Fet version of the JCM800

[mhelin]

2. Why do the circuits need a negative supply on the source and why do that supply need to be buffered?

Because with autobias with source network tied to GND I can`t get stage gain, frequency response & top/botton clipping points same as for tube prototype.

I take it that you don't have to use negative supply if you are NOT trying to emulate some known amp. In fact in this JCM800 emulator only three first gain stages actually use additional negative biasing.

How about using a lower supply voltage in range of 48-100V?

[blackcorvo]

I take it that you don't have to use negative supply if you are NOT trying to emulate some known amp. In fact in this JCM800 emulator only three first gain stages actually use additional negative biasing.

I believe the last two stages are just buffers (source followers), so they don't really affect the distortion sound, right?

How about using a lower supply voltage in range of 48-100V?

I tried this at 120v and seems to work OK with about -2v bias, but I don't have a scope to check curves:

Rg 1M+47k, Rp 100k, R2/D1 1k/I.R. LED, R3 330R (but I'll probably change to 220R later), D2-D4 1x 1N4007, LeftR 1k5, RightR/C2 1k/220nF.

Wonder if an I.R. LED for D2-D4 would work OK as well? I'm not really looking for an authentic replication of a certain amp's sound, I just want decent overdrive/good voltage swing. Most of the distortion I want will come from the power tubes anyways (6AS5, 8K P-P, fixed-bias -13v).

I also would like to point out that I used a "sourcedyne" (split-load) inverter in my prototype, with 47k for both upper and lower loads, 1k source resistor, 10k stopper and 470k Rg. B+ at 125v.

Maybe there's a way to get a more plesant distortion curve in that inverter as well?

[tomfiggy]

How many people have built one of these on here? I think I'm going to give it a shot. Does the PCB work OK? I was going to make a one off with the artwork in this thread. Any updates to that or the schematic?
Thanks You

[juanMotta]

Hi, any one knows who have build this?

Juan Motta

[phatt]

Just PM KMG as I'm sure he can point you to people who have built this unit.
Be aware it is not a simple build,, Also It will likely require the exact parts if you want to use his PCB layouts.
Depending where you live you may not be able to access the same parts and too late if you already printed the PCB.

I strongly recommend you breadboard the circuit before you commit to a build. Not because I doubt KMG but it's complex and better to find any issues before you build stuff like this.
Phil.

[juanMotta]

Just PM KMG as I'm sure he can point you to people who have built this unit.
Be aware it is not a simple build,, Also It will likely require the exact parts if you want to use his PCB layouts.
Depending where you live you may not be able to access the same parts and too late if you already printed the PCB.

I strongly recommend you breadboard the circuit before you commit to a build. Not because I doubt KMG but it's complex and better to find any issues before you build stuff like this.
Phil.

Thanks for your answer!! yes, i already pm Kmg but i don´t have any answer i also sent an email to the 2 email address of his web page.

[juanMotta]

Sorry for the question... but can i build the 15 watt version of the kmg fet amp with 1 amp transformer? is not specifed in the layout and schematic Just said the 80 v +-.

[rankot]

I started low voltage fet experiments with Victor Kampf idea.
http://www.sugardas.lt/~igoramps/article68/article.htm
But sound was not close enough to tube one & I moved to high voltage experiments.

I tried to read Victor's article, but I am not sure what is Rc value on this page. Could you please explain it?

[phatt]

I tried to read it also but it's in Ruski or some thing so I'm stumped.
Phil.

[J M Fahey]

https://translate.google.com/translate?sl=ru&tl=en&js=y&prev=_t&hl=es-419&ie=UTF-8&u=http%3A%2F%2Fwww.sugardas.lt%2F~igoramps%2Farticle68%2Farticle.htm&edit-text=

Mind you: D1, D2 and D3 are not "diodes" per se but Russian Germanium transistors "connected as"  diodes, they add Germanium roundness and smoothness to what would otherwise very harsh FET clipping.

They also tried Schottky diodes, which clip at a level similar to germanium, but although the clipping level was close, edges were sharp.
Of course, Russians can still easily buy Germanium transistors at their Electronics shops.

[KMG]

Hi guys.
Unfortunately my IP was erroneously banned for a long time.
Now I returned back.
About

Hi, any one knows who have build this?

Many people build this preamp by youself, but unfortunately not in in English speaking countries (Russia, Ukraine, Israel). Many peoples purchased full version (F-800) with 5W poweramp.
I strongly recommend you to build LND150 version of preamp by the two reasons:
1. 2SK216 is very difficult to buy (they end of production, there are many fake ones at ebay)
2. LND150 more close by sound to the tube one.
Also this version does not require any trimming after assembling.
Some responces about sound of the F-800 you can read in this thread http://forum.guitarplayer.ru/index.php?topic=273237
Unfortunately in Russian, but you can use Google translate.

[joecool85]

Hi guys.
Unfortunately my IP was erroneously banned for a long time.
Now I returned back.
About

Hi, any one knows who have build this?

Many people build this preamp by youself, but unfortunately not in in English speaking countries (Russia, Ukraine, Israel). Many peoples purchased full version (F-800) with 5W poweramp.
I strongly recommend you to build LND150 version of preamp by the two reasons:
1. 2SK216 is very difficult to buy (they end of production, there are many fake ones at ebay)
2. LND150 more close by sound to the tube one.
Also this version does not require any trimming after assembling.
Some responces about sound of the F-800 you can read in this thread http://forum.guitarplayer.ru/index.php?topic=273237
Unfortunately in Russian, but you can use Google translate.

Sorry about the block, KMG, and welcome back!

Also, is there a kit for this available?  Parts and PCB?

[KMG]

Now I have only "full version" boards (preamp+5WPA+PS)

For separate preamp I can offer gerber/drill files:
jcm800mvfGerber.zip

[joecool85]

Now I have only "full version" boards (preamp+5WPA+PS)
For separate preamp I can offer gerber/drill files:
jcm800mvfGerber.zip

That's perfect!  How expensive is it to get the boards?  Do you sell a kit with the components as well?

[rankot]

Is it possible to use any other combination of diodes? ES1J is not available in my country. Also, can I use two BAT85 instead of BAT54, they seem to be almost identical?