Got a Marshall 2204 in for a modification

[greekie]

Hi guys,

Just wanted to share my experience about an 80s Marshall 2204 I had in for a modification.

The owner wanted a more vintage-y vibe from the two channels. Basically it was too tight sounding, and too hissy on the high-gain channel.

What I did was first converting the low-channel to a JTM45 with a 250pF/56k tone stack and 32uf+32uf for both the screens and the preamp. On a 2204 the low-channel is a cold biased 100k/10k triode stage, and I changed this to a 100k/1k5 with a 5uF bypass capacitor. If your preamps HT is around 200V, those values will work well almost everywhere. I kept the 50uf+50uf for the B+, as it keeps everything nice and quiet.

The high-channel triode was set up with a 150k/2k7 with a 0,68uF bypass. This keeps the bass nice and tight, as this stage feeds into the 100k/1k5-stage, and works well with humbuckers as well as single coils. I put a 200pF/1kV capacitor from the plate to cathode of this triode, as the amp had plenty of highs and I didn't want to mess with the tone stack further.

All bright caps were clipped, I installed shielded cable for the high-input, and made sure that was grounded when the low channel was used.


Now, the preamp gain-pot was changed to a dual 1MA potentiometer. This gave me the opportunity to maintain the original look of the amp, as well as being able to control both channels gain with the same potentiometer. The first part of the potentiometer fed the low triode with the signal of the high plate, and the second part fed the V2.

This setup worked well with the pre-PI master volume, and I installed a trimmer on the PI so this could be balanced, as Marshalls usually are problematic, having one of the EL34s red plating when you're hammering away with everything at 11.


I hope this is useful to some of you guys. What are your favorite Marshall tips?

[Reeltarded]

Excellent report! What you did sounds good to me, bet!

I do whatever I can think of depending on where I want to end up. I like lower filtering, generally, and I like raised plates. 50k or so CF instead of 100k, resonance.. I like what everyone likes just in different amounts for what the amp is begging for. heh..

They all sound so different.

[LeftyStrat]

Nice. I also like 32uf+32uf on the preamp.


Miles, never thought of dropping the 100K on the CF. I'm going to have to try that.

[Reeltarded]

It can be hard on the tube in the startup there. I have never had a meltdown, but I have seen a couple claims. Merlin has a modification to save the CF tube. Needs a cap and a diode to clamp the current til the thing warms up.

The problem/good thing about the Marshall topology is that damned near everything sounds good. I swear. Never been so hard to stop tinkering with anything. All stages can be 820r or 2k7 or 1k5 or any of those variations in most any order and.. when to stop spending solder? heh

I like smaller value bright caps. I used to like no peakers but that depends too these days. If you get a lot of gain going on it is good to peak a stage to keep it forward with a big mix.

Dave opened me up to a whole new world with PI values. .22u both sides and 100k vs the 1M/s. you can hit that WAY harder than the stock one with a flatter response. Noise floor drops off. Worthy.