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Author Topic: Mosfet JCM800 by KMG  (Read 3138 times)

jcmbro

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Mosfet JCM800 by KMG
« on: September 21, 2018, 02:04:44 PM »

http://milas.spb.ru/~kmg/jcm800fetLnd150_en.html

Has anyone attempted a build of this project by our fellow forum member KMG?
Will also build this 40w poweramp into the same chassis.

http://milas.spb.ru/~kmg/fetpa_en.html

I will start working on it in a few days and would appreciate any tips or advice! :D
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Cpt. FixIt

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Re: Mosfet JCM800 by KMG
« Reply #1 on: September 26, 2018, 06:19:08 AM »

I have built KMG's LND150 JCM800 as well as his E530 and his microamp for a friend.
Aquiring the components was easy, except for a few semiconductors that I had to order from Mouser(together with the transformer for the microamp).
You will have to make your own design for the HV-supply, if you don't have any experience designing proper layouts for SMPSs I recommend you use the 12VAC-into-backwards-transformer variety. It is a bit bulkier and less efficient, but you only need a few milliamps anyway.
Also, you will have to etch and drill the PCBs yourself, which is rather easy with KMGs single-sided designs, but certainly is some effort compared to just sending away gerber files and waiting for the PCB to show up in the mailbox.
Soldering SMDs was a real pain until I realised I needed new glasses and a finer tip for my soldering iron. ::)

Don't forget that you will have to have the output transformer manufactured (or wind it yourself), it is a good idea to get in contact with the transformer guys before you start! In my experience they are usually very helpful, and having transformers wound to spec is often cheaper than buying off the shelf (...and you won't find anything for your PA there anyway).
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jcmbro

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Re: Mosfet JCM800 by KMG
« Reply #2 on: September 27, 2018, 05:51:20 PM »

I have built KMG's LND150 JCM800 as well as his E530 and his microamp for a friend.
Aquiring the components was easy, except for a few semiconductors that I had to order from Mouser(together with the transformer for the microamp).
You will have to make your own design for the HV-supply, if you don't have any experience designing proper layouts for SMPSs I recommend you use the 12VAC-into-backwards-transformer variety. It is a bit bulkier and less efficient, but you only need a few milliamps anyway.
Also, you will have to etch and drill the PCBs yourself, which is rather easy with KMGs single-sided designs, but certainly is some effort compared to just sending away gerber files and waiting for the PCB to show up in the mailbox.
Soldering SMDs was a real pain until I realised I needed new glasses and a finer tip for my soldering iron. ::)

Don't forget that you will have to have the output transformer manufactured (or wind it yourself), it is a good idea to get in contact with the transformer guys before you start! In my experience they are usually very helpful, and having transformers wound to spec is often cheaper than buying off the shelf (...and you won't find anything for your PA there anyway).

Thanks for the response! Also apologies for not seeing it sooner.

I have the gerber files for the preamp so i had those done by a friend and etched the poweramp PCB myself. Don't have any experience with SMD soldering but there's a first for everything right? :P

What did you use for a power supply? I'm having a hard time finding something that can supply +80V,-80V,-12V,+340V on the cheap.

Also what are your output transformers specs? I got some trannies from a broken SS poweramp that i'm going to have rewound by a buddy of mine.

Didn't have to worry about high voltages with building pedals :D
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Cpt. FixIt

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Re: Mosfet JCM800 by KMG
« Reply #3 on: September 28, 2018, 06:43:42 AM »

Don't have any experience with SMD soldering but there's a first for everything right? :P
It is actually pretty easy once you get the hang of it. And the SMDs Mikhail chose are not the gazillion-of-tightly-spaced-micro-legs type anyway, so everybody should be able to do it.

What did you use for a power supply? I'm having a hard time finding something that can supply +80V,-80V,-12V,+340V on the cheap.
You have several optios:
- Have a transformer custom made
- Look for a transformer with 2x~60V sec and get an extra one with 12V sec. Add a third small 230V(2x115V)/9-10V in reverse, secondary connected to 12V. The preamp draws only a few mA, so that would be an acceptable solution.
Or use the step-up powersupply Mikhail suggested and adapt it for the higher voltage. But be warned, the devil is in the PCB layout, you may encounter all kinds of noise and oscillation if you have no prior experience with that kind of stuff.

Also what are your output transformers specs? I got some trannies from a broken SS poweramp that i'm going to have rewound by a buddy of mine.
I only built the MicroPA and used the suggested Xicon trannie, which is available from Mouser, for about 5 bucks IIRC.
If you are going to rewind an existing trannie, make sure to get the measurements done to guestimate the core's specs before you take it apart!
If you need help with that, KMG made some excellent posts and also has software to make the calculations more convenient on his webpage.
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Zozobra

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Re: Mosfet JCM800 by KMG
« Reply #4 on: September 28, 2018, 06:47:50 AM »

For the B+ you can use a ZVS DC boost converter perhaps?  They're only a few bucks on ebay.  I've built an ecc99 poweramp with one run from a laptop PSU.  I also used a similar bucking smps to supply the heaters.

https://www.ebay.co.uk/itm/142934951063

Coincidentally I've just ordered a bunch of PCBs for this preamp from JLCPCB and this will be my first foray into SMD soldering!  If I like how it sounds I may we knock together a design of my own with fewer SMD components and then make the project public on EasyEDA.  I'm happy to have collaborators check on the project if I go through with it as I'm a bit of a PCB layout noob  ;)
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jcmbro

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Re: Mosfet JCM800 by KMG
« Reply #5 on: September 28, 2018, 12:06:33 PM »

- Have a transformer custom made
- Look for a transformer with 2x~60V sec and get an extra one with 12V sec. Add a third small 230V(2x115V)/9-10V in reverse, secondary connected to 12V. The preamp draws only a few mA, so that would be an acceptable solution.
Or use the step-up powersupply Mikhail suggested and adapt it for the higher voltage. But be warned, the devil is in the PCB layout, you may encounter all kinds of noise and oscillation if you have no prior experience with that kind of stuff.

