Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dinkyguitar on March 24, 2020, 05:30:18 pm
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Hi All,
I've got an itch to build a JCM800, but I haven't built an amp before. I've built other small circuits have soldered so I'm confident I can put an amp together, hopefully one that will work :).
For the board i found this:
https://grangeramp.com/product/jmp-2203-2204-st1-circuit-board-green-2/
Using this schematic and reference pics.
https://mhuss.com/MyJCM/index.html
On the schematic:
https://mhuss.com/MyJCM/JCM800_2204.pdf
I see the presence is 22k where as other schematics show 5k, then there's the bias pot. This is also 22k instead of 10k like I see on other schematics.
So, if I'm going to use the PCB I referenced, I should be ok following the above schematics from mhuss right?
I'm also planning on using el34's and i see the notes in that schematic saying RG, RB1 & RB2 need to change values.
Has anyone built a JCM800 using a PCB?
dinky
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Dinky,
If you're inexperienced then you're in for making a few mistakes. It's normal as there's many things to go wrong in any amp build anyways among the experienced. It is never a good idea to use a pcb or any layout and not have the exact matching schematic to use that things are based on. My suggestion to you is take the schematic and slowly and systematically start at the input jacks and trace one by one each part and signal/voltage path for your all jacks, pots, switches, sockets, transformers, etc... you need to verify the entire circuit or else you are set up for frustration and failure. If you don't you will be forced to do this anyway to find out what doesn't match to what and hopefully if you're lucky you won't blow expensive replacements. Good luck.
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I've got an itch to build a JCM800
so you have Marks version, I'd have no problem following his version
I would have no problem following Marshalls original version
I'd get confused if I tried building them both :icon_biggrin:
If you're just starting out, focus on the how, leave the "why" til you finish jambing a couple hours with it.
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Thanks...well that's kind of why I'm asking now.
I've had the itch for a while now, and in the beginning I read how amps works, starting with this:
https://robrobinette.com/How_the_Marshall_JCM800_Works.htm
Followed by videos and such.
That schematic from Mark, I compared it to the StewMac building instructions for a JCM800 and they are very, very close, with the exceptions I pointed out.
I also compared both those to:
https://drtube.com/library/schematics/69-marshall-schemas#JCM800
So I can read schematics as I did go to electronics school...
Anyway, I just has those opening questions so I can understand better why the differences.
dinky
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Hi !
I just finished my second jcm800 (2204) and maybe the 50th amp in my ''career'' (point to point). I had a bug due to a coupling cap physically misplaced, that I had to turn 90 deg. to give the amp its stability. If you have the right pc board with all components of the right size, MAYBE you'll be able to build it without a bug. But a PC board is easy to burn if you have to swap parts many times, if you see what I mean. You have to take your time, making sure each and every solder is well done. I would start by the B+ supply section. From time to time, you can fire it up and take the DC readings, making sure this section is good. this is the way I do. So when the supply is achieved, you can relax taking your time for the rest, confident in your abilities….Good luck
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I would NEVER advise someone early in their amp building experience to use a PCB for a JCM800.
I would like your early attempt at building this to be successful. I think you will be significantly lowering your chances for success taking this approach.
I think even with a solid turret build (which I would view as much much more reliable), you should be prepared for trouble shooting an early build. Trouble shooting a PCB board is going to be a tremendous challenge for you.
with respect, Tubenit
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ok...so the consensus is torrent.
Everyone has mentioned "troubleshooting".....if you follow the schematics, and choose the right parts why would you have issues?
Is it more "human" error? Or does every amp built like to be built a certain way?
Meaning, even if you have the right parts, the routing of wires might not work everytime?
dinky
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Why issues ?
Because of a bad solder, bad part value, defective part, bad lead dress, parasitic from you dont know where , too high the heater voltage , bias tweaking problems and more. These issues are much more easy to tame in a p to p configuration. It is one thing to get the parts together and one other to make them work. Even the most experienced guy here would probably have to modify or tweak the amplifier to make it work properly.
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Why have issue ?
Read this post from a experienced amp builder here in the forum;
https://el34world.com/Forum/index.php?topic=25715.0
No experience in building amp ; I'll start with a basic one; one volume , one tone and single end tube output.
Building a amp is more than assembling a puzzle, you will have some building issue for sure. Some easy to fix or some not !
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+1 on turret board build.
If (when) you need (want) to swap parts, turret board will be easier platform for doing so.
And for a first amp build, you're gonna have lots of "irons in the fire" with the power supply, preamp, poweramp, lead dress, all at one time, and, no offense to Granger, taking out the possibility of any errors on their part will make your build easier.
Yes, the "human error" will probably pop up. Even after you check through your wiring multiple times, that one little whoopsie will still be hiding in there *somewhere *.
Also bad components happen, even from reputable component sellers.
22k for bias and presence sound more in line with schematics I've seen. I don't recall ever seeing 5k in either position. I've seen 50K for bias pot, but I've not seen 5K.
The stock jcm800 2204 preamp sounds great, I've built a few with and without power sections. Great "vintage rock" tones, and put a tubescreamer or similar in front of it, and get those violin like lead tones.
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the "human error" will
I'm up to ~20 amps, 16 I call my proto-type period :laugh:
My current one had 4 human errors, all would be guaranteed "problems" for a new builder simply because you're new, nothing to do with skill sets per se.
understanding "the differences", the builder thought it was "better", why, only they know, we can guess; they wanted more highs, gave a nice bump in gain, smoothed out.......
building from original, everyone knows "it", well documented........
then, when you get it working, most of us "tweak" to our personal taste. That is when you can hear "the why", or if you're a geek like me, see it on the scope :icon_biggrin: