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I have a JCM800 labeled version of this amp, and I'm curious as it doesn't really seem to have that much gain as they seem to always have available.

I wonder if it has been modded more than the ppimv knob added at the back...there's an extra unlabeled jack in the back too I don't know what is it.

I guess I have to take it to an amp tech to find out, but does anyone know if there are mods that would include adding some sort of foot switch gain selector or something...I am leery of just trying a switch in that jack...could it be something like that?

Even floored with all the volumes up, it barely gets into ACDC territory really...so what might be the reason?

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Different amps, a jcm800 is not a plexi. Jcm is cold biased in the first stage while a plexi is hot biased.

This isn't a JCM800, it's merely built into JCM800 case. They did that for the last 1956 builds having ran out of old school cases in the 80s. In the front it says JCM800, but in the back model 1956 Super Lead 100, and has the 4 inputs too etc controls. It is technically a plexi, a so called "metal face plexi". Exactly the same as my buddy's late 60s one save for the front panel.

Been looking to trade it for a real 800 though, as I could use the gain available on those.

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deeaa wrote: ↑Mon Jul 05, 2021 10:01 am This isn't a JCM800, it's merely built into JCM800 case. They did that for the last 1956 builds having ran out of old school cases in the 80s. In the front it says JCM800, but in the back model 1956 Super Lead 100, and has the 4 inputs too etc controls. It is technically a plexi, a so called "metal face plexi". Exactly the same as my buddy's late 60s one save for the front panel.

Been looking to trade it for a real 800 though, as I could use the gain available on those.

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This may sound stupid, because I don't know a whole lot about amp circuits, but couldn't putting in hotter preamp tubes significantly increase the gain?

Yeah, but they are regular 12ax7s and quite normal too, tried several Marshall ones and a few others...it sounds the same. I do suspect there is some mod going on there that keeps it cleaner than stock. Cranked up, it sounds exactly like All Right Now by Free though but seems to me based ikon YouTube many people get a lot more gain than that cranking it up.

I guess I have to take it to an amp tech to see. It works great zero hiss or noise too, a superb lower gain crunch...with a weak guitar it is instant RHCP and and with a strong bucker instant ACDC rhythm sound, but...well maybe it's the other way around and those who get more gain do some extra tricks.

Will have to look into it more. I want to find out what is that extra jack there on the back. That is clearly not stock, and it isn't a loop because there is only one.

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Have you tried jumpering the channels?
Many variations i see.
"A note here is that the name “JCM800” itself is, many times, related just to the 2203/ 2204 models, but the JCM800 was a series. This means there were other models. Altough rarer, the non-Master Volume amps (#1959/#1987/ #1992/ #1986) were still being made under the JCM name."

I guess who ever had it back then added a master on the back as you said. It really depends on the circuit used and if/any mods were done to see what's going on.

Yep. The 2203 is my holy grail of amps. That's the original single channel JCM. Never owned one, except a 2203 2x12" combo I had briefly, and stupidly sold off, because back then I felt it too much of a one trick pony.

I've owned several jcm800 2210 heads though, which is the 2 channel version. But I never liked the drive channel much and always ended up just using the clean channel anyway and an OD pedal.

There are some other variations as well, but those are the most common ones.

I've never seen another 1956 model with a JCM front panel, and that's why I would like to keep this as well - they are rare. I got it quite cheaply and I figure I could sell it for a nice profit already but after another decade it may really be pretty valuable. Or not, you never know. At any rate I know if I do sell it I will never get my hands on one again. The new handwired versions of it cost nearly two grand.

I may end up ordering the SC20H which is a 20W mini version of the 2203 head, and it sounds just the same too. It's just that it costs nearly a grand...

In a perfect world I would have this 1956 for cleans and A/B switch with a 2203 for drives...but no way I will ever lug around two heads and two 2x12" cabs.

In any case I feel like I have to buy that mini JCM as well though. At 20W it would be better suited to band volumes than this 100w monster which is loud as hell even if I use a power brake with it.