Looks like custom trannies are the way to go then.
Quote
I only built the MicroPA and used the suggested Xicon trannie, which is available from Mouser, for about 5 bucks IIRC.
If you are going to rewind an existing trannie, make sure to get the measurements done to guestimate the core's specs before you take it apart!
If you need help with that, KMG made some excellent posts and also has software to make the calculations more convenient on his webpage.

How loud can the 250mW poweramp get? Is it as loud as say a pair of PC speakers at full volume?
Will measure twice,cut once for sure!

For the B+ you can use a ZVS DC boost converter perhaps?  They're only a few bucks on ebay.  I've built an ecc99 poweramp with one run from a laptop PSU.  I also used a similar bucking smps to supply the heaters.

https://www.ebay.co.uk/itm/142934951063

Coincidentally I've just ordered a bunch of PCBs for this preamp from JLCPCB and this will be my first foray into SMD soldering!  If I like how it sounds I may we knock together a design of my own with fewer SMD components and then make the project public on EasyEDA.  I'm happy to have collaborators check on the project if I go through with it as I'm a bit of a PCB layout noob  ;)

It's good to see others also attempt this project! I'm probably going to use a custom transformer but thanks for the advice. Be sure to update yeah? :D
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phatt

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Re: Mosfet JCM800 by KMG
« Reply #6 on: September 29, 2018, 06:48:06 AM »

For the B+ you can use a ZVS DC boost converter perhaps?  They're only a few bucks on ebay.  I've built an ecc99 poweramp with one run from a laptop PSU.  I also used a similar bucking smps to supply the heaters.

https://www.ebay.co.uk/itm/142934951063

Coincidentally I've just ordered a bunch of PCBs for this preamp from JLCPCB and this will be my first foray into SMD soldering!  If I like how it sounds I may we knock together a design of my own with fewer SMD components and then make the project public on EasyEDA.  I'm happy to have collaborators check on the project if I go through with it as I'm a bit of a PCB layout noob  ;)

Why yes that makes more sense as us old fellas can't even see SMD's let alone solder them in place. :dbtu:

Yes PCB software does take a while to learn but doable.
The latest version Kicad is a bit easier to use.
Whatever CAd you use Just start with something simple to learn the basics. 8)
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Zozobra

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Re: Mosfet JCM800 by KMG
« Reply #7 on: September 29, 2018, 02:23:10 PM »

I'll probably just use the EasyEDA online editor.  There is a full version too but the online one seems more than powerful enough for my needs :)  It hooks straight into a Chinese PCB fab house that offer silly cheap prices on boards 100x100 or smaller.  I just need to sit down and make a few of my own generic component types when I get back from my holidays. 

I'm kinda curious about the mini poweramp project too.  Shame there isn't a gerber for that (or if there is I missed it) otherwise I'd have ordered some too.  I was thinking it would be kinda neat to maybe run one into a dummy load to feed into a real big SS amp for live use :)
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Katoda

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Re: Mosfet JCM800 by KMG
« Reply #8 on: October 02, 2018, 02:16:41 AM »

If you want to use the pcb layout from the project site, there is a way to transform that pdf into an image and then turn that image into a gerber.
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Zozobra

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Re: Mosfet JCM800 by KMG
« Reply #9 on: October 06, 2018, 04:58:48 AM »

I think the easyEDA package can do that. I'll take a look when I get back to a computer.
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jcmbro

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Re: Mosfet JCM800 by KMG
« Reply #10 on: October 07, 2018, 05:05:15 PM »

Does anyone know why the -12v bias is passed through BC857 transistors at Q1,Q3 and Q5?
I'm curious as to why that circuit is necessary,i believe the valve jcm800 doesn't have a bias control circuit?
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phatt

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Re: Mosfet JCM800 by KMG
« Reply #11 on: October 08, 2018, 09:15:04 AM »

Does anyone know why the -12v bias is passed through BC857 transistors at Q1,Q3 and Q5?
I'm curious as to why that circuit is necessary,i believe the valve jcm800 doesn't have a bias control circuit?

Well I think you will find it's there to replicate grid current like a triode. :tu:

 google,,  forward grid current or grid current limiting.
It plays a big part in the way valves react when over driven.

You can download Chapter one of Valve Wizards book for some in depth info here; http://www.valvewizard.co.uk/gainstage.html
Phil.
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jcmbro

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Re: Mosfet JCM800 by KMG
« Reply #12 on: October 08, 2018, 04:57:02 PM »

Thanks phil!

After much reading i think i kind of understand what it does.
I also imagine if one were to "Mosfetize" a different model of amp,say a Fender or a Soldano the circuit might need fine tuning?

Also i realized that the same circuit is not used for the last two mosfets,is this because distortion does not occur at these last stages and therefore unnecessary?
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Zozobra

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Re: Mosfet JCM800 by KMG
« Reply #13 on: October 09, 2018, 02:02:11 AM »

The 2nd to last MOSFET is replicating a cathode follower and the last one is an output buffer/FX send to pad the signal level down is my guess.
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Zozobra

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Re: Mosfet JCM800 by KMG
« Reply #14 on: October 16, 2018, 11:19:21 AM »

I should have a set of 10 PCBs in the mail from easyEDA pretty soon.  I've decided that I'm going to rework the design to use through hole components rather than buy a bunch of SMD stuff I'll use once, so if anyone wants one (or more) then I'm happy to send them your way for the cost of shipping.
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