High voltage FET stage for high-gain channel question.

[psychepool]

I want to make footswitchable 2-channeled JCM800 that has clean channel.
It will base on AX84 Renegade but would be add some mod.
http://ax84.com/media/ax84_m175.pdf

The one thing that worry about is additional stage for JCM800 high-gain.
Frankly, I don't need extreme high-gain so I could get enough distortion with JCM800 original circuit, but sometimes need extra high-gain just for enjoy.
So I want to add one gain stage if I can add it simply.

Bogner Ecatasy has one additional stage that can on/off switchable for high-gain.(Look at V3B on this scheamtic)
http://milas.spb.ru/~kmg/files/schemati ... Preamp.pdf

But if make it bassed on Renegade, there's no tube stage left.
I feel very uncomfortable for add a tube, and even the half of tube is waste.

So I planed to add a stage with high voltaged FET.(like LND150...and etc.) But I never tried it.
Is it will be alright? Has no trouble?
Can I make it with same way that I making with tube?

And is there any popping noise to switching the stage that way without muting circuit?


I don't mind if there's no additional gain stage but if the result would be effcetive, I want to try it.

Waiting for the experiended advise.

[strelok]

A few suggestions:

1. Post phase inverter master volume. This will essentially give you another gain stage to work with. Unless of course you already find that you want more gain with a JCM800 fully dimed. IMO it really improves the sound of this circuit when its at moderate volume levels when compared to the pre-phase master. The distortion is smoother, fuller and not as buzzy.

2. Play around with the plate resistor values. Especially on the first gain stage of the distortion channel. Increasing it will give you more signal output from that stage and drive the following stages harder.

3. Use a clean boost or an overdrive pedal. If you're just looking to get the occasional extra push over the cliff, this would be the easiest way to do it.

As to the mosfet, IMO its not really the best solution. SS devices for added distortion generally just don't sound all that good, and they can be finicky to get working right in this type of situation. My advice would be to build the circuit stock, try some of the above as they're cheap and easy to do and see if that doesn't get you what you want.

[psychepool]

Thanks for your advise.

I'm also a kind of man who feel uncomfortable when add SS into tube amp. It feels like lost purity.
(That is the reason I don't try to make a Marshall 2550 clone, nevertheless I like the sound of it.)
I don't want to economize by myself on DIY amp.

But I thought add some trick for rarely use mod wouldn't feel very uncomfortable for me.
And if turn off the added FET stage, it would be perfectly same with original JCM800.


The ppimv is not proper for my plan because I planed footswitchable 2-channel(Clean/Dirty).
Preamp master volume is vitally need for volume balance each channel.
It will not bad idea if there is non pop-noised switch for ppimv.

Plate resistors can be changed too because I will try many other 800ish schematics.(Bogner Shiva, Soldano atomic...etc.)


My question is just "That FET stage would be operate and does it efficient?", not about "How to increase JCM800 gain?".


Your opinion is FET and other SS devices for add gain stage dosen't good idea for sound.
By the way, I readed this article.
http://www.geofex.com/Article_Folders/m ... tfolly.htm

As I have told you, I'm also a kind of a man that this article says "Quit reading this now and save your time and outrage.".
But use the FET for cathode-follower is has some interest for me.

If using SS devices is bad choice for gain stage, how do you think about use it for cathode-follower?

[Littlewyan]

You have a few stages in the JCM800 pre amp that have unbypassed cathodes. Maybe try switching a bypass capacitor over the 820Ohm Cathode Resistor on the 3rd stage?

Or maybe switch a resistor in parallel with the 2nd stage 10K cathode resistor so it goes down to 5K in value or something.

These are just two simple options for simply increasing gain adn they can be put onto a switch. Push pull pot would do the trick. You'll need to experiment as for all I know it might sound terrible with those mods. Or it might sound awesome .

[xtian]

+1 cat bypass for extra gain. That's my favorite setting for Malcolm Young crunch on my '83 2203: Low gain channel, added cat bypass (1uF on V1b).

Also +1 on Mr. Keen's MOSFET follies article. MOSFET as cathode follower works great, frees up a glass valve for something else.

[Littlewyan]

The only thing to remember with using a MOSFET is it won't satuate like a 12AX7 will. The Cathode Follower stages in Marshalls provide nice asymmetrical distortion which equals second order harmonics. Thats why they still use them . The MOSFET won't satuate at all so you'll lose that distortion.

I think a nice simple cathode bypass capacitor on V2a's 820Ohm Cathode Resistor is a good place to start. Maybe start with .68uF, if thats too bright then go up to 10uF or maybe even 25uF.

I am slightly jealous that you both have JCM800s btw

[xtian]

The only thing to remember with using a MOSFET is it won't satuate like a 12AX7 will. The Cathode Follower stages in Marshalls provide nice asymmetrical distortion which equals second order harmonics. Thats why they still use them . The MOSFET won't satuate at all so you'll lose that distortion.

Good point.

[John_P_WI]

The only thing to remember with using a MOSFET is it won't satuate like a 12AX7 will. The Cathode Follower stages in Marshalls provide nice asymmetrical distortion which equals second order harmonics. Thats why they still use them . The MOSFET won't satuate at all so you'll lose that distortion.

Good point.

I'm not so sure. KMG did some amazing work with the fets and diodes replicating the characteristics of the tube. Somewhere I have a pcb half designed to explore his fet studies.

Check out

https://youtu.be/cB5XaE_vEJk and

http://www.ssguitar.com/index.php?topic ... 1#msg20371

I wish Mr J M Fahey would chime in and set us all straight, I'm definitely interested.

John

[psychepool]

As I wrote upper reply, My question is not about 'boost the gain'. It's about the MOSFET stage whether work or not.

If it works well and sounds good, it could be used many purpose.
There's many good-appraised MOSFET fx loop circuits. So I thought that use the MOSFET in gain stage and cathode-follower would be also good choice.

But as Littlewyan says, It wouldn't be effective as tubes.
It would be good if I can compare both by my ears, but it will be impossible before make it by myself. And It would be very tired work.


It will better to use stock tube stage. Thanks for advise!

[psychepool]

Oh, John makes me agonized.

I couldn't gratify my curiosity until I try it by my self. hahaha...

[pdf64]

As I wrote upper reply, My question is not about 'boost the gain'. It's about the MOSFET stage whether work or not.

If it works well and sounds good, it could be used many purpose.
There's many good-appraised MOSFET fx loop circuits. So I thought that use the MOSFET in gain stage and cathode-follower would be also good choice.

But as Littlewyan says, It wouldn't be effective as tubes.
It would be good if I can compare both by my ears, but it will be impossible before make it by myself. And It would be very tired work.


It will better to use stock tube stage. Thanks for advise!

There's no doubt that MOSFETs work!
FX loops are generally designed to be as transparent, ie linear, as feasible.
SS devices almost invariably have to use negative feedback to achieve that, which tends to make their transition into overdrive harsh and abrupt.
We tend to like tubes for their non linearities; even though they have 100% negative feedback, a quirk of the dc coupled cathode follower's performance acts to shape the waveform in a pleasing way when it is overdriven.

[stephenl]

Ron Worly over at SonicDude Ranch
http://sonicduderanch.com/
Has some nice " JackBoost" LND150 boards for sale that replace your input jack. It has a B+ Filter node built in - so no amp mods. It' nicely done and sounds transparent.
He also has an LND150 effects loop that is jack mounted.