'82 JCM800 2204

[gldtp99]

I recently serviced this amp for a friend --- It is a 6550 spec amp that had EL34's installed (which biased the EL34's so cold that it barely operated at all)

I quickly measured all the resistors in the amp (initially looking for the "problem" before I realized it was a 6550 amp) and I found that the V1b Plate Load resistor had drifted high (measured @200k --- stock is 100k)

It is an original Iskra carbon film resistor and it looks original to the amp (it's marked 100k) --- but it measures @200k

After I installed a set of 6550's in the amp and biased it up I tested it (at high volume) and realized that this amp is a F'ckn BEAST --- It is the most "alive" and the loudest 2204 I've ever played

The owner (who runs a pro backline rental service) said this amp used to be a favorite rental for touring pro musicians for many years --- then it quit "sounding right" and was put aside (EL34's in a 6550 amp will do that)

Switching Plate Load resistors from the stock 100k to 220k used to be a "quick and dirty" mod that some people did to get more preamp distortion (it does work) --- but this amp modded itself through Resistor Drift

The matching original V1a Plate Load resistor still measured 100.3k (still in spec -- not drifted)

I have another '79 Marshall 2104 head (used to be a 2x12 combo -- but same amp as a 2204) so I pulled it out and measured the V1a and V1b Plate Load resistors (both original Iskra's) --- and on the '79 the V1b resistor was still in spec (100.8k) but the other V1a resistor had drifted up to 134.7k (from stock 100k)

These are the original type Iskra carbon film resistors that some 'purist" Marshall experts will search for to do "Exact Marshall Builds" ---- but, in the real world, they tend to drift all over the place (maybe improving the sound of the amp, depending on your viewpoint)

But that '82 2204 (with the self-drifted @200k V1b plate resistor) sounds better and more alive that most 2204's (and much better than an '81 I used to own)

I think I'll try this same combination in my next 2204 build ---

[Roe]

200k works well with the 2k7 cathode.

cf. https://www.ampbooks.com/mobile/amplifi ... alculator/

[pjd3]

Glad I came across this thread.

Will be getting back to a 2204 build that's almost done and looking to experiment with mods to see which ones win.

This certainly looks to be something to try out.

Thank you,

PJD3

[WhopperPlate]

200k works well with the 2k7 cathode.

cf. https://www.ampbooks.com/mobile/amplifi ... alculator/

Jose Arredondo agrees

[gldtp99]

I did some tweaking to a 25 watt 2204-type build I did recently --- I put the 470k/470pF !st Treble Peaker back in (i had a 68k in there) to give it more treble response when the MV is cranked above 5 --- this worked well, I'd gotten into the habit of building 2204-type amps without the 1st Treble Peaker (subbing a 47k or 68k resistor) to make them smoother sounding at lower volume levels (less ice pick) --- but with the MV cranked above 5 the amps were too muddy/undefined sounding for me

I also (and more importantly) removed the stock 100k V1b plate resistor and replaced it with a 150k --- This served to "wake up" the amp when cranked but still retain enough Clean Headroom (when set Clean) to use the amp as a "General Purpose" amplifier when using cleaner Pre Gain settings

I also have some 180k's on hand but I went with the 150k plate load resistor on just V1b

It is amazing how loud a 25 watt 2204 build (w/6L6GC x 2) can get when it's cranked up all the way --- the lower voltage @120mA PT and ClassicTone Deluxe Reverb "Fat Stack" 4/8/16 ohm OT (6.6k primary) really put out the sound

No, it's not loud like a cranked 2203 but how loud do I really play anymore ? It seems like most people are using attenuators, load boxes, iso cabinets, etc, etc to control volume levels anymore

I built two 25 watt 2204 type heads, as experiments, and they both put out more sound/volume than I ever thought they were capable of ---- the other one has a Hi Gain Roccaforte HG100 type preamp and it sings and wails (but the amp has no usable clean tone)