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PostPosted: Sun, Nov 10, 2019 5:23pm 
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Picked up a 1980 JMP 2204 and the same Era JCM800 cab with stock G12-65's. It sound pretty good. It came loaded with El34's. The guy told me it had the conversion done years ago. At any rate I break it ou of the head shell to check it out and bias it.The bias adjustment was maxed out and I was getting 7ma's on one tube and 11 on the other. I was thinking maybe they were cold set of tubes. So I dropped some other el35's in. I wasn't hardly getting a reading on them. Then it occurred to me, I bet it didn't ever have the conversion done! So I dropped a pair 6550's in and i was getting 37ma. This still being maxed out. Still cold.

I'm not sure I want to commit to a conversion yet. I'd like to hear the El34's in the appropriate bias range. Anybody have any suggestions? Maybe alligator clip some different value resistors on top of the bias resistor? Any input would be great. Thanks!


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PostPosted: Sun, Nov 10, 2019 6:19pm 
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Ideally you'd want to swap the grid bias resistors from 150k to 220k as well. You need less -DC Voltage. Factory it would keep the 47k near the bias pot and swap the 150k before the bias diode with a 220k. If you were to to piggy back anywhere, it'd have to be the 47k to drop negative voltage. I don't remember what I've used in the past, bit I'd start with a high value and work down

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PostPosted: Sun, Nov 10, 2019 6:34pm 
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Changing the resistors is a quicky. I would make the change and then play the amp for a while to see what you think. Going back would be easy. Personally I’ve never liked 6550’s in a Marshall so I would say square away EL34 use all day long.


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PostPosted: Sun, Nov 10, 2019 6:55pm 
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Been a Marshall player for the past 30 years. I loved the non master volume amps and hated the master volume models. I had it told to me that Marshall’s sound best with EL34’s. Almost a year ago I picked up a 2203 Marshall with 6550’s and was blow away. I realized I hated Master volume Marshall’s because of the EL34’s. Hahaha

Regarding the conversion. I would just put a 47k resistor on top of the 47k to get around 22k. EL34’s should bias up. I’m too lazy to change the 150k bias resistors.

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PostPosted: Sun, Nov 10, 2019 7:13pm 
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That sounds right for 6550's... they are to be biased like you had el34's in there not what a 6550 should actually see... that's what was intended by Marshall. Some like the tone of 6550 marshall's like me... others dont, just personal preference.

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PostPosted: Sun, Nov 10, 2019 8:00pm 
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I like them both; never felt the need to change to one or the other when I've bought a 2204/3. Whatever it came with, I roll with..I have a pretty good tube collection now and have a nice supply of vintage 34s and 6550s for either occasion.

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PostPosted: Sun, Nov 10, 2019 10:24pm 
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I have an even split of amps with the two right now...

'73 Super Lead (100W) w/'90s EL34 Winged Cs
'74 Super Lead (100W) w/'70s USA Tung Sol 6550s
'79 JMP 2203 w/'70s Sylvania 6CA7s
'81 JCM 800 2203 w/'70s Sylvania 6550s

I like them all for different reasons. Certainly a difference, no doubt about that. Just realized my '73 is the only one without '70s USA made tubes. Maybe I'll grab some to swap in there.


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PostPosted: Mon, Nov 11, 2019 1:00am 
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What is the actual tonal difference between EL34s and 6550s? Is it a midrange frequency thing? Or a low end 'tightness' thing?


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PostPosted: Mon, Nov 11, 2019 7:24am 
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Thanks for the input fellas. I will say the 6550's had a much more bolder feel and sound. Really clear and tighter feeling compared to the 34's. Granted they were not in the bias range. But you can certainly hear and feel the difference. The el34's are quite a bit more "dirty" and the midrange is quite pleasant.


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PostPosted: Mon, Nov 11, 2019 10:45am 
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I'm looking at buying a JCM 800 2204.

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PostPosted: Mon, Nov 11, 2019 11:04am 
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petejt wrote:
What is the actual tonal difference between EL34s and 6550s? Is it a midrange frequency thing? Or a low end 'tightness' thing?

My take is: 6550s have more tight low end, good mids and highs...EL34s have decent low end and sizzling upper mids/highs. Both sound great but if you want more low end 6550s are the ticket; want screaming upper mids get EL34s.

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PostPosted: Mon, Nov 11, 2019 2:43pm 
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EL34 break up pretty soon. 6550 stay clean till really pushed. Then they get the woody tone.

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PostPosted: Mon, Nov 11, 2019 8:50pm 
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From my experience, even when pushed hard, the 6550’s aren’t as warm, growly or tonally complex as the EL34’s. They have less personality and less grit/smoother to my ears, but have more robust, tighter lows and a more efficient sound, which I like. I guess there are pros and cons to both, but in general I prefer EL34’s, especially in Marshall type amps since it really plays to their inherent strengths of being raw, upper midrangey and growly. Maybe for amps that sound very tight and not as warm I can see it doing well and playing to it’s strengths like in a Wizard MTL or CCV.

Also, from my experience trying some identical amps with 6550 vs EL34 vs also 6L6, I find the latter two to feel better to play and they have a nice compression in comparison to 6550’s, which felt a bit stiffer to me. Some amps I’ve tried with 6550’s or KT88’s also seemed a bit hollow sounding to me, but in some cases it was probably just the voicing of the amp itself


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PostPosted: Mon, Nov 11, 2019 10:30pm 
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I prefer KT88 over 6550 in most cases.

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