msch997

Hi Folks, I am from Johannesburg in good old sunny South Africa, nice to make all your acquaintances.

I would like the benefit of your considerable experience if you don't mind to help me with a decision I need to make.

I recently acquired a pristine JCM800 which from the serial number is a 1981 model. Opening up the back I see that it is to the USA spec i.e 5881/6550 Sovtek power tubes and not the EL34s which to me is the Marshall sound.

My options seem to be
1) Leave as is
2) Convert to EL34 spec <not sure what is involved here>
3) replace the 6550's with JJ KT88's <thats assuming I can get them here>

What would the body of opinion suggest and if I go the EL34 route does anyone know what is involved with the conversion and if its a major conversion or something simple for a qualified tech.

Thanking you all in advance

Mike Schneider

steelhorse

Hi and welcome!

Do you have any pics of this amp?

PaoloJM

Changing to EL34s requires the altering of bias voltage divider resistors to 220k, I believe their value is around 100k for 6550s. Any competant tech could do this.
Most people seem to agree that the distorted tone of an EL34 is superior to that of a 6550, though there are also a lot of 6550 fans out there. I would suggest trying to get to hear both in person first or, failing that, there are many samples of the tone of each available on uTube and other web-sites if you search around.
As far as KT88 are concerned I would say they're getting rarer and more expensive, I think JJ and a few others still do them, and for this reason I would steer clear of them. Stevie Ray Vaughan swore by them though so who am I to say....

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bigragu63

Hi Mike,

In addition to PaoloJM's comments about the technical work of changing resistor values to convert to EL34's, I use this schematic, courtesy of Mark Huss... http://mhuss.com/MyJCM/JCM800_2204.pdf It lists the different resitor values for both tube types.

Regarding suggestions between the 6550's and EL34's...I had a 4104 2x12 combo (same chassis as the 2204) USA model that I converted from 6550's to EL34's, and I did like the tone better. Although, I have no idea how old the 6550's were that were in it when I bought it. They could've been old and tired.

I now have a 4010 1x12 (again, same chassis as 2204) USA model that I did some tone tweaks from this site: Tales From The Tone Lounge; The Ultimate JCM800? I'm using SED Winged C, SV6550C power tubes, with a JAN philips 5751 in V1, and JJ ECC83S gold pin tubes in V2 and V3, and I love it! All tubes purchaced from Tube Depot Welcome to TubeDepot.com!

I also changed the speaker in the combo to a Celestion Vintage 30, which improved the overall sound of the amp. Whatever speakers you use in your extension cab can make a big difference in tone as well.

Good luck,
Mark

st.bede

all I know is a lot of people did not like 6550s, but as pointed out some did....my two cents is live with the amp for a while unless you have a paticular knowledgeable idea of what you like and dislike....and how to get there..

it should become clear as time goes on....I get so excited at first that I just want x to be it's best, then I settle down and only make those changes after a while

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mmorse

I have an '83 2204 that originally came with 6550s. If you want to get an idea of what the amp will sound like with 34s, you can do the voodoo amps mod. Just piggyback a 47k resistor on to the bias resistor. That should get the bias trim pot in range to properly bias 34s. That's all I did. I've run 34s, KT88s and now JJ6v6s in mine.

The KT88s really make the amp roar but, because of the increased headroom, the amp needs to be pretty loud to get a nice Marshall crunch. I think my amp sounds best with 88s but they aren't practical from a volume standpoint.

The JJ6v6s sound very much like 34s but at a lower volume. The amp puts out 22 watts with 6v6s. Still plenty loud for my needs but now gets crunchy a lot faster.

msch997

Thank you for all the sound advice.

I am going to follow the gist of it which is leave the amplifier as it is for a while and then decide. It sounds pretty fine the way it is I must say.


Regards and Thank You



Mike Schneider