Plans for JCM800 2204

[patch]

Hi All,

Are there any plans on this site for a JCM800 50 watt?

And can you actually build this amp:

http://ceriatone.com/images/layoutPic/H ... hupa50.jpg" onclick="window.open(this.href);return false;

Using this information in the layout?

patch,

[hopkinWFG]

I say stick to a metro amp layout.. I do have the 2204 50watts layout if you want it? if you have basic electronic know how it be easy but it be a hard knowledge to know what kinda resistors, filter caps, coupling caps not forgetting which tranny to go for and chassis to go for a more modern type or classic type... I am also currently trying to understand more and hopefully I could purchase by end of this month batch by batch till I complete it..

[demonufo]

The metroamp layout below is the one I would go by, however, there are several things wrong with it.
http://metroamp.com/wiki/index.php/2204_chassis_layout" onclick="window.open(this.href);return false;


First and most importantly...
The white wire on the standby switch feeding the bias circuit. This needs to be removed from the bottom terminal on the standby and wired to one of the top terminals which go directly to the PT HT supply. Doesn't matter which one. This way, you have a constant bias supply to the power tubes when on standby. Crucial.


There is an NFB (negative feedback lead) missing. See the purple wire on the layout? There should be another one on the other end of that resistor, going directly to the 4ohm tap of the OT on the impedance selector.


There are two missing wires for the tone stack. If you look at the edge of the turretboard on the diagram, you can see the start of two pink wires that disappear at the edge of the board. The leftmost one connected to the .022uF coupling cap should be connected to the left leg of the treble pot (that jumps across to the bass pot wiper) and the rightmost one connected to the mica cap in the tonestack should go to the right leg of the treble pot.


There is a jumper wire missing from the input jacks. Not crucial, but without it the amp will hiss when nothing is plugged in.
This jumper wire should go from the top right terminal on the HI jack, to the top left terminal of the LO jack.

Edit: Also, the treble bypass cap is missing from the preamp gain pot. There should be a 1000pF 500V cap (usually a ceramic disc, silver mica will do nicely) across the centre terminal and the right hand terminal. Of course, many player cut these off, but personally I don't find the amp cleans up very nicely on the guitars volume pot without it.




That's all the important bits. The rest of this info is all suggestions and improvements.


The NFB resistor on the layout is marked as an option between 47K and 27K. Actually, these were 100K as stock (less negative feedback) but this may be a little aggressive and even too gainy for some tastes. Get all three values and experiment yourself. It's subtle at lower volumes, but a worthy tweak.


The tonestack on early 2204's would be 33K and 500pF. The later ones 56K and 250pF. Personally I prefer the 33K/500pF setup as it's a bit thicker in the mids and less sharp.


The bias caps are marked as an option between 8uF and 10uF. 10uF will be a more stable supply by far. Normally these would be 160V caps, but there is no disadvantage of fitting 200V rated caps here. This is one of the areas where you want a quality spec. cap, and not worry about things like tonal effects. I find Rifa's work very well, and measure superbly, unlike some modern Sprague offerings...


Presence control. Many people earth the presence to the same buss as the rest of the pots. It can be of great benefit to give the presence control its own dedicated earth (definitely NOT to the transformer bolts. You do not want to ground ANYTHING to transformer laminations). If you can route it neatly without going near to any other cable runs (although passing at right angles is perfectly acceptable), it can be beneficial to ground it at the speaker jacks. If you are ever likely to consider fitting a PPIMV to this amp in addition to the pre PIMV master, you may want to consider fitting the PPIMV in the Presence hole, and fitting the presence control in one of the speaker jack holes.


Grid blocking resistors are advisable on the power tubes to prevent oscillations. Remove the orange and green wires from the power tube sockets, and install a 5.6K resistor in series. Wire the resistor directly to the socket, put some heatshrink on the wire, solder the wire directly to the resistor and heatshrink the exposed connection. Do this for each wire. Be careful not to switch the wires around, or she'll howl like a banshee.


The 68K v1b grid resistor that is mounted from the HI input jack would be better off mounted directly at the socket, where the cable goes to. Those dark green cables are actually shielded cable. RG174U is the best for this purpose. The resistor should be mounted direct to the v1 socket on pin 7 (v1b grid). The core of the coax should be soldered directly to this and heatshrunk, and the shield should be heatshrunk and NOT connected to anything at this end of the cable. Then connect the other core of the shielded cable to the bottom left lug of the HI input (where the resistor used to be) and connect the shield of the cable to the bottom right of the HI input jack, which directly grounds it. This will help prevent radio interference and parasitic oscillation being picked up in the first gain stage.

There is also another shielded cable between the master volume and v1a grid (pin 2). This should again have the shield heatshrunk off at the tube socket end and not connected to anything, the core of the cable should be connected directly to pin 2 of v1. The other end should have the core of the cable connected to the wiper (centre lug) of the master volume as shown, and the shield should connect to the left leg, which is grounded. This will help prevent oscillations for the second gain stage. Further down the circuit there isn't a great deal of positive gain, so it is not really of concern.


Personally, I prefer omitting the biasing resistors from the power tube sockets and using decent probes to bias. If omitting these, you should connect the two pins together that had the resistor between. If using them, you will find it more reliable to still short these pins together, and put the resistors in series with the ground connection instead.


As is the case for most of the 2204 layouts in Cyberspace (except for SDM's layout and maybe a couple of others), the bright cap seems to be omitted. Personally I think the 2204 benefits from it, as it can get muddy when rolling back the guitar volume pot without it. It should be a 1000pF 500V cap, mounted directly across the preamp gain wiper (centre leg) and the right hand leg.


Between the LO input and the preamp volume pot, you will see two wires going to a treble peaking circuit (470K resistor with a 500pF cap in parallel) on the turretboard. This is much neater if you take them off the board and mount them directly between the two points. Green wires not necessary.


That's about all the important stuff I can think of for a bone stock 2204 build using the Metroamp/Billy Batz layout.
I hope you find this useful, else I've just wasted 10 minutes.

Edited for obvious omission...

[demonufo]

Clearly it was in vain, after all.


Oh well, I can bookmark it and link straight to it next time a 2204 build question comes up, since I seem to be rewriting mostly the same stuff over and over.

[neikeel]

Clearly it was in vain, after all.


Oh well, I can bookmark it and link straight to it next time a 2204 build question comes up, since I seem to be rewriting mostly the same stuff over and over.

Exactly!

BB still drops by here but does not seem to post. Updates of those layouts would be cool, I had reason to look for a 12 series one the other day and found the mash up lead/bass amp allegedly from 68.....

[demonufo]

BB still drops by here but does not seem to post. Updates of those layouts would be cool

Yeah, I'd do it myself if I knew my way around a mouse, or any relevant software package.

I wonder how young Dan is doing these days. Miss that guy. And MacG. And T-shirts. and so on and so on etc.

[danman]

This is great information! I just finished my first 50 watt plexi build from the Metro layout and now I want to do a 2204 next. The plexi fired up perfect on the first try and sounds great and now with all this info and tweaks for the 2204 i feel pretty confident I can tackle it successfully. Thanks for taking the time to share this!

[demonufo]

Edited my essay. Missed the bright cap off...