2204 mods.txt

7/27/2019 2204 mods.txt

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Here they are. R5 is 470k and about to be replaced with 68k, C4 and C5 are aboutto be removed.

Marshall used lower voltage power transformers in the late 70s. My 1981 JCM800 2204 has 365V on the plates, so it's closer to the early master volume JMPs in that respect. In late 81 or early 82 Marshall changed to higher voltage trannys. Most JCM800s have more than 450V on the plates, that's at least 50V higher in most cases, which will definitely change the tone. More volts equal more clarity, atighter, punchier sound but it's also brighter, which in combination with the treble peaking circuit and bright caps might sound harsh and piercing. Lower voltage gives a browner sound, softer, warmer and more creamy.

So there is a difference between the two. Then there's also the change in components (caps) which will also influence the sound to a degree.

To conclude, If you're looking for a crisp, tight and pucnhy sound (if you playmetal for instance and you're boosting it with a pedal), the later JCM800 era amps might be more appropriate. If you're looking for a creamy, warm overdrive sound you should go for an early JMP. But it's all personal taste in the end, I usemy low voltage amp for metal, but I like a slightly mushy low end for riffing.

There was also a difference in transformers which took place around the summer of 80 which resulted in different palte voltage especially the 50 watters.My '74 JMP 50W has a B+ of around 360VMy '85 2204 is around 430VAnd it definitely shows as far as volume goes.The JMP is quite a bit louder than the MK II ;)

A common mod to tame brightness in a Marshall 2204 is to replace the470k/470pf at pre-amp pot with a 68k, the "classic" Marshall input value. Weare told this correction is necessary to arrive at the normal input of 68kohms.The fallacy: 68k is, yes, the classic input value, but the amp already HAS 68kat the input...the hi input...the one most people use. And if we add another68k, we have two in the same signal path. Treble's reduced, but the signalhas become so fat, the bottom sounds mushy.

If one wishes to decrease treble via replacing 470k/470pf which areactually located AFTER the first gain stage (assuming we use the

hi input, which is the usual case) a better choice is 220k/470pf or,maybe, 470k/.0022uf. Either will diminish the relative treble, withoutcreating a mushy bottom end, and both add a little gain, as well.

Having said this, there are, of course, a few players who use the lowgain channel of a 2204. For them, replacing the 470k/470pf with a 68kwill change the input resistance to the classic 68kohms. Everybodyelse is already there. So don't be fooled!

7/27/2019 2204 mods.txt


Thanks, Drewl. Cutting the bright cap is very common and works well. I cut mineseveral years ago. It helped a lot but, for me, the amp wasstill too bright. Part of this is because my lead pickup is mounted very close to the bridge. The mods I mentioned in the last thread are inaddition to clipping the bright cap. I like both the 220k/470pf and 470k/.0022uf....the first is a little crisper, the second is a little punchier.

Most Marshalls are not considered to be too bright, but many players complain that their JMP or JCM 2204's are. One reason for the brightness is the bright capwhich is often removed. But another reason is that these amps, due to the cascade design, have not one, but two 470k/470pf treble peaking networks, oneat the pre-amp and one on the pcb. This is anything but "classic", and creates aLOT of treble. However, the popular mod of replacing the first 470k/470pfwith 68k means the guitar signal will see the STOCK 68k resistor at hi input andthen see the NEW 68k after the first preamp, too. This new cascade isanything but "classic", as well. I have read several mods where the 470k/470pf are replaced with something not so extremely different as the relatively lowresistance 68k, and I think these ideas are the best.

Here are some treble peak combinations I've been experimenting with in live situ

ations: 470K/.0022uf, 220K/470pf, 220K/.0022uf. These are all chunkierthan the original 470K/470pf, but not as chunky as 68K/NO CAP which is a popularmod (and the original subject of this thread), but (opinion here) assomeone who usually plays humbuckers, I feel is too chunky. Also, I don'tlike continually soldering and unsoldering at the lug of my preamp pot....the repeated changes in temp are not good for it, so I left an inch longlength of wire attached to the lug and solder to THAT. And beware ofthe potential high voltages!

I know this is a bit late . . like 6 months. But I've been reading this thread and I can't help but adding a little information here. The 68K resistor on the Hi

gh input is not for signal resistance. It is the "grid stopper" for the preamp stage. Its purpose has nothing to do with tone. It is there to filter out radio frequencies. Lower values can be used there but with some risk of radio creepingin so Marshall (and most amp makers) use 68K for complete filtering. Any highervalue will do the job so the 470K stock value for the Low input works just fine.That one is indeed at that value for tone shaping (along with its 470pF bypasscap) but it works as a grid stopper as well.

That's why Tone Lizard chose 68K to replace it: if the guitar is plugged into the Low input and the player turns the Volume knob all the way up, the tube (V1b in this case) will still see the necessary 68K and not be tuning in any radio stations, phones, etc.

Also, if using a 68K in place of the 470K/470pF combo going to the Volume knob makes for mushy sound it's because there is too much signal getting to V1b, not because the signal is seeing two 68K resistors.

As indicated above a better approach would be to use a smaller value bypass capon the 470K. The same goes for the Volume knob bypass cap. But this is if you want less treble at lower volumes. There are other things to do as well, like a 500pF bypass cap on the plate resistor of one of the stages (attenuates frequencies above 3.1K). But you should realize that when that treble is blocked it will be missing when you really crank the amp up, not just the Volume knob the Master

7/27/2019 2204 mods.txt


too. Because if this I'm considering putting a "Bright" switch that controls what value is being used for one, or even both of the bypass caps - modded for lowvolume and stock for blasting.

If you want more gain try lowering the cathode resistor on V1b - the 10K one, but I wouldn't go any less than 4.7k. I think 8.2K or maybe 6.8K would be good choices though I have not done this on my amp. YMMV and it depends on the sound you're looking for. Note that this mod boosts gain at all frequencies, not just treble or bass.

Okay, 'nuff said.Ken

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2204 mods.txt