I just finished repairing a JCM 900 4100 Dual Reverb amp for a friend. He bought it and the cabinet really right I think about $600 or so and it's very clean and it looks like brand new.

He brought it to me at a gig a week or so ago and said that it sounded bad and the sound was dropping out. It turned out that it had some nasty pots that needed cleaning and that cured the drop out problem. The output tubes were a new looking set of Groove Tubes EL34S tubes. I checked the bias on the amp and they were running at about 15ma idle! According to the book this should be about 38ma at this B+ voltage(475 volts). I noticed then that one of the tubes was gassy. I checked it out and found a crack down close to the base!

What I think happened was that someone stuck the EL34's into this amp which being a 1993 production must have come with 5881's from the factory. The tube retainers were EXTREMELY tight with king kong springs. I don't know if this could have cracked the tube or maybe somebody fumbling around in the amp putting the tubes into the sockets might have done that, who knows.

I happened to have 4 brand new EL34EH tubes on hand so I put those into the amp at 32ma and it sounds MUCH better.

I wouldn't say that this amp has much of a warm tone in either channel but it's not horrible. I know Myles(guitar player in my band) could get by on a gig with this amp. It's very much a hot rodded tone and it comes from that philosophy. Is it worse sounding than say a Peavey 5150? Probably not.

We'll see how he likes the amp now that it's at least working as intended. My question is, how do you tell what output tube a Marshall from this area used originally? Were there any circuit changes? I'm tempted to try it with a 6L6GC and see how the voicing sounds. Right now with the EL34 it's pretty darn bright but maybe the customer will like that in the amp. Do you think Marshall voiced this amp with the Russian 5881 Sovtek in mind? If so it might be the superior choice in a JCM 900 head like this one...

I've read many times that unless you gamble with allegedly NOS tubes, it doesnt matter how it originally sounded; tubes today aren't what they were 20 years ago (before it became less profitable to manufacture them in developed countries).


Incidentally, doesn't the 900 use clipping diodes for at least part of the distortion? (just curious, not related to discussion).

I've always liked 6L6 for the power section anyway. I suppose 5881 would sound close too.


I've also read theories that power amp distortion is actually phase inverter distortion....if that's true, a post phase inverter master volume would be cool. I know it sounds good in my 100 watter. Kinda makes me want to try a transformer after the PPIMV and a resistor network to go line out to PA, though I expect speaker overload (physical, not electrical) is part of the "secret sauce" LOL.


Maybe a PPIMV and 6550's or even KT88's feeding 10's LOL

The JCM 900 circuit really has a distortion box built in. It uses op amps to do most of the work, two diode clipping circuits one LED and one using rectifier diodes. Then it runs the signal through a couple of tube stages before the effects loop. Reverb is also driven via solid state op amps.

I'm not entirely sure the approach taken by Marshall in this amp is a bad way to go really and it was certainly less expensive to build. I mean if I put my TS808 Tube Screamer in front of my Marshall 50 watt non master volume head it sounds good with the pedal giving the amp a little extra "kick". This is just built into the JCM900 and you can't bypass or remove it from the circuit. It will NOT give you the non master volume Marshall sound but I don't think it intends to anyway. It's also kind of cool how it uses what looks like a preset tone stack in the lead channel that changes the frequency emphasis on that channel while still retaining the tone controls that are shared by each channel.

In an amp like this I really think you could choose the output tube based almost entirely on reliability and longevity since in the case of most users it will contribute little to the tone. I'd say a KT88, 6550, 6L6 or EL34 could all be used without trouble or a whole heck of a lot of difference in the sound. What is interesting is that with the clean channel master volume all the way up the amp seems to have nice dynamics and the tone is not bad.

A lot of power amp distortion is in the phase inverter section, yes. But with that sort of master volume you still remove the output transformer from the picture and it does play a role at least when it comes to sculpting the frequency response of the amp. It'll still do this but since it's never driven to saturation it won't sound quite the same.

The transformer saturation is a good point....


