Marshall JMP - 6550 vs EL34? Give my your input!

[ronmail65]

I have 2 JMP heads (a 50 and a 100) made in 1980 / 81. They're both equipped with 6550's, but I've been hearing more about EL34 conversions lately.

What does it take to convert to EL34s? (cost and work effort)

What is the difference in tone and performance between EL34s and 6550s?

Ron

[Wakarusa]

I have 2 JMP heads (a 50 and a 100) made in 1980 / 81. They're both equipped with 6550's, but I've been hearing more about EL34 conversions lately.

What does it take to convert to EL34s? (cost and work effort)

On most amps you need to change a single resistor. Before you get excited, to change it correctly you have to lift the circuit board to desolder the old one and solder in the new one. After that it's install tubes and bias to taste. An hour or two at most. Don't let someone just clip out the old resistor and solder a new one to the old leads sticking out of the top of the board.

On some amps you also should move the feedback wire to a different pin on the speaker impedance selector. This adds 10 minutes or so to the project ;)

What is the difference in tone and performance between EL34s and 6550s?


EL-34 are typically more midrangey (and often a bit brighter) with an earlier onset of compression in these amps. The easiest way I know of to describe the difference is 6550 = 80's American rock, EL-34 = 60's & 70's British rock.

[jlummaa]

EL34 = more mid range, earlier break up
6550 = big bottom end, more headroom

I converted my JMP superbass (with metro amp PTP board) to 6550s and just changed the resistor which is in series with the single bias diode. It was 27k with El34s and now it's 22k with 6550s.

You may also want to adjust your NFB to taste since the amp's headroom will change quite a lot. I changed the NFB resistor to 100k with 6550s but you may want to make it lower value (higher NFB) with EL34s.

[mmorse]

Well, if your amps are MV amps (2203/04) you only need to change the bias resistor. My amp was a 6550 head and I piggybacked a 47k resistor on to the existing resistor and I was able to bias 34s. May not be the proper way to do it but it worked fine. Never had an issue. Got that trick off the Voodoo amps site. At that time, Marshall put the NFB wire in 6550 amps on the 4 ohm tap of the impedance selector switch. They put it on the 8 ohm tap in 34 based heads.

You don't have to change the NFB wire. If you find the amp is too gainy with 34s, move the wire to the 8 ohm tap.

I ran Mullard 34s for a long time in my 2204 but have since gone to KT88s. I like the extra low end heft from the bigger bottles. And I find 88s are more aggressive sounding to my ears than 34s. I find I get a better low end and lower mid response and still get enough gain for classic through 80s hair band rock.

If your amps are 1987/1959, then things may be a little different.

[Wakarusa]

A bit more on the negative feedback connection...

As you dig through the Marshall 2203/2204 (both MkII and JCM800) schematics, you can see that the NFB wire moves around a bit. There's drawing 78331-3 for MkII master and lead circuits that only shows 4 and 8 ohm secondaries (Canadian models?). In these, the NFB always goes to the 4 ohm tap. In the JCM800 schematics the NFB connection is always shown to the 4 ohm tap. In the Unicord drawings dated 1976 and showing 6550 outputs and 4, 8, and 16 ohm secondaries the 50W 2204 connects NFB to the 4 ohm tap while the 100W 2203 connects NFB to the 8 ohm tap.

As mmorse points out, you can use whichever you prefer. The amps I've seen with "original" EL34 were all tapped from the 4 ohm winding.

[John Phillips]

You also need to change the two 82K bias feed resistors to 220Ks, or the amp will sound a bit choked (kind of like having a cross-line post-PI MV permanently turned down a bit). It's essential that this is done properly by lifting the board, not snipping and soldering above it, since if you cook the solder below and cause a cold joint - easy to do when you heat the ends of the clipped wires - you could cause a bias failure and fry the power tubes.

BTW, lifting the board is much easier than it sounds - just undo all the pots and jacks, and the board will lift up while still attached to the tube sockets. Sometimes the two wires to the cap can under the board are too short to let you get good access though, so you may need to unsolder those if you're not good at working under the edge of a tilted board.

As Todd said, a lot of the stock EL34 models had the feedback wire connected to the 4-ohm tap as well. Personally I think this makes them too harsh and bright, and makes the Presence control a bit too limited in range.

[mmorse]

I know that at some point, Marshall standardized on the 4 ohm tap but it was my understanding that previous to that they used the 8 ohm for 34s and the 4 ohm for 6550s. But, that might not be accurate.

When I switched to 34s, I moved the NFB to the 8 ohm tap but the amp cleaned up too much for me. Had to crank the amp much more to get any break up so I went back to the 4 ohm and have left it there ever since.

I would guess that if you wanted a really clean Marshall, you could move the NFB to the 16 ohm tap. But what fun would that be. :)

[Voxy Foxy]

My friend had an 800 with 6550's, and it sounded good. He got JJ E34L's (Their EL34) and it made the 6550's sound anemic and sterile...EL34s really put the balls in a Marshall, IMO.

:dude