JCM800 Point to Point Part II – Xtra Gain Choices

Check part I here for general info as to how and why I refurbished this 2203 head.


In this longer than usual article we’ll discuss how to squeeze the juice out of a Marshall 2203.
But first a little bit of ampology. The Marshall Master Volume also known as ‘2203’ or improperly as the ‘JCM800’ is actually a not so high-gain, 1974 designed amp first introduced in the JMP lineup in 1975 and made famous by the likes of Status Quo, AC/DC, Zakk Wylde, Slash or Metallica, all of which used a (heavily) modified version of this amp. I don’t recon there could have been more than a few unmodified JCM800 ever used on any major heavy-metal recording. The much sought after balls-to-the-wall ‘JCM800 sound’ that many guitarists hope to achieve by purchasing an old stock 2203/2204 head or god forbid a $2000 reissue is a myth IMHO. The 3 gain-stage amp produces its optimal distortion only at high loudness, past the point where the power tubes start to distort and compress. And even past then we are far from the gain level and bass tightness required by your garden variety modern days rock/metal guitarist. And some JCM800 fart out big time at high volume, for want of new filter caps or better tubes, or just about anything else.
The ‘JCM800 sound’ needs to be re-branded urgently as the ‘JCM800 Modded Sound‘. So how does one go about transforming the somewhat mild-mannered English gentleman with his ways stuck in the 70’s into a mountain of glorious gain? Bellow is an overview of the various solutions as I have come to understand them, ranked from simple to complex:

  1. **Easy**$** Triode switch
    A little feature based on the idea that a 100W amp in triode mod behaves in the same way as a 50W pentode and therefore breaks up sooner. It’s a bold marketing claim.
  2. **Easy**$** Cold bias
    Biasing a Marshall down to 15mA-20mA, instead of the customary 38mA may produce a surprising amount of added warm, lively and spongy signal clipping. Hum is reduced as well. On my EL34 Superlead per example it works well for bedroom level playing. But for loud rehearsal conditions the signal is too degraded to my liking. I must have read somewhere that EVH used this technique a lot back in his plexi days along or as a result of lowering main power voltage with a variac. Crossover distortion can develop in cold biased amps, meaning that what one plays is not what one hears. Which for some guitar players could turn into a useful functionality… somehow I find myself reflecting on Ace Frehley and Phil Collen’s guitar skills right now.
  3. **Easy**$** Diode mod
    Been there, done that, quickly removed it. Although diodes do sound good on the Silver Jub and the JCM800 2210, who are we really kidding? Both these amps owe their fierceness to the fact that they are plate driven and therefore have 1 more gain stage. If adding a couples of 5 cents diode or led here and there could really produce way more gain without affecting the harmonics and the signal frequency band, then every amp would be stuffed with it up to the rafters, which is not the case. People have actually been known to remove diodes in their amp (JCM900 DR).
  4. **Intermediate**$** PPIMV
    A Post Phase-Inverter Master Volume is something I would otherwise describe as a Symmetric Low Impedance Extra Gain Stage. I intuit that it turns the otherwise gain-neutral PI stage into a distortion engine by overloading it, though would I need to perform a scope reading on that one so I don’t talk out of my ass (again). We can perceive right away the risks associated with using a low quality stereo pot in this application, kinda majorly affect the ‘symetric’ aspect here. I personally  favor the Rich Mod variation over the LAR/MAR or the Mission Amp/Bruce Collins Mods, as it does not filter the high quite as much. But the Rich Mod affects the bias and as such cannot be adjoined with a simple quadruple-pole bypass switch unlike the other two. Stay tuned for the upcoming post comparing the 3 most common types of PPIMV.
  5. **Lazy**$$$** Attenuators and power suckers
    Playing through a Superlead without attenuation sounds silly nowadays. Such common sense might apply to the 50W and  100W Master Volume Marshall, although reactive attenuators – Marshall SE-100, Rivera RockCrusher – are expensive and bulky, while purely resistive attenuator – Hot Plate, Palmers notably – do not sound any better than the much cheaper and simpler PPIMV mod. Plus they destroy the output tubes & OT, and affect the tone. So I would say go for it only if you have the dough for a top notch unit. And don’t expect miracles. This won’t replace an iso speaker mic’ed and re-amp’ed by a large p.a. type system, the kind of set-up the pros favor.
  6. **Intermediate**$$* Transistor Boost Entry Stage
    Most guitarists use boost/OD pedals in their signal chain before an already cranked-up/breaking-up amp so no need to linger on the advantages – cost, versatility, possibilities – of such a solution. While transistors add their own color to the sound and lack in transparency with respect to real tubes, they make for a cheap and easy way to overload the entry gain stage. Integrating the functionality straight into the amp is absolutely feasible (Marshall 2203KK, YJM100, Ibanez TSA, Rivera TBR). I favor a basic TL072 amplifier circuit. Food for thoughts..
  7. **Intermediate**No extra tube**$** Working with the existing amp lay-out
    I am not referring to the well-documented “José mod” which essentially tweaks a Plexi style circuit into a 2203 +/- a few diodes and component value changes. Check out the Ceriatone Yeti and Chupacabra for further info. The idea here is to push the  JCM800 over the edge by performing any or all of the most common component value changes. Please refer to this Tone Lizard article, this guy comes up with a shitload of possibilities. The V1b cathode bypass cap mod (which I personally couple with larger 0.47µf PI caps à la Orange and a hum balance pot) is a recommended by me option.
  8. **Advanced**No extra tube**$** 4 Gain Stages Mod: Driver Stage Tweak
    Now this is where the rubber meets the road. A bog-standard DC coupled cathode follower stage occupies 2 cascaded triodes: the first triode is an amplifier that feeds a ‘cold’ triode (100k cathode resistance). The latter does not provide any voltage gain by itself and acts as a buffer to spew out an amplitude-unchanged low-impedance signal by way of its cathode. It is also described as an “impedance conversion stage”. Such an arrangement explains why the 2 triodes count as only 1 gain stage in the amp gain structure. The second stage drags heavily on the first stage anode current, which causes added compression and lends the tone what has been described as a classic beefier flavor.
    One can convert the cathode follower stage into a high impedance plate driven gain stage that uses only 1 triode i.e. go the way of the dual channel JCM800 mod.2210, Silver Jub or Splawn Nitro. By Plate-loading the tone circuit, we will get a lot of extra gain: of course we have just freed up 1/2 tube (V3b) which is available for an added pre-EQ gain stage (Silver Jub) or EQ-Recovery stage (2210) to make up for the losses inside the EQ, as with any high impedance signal that finds a path to ground. Splawn Amplification has apparently performed this mod on their customer’s Marshall for years before launching their own product using the very same output circuit. I haven’t tried it yet on a stock jcm800 but I am extremely interested, my first Marshall amp was a 2210 dual channel , back in 1994, and I loved its punch and quick dynamic response. Bellow an implementation example, as per UK amp maker Martin Kidd.