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Can you provide inside pics of the items which appear 'modified,' which you mentioned? Be sure to include where the wires for the back of the amp, end up. Also include the input jacks to the turret board. I'll let you know what mods you may have.

I've repaired a few of the JCM800 1987's (50w) and I believe one or two of the 1959 (100w) and have seen more than my fair share of awful modifications to these circuits. The JCM800 versions are very true to the early to mid-70's Super Leads, except the power and output transformers. They moved from Partridge to Drake. Results were a drop of B+ voltage from as much as 520VDC to about 380VDC. Marshall engineers figured out that average available household power had increased about 5% since the initial design, so they redesigned the Drakes with less windings to place less voltage on the tube plates and increase the reliability. The Drake output trans had less iron and higher tolerance for winding variance.

Stock, your JCM800 1959 should be loud and clear when plugged into the normal channel (input 2) with the volume on '3.5.' '2.5' should rip your head off in the bright channel (input 1). 'All Right Now' tone happens when you plug nito the upper jack of Input 2, jumper its lower jack to input 1 upper jack, volume 1 on '4' and volume 2 on '5'. Without a speaker soak, this should be a painful volume.

Another thing to watch for and is common with many amp brands in this era, is an over-biased (cold) output tube section. Most vacuum tube manufactures were out of business, most of the world were not doing business with the communist countries still making tubes and amp companies were experiencing costly tube related sales (failure at the stores) and warranty problems. Rose Morris demanded that Marshall overbias the output tubes (40% to 45% of max plate wattage) and use GE6550 types in their USA exports. These amps do not sing when under biased, especially when with those GE tubes. Which led to players seeking out the old Marshalls.

Have your amp tech bias at 62% to 65% of max plate wattage and your amp will wake up. Even using it as a clean amp, it will sound more musical.

Thanks...the amp is at band room, I would have to lug it home to open it up.

The amp sounds just like you said, it's basically clean and VERY loud with a huge punch when you turn it up....not driven but still kind of, like how a good acoustic guitar sounds huge and almost driven when strummed.

When I combine the channels it gets that Free sound with ease, but it is indeed excruciatingly loud unless I lower the PPIM volume on the back and/or use a power brake.

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I made this video when I first got the amp.

I have a biasing tool so I may have to bring it home open it up and take some photos and check the bias...

But I'm thinking it probably does sound like it should and those who get more driven sounds from theirs have something to achieve that.

The nice thing about it is that it is super quiet when not playing. It's almost like is this thing on? And then you hit a chord and it's like that opening scene in Back To The Future.

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@andrewsrea here's a few pictures.

Looks to me besides the master volume added, the mystery 3rd jack is for 3rd speaker. I don't know why, or how should I calculate impedances if I were to use it.

Otherwise, I sure can't tell much about what's been done to it.

Closeup on the 3rd speaker jack Old service slip and nameplate This already has the drake transformers despite being 1956 model

Front and rear views

Looking inside the amp I can only see one trimmable potentiometer, so I haven't the faintest how are you supposed to adjust bias on these...

BUT alas, I think I may have found the reason it sounds curiously clean and lots of low end...it has C6A7 tubes in place of EL34's!

Those are compatible, yes, but I do believe if I get a quartet of EL34's rated to break up extra early, I can get a lot more gain on the tap then.

So off to the amp man it is!

The ones with 6550 tubes it would make a difference but 6ca7 and el34 are basically the same. They still may be biased too cold.

Great pics, videos and descriptions, @deeaa !

1.) The mystery jack appears to be a factory 'Direct Out,' for the purpose of recording or slaving other amps. This type takes the signal from the output transformer, rather than along the preamp chain as other designs do. It is not for additional speakers or headphones.

2.) The Master Volume mod looks copasetic and is the same type found in the JCM800 2203 model. It has been inserted between the Treble variable lug #2 and the blocking cap, just before the phase inverter tube #3. The reason this amp does not sound like a 2203, is the 2203 adds one cascading preamp, has more gain in the initial gain stage (higher plate resistor) and has a bias resistor in the 2nd stage that offsets the 'zero line' which compresses and clips asymmetrically. FYI: I prefer the Ken Fischer MV, which places a dual 1M pot MV between the inverter tube and the output tubes. A much more complicated install, though.

