Marshall JCM 800

SidNewton · August 2013

Today I took delivery of a Marshall JCM 800 50 watt head. The amp seems to be in full working order despite the fact it's been lying unused for a few years.

The only immediate problems seem to be a few scratchy pots and noisy preamp valves.

So I'm looking at replacing the 3x ecc83's. Also, if I want to replace the 2x EL34's in this amp I'm guessing I will need to take it to a tech for rebiasing. Any recommendations on the best ecc83's & EL34's for this amp plus best way to clean up the scratcy pots would be appreciated.

matone · August 2013

Probably a good idea to get it checked over by a proper tech.

photek · August 2013

I spent a long time A/B ing valves in my 800 and really liked svetlana winged c for power and just simple JJ's in the pre. Of course decent old mullards are probably going to be cool but couldn't get hold of any.

ICBM · August 2013

Cleaning the pots is easy - you need Servisol Switch Cleaner or an equivalent, and in stubborn cases WD40.

Try the Servisol first, squirt it in through the slot in the pot inside the amp and rotate the pot fully a few times. Usually that cures it, but if not try WD40 afterwards. Yes, I know WD40 is not pot cleaner! But this works. For some reason JCM800 pots seem especially prone to going extremely crackly, much worse than most other amps, and the crackle often comes back after just using contact cleaner.

Valves - I would get RFT (old East German made, came under many different brand/label names) ECC83s - they're not that expensive by old valve standards, and sound by far the best in late-70s and 80s Marshalls in my opinion. Possibly because they were the OEM type when the amps were built. They have a very rich, full tone but still with plenty of crunch and bite.

Whatever you get, avoid using any Russian-made ECC83/12AX7 in the middle preamp valve position, they have a high failure rate due to the high cathode voltage - the valves aren't made to the proper spec and won't always take it. This includes all Sovtek, EH, Tung-Sol 'reissue' and others. I've even had a few JJs fail in these positions. All old-production valves and Chinese-made ones are fine.

Power valves - JJs seem reliable. If you can find old Siemens EL34s, again they're better and not as expensive as Mullards; they were also the usual OEM valves in the 80s.

SidNewton · August 2013

Cheer fellas, might have to be JJ's all round for now until I can stretch to some RFT preamps. Will the EL34's have to be rebiased or can I just drop in a matched pair?

ICBM · August 2013

It's unwise just to put them in. Not only do valves vary quite a lot so you really don't know whether they'll bias OK or not, one of the few problems JCM800s regularly suffer from is leaking bias supply caps, which results in the valves running too hot. You could easily cook your new valves. At least have the bias checked and adjusted.

If it can't be adjusted into the right range, make sure your tech knows that the caps need changing and not as many seem to do, bodging the resistor values to force the voltage higher.

SidNewton · August 2013

Thankyou. Much appreciated.

John_A · August 2013

I would say before you resort to cervisol, just try operating the pots for a few minutes, they may just fix themselves

EdGrip · August 2013

John_A said:

...try operating* the pots for a few minutes...

*twiddling.

I've found Mullard Magic to be an interesting source of good old valves, and he's helped me identify some stuff in the past too.

ICBM · August 2013

John_A said:

I would say before you resort to cervisol, just try operating the pots for a few minutes, they may just fix themselves

Highly unlikely with a JCM800.

I don't know why the pots in them seem unusually prone to going crackly, but they are.

SidNewton · August 2013

Just an quick update on this. I had the amp revalved with JJ's all round and the pots cleaned up at the same time. It's now sounding and performing rather splendid. I used it for the first time at band practice last night and am chuffed to bits. I have to use the low input in combination with an overdrive pedal as the high input is so damned loud. Shame, because it sounds fantastic. Still the low input provides a nice 'clean on the edge of breaking up' sound. Who said Marshall can't do cleans?

So here it is.

I'm using it with this

Via this

ICBM · August 2013

Marshalls do excellent cleans, I've never understood why there is a popular belief that they don't.

If you can't get the High input to work at a low enough volume, you really need an attenuator though - it's really worth having the option of that side of it. You don't need to crank it fully, just use the attenuator to open it up to the point it kicks in. Unfortunately one of the features of these 'horizontal input' JCM800s - which snobs will put down as being inferior to the 'vertical input' type - is that the pot taper is rather abrupt. In fact that may be one of the reasons they're not thought to sound as good, since it's harder to dial in that 'just right' point without blowing your head off

.

And yes, these are some of the loudest 50 watt amps ever made...!

Moltisanti · August 2013

very nice, did you buy it on ebay or from a shop ?

photek · August 2013

If you haven't already tried one, borrow a zvex box of rock or a distortron to try into the low input instead of your TS, they sound epic. You can borrow mine if you are in the cambs area.

John_A · August 2013

photek said:

If you haven't already tried one, borrow a zvex box of rock or a distortron to try into the low input instead of your TS, they sound epic. You can borrow mine if you are in the cambs area.