  9. **Advanced**Added 12AX7**$$** 4 Gain Stages Mod : Intermediary Gain Stage between V1a and V1b:  This mod, which preserves much of the JCM800 tone characteristics, consists in adding an extra 12AX7, wire only 1 half of it to duplicate the 2nd gain stage, leave the rest of the preamp unmodified and change the tubes to 6550s. This is the Caswell SIR #39 mod, another extremely thoroughly documented circuit. Tons of Schematics online. Ceriatone offers a lay-out on that one. I have currently installed the mod on my JCM800 PTP. The main advantage here is the ease with which one can perform that mod, if we overlook the drudgery associated with drilling a clean whole inside a steel chassis. It is not necessarily the cheapest mod though, you need 1x tube socket + 1x 12AX7 + 3x resistors + 3x caps + 1x potentiometer on the preamp side providing you can live without channel switching, 4x 5W screen resistors + tweak the bias to accommodate for 6550s + 4x 6650 tubes. All told around 120 euros/$ minimum in parts. The mod sounds extremely thin while keeping the existing poweramp, but you may still want to give it a try before modifying the power section altogether, it pushes a metric ton of gain with EL34 and PPIMV. The mod still sounds pretty thin with 6550, well everyone knows slash’s preferences when it comes to cutting through the mix. Good mod for soloists IMHO. I would suspect that the added gain stage positioned right behind the entry stage bolsters its treble-ish tonal character.AFD Preamp.gif
  10. **Advanced**Added 12AX7**$$** 4 Gain Stages Mod : Intermediary Gain Stage between V2a and V2b: The Soldano Hot Modjcm800-hotmod 
  11. **Advanced**Added 12AX7**$$** 4 Gain Stages Mod: Preamp Overhaul with EL34
    Work in Progress (Friedman BE100)…
  12. **Advanced**Added 12AX7**$$** 4 Gain Stages Mod: Preamp Overhaul + 6L6 mod
    Work in Progress (Cornford MK50)… Somehow it seems that the more gain you extract from the preamp side, the cleaner and warmer the poweramp stage has to be. Which is why most 4,5 and 6 gain stages amp have 6L6 or 6550 powertubes. Otherwise you end up with a very thin and nasal amp tone.
  13. **Advanced**Added 12AX7**$$** 5 Gain Stages Mod: Amp Overhaul
    There I basically recommend copying the well named Soldano Hot-Rod single channel MKI or the similar SLO-60. It is a synopsis of Soldano’s modifications in a Marshall package with the simplicity that renders the whole thing somewhat feasible for amateurs like me. 5 gain stages instead of 3, single channel, no tube buffered fx loop, cold stage cathode follower and 6L6 tubes. Schematic available online and bellow (for private use only). Thank you NicoSonic. soldano-hr50-sl60-xl-mod
  14. **Expert Mod**2x Added 12AX7**$$$** 6 Gain Stages Mod: Amp Redesign
    We are getting down to some serious biz and some serious hum too! While keeping it as simple as possible, single channel / no fx loop, let us go about grafting the 5150 lead channel circuit (plate-driven EQ) into a Marshall JCM800 chassis. Basically, the idea would be to remove every bit of electronic inside the amp barring transformers and possibly filter caps, drill holes for 1x extra 12AX7 and wire a 5150 red channel preamp circuit on a PTP board. The circuitry would look approximately as such:

5150 single channel preamp.png

A standard 6L6 or 6550 Poweramp/PSU would have to be added along with new or existing filter caps. Thank you A-Wai Amps for the preamp schematics. This modification is difficult to implement on a point to point board… 6 gain stages + plate fed driver + big tubes = mega 50/60hz hum. Check out amp wizard heater circuit mods. Consider elevating the 6.3V AC and rectify it for the first 2 gain stages. Cold bias is recommended as in the original amp. A well designed ground plane is required.

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