JMP 2203 Run at 50W

HHwarner

I want to replace the Output Valves on my 2203 JMP but not sure what to get. Iv got various Matched pairs ie EH, JJ &  =C= which iv used in my 50w 2204 JCM. I would like to try em in my 2203 just to see which sound the best. I know Marshall dont recomend it but I know a few people who run 100 watters at half power by pulling 2 valves. Im not sure what Impedance to set my amp to. My Cab is 16 or 4 ohms Mono. For my main tone I push the power section hard so would it be ok to run for a short test with the Powerbreak. Cheers,

Scott

ICBM

Fit just the two outer power valves - it shouldn't actually matter in a 2203 but this is good practice for some older JMP Marshalls which don't have grid stopper resistors on the inner pair of sockets* - and set the impedance selector to half that of the cabinet.

ie in this case, set the selector to 8 ohms and use the 16-ohm input on the cab, or 16-ohm setting on the Powerbrake. It's safe, don't worry about cranking it.

(*Although in a DSL/TSL100 it's the other way round - use the inner pair or the "1 and 3" positions leaving out the one nearest the power transformer, which has a suppressor cap on it which can cause trouble.)

HHwarner

Thanks, I'll have a play later. Not sure what valves are in there at the mo they are non branded.

HHwarner

MMmmmm, Just taken the unbranded ones out. Think they may be Mullard made XF4. I think ill leave these in!!!!

http://s879.photobucket.com/user/hhwarner/library/Valves

ICBM

HHwarner said:

MMmmmm, Just taken the unbranded ones out. Think they may be Mullard made XF4. I think ill leave these in!!!!

Indeed they are. Quite likely original to the amp as well!

If you're really lucky it will have Marshall-labellel RFT ECC83s too - these are by far the best-sounding preamp valves in 70s Marshalls in my opinion, perhaps because the amps were voiced with them in! They're really aggressive and slightly hairy-sounding but not harsh, which these amps can sometimes be with other valves.

HHwarner

Ahhh Bugger!!!!! One just rolled off the bench. Now I have 3 Mullards. Might have to run it at 50 w for a while now. Do I need to do anything with the bias. Both running at 32ma. Is there any down side to running at 50w until the tubes have seen better days?

SidNewton

ICBM said:

HHwarner said:

MMmmmm, Just taken the unbranded ones out. Think they may be Mullard made XF4. I think ill leave these in!!!!

Indeed they are. Quite likely original to the amp as well!

If you're really lucky it will have Marshall-labellel RFT ECC83s too - these are by far the best-sounding preamp valves in 70s Marshalls in my opinion, perhaps because the amps were voiced with them in! They're really aggressive and slightly hairy-sounding but not harsh, which these amps can sometimes be with other valves.

RFT's seem a little harder to find just recently. I've been casually looking for some ecc83's to pop in my JCM 800. Anybody know of a reliable source? I've seen the ones on ebay from Bulgaria but they seem to have an 'endless' supply'.

ICBM

SidNewton said:

Anybody know of a reliable source? I've seen the ones on ebay from Bulgaria but they seem to have an 'endless' supply'.

Quite likely.

There were used as a form of Soviet Bloc internal currency, it appears! They certainly can be found with "Made In" labels from most of the former Iron Curtain countries, regardless of not actually being made there.

HHwarner

First Pre amp ecc83 sure looks like an RFT. Not Marshall branded but another brand. Cant quite make it out. Next 2 pre amp are Mullard ecc83

http://i879.photobucket.com/albums/ab355/hhwarner/Valves/IMG_06561.jpg

http://i879.photobucket.com/albums/ab355/hhwarner/Valves/IMG_06551.jpg

ICBM

Edicron. Definitely an RFT, the single-piece folded 'wing' plates and the 'star' micas are distinctive. Might be original too, I can't remember when Marshall stopped using the Marshall-logo ones.

HHwarner

Tis a shame Iv only got 3 EL34s now, bugger!!!!!

ICBM

I have a few spare XF4s. They're all old pulls so I wouldn't consider them saleable normally, but if it's to make a set with three other equally worn valves, you're welcome to one. The difficulty would be to work out which (if any) would be a good match for yours...

HHwarner

I suppose it would have to be try some and see if I can match em like that. There is an old junk shop local to me which had some odd Mullards. They might let me try a few. The set iv got were not perfectly matched bias wise. About 6ma difference. The 2 iv got in there now are both bang on 32ma. If I have no joy at the junk shop I might take you up on the offer if that's ok. Cheers, Scott

ICBM

No problem. Probably the best way is to measure the plate voltage and bias voltage on your amp, since they affect the current draw, and see what I can find. I have a test rig which can adjust both voltages fairly widely so I should be able to set up a duplicate of your amp's voltages, at least close enough to get into the right ballpark.

HHwarner

That's great. Ill check the bias voltage tomorrow, Plate voltage was 476v with 2 valves fitted and biased at 32ma. I didn't measure bias voltage. Ill let you know what it is. Thanks, Scott

HHwarner

Right then, Iv just had the 2203 on the bench warming up and it has stabilised. Plate voltage is 471v, Bias Voltage is -41v and bias current is 33ma (measured as mv so in reality 28ma?). Is that any help ICBM?

ICBM

Nearly - where did you measure the bias voltage, at the valve base pin 5 or at the bias cap? The feed resistor is a large enough value to lower it slightly at the valve.

For what it's worth, Marshall's original biasing instructions were to set the voltage to -42V at the cap, not measure the current - Mullard EL34s were consistent enough that it was always pretty good!

HHwarner

That was at pin 5. Just measured again at bias cap -43.1v

ICBM

Ok, that should do! I'll let you know what I've got in a couple of days hopefully.

HHwarner

Thanks Dude, your a good un