want to mod JCM800 (2210), more gain needed

[HTH]

I've got a dual channel JCM800 (with reverb, model number 2210) that I want to increase the gain in. Got a few ideas myself, but I'm wondering if anyone else has experience with these amps and/or has some mods that work well.

I've already put some Svetlana 'winged-C-' 6550s in the amp - the bass and overall tone has improved.

I've increased the plate resistor on the first stage to 220k - more gain, but it's not quite got the attack that it did with the 100k. 150k might be spot on.

Tried increasing the cathode bypass cap on the first gain stage too, but I thought the low end clarity/focus was compromised (even with a 0.68uF cap cersus the stock .47uF cap).

Looks like the reverb recovery stage (V4a) could give more gain by adding a cap across the (currently unbypassed) cathode resistor - was thinking 22uF to start off with.

The PI values are a bit wierd too - the 330k and 120k will give less bass than the usual 1M resistors used here, so I'm gonna change them both to 1M. Will also try changing the 470ohm PI resistor to 820ohms.

A 0.68uF cap on the presence pot should give more midrange grunt, will have to see.

Planning on increasing the PI coupling caps from 22nF to 47nF for more bass as this amp is a bit light in that department. A 1000pF cap across the feedback resistor should also help with the bass end too.

[Robert M. Martinelli]

Planning on increasing the PI coupling caps from 22nF to 47nF for more bass as this amp is a bit light in that department. A 1000pF cap across the feedback resistor should also help with the bass end too.

Hi HTH,
the 1000 pf cap across the feedback resistor makes for a HPF, and, being that a NFB, more highs would "go back", so you would probably end up cutting highs instead of boosting lows IMHO.

Hope this helps

Best regards

Bob

[HTH]

Hi HTH,
the 1000 pf cap across the feedback resistor makes for a HPF, and, being that a NFB, more highs would "go back", so you would probably end up cutting highs instead of boosting lows IMHO.

Hope this helps

Best regards

Bob

ahh, yep that cap should be in series (whoops)

any mods for this one Mr Vox?

[Robert M. Martinelli]

As to gain, as usual I think it's all about finding that "sweet spot" between headroom and gain, so you' re probably right, 150 K could be dead on; I think it' s better to increase gain a little on several stages instead of trying to achieve a massive increase on one single stage, so a little tweaking on that V4 could be also good.

As to the tone, I've seen some people adding a combination of an LPF and an HPF at the first gain stage cathode, especially in the '90s when people was trying to cut mids as much as possible ( but it can enhance mids as well simply moving the cutoff frequencies ).

You could try to split the cathode resistor in two ( e.g. substitute a 2.7 K with 1,5K in series with a 1,2 K to keep the DC bias voltage unchanged ) and add two caps, one in parallel with each resistor, to tailor the frequency response to suit your taste. If the HPF cutoff frequency is higher than the LPF's you get a notch, or band-reject ( good for cutting mids ) while if the HPF cutoff frequency is lower than the LPF you get a band-pass ( good for a mid-boost ).

Changing the first cap at the input from .047 mF to 0.1 could help on the low side.

Hope this helps

Best regards

Bob

[Dave Curtis, dB AudioTech]

Check this out:

http://tone-lizard.com/Ultimate_JCM800.htm

[Gtr_tech]

Check this out:

http://tone-lizard.com/Ultimate_JCM800.htm

He has the split channel type....that page refers (as do most JCM 800 mods) to the regular 2203/2204 style. 2210 front end is a whole different animal.
http://www.drtube.com/schematics/marshall/2210prem.gif

[drewl]

You want to tweak both channels or just the "boost" channel?
I had both the 2210 head and the 4210 combo and played around with them alot.
I increased the gain of the reverb return stage so with the reverb you also got a nice volume/distortion boost.

Changing the clipping diodes to LEDS or germaniums will give you more grind.
you can lower the value of V1B's 10k cathode and remove the diode.

[HTH]

You want to tweak both channels or just the "boost" channel?
I had both the 2210 head and the 4210 combo and played around with them alot.
I increased the gain of the reverb return stage so with the reverb you also got a nice volume/distortion boost.

Changing the clipping diodes to LEDS or germaniums will give you more grind.
you can lower the value of V1B's 10k cathode and remove the diode.

cheers Voxrules, will try the split/stacked cathodes -sounds interesting, any suggestions for good cap values to start off with?

Drewl, both channels need tweaking really. I've lowered the 150k plate load on the second stage of the clean channel which gave a little more headroom. The clean channel could do with a dip in the mids for more of a 'Fender' clean. Not familiar with that kind of tonestack though so unsure of how to tweak it for a mid scoop.

The gain/boost channel is sounding much better now... V1a plate resistor sounds good at 220k. bypassed V4a (reverb recovery) cathode with 10uF cap - much more gain and bass end. Have put a 2200pF 'resonance' cap in the NFB loop but it's probably a bit OTT fixed on full - a variable version will be better to 'tune it in'.