+1 I know on paper it seems a bit daft getting a pedal to make a Marshall sound like a Marshall, but the BOR's combined with a clean Marshall really sound more like a cranked Marshall than the same pedal in to a non-marshall amp.

Love my JCM800, it's getting the most use out of all my amps these days

Cirrus · August 2013

I'm glad you like the cleans - our singer brought a 1980 jmp 50 combo to our lockup, that's basically the same circuit as a jcm800, and I was really impressed by the cleans on the low input. Takes pedals really well, it's killer with a Rat. To be honest, I've hardly spent any time with the high input side since it's got basically no headroom/ cleans to speak of.

koneguitarist · August 2013

SidNewton said:

Just an quick update on this. I had the amp revalved with JJ's all round and the pots cleaned up at the same time. It's now sounding and performing rather splendid. I used it for the first time at band practice last night and am chuffed to bits. I have to use the low input in combination with an overdrive pedal as the high input is so damned loud. Shame, because it sounds fantastic. Still the low input provides a nice 'clean on the edge of breaking up' sound. Who said Marshall can't do cleans?
So here it is.
I'm using it with this
Via this

Exactly what I used to do with my old JCM800, small gigs just crank it without pedals, big gigs, low input and a good OD pedal in High input. One of my all time favourite Marshalls.

SidNewton · August 2013

Moltisanti said:

very nice, did you buy it on ebay or from a shop?

It was originally on ebay. I was in 2 minds whether to bid on it but didn't in the end. I made the guy an offer a day or 2 later which he accepted, so we done the deal outside of ebay. He hadn't used it for years and had a nice coating of dust inside and out.

photek said:

If you haven't already tried one, borrow a zvex box of rock or a distortron to try into the low input instead of your TS, they sound epic. You can borrow mine if you are in the cambs area.

That's interesting. At the moment I have the OCD in one loop for a crunch rhythm sound with the delay and TS in the other loop for a lead sound. The OCD sounds great but I'm not completely sold on the TS.

The thread in the FX section about overdrive and distortion pedals singles out the Zvex as a pedal which works well with either a clean or driven amp. I might have to check one out. Thanks for your offer but unfortunately I'm in Gloucester. I also like the look of the new Xotic SL drive. Some nice sounds on the Youtube demos.

I will still have to explore ICBMs suggestion at some point of using the high input with an attenuator. It does sound glorious. I guess I could clean things up on the guitars volume control and simplify my board even further.

Cirrus said:

I'm glad you like the cleans - our singer brought a 1980 jmp 50 combo to our lockup, that's basically the same circuit as a jcm800, and I was really impressed by the cleans on the low input. Takes pedals really well, it's killer with a Rat. To be honest, I've hardly spent any time with the high input side since it's got basically no headroom/ cleans to speak of.

Nice. My drummer announced at the last rehearsal that he's found a Marshall 1987 head in his attic. ICBM will correct me if I'm wrong but I think it's a 50 watt 60's or 70's model. Belonged to an old boyfriend of his wifes sister who has long since disappeared. Unfortunately the input for the mains cable is a vintage style, which he hasn't got so we can't fire it up at the moment. Hopefully we can find one. If so I let you know if it's still in working order and get some pics up.

ThePrettyDamned · August 2013

This, along with a USA 6505 and the amt stonehead, is my all time dream rig amp.

I like Marshall cleans, so long as you don't expect fender cleans. It's a very different character, and lends itself to the less tasteful side of blues - not those guys who sustain a note forever and sound smooth, jazzy and cool, bit those raucous guys who want to make a racket. Scoop the mids, it sounds cool

Never really looked into the difference between different jcm800s. They all sound awesome! I'd like a mosfet lead 100, too, they're supposed to sound immense. And loud

ICBM · August 2013

SidNewton said:
Nice. My drummer announced at the last rehearsal that he's found a Marshall 1987 head in his attic. ICBM will correct me if I'm wrong but I think it's a 50 watt 60's or 70's model. Belonged to an old boyfriend of his wifes sister who has long since disappeared. Unfortunately the input for the mains cable is a vintage style, which he hasn't got so we can't fire it up at the moment. Hopefully we can find one. If so I let you know if it's still in working order and get some pics up.

It's a good thing he hasn't got the cable... DO NOT just fire that amp up without a professional check-over first. If you're unlucky just turning an old amp on after years or decades unused can cause expensive damage which will permanently reduce the 'vintage value' of the amp as well.

It's quite likely it will need all the electrolytic caps replaced, the voltage selector hardwired and the impedance selector at least cleaned and re-tensioned (preferably bypassed or replaced) in order to make it reliable. The perhaps surprising thing is that the valves may well be fine.

If you're going to get it properly serviced it's worth replacing the input socket with one of the Tube Amp Doctor IEC (kettle) sockets so you don't have this issue again.

SidNewton · September 2013

Cheers fella but I don't think I'll be seeing the amp anytime soon. The drummer had a paddy at the weekend and quit the band. Shame, I would have liked to have a look.

Howdy, Stranger!

Categories

In this Discussion

Become a Subscriber!

Marshall JCM 800

Comments