JCM too much Bass

[Tage]

My brother feels his JCM 800 (2204, from 1984? I think, 6550 tubes) has too much bass when he turns up his preamp gain along with an almost cranked (around 7) master volume.

I was thinking that changing the V1 cathode bypass cap to a lower value, say 470n or 330n, will help tame some of the lower frequencies in the amp.

I should mention that the amp has been modified quite a bit by the previous owner, so I will check component values before going at it as though it's stock.

Anyone else have some tricks that may work for this perceived issue?

[Chuck H]

Too much bass is a relative term... Let me 'splain what I mean...

Does he think there's too much bass in the final EQ? Not likely as every heavy distortion player likes tons of thump if they can get it. But I have to ask.?.

Or does he mean that the definition of the bass notes is too muddy?

In the latter case I would say that trimming bass in the early preamp stages is good. However, I don't think there is a need to go lower than 680n for the first stage bypass cap. I'm guessing that's the value that is there now. Is it??? May be better to reduce the first stage (and possibly the second stage) coupling cap.

I also think that if he's overdriving the power amp that there is some magic being lost by not switching to EL34 tubes. IMHE 6550's are best used for amplifying chores and not the best for clipping character. So, since he's clipping the power amp I would say start by changing to EL34's and adjust early stage coupling caps after that if you still need more bass attenuation.

It would help to know what kind of tone he's after and what mods have been done to the circuit thus far. So this is a very macro analysis.

[Tage]

Yeah I know it's a poor description, he feels the bass starts to over power and fizzles out. Muddy would also be an a good description.
He want's to keep the 6550's because he want's a more linear sound with that extra bit of power. As far as tone he realy has the wrong amp in my opinion, he lie a straight up loud amp with some mild Power stage clipping and just enough drive from the pre-amp, I know that is a generic tonal description, he plays along the hard core line of things, not exactly high gain metal, high power is what he likes.

I hadn't considered the coupling caps though, that is a good idea just down to a 10n?

[Chuck H]

800's sound kind of "fizzy" anyway. So you may be right that it's not he ideal amp for him.

Too much bass does cause fizziness (is that a word?). I think it's because the waveform clips so abruptly. The 2204 uses a combination of hot and cold biasing in the preamp that seems accentuate this effect. They also use large coupling caps and two high pass filters that makes for a real boomy/fizzy sound so popular in the 80's. Also making it worse is that the 2204 has two of three cascade gain stages unbypassed so there is a lot of local negative feedback. This can make for a more sterile cold feeling amp, which seem to me to make boomy/fizzy sound even worse. If you can't tell by now I'm not a big 2204 fan. But there are a lot of guys who wouldn't play through anything else. So it's ALL subjective. Here are a couple of changes that should give good crunchy gain and get the sound out of the 80's. It's just a few components so it should be easy to reverse if he doesn't like it.

[Tage]

Thanks a lot Chuck,
that looks good, those cathode resistors are more "F" style, they will give a hotter bias won't they?

I'm gonna give those mods a try, one thing at a time and get him to try it with each change.

[Chuck H]

Well now, it's just a few components but it's an overall voicing thing. Each change on it's own isn't idealized for the stock circuit so changing one at a time may do strange things. Then you end up undoing the changes he doesn't like before you get to hear how it's suppose to sound together. That and who want's to lift that board repeatedly???

The bias resistor on V1b is hotter which will increase gain, but we've reduced bass with the smaller coupling cap on the previous stage and reduced treble by taking out one of the high pass caps. So the perception will be more mids. AND the added cathode bypass cap will raise mids and highs an additional 6dB. The bias resistor on V2a is cooler than the stock value. IMHE running the preamp bias closer to center sounds less bass fizzy. Overall there will be an increase in preamp gain above the stock circuit but it should be more useable, better sounding gain. Of course there are ways to reduce gain too if your guy want's less gain, or he could just turn down the channel volume and turn up the master volume.

[Tage]

Good call on pulling that circuit board more than once will be a hassle. He said he wanted less gain, but these changes look like they will for sure give a better pre-amp voicing.

I am going to be building a 100S inspired amp for him soon, that should be more along the lines of what he is going for.

P.S. That is a 2.2 Micro-Farads right not 2.2n? 2.2n seems low for a 1.5K

[Chuck H]

Yes that's 2.2uf, not nf.

Most of the preamp distortion will happen at V2a. See attached. No reason you can't leave the potentiometer there as the load resistor. This will allow to tweak the gain during testing so your friend gets what he wants and you don't have to lift the board again. He may find that the extra gain is a good thing with the new voicing. Like I said, he does have the option of turning down the gain with the channel volume. Everything above his "normal" setting is a boost.

[Alex R]

I've seen a few of these amps with muddy bass following mods designed to add bass to what is by its nature a toppy crunchy amp. The result is often something that doesn't sound like a Marshall at all. Sometimes people mess around with the R5/C4 filter, perhaps because it is so obvious going across to the volume control in the air as it were - but it is there partly to reduce bass in order to stop the amp sounding muddy.

[Tage]

Yeah some one added an Orange style FAC control to this amp, But I have yet to see were they added it into the circuit. I'm gonna remove it when I do these mods, it gives a pretty weak tonal variation with only 5 positions.

[Pedro Vecino]

I should mention that the amp has been modified quite a bit by the previous owner, so I will check component values before going at it as though it's stock.

It is very possible that this is the origin. If you use moderate gain settings, verify that there is a 1n capacitor on the gain potentiometer. Another important point is the 470K resistors with the 470p capacitors in parallel. Both are necessary filters and are sometimes removed or changed in value.
Compare the whole circuit with the original and retrieve it if necessary. An original 2204 never have too much bass
Regards

[Tage]

It is very possible that this is the origin. If you use moderate gain settings, verify that there is a 1n capacitor on the gain potentiometer.
Regards

Yeah I think that might have gotten upped to a 2.2n I'll check that soon and let you know if it has been changed

[Gaz]

2.2n is relatively large, and would definitely give the amp an impression of too much gain, with the gain coming on really quick.

It's curious the amp has too much bass with the FAC type control, which is an Orange can change a stage's coupling cap from about .1uf to 500pf. I wonder what the values are?

[Bernd]

What about changing tubes? I have a 2204 from 1990 and by putting in different tubes you can alter the tonal character a lot. Meaning preamp tubes and of course power tubes. I haven't heard this amp with 6550s but I remember that another marshall with the 6550s I had was much bassier before turning to EL34s.
I have quite a collection of different preamp tubes and they sound very different one to another. Some of those Marshall labelled tubes have a strong bass response.
Mine had rather too much of the ear biting highs when it was in its original status.

Bernd