JCM 800 mods

Fatfingers

Has anyone done any mods to get a bit more gain out of a JCM 800? I was thinking along the lines of the preamp changes listed here

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

Any views on these? Or any other suggestions? Ideally I'd like to make a switchable gain boost, although I'm not after anything too shredtastic, just a bit more oomph for single note stuff.

ICBM

Best footswitchable gain boost mod for a Marshall JCM800.

http://images.thomann.de/pics/prod/103975.jpg

£39 new, including footswitch.

Fatfingers

Funnily enough I have one of those and used it for years!

ThePrettyDamned

With ICBM. If you don't like the bad bypass, I think you run a buffered pedal before it - regurgitating info rather than first hand experience.

Another good one (well, I thought so) was a ds-1 set right. Crazy lead tones. The mxr dist iii is amazing through a jcm, too. Loads of boost and adds extra overdrive of its own wonderful pedal.

ICBM

Fatfingers said:

Funnily enough I have one of those and used it for years!

You really aren't going to get a lot more by modding the amp. If you do, do it to change the basic tone and gain level slightly - not extensively - if they're not exactly as you want, and don't bother with the footswitching. If you need to change the amp more than that you've got the wrong amp.

Back in the old days there wasn't much alternative to modding your Marshall, but these days there are so many great amps and pedals it really isn't the best way. The amp is a classic for a reason, and does actually sound best unmodified, in my opinion. (Maybe with the exception of taming the bright cap network a touch, it really is quite extreme.)

Even more so with the older ones - quite a lot of my work these days is in restoring old Marshalls, and in many cases that involves removing previous mods. Every single one sounds better as it was meant to be originally.

I also nearly posted the DS-1 instead of the SD-1 - I prefer them, but I know they're more of an acquired taste, and they can be very fussy with an 800 if you don't hit the exact spot between mud and shattering glass .

Fatfingers

I've already snipped the treble bleed cap on the gain pot and might also replace the other bright network stuff on the input socket. Maybe I'll leave it at that and dig my old Boss SD-1 out.

Any experience of using an Airbrake with one? Or adding VVR?

ICBM

Fatfingers said:

I've already snipped the treble bleed cap on the gain pot and might also replace the other bright network stuff on the input socket. Maybe I'll leave it at that and dig my old Boss SD-1 out.

Any experience of using an Airbrake with one? Or adding VVR?

Be careful, the Airbrake might not have the power handling for a cranked JCM800 - even a 50W, they put out up to 90W in full power stage distortion. The 100W ones are completely mental, way over 200W usually.

I've never tried any form of power scaling (is that what that is?) on one, I don't like doing major mods like that. My experience of power scaling in general is that I prefer conventional attenuation... power scaling sounds more like a very good master volume than like a power amp working really hard, to me.

Fatfingers

I'm not surprised about those power figures. I took the JCM to a mate's workshop the other day. Jesus it's loud. Possibly the loudest amp I've ever had, with the possible exception of an old Hiwatt Custom 100, which was effectively useless unless you were in a stadium.

The Airbrake I'm building is rated at 100 watts, so I should be okay. 

I put VVR (power scaling) on a little Ampmaker SE5 that I built and it works very well. However, I also put it on an old Selmer Treble 'n' Bass 50 and it doesn't sound anything like as good.

Fatfingers

What's your view on a post phase inverter master volume. Any appreciable improvement over the standard master volume?

ecc83

If it is gain you are after (and this might be heresy!) try splitting the anode load to the 83 driving the cathode follower into 2x 47k and bootstrapping the junction from the CF with ~100nF 630V.

Dave.

ICBM

Fatfingers said:

What's your view on a post phase inverter master volume. Any appreciable improvement over the standard master volume?

Depends what you want - they sound different. The PPIMV is closer to the sound of a non-MV amp, the normal MV gives that tighter 'rock' crunch. Bear in mind that the PPIMV progressively defeats the NFB as it's turned down as well, so the presence control becomes less effective.

You can fit both if you want to!

ecc83 said:

If it is gain you are after (and this might be heresy!) try splitting the anode load to the 83 driving the cathode follower into 2x 47k and bootstrapping the junction from the CF with ~100nF 630V.

I might try that next time I have to change one of the 100Ks… I have a suspicion it might sound way too 'spiky', but I don't really know!

Fatfingers

Good point about the NFB being affected by the PPIMV. I'll pass on that too then.

Thanks ECC83, although I suspect you may be right about it sounding too spiky.

The parts for my Airbrake have just arrived so I'll put that together and go from there, I think.