I'm planning on changing the V1b stage altogether - 100k plate, 2k7 cathode with 0.68uF bypass to start off with. Like others have noted, the diode clipping is fun to play for a while but the tone soon gets tiring, gonna remove all diode clipping and make notes on the tonal changes. Want the overdrive more organic than it is right now as it seems a bit processed/transistory.

I'm starting to hear the clean channel bleeding through the more I play the amp and once you notice you can't ignore it. Any fixes for this? (its got the IC switching, not the earlier discrete switching)

[drewl]

Some of the later models I've noticed a resistor/cap combo on the switching chip which was probably there to prevent the channel bleed.
I'll have to draw it out next time I see one.

[cbarrow7625]

HTH, here's my 2 cents....

First off, with just 3 gain stages you're only going to coax so much gain out of this thing if you want to remove the diodes & clippers (a good move, BTW).

I always look to remove things that are limiting gain to start with, i.e., things that load down tubes or cause unnecessary voltage dividers. The first thing I would do is get rid of C39 & R12 (.047uf & 220k resistor before 2nd stage). It's possible it will make the channel switching noisier though.

Second, get rid of that whole mess between the 2nd & 3rd stage and replace it with components with more traditional Marshall values (.022uf cap, 470K/470K divider w/ 500pF cap around the series resistor). The second & 3rd volume controls are completely unnecessary. You might want to just use the second 1M pot to make the 470k / 470k voltage divider variable.

Get rid of C22 (.002 cap around 3rd stage plate resistor) unless it make the thing oscillate like crazy. You can replace it with a smaller value cap if this is the case. This should help bring out the harmonic content a bit.

Next thing is to look at that 2nd stage. What's up with that 470k plate resistor (R7)? It will increase the gain of the stage but also make it sound less dynamic & mid-rangey (IMHO). I would change that out to 100k-200k (season to taste). Get rid of the diode in the cathode. Try replacing it with a 1-5uF capacitor. That should liven things up a bit. You can also try reducing the 10K resistor to 4.7k if you still want a little more & the amp will let you (no oscillations, etc.)

To tighten up the bass a little I would reduce the value of the 3rd stage cathode cap (C18) from 22uF down to 5-10uF. I'm not a fan of cathode followers, so I use these values quite often in my amps with good results. A 22uF cap in that position usually just sounds to flabby / farty when the stage is driving a tone stack. Too small a cap there and you will get a high pass right in the middle of your midrange which ends up sounding funky.

One last thing on the dirty channel, try getting rid of C20 (.002uF) cap in the tone stack. Sometimes a cap in that position can be useful, but this amp ain't one of them. It should make the mid/treble sound a little more "normal" and less constrained. You could also try to re-wire the Mid & bass pots in the "normal" Marshall configuration (all stacked together). It does have a different sound. I find the arrangement they have to be useful in extremely high gain amps but not in a 3-stager like this one.

So I think I have basically told you to re-build this thing to about the same design as a Marshall master volume amp (with the addition of a bypass cap on the second stage). I have an almost identical channel in a project amp and it has a decent amount of gain (it's no dual rectifier in the gain department, but it'll get the job done).

As far as more "Fendery" clean tones, you might be out of luck a little. However, you will get a little more mid-scoop if you reduce the value of C5 (.01uF). Try cutting it in half at first (.0047uF) and adjust to taste from there. You can also reduce the value of C23 (.002uF), the treble bleed cap. It won't affect the mid-scoop but it should give you a little more top end on the treble.

I'm also partial to a 1.5k cathode resistor on the first stage for both clean & dirty channels. The distortion will lose a little of the Marshall "flavor" but the attack may improve a little. It should definitely help out the clean channel.

[HTH]

HTH, here's my 2 cents....

First off, with just 3 gain stages you're only going to coax so much gain out of this thing if you want to remove the diodes & clippers (a good move, BTW).

I always look to remove things that are limiting gain to start with, i.e., things that load down tubes or cause unnecessary voltage dividers. The first thing I would do is get rid of C39 & R12 (.047uf & 220k resistor before 2nd stage). It's possible it will make the channel switching noisier though.

I'd considered getting rid of R12 for sure, but I thought that it would be needed to reference V1b's grid to ground. If I got rid of both C39 and R12 together, wouldn't that screw up the channel switching?

Second, get rid of that whole mess between the 2nd & 3rd stage and replace it with components with more traditional Marshall values (.022uf cap, 470K/470K divider w/ 500pF cap around the series resistor). The second & 3rd volume controls are completely unnecessary. You might want to just use the second 1M pot to make the 470k / 470k voltage divider variable.

my plan was to certainly try removing the diode clipping here, but I'll keep the gain pot and volume as they serve to match the volume to the clean channel. I'm gonna put some 1000pF - 4700pF bright caps across the gain pots though for some extra hi-mid snarl. with this being an 80s Marshall, all the pots are mounted to the PCB so it makes fairly heavy changes a bit tough.

Get rid of C22 (.002 cap around 3rd stage plate resistor) unless it make the thing oscillate like crazy. You can replace it with a smaller value cap if this is the case. This should help bring out the harmonic content a bit.