OK, I want to someday build a marshallesque all tube amp, fairly straightforward with;

1. four preamp gain stages in a single channel
2. "normal" MV
3. FMV tone stack
4. a PPIMV that's somewhat limited in how much it shorts the two outputs together
5. two 6V6
6. a middle of the road sized OT
7. a NFB loop with component values selected for my ear that isn't affected by the impedance selector
8. diode rectification (just far more reliable and I can live without the sag)

I don't need a bunch of features, and believe it or not though I like truckloads of distortion I do NOT want it to sound like it has a rat pedal in front of it...I'd just like for it to grind at sane volume levels without being shrill....several slightly toned down gain stages sounds like a good idea.


I'd also like it to be able to wash the dishes, fix the global economy, and cure cancer.

I've tested a Marshall with 6V6's in the output section and I put a post on this forum about that exercise. It was interesting. I used one of my 1974 4 input 50 watt Marshall amps and I just biased the amp so that it could use a 6V6. I used an Electro Harmonix 6V6 which could easily handle the voltages(about 400) of this amp. Myles the rhythm guitarist in my band www.polarisband.com uses one of these amps(I have two that are identical) as his amp and he's used it since 1993, no effects, nothing... Anyway we've had the KT66's in the amps for about the last 5 years either the Groove Tubes Saratov or the Valve Art. This made swapping the 6V6 even easier.

He tried the amp at a gig and he felt that it was just a little too soft and it didn't have enough headroom and dynamics when compared to the KT66 powered amp. The tone was very similar and it's something you might like.

Thats interesting....I would have thought with the same voltage but less current it would have MORE headroom. I guess I havent thought it out. I also thought KT66 tubes were almost like 6L6 tubes.

The KT66 is sort of a cross between a 6L6GC and an EL34. For many years we had used the JJ/Tesla E34L's in those amps with great results. But just for fun I decided to try something else and the KT66 is really a great tube. It's somewhat cleaner than the EL34 with more mids yet when it breaks up it's more like an EL34 with a better bottom than the 6L6 at least in our opinion. They are worth trying and it's an easy swap.

6V6 puts out less power than the 6L6 or EL34 so it tends to run out of steam sooner given the same voltages and loading. This can be good if your amp is overly loud and you don't run an attenuator like we do(Marshall Power Brake).

It's so weird to me to hear folks say the '34 has better low end....the 6L6 always seemed to have better low end to y ears. As for distortion, maybe I'm just tuned to preamp distortion

I think that the 6L6's bottom just has something that Myles doesn't like. The EL 34 has bottom but the mid range is less pronounced. The highs are more present and they have more of a raspy edge which tends to work well in a rock setting. My favorite is to combine the tones when we're recording. Do a track with say my '59 Bassman reissue and then another with the Marshall or a HiWatt.

Myles has never been a big fan of a tube rectifier for rhythm guitar which is what he does about 90% of the time. He thinks it's ok for leads. He plays a 50 watt Marshall through a Power Brake into a Peavey Classic Series 4x10 open backed cabinet. He runs both channels jumpered with the volumes on about 4. Power Brake is down about 2 or 3 clicks in most rooms where we play. Robby who's our lead player likes an amp with lots of headroom and an aggressive attack. He's a big fan of Hiwatt amps and his live rig consists of a Sovtek Mig 60 into a Mesa 1-12 closed back wedge cabinet with an Electro Harmonix 60watt ceramic magnet 12. Robby uses a Vox tonelab pedal so that he can get different sounds, effects, wha etc. We've experimented with output tubes in the Sovtek and while for years we used the JJ E34L we've tried the Electro Harmonix KT88's and 6CA7's. He's been using the 6CA7's for the last few months and they seem to have better headroom yet a similar tone to the E34L in this amp. They are very good. I've heard that they are basically 6L6 guts with some minor changes to the grid spacing to get it to have characteristics more like those of the EL34.

For preamp tubes whatever's quiet seems to work best in Robby's amp. He's using it for clean power anyway. He doesn't care for a Marshall 50 watt for this purpose, too mushy for him. Myles has been using the JJ 12AX7 from Groove Tubes in positions 1 and 2 and their 12AX7M as the phase inverter. This seems to be about the best.

I'm fortunate to have a nice collection of good amps to experiment with along with two good guitar players who are very different.