3.) The trimmer (variable resistor) pictured behind the Bass pot, is the bias control. It has been a while, but I believe fully clockwise = overbiased and counter clockwise reduces the bias, increasing the plate current and idle wattage. You can check if I have my direction correct with a digital volt meter (DVM). Have the DVM in a DC mode to measure a range of 90 volts or less (range will be 30VDC to 65VDC). Don't choose too high or you'll lose the detail of tenths of volts. Connect the black wire to ground (chassis is fine). Mark your trimmer position with a grease pencil or a dab of light color paint. With the amp plugged into a speaker, 'on' and in stand-by mode, apply the red probe to pin #5 (control grid) of an easy to access output tube. Write down the result, which will be a negative voltage. Using a plastic screwdriver, turn the trimmer a hair clockwise and recheck the voltage on pin #5 of the output tube. Compare to the first record and if it got more negative - I was right. If it got less negative, then counter-clockwise is the underbias position. I'll take you through biasing below.

4.) IMHO, @mozz could be right that 6CA7 could be identical to a brand's EL-34. There are some brands which are not and land in between a 6L6 and EL-34 as far as bias requirements. I know for sure that current production EHX 6CA7, 70's Sylvania 6CA7 and some GE 6CA7 (sometimes branded as RCA or Phillips) are not drop-in replacements for EL-34. The tell is typically a skinny EL-34 looking 6CA7 is equivalent and the 6CA7s with the tubby glass, are not. A true EL-34 is a 'kinkless-pentode' design and some 6CA7s are 'beam-tetrode' design. Many 6CA7s of this design can go higher than the max plate wattage of an EL-34, but I ignore this and treat it as extra reliability.

Bias: I suspect you have a Bias King type of device, to which you place in between an output tube socket and the actual tube. So I will give you my process accordingly:

Warning: amps have voltage with can kill you. Please proceed carefully. Be scripted with your moves in the amp.

1.) Calculate the wattage max and target. The Bias King tool inserts a small value, accurate resistor in series with the tube's cathode and ground. It measures the voltage drop across the resistor and does the math for the display, Which is the total tube current (plate+ screen).
It is safe to use 30w per tube for 6CA7as the max wattage (25w plate + 5w screen).
Max idle wattage = 21w (70% of max)
Target idle wattage = between 17w and 21w. The former is enough to mitigate bad sounding crossover distortion. As you apply less bias (running 'hotter'), the amp may get louder and less sterile (changes in the midrange & compression).
Wattage is calculated as: Plate voltage (pin #3) x milliamp reading from bias tool / 1000. Example: 405VDC x 55ma /1000 = 22.3w
note: plate voltage will decrease as you remove negative bias voltage, while increasing tube current. So, you have to adjust both the current and voltage in your calculation, each time you adjust.
2.) Connect the bias device and connect the black probe of your DVM to ground, with the DVM range set to 500v or less.
3.) Warm the amp up on standby, all knobs to zero and a speaker or 100w load plugged in, with the proper ohms selected for that load.
4.) Begin fully over-biased, put the amp in play mode (remove standby), record the plate voltage (red DVM probe to pin #3 of the output tube) and milliamps from the bias tool. Calculate the tube wattage.
5.) Reduce the bias by making a small turn on the trimmer. Watch for the tubes' grey plate getting an oval red glow in the middle of them - increase the bias if you see this. Repeat the notating and calculation.
6.) When you get to 17w, stop and play the amp. Play with the settings and listen. If you love the tone - stop. If not, set the controls to zero, take it to 18w and repeat. Caution: your ears like loud and early on, it may be the same tone - louder. So, listen critically.

I rarely go above 65% of max wattage with my amps. I've had 70% sound different, but great and then hours later it sounds like the tubes went dull.

I realize this is long and I may have distracted from your post, so my apologies and hope you found this useful.

@andrewsrea whoa, awesome and detailed info & guide! Thank you so much - I'm definitely going to save this up for future reference too.

