Old Marshall amp

[8bit]

I've had this since I was about 16 I think, not this actual one but this model I think:



From what I can make out, it's a Marshall JMP made in 1981 (according to the inspection date scribbled on the QC sticker inside at the back). It's a 100w model, 3x 12AX7 and 4x EL34 valves. I think my mum paid like £100 for that plus a tatty old Arbiter 2x 12" cabinet back then.

I've not picked up a guitar in about 25 years (god that makes me feel old) but our eldest lad has expressed an interest so I dug the old guitar out and fired it up. It didn't work, looking in the back, one of the EL34s doesn't glow when warmed up so I guess it's goosed and needs replaced.

So do I just buy an EL34 valve from basically anywhere and plug it in and hey presto? Or are they not all created equal, i.e. should I get a matching set? Once some/all valves are replaced, is it just power on and play or do they need burned in or anything of that sort?

Also, from a very quick Google/eBay search, looks like the amp itself might be worth a few quid these days? Not necessarily thinking of selling it but it'll depend on whether I get the interest in playing back again, and/or find the time...

[jackofall84]

You can use pretty much any EL34 valves, but they all have their own tell tale sounds. Yes you can just swap the valves out and in but you also need to set the BIAS, I've never done it myself as there's plenty of guitar techies in Bristol who didn't charge much for changing and BIASING valves but I don't believe it's that hard to do.

JJ are a pretty common brand for Marshalls and you can get them from hot rox.

I reckon that amp would be worth a few quid these days!

[Turn7]

There used to be a compnay advertise iin Guitarist that sold specific valves for amps, but I cant for th elife of me, remember their name.

Sorry for useless post.

As others have said, worth its weight in Gold these days,

[NapierDeltic]

By all means swap out the valve; likely all four would benefit from replacing. However I would consider paying to have the amp thoroughly inspected. Being 40 years old it may well need re-capped and the tube sockets would benefit from an inspection and clean as well. Having had this done, a re-bias (as suggested prior in this thread) would be the next step.

You could put a new valve in there and then release the magic smoke when you turn it back on. Only ever turn the amp on with a load connected to it as well. I don't know if old Marshalls are clever enough to know when there isn't any speakers connected, or whether that switching (if it does exist) would necessarily work on your amp.

Also consider upgrading that Arbiter 212!

[Stan the Bat]

As others have said first thing is to get it checked out, then onward and upward.

[gazza285]

As said, get it checked out professionally, the electrolytic capacitors dry out over time and could well need replacing.

Nice amp though, worth money these days, selling from £600 up on eBay, so well worth getting it checked out.

[Grrbang]

Nice amp that, bring it to an amp tech and they will sort it out for you.

Usually it’s just the tubes, so shouldn’t cost much on top of the tubes.

Agree with the cab comments, grab a used or new speaker cabinet with Celestion greenback speakers. Make sure the speakers aren’t ripped. The amp is powerful enough for 4 Speakers which sounds great, but 2 is ok too. Make sure the speakers are plugged into the right speaker socket, matching the ‘impedance’.

[8bit]

Thanks for the responses, all.

I had another look at the amp at the weekend, all four of the power tubes are different brands/labelling from each other. I don't recall ever changing a tube back when I used the amp a lot so I guess the previous owner did them at various points, or it was supplied like that from the factory. Anyway, I've never had it biased either so I'll look into that.

Thanks for the advice re. speakers, I was aware not to turn it on with no speakers attached but worthwhile advice for anyone else how happens upon this thread and didn't know. I'll need to see if I can get a new Marshall logo for the front as half of the current one is broken off...

[Some Guy]

https://www.proaudioservice.co.uk/guitar-amplifier...