Marshall Advice?

[Gibsonman63]

I have a 1985 JCM800 Marshall 4210 combo. Basically a 2205 in a 1x12 combo. I have some hum issues that I am working out and I need to replace one pot that I managed to break, strapping my amp to the inside of an armadillo art car. (hey, a gig is a gig!) I am contemplateing replacing the coupling caps with Sozos or Jupiters to try to squeeze a bit more mojo out of it while I am in there.

What really bothers me about the amp is the lead dress. The way the amp is built, the wiring between the preamp board and the tube sockets needs to be long enough to allow the entire board to be pushed backwards to allow the pots to clear the holes so you can flip the board over to work on it. They have done that and then some, so those wires are a bit long and are all tucked up under the circuit board.

I would really like to replace the entire thing with a turret board, to make tweaking easier and so I can do some proper lead dress, but with the channel switching and effects loop using chips and transistors it becomes a design problem for me.

My preference would be to make this amp a bit of a sleeper, stock from outside appearances, but with the magic mojo under the hood.

Suggestions?

[xtian]

I have an '83 2203 (single channel, no loop, very simple). Have you replaced the power filter caps? Mine are still original, and I have no hum issues, but I know they're near the end of their life.

I think replacing the main PCB with turret board won't improve your hum, nor make a significant upgrade in tone. I'd rule out the caps first.

[Gibsonman63]

I replaced the filter caps four or five years ago along with the rest of the electrolytics on the board.

I plan to trace through with the scope for good measure, but these amps are known to be a little hummy. I don't think it is a totally unreasonable amount of hum, but my builds so far have been much quieter. I am looking for ways to clean up the lead dress, but still have the amp servicable. It looks like the knob layout dictated the PC layout so the wiring under the board crosses back over itself to switch in the extra gain stage.

It would be nice if someone made a turret with a pin on one end so you could solder it onto a PC board for retrofits.

Can someone educate me a bit on capacitors? Most of the capacitors on on the board are the square molded type. There is not voltage rating given on the schematics, but on the caps there are suffixes, M100, M400 and K100. I would think that the coupling caps would be rated for 400 volts, but then the other two values don't make sense to me. Also, the first stage cathode bypass cap is one of these molded types. I would have expected it to be an electrolytic.

[Gibsonman63]

This should give you an idea of what I am trying to clean up. These are the wires running from the circuit board to the tube sockets. Now I understand why a lot of amp companies went with connectors, but I really don't want to go there.

I am still checking things out, but the grounding is a bit strange. The jack for effect send and return have separate grounds in different places and they are tied together at the jacks. It looks like someone ran another ground from the reverb pot to the ground on the reverb transformer.

[Gibsonman63]

I couldn't help but break out the soldering iron. I have separated the incoming AC from my B+ and re-ran all of the heater wiring. I still have some lead dress to do on the preamp tube wiring.

The reverb transformer is open and is mounted inside the chassis. I would like to get an enclosed one that I can mount on the outside, so I can clean up the wiring going to the PI and reverb driver tube. From what I understand, this one drives an 8 Ohm tank. Can anyone recommend a suitable reverb transformer that would be safe to mount on the outside of the chassis?

More pics soon.

[Phil_S]

I think the Fender reverb drives an 8 ohm tank. It is often used to take SE output from a 12AU7 to a speaker, like in an AX84 Firefly. Here's eBait, make an offer... I'd offer $10. http://www.ebay.com/itm/250392815108?ss ... 1423.l2649

[Structo]

I would rather work on that bird nest of wiring than an amp with ribbon cable and connectors.

I'll never understand why these amp companies moved so far away from simple designs.
I can see using a solid state effects loop for cost saving but to add all the transistors and other chips to what used to be a pretty basic 100w amp just baffles me.
Then in addition the delicate ribbon cables and multiple circuit boards just adds ways for something to go wrong.
I guess some people have to justify their jobs by submitting new designs all the time.

[Reeltarded]

They do it because it because it's what is cheapest.

Those new ones with the tic-tac style pots really piss me off.

[Gibsonman63]

I agree about the ribbon cables. I have been inside a couple of newer Boogies that had them everywhere.

This amp is actually starting to clean up a bit and look like something except for the boost circuit that crosses back over itself. Of course every time you access the back of the board, you have to dress out all of the preamp wiring again.

I spent a couple of weeks floundering contemplating various "wreck-ish" designs for this amp before deciding to just make it what it is, only better. Then if I still am not crazy about it, I can always sell it and build a Rocket.