JMP 2203 - EL34 vs 6550

peterc52

peterc52

Well-known member
Never tried a 2203 with 6550 like many of you US citizen have as they never were in Europe.

Is it worth trying? How different?
 
Bigger, deeper lows....maybe a touch 'glassier' with a tiny bit less mids...but still great midrange. KT 88s are similar but a good bit 'more' in that same direction. If you have a 6550 amp you can also (for the most part) run KT88s, and maybe KT90s which to me sound like a huge EL34.

If I got another Marshall that came with 6550s, I'd leave it that way for a cool change, even if slight, tone wise. Plus I can then use my GE 6550s :giggle:
 
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6550's are not a true "audio" tube they were used in servo amps for B-52 Bombers. But were plentiful in the 70's as Marshalls
coming across the pond from Bletchley UK w/ EL-34' / 6CA7's the tubes wern't surviving the trip so Marshall started shipping them
w/o tubes for USA and installing 6550's when getting to the States.
Rock amps sound best with good EL-34's the 6550's are stiff and don't really break-up. They can sound good but you'll go deaf.
50 watt JMP's tend to work well w/ 6550's but 100 watters are just brutal.
 
Only had them in a Plexi RI. Pretty much what Racerxrated said applies although I would add they do not break up nearly as easily as an EL34. You have to get deep into the volume control before that happens. The upside to that is they are extremely rugged and reliable. At the time i was using GT branded tubes and I found the Chinese versions to be warmer with earlier breakup than the Russian variety plus they were cheaper.

I a/b'ed that amp next to a triple rec once and it utterly demolished the MESA with cut, punch, presence, and volume.
 
One other point to add....Marshall had to use 6550s in the 70s for overseas Marshalls because they were showing up with broken tubes..and in some cases, the reliability of the EL34s were starting to wane(in the 80s I think). But, the transformers are still designed around EL34s..so the recommendation is to bias them as EL34s. In a Marshall that ends up somewhere between 30-40ma. Typically, a 6550/KT88 would be biased much higher, like 50-60ma in a VHT D60. But, to be safe I bias them lower like a 34. Saves on tube life as well.
Just a piece of advice I read on the Marshall forum, from a few expert techs that restore old (1965-present) vintage Marshalls.
 

13 yrs ago. Both amps have SED Winged =C= [not svetlana] power tubes & NOS Mullard pre's.
EL-34's in general just sound warmer to me. YMMV.
If you're gigging with a 100 watt 6550's Marshall you'll wanna move the front row back about 30 yds or risk a mutiny.
 
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Back in the day I always ran 6550’s in my old Marshalls. Never got asked to turn down. Putting in 6550’s doesn’t make the amps any louder. Just more solid lowend and abit more clarity.
Once we got to the bottom of the GE6550 barrel, those were no good and the chinese always blew up on me. Ended up using Sovtek KT88’s which were pretty good. No idea what are any good now.
 
VonBonfire said:
Only had them in a Plexi RI. Pretty much what Racerxrated said applies although I would add they do not break up nearly as easily as an EL34. You have to get deep into the volume control before that happens. The upside to that is they are extremely rugged and reliable. At the time i was using GT branded tubes and I found the Chinese versions to be warmer with earlier breakup than the Russian variety plus they were cheaper.

I a/b'ed that amp next to a triple rec once and it utterly demolished the MESA with cut, punch, presence, and volume.
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That would happen still with any powertube type it can take
 
I far prefer EL34’s in them (or even 6L6GC’s like in my ‘69 plexi, which sound awesome btw). With 6550’s the response is slower than with EL34’s, the attack is slightly rounder (even more so with KT88’s) and you sacrifice center and uppermidrange growl and kerrang (both qualities that really define much of the signature Marshall sound to me). Those frequencies will get smoothed out. You do, like others said though, get a deeper low end and more headroom. To me it’s a poor trade off for where Marshall’s excel