Next thing is to look at that 2nd stage. What's up with that 470k plate resistor (R7)? It will increase the gain of the stage but also make it sound less dynamic & mid-rangey (IMHO). I would change that out to 100k-200k (season to taste). Get rid of the diode in the cathode. Try replacing it with a 1-5uF capacitor. That should liven things up a bit. You can also try reducing the 10K resistor to 4.7k if you still want a little more & the amp will let you (no oscillations, etc.)

I was gonna try 100k plate and 2k7/0.68uF cathode. The amp is already quite bright and snarly through the mids, so I may be able to go higher on the cathode bypass cap for a wider frequency boost.

To tighten up the bass a little I would reduce the value of the 3rd stage cathode cap (C18) from 22uF down to 5-10uF. I'm not a fan of cathode followers, so I use these values quite often in my amps with good results. A 22uF cap in that position usually just sounds to flabby / farty when the stage is driving a tone stack. Too small a cap there and you will get a high pass right in the middle of your midrange which ends up sounding funky.

agreed, 22uF is a bit big - 5uF is typically more than enough. will start with 10uF and take things from there.

One last thing on the dirty channel, try getting rid of C20 (.002uF) cap in the tone stack. Sometimes a cap in that position can be useful, but this amp ain't one of them. It should make the mid/treble sound a little more "normal" and less constrained. You could also try to re-wire the Mid & bass pots in the "normal" Marshall configuration (all stacked together). It does have a different sound. I find the arrangement they have to be useful in extremely high gain amps but not in a 3-stager like this one.

I can get rid of the cap to ground off the bass pot (this is scooping the mids right?), but the full rewire to a typical old-school plexi type isn't going to be possible because of the pcb-mounted pots.

So I think I have basically told you to re-build this thing to about the same design as a Marshall master volume amp (with the addition of a bypass cap on the second stage). I have an almost identical channel in a project amp and it has a decent amount of gain (it's no dual rectifier in the gain department, but it'll get the job done).

As far as more "Fendery" clean tones, you might be out of luck a little. However, you will get a little more mid-scoop if you reduce the value of C5 (.01uF). Try cutting it in half at first (.0047uF) and adjust to taste from there. You can also reduce the value of C23 (.002uF), the treble bleed cap. It won't affect the mid-scoop but it should give you a little more top end on the treble.

cheers, will try that.

I'm also partial to a 1.5k cathode resistor on the first stage for both clean & dirty channels. The distortion will lose a little of the Marshall "flavor" but the attack may improve a little. It should definitely help out the clean channel.

will try that too, though I'm trying to keep the OD as Marshally as possible - the clean channel is a secondary consideration, but I'd still like to improve it where possible so long as it doesn't affect the OD tone.

With so many preamp tubes, the best thing to do with this amp really would be to ditch the PCB, put some new pots in there and wire it up PTP with some tagboards. I'd leave out the reverb so the OD channel can be optimised using the extra gain stages. BUT, that would be way too much work when the amp already sounds pretty good and can hopefully be modded to give a decent enough tone with fairly little work.

Cheers, will post with final mods I settle on so others can benefit from this in the future.

[Robert M. Martinelli]

cheers Voxrules, will try the split/stacked cathodes -sounds interesting, any suggestions for good cap values to start off with?

I'm starting to hear the clean channel bleeding through the more I play the amp and once you notice you can't ignore it. Any fixes for this? (its got the IC switching, not the earlier discrete switching)

Hi again HTH,
for starters you could try a .22 mF paralleled with a 1.5 K resistor, both of them in series with a .1 mF paralleled with a 1.2 K resistor, then try slightly larger caps ( say .47 mF and .15 mF ). It' s a matter of taste, so you' ll probably end up with different values, you can change the resistors' values as well provided that their sum is about 2.7 K to keep the DC bias point unchanged.

As to the clean channel bleeding, I would change back the IC switching to a relay switching.

Best regards

Bob

[HTH]

Hi again HTH,
for starters you could try a .22 mF paralleled with a 1.5 K resistor, both of them in series with a .1 mF paralleled with a 1.2 K resistor, then try slightly larger caps ( say .47 mF and .15 mF ). It' s a matter of taste, so you' ll probably end up with different values, you can change the resistors' values as well provided that their sum is about 2.7 K to keep the DC bias point unchanged.

As to the clean channel bleeding, I would change back the IC switching to a relay switching.

Best regards

Bob

thanks Bob, appreciated - I'll try that out.

as for the relay switching, it's way too invasive a mod for this amp - looking for a tweak/fix for the IC switching if possible.

[drewl]

After all this nonsense, you'd be better off selling this amp and buying 2204 or a kit that you could mod the way you want and leave this one stock since you can get more money for it that way.

[HTH]

After all this nonsense, you'd be better off selling this amp and buying 2204 or a kit that you could mod the way you want and leave this one stock since you can get more money for it that way.

I've got a 2203 so I'm fixed for that kind of tone, the 2210 has it's own thing going on just it could do with a bit help getting the best from it (just like the 2203)

to be honest, just adding a 10uF cap across R25 fixed most of the amp's shortcomings, but I think it can still be better with some tweaking. this is a DIY site afterall, not looking to buy something off the shelf or put a kit together.