators with all 2N5457s. All I can say is that I'm very happy with the sound.

[R.G.]

Both circuit fragments that bwana shows have the unbypassed source resistance that would lower gain. The Sparkle fragment is a more useful version, as this one does not change bias as you turn it. Another one is to use a trimmer pot for the source resistor with the source and ground connected to the ends of the pot and a bypass cap connected to the wiper and to ground. The sparkle version separates AC and DC conditions for the JFET better.

Doing some simulation, it's hard to get gains matching a tube without clipping because tubes have such high voltages before clipping. That's probably what's behind the sims distorting so easily.

[puretube]

http://l2.espacenet.com/espacenet/bnsviewer?CY=de&LG=de&DB=EPD&PN=US5434536&ID=US+++5434536A1+I+...

[RDV]

...and my Thuderchief would probably be more to you guy's liking as it doesn't have screaming gain at all. I just use it like a mild boost/drive for the low(er) gain channel in my Marshall. I may have made a mistake in me perfing, but I couldn't find it.

RDV

[B Tremblay]

Another one is to use a trimmer pot for the source resistor with the source and ground connected to the ends of the pot and a bypass cap connected to the wiper and to ground.

We chose that approach, but with a panel pot, for the input stage of the Condor and it works nicely.

[bwanasonic]

Doing some simulation, it's hard to get gains matching a tube without clipping because tubes have such high voltages before clipping. That's probably what's behind the sims distorting so easily.

I was wondering how running these types of circuits at 18v would effect headroom.

Kerry M

[cd]

(2) the pre- and post-clipping EQ is as important if not more important than the actual clipping that goes on.

Ya know, I've said this to most guitarist friends I know millions of times (i.e. use two EQ pedals for tone shaping), but almost all of them are content to go through pedal after pedal until they find "their" sound.

[electrictabs]

1)the pre- and post-clipping EQ is as important if not more important than the actual clipping that goes on.


2)However, it's simpler to change a JFET stage's gain by changing the unbypassed source resistance for the stage than it is to deal with the changes in bias point of subbing in unlike JFET types.
So yes, mess with the JFETs. You can change gain easily by inserting unbypassed source resistance, and probably should do it that way instead of swapping JFETs at random. The J201 is a high gain device, so it's easy to turn the gain down.

1)for that reason i always prefer putting the eq in all my emulations than doing the all knobs to 10 mod

2)if you don't like too many pots you can do this(as in some amps)
i believe that any value less than 100k (according to your ears of course) will do the job
if you have more than one tube or fet like this you can use a dpdt,3pdt,etc
to lower the gain of the circuit a lot...

[Satch12879]

Ya know, I've said this to most guitarist friends I know millions of times (i.e. use two EQ pedals for tone shaping), but almost all of them are content to go through pedal after pedal until they find "their" sound.

David Gilmour used to use two Boss EQs in his pedal board for just such a reason.