I have currently a 1979 JMP2203 with GE 6550A’s now in for a mod and getting switched to EL34’s. Should come back much improved
 
braintheory said:
I far prefer EL34’s in them (or even 6L6GC’s like in my ‘69 plexi, which sound awesome btw). With 6550’s the response is slower than with EL34’s, the attack is slightly rounder (even more so with KT88’s) and you sacrifice center and uppermidrange growl and kerrang (both qualities that really define much of the signature Marshall sound to me). Those frequencies will get smoothed out. You do, like others said though, get a deeper low end and more headroom. To me it’s a poor trade off for where Marshall’s excel

I have currently a 1979 JMP2203 with GE 6550A’s now in for a mod and getting switched to EL34’s. Should come back much improved
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The response has always been slower with el34’s ime. They compress and mush out making the attack slower in every Marshall I have had. Another reason I ran 6550’s back in the day.
 
Greazygeo said:
The response has always been slower with el34’s ime. They compress and mush out making the attack slower in every Marshall I have had. Another reason I ran 6550’s back in the day.
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They do compress and mush out sooner when pushing the poweramp hard since they have less headroom/wattage, but the response itself I find to still be faster/less delay with a less round attack and when you’re not pushing the poweramp as hard like on a 2203 or modded Marshall the difference is more pronounced in what I’m saying and this thread is about 2203’s specifically fwiw. I find it to be a little bit like comparing 10, 12 and 15” speakers or also lower vs higher speakers. The bigger ones have slower responses and sound less tight/sharp, but bigger sounds and takes more to overly distort them and mush out
 
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The attack is a lot faster with 6550s, which makes it feel more percussive and punchier. Like a Marshall running through a Mesa power section. I think a JCM800 2203/4 would benefit more with them than a JMP style amp. I'm not sure if the JMP era 2203s have the pre-amp knob though.

If you play hard rock and heavy riff based metal (think Tool, Mastadon etc) 6550s work better IMO.
 
braintheory said:
I far prefer EL34’s in them (or even 6L6GC’s like in my ‘69 plexi, which sound awesome btw). With 6550’s the response is slower than with EL34’s, the attack is slightly rounder (even more so with KT88’s) and you sacrifice center and uppermidrange growl and kerrang (both qualities that really define much of the signature Marshall sound to me). Those frequencies will get smoothed out. You do, like others said though, get a deeper low end and more headroom. To me it’s a poor trade off for where Marshall’s excel

I have currently a 1979 JMP2203 with GE 6550A’s now in for a mod and getting switched to EL34’s. Should come back much improved
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The Tung Sol EL34 sound pretty big.
 
braintheory said:
They do compress and mush out sooner when pushing the poweramp hard since they have less headroom/wattage, but the response itself I find to still be faster/less delay with a less round attack and when you’re not pushing the poweramp as hard like on a 2203 or modded Marshall the difference is more pronounced in what I’m saying and this thread is about 2203’s specifically fwiw.
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I don’t agree with that at all. An el34 has a less round attack? No way. Yes 2203’s…had plenty of them, modded many 4 inputs to 03’s. They all ended up with 6550’s, so they would have a solid low end that would keep up. Still was never 100% happy, but it was the best I could find at the time. There just were not much in the way of higain amps like now. The rest of the tone is very similar between the two tube types. Stronger lows and clearer note definition in chords. Not that one is better or worse, just different and only by alittle.

All my remaining Marshalls are back to mostly stock 4 inputs with el34’s. They get used for what they are now.
 
I prefer EL34s for Marshalls and 6550s for SLO circuits with dominant mids. EL34s can throw dumb low end and still retain mids. All of my recent videos use a Svetlana EL34s that aren’t even known for good low end. It’s all in amp circuit design.

6550s have a bigger sound to them but are less musical - it’s like a V shaped tube EQ. In the room it’s a broader sound but under a microphone they’re shrill and hollow.
 
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