I've adjusted the bias years ago a few times on amps where it was easy - but you're right I have a 4-connector metering tool that goes between the tubes and sockets and allows to measure easily with a multimeter.

I'm not gonna do it today - just spent hours soldering new pickups for my guitars as per my other post on parallel wiring...but maybe tomorrow. I will have to go and see if the local store has EL34's that would be good too, first.

Again, thank you VERY much for all the info, very appreciated!

Well, for now I decided I'm too busy to do stuff to it, so I took it to a local amp tech. He will put in a full matched el34 quartet and bias it, and modify the gain structure Closer to JMP for a little more gain on tap. Also he told me it looks like some of the filtering caps aren't to specs and he will restore them to original values. We chatted a good half an hour discussing my tone preferences and various options without entirely rebuilding the amp.

It's a bit much but I'm hoping it will be no more than circa $400 for the whole thing, and then it should be even more of what I desire it to be and last me next to forever. But, he only did a cursory overlook for now, once he tests and measures everything there may arise a need for some more new parts.

Next time it needs new tubes I will change and bias them myself, but for now I'm happy to have it properly restored and tweaked as per my sonic desires by a pro.

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Ho-lee great balls of fire...

Just got the amp back from the Amp Wizard (which is how I shall henceforth call him)....

When I took the amp to him, we talked for a good while about my sound preferences and uses, and then he did his tricks.

He explained everything in detail when I picked it up, hopefully I got it right...here it goes:

First of all, it was made 100W again with a matched set of quality EL34's biased a wee bit on the warm side.

Then the filter caps were redone to correct specs.

The input stages were remade in what is close to 2203 style; there is now a totally clean input that bypasses one tube and gives a really attacky spanky plexi clean, exactly like RHCP funky clean really.

Then there is a lower gain input, which sounds about just the same as the amp did before, just a little better with proper bias, and goes into old ACDC territory when cranked...great for getting that Powerage sound and cranking the power tube drive.

But the 3rd input....whoa whoa. Just about normal newer JCM800 but not quite as piercing and top heavy. It's downright incredible. No matter what I play from Metallica riffs to Back In Black or some 80's hair metal riffs, I dunno, Kingdom Come or Van Halen, it's perfect. Just a little turn of the gain or guitar volume, what an incredible crunch and murderous chugga chugga and absolutely screaming high notes. No need for OD, nothing...just a straight line to amp and it's like on all my favorite records, only better experienced right at the amp.

I'm so happy I'm nearly in tears. Can't believe how fun this is to play now. It's hands down not only the best sounding amp I ever owned, it is the best I ever played, period. Bloody 30 years that I've coveted for this sound and never got around to getting or could afford one.

There was other stuff too, it has a 2203 style master volume ad well as a PPIMV now, and you can play with their ratios to interesting results....doesn't need to be insanely loud to sound stupid good. Probably won't need my power brake with the band any more.

Also there were many spots he fixed future proof, replaced some frayed leads and such, removed many connections originally done with connectors and soldered them point to point instead, stuff like that.

WELL worth the 460€ he charged for the job and parts. Money well spent.

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Nice! Maybe we’ll get to hear some samples?

I’m relatively new to guitars, and especially amps, but I have recently realized that I should just get a Marshall SC20. All of my favourite tones are made with JCMs or hot-rodded Plexis, so why beat around the bush? I don’t think I need, or could properly use a 2204, so SC20 it is.

@Milkman the mini JCM is absolutely stellar. I was having a really hard time whether I should serviced and keep my amp or sell it and get an SC20H. I've played it many times and it is GREAT.

I don't love the upright 2x12" they have for it, though...it's not bad but when I hooked the head up to a normal 2x12" cab, I think it was a Fortin cab with V30s, the thing came alive a LOT better.

If you love the iconic Marshall sounds, can't go wrong with that head for sure.

Video coming up...I only have a call phone to record with.

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Better demo now that I realized I can shoot from a distance and zoom in...



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Sounds great. When are you taking it back to the band room and how much time are you gonna spend there?
I like what you've done with the parallel neck pickup too, I'm insprired.