Do I need to exchange the 6550s for EL34 power tubes in my JCM 800?
Ez Nut
I just got a stellar deal on a very old JCM 800 Mk2 2204 50 watt head, but I
was wondering if I need to convert the power tubes from 6550 to EL34s. I
know EL34 amps are reccomended, but I couldn't pass the deal up. Is the mod
worth it? What are the advantages to EL34s? Lastly, why do some JCM 800s
have El34s and others 6550?
Thanks,
Erik Person
The Fabulous One
In the mid 70's, the U.S. Marshall importer, Unicord, decided to start
putting 6550's in place of EL-34's in the amps, apparently for reliability
reasons (I'm told that 6550's are a little more robust than EL-34's, but I
can't confirm or deny this) thereby reducing warranty claims.
Unfortunately, in order to use 6550's properly, the bias supply must be
increased by approximately 10 volts and the distributor didn't bother doing
this initially, which resulted in the amps sounding very harsh indeed as
they couldn't be biased properly. Eventually (I don't know when) the amps
started being modified properly to use the 6550's. Your Marshall may already
be correctly modified to use 6550's, but you need to get an experienced
valve amp dude to look at it.
The 6550's are a cleaner (some would say harsher or more sterile sounding)
tube as far as crankin' the amp up goes, i.e. don't distort as much. EL-34's
will distort quicker and sooner and thus sound 'warmer' (purely
subjectively ) to our ears. Some players like an amp which stays cleaner at
higher volumes and thus prefer the 6550 sound, but if you want the amp to
start breaking up at lower volumes try the EL-34's. If the bias supply is
already set up for 6550's, you will need to get it modified back for
EL-34's. Setting the bias correctly is critical for getting amps to sound
their best and get longest life from the tubes.
Changing a Marshall from 6550's to use EL-34's is a common modification so
any competent valve amp tech should be able to do it easily. If you want the
mods, email me and I'll let you know but don't work on the amp yourself as
some of the voltages inside a valve amp are lethal. Also, only let people
who are experienced specialists with valve amps work on it - the local Hi-Fi
repairman won't know what he's doing.
Regards,
Big Mal
Ez Nut <eeez...@yahoomail.com> wrote in message
news:z9Ma5.3222$5e2.1...@news1.rdc1.ga.home.com...
RonSonic
wrote:
>Everyone,
>
>I just got a stellar deal on a very old JCM 800 Mk2 2204 50 watt head, but I
>was wondering if I need to convert the power tubes from 6550 to EL34s. I
>know EL34 amps are reccomended, but I couldn't pass the deal up. Is the mod
>worth it? What are the advantages to EL34s? Lastly, why do some JCM 800s
>have El34s and others 6550?
Good acquisition - has it got the over-under or side by side inputs?
6550 or EL34 is really a question of taste. If you're looking for a
large metal tone with a big clearly defined bottom stick with the
6550's. Need more compression and midrange grind you want the EL34's.
It's an easy swap, change the feedback, verify the screen R's and
maybe a resistor or three in the bias circuit.
Might not be you though and if it's got the original GE 6550, they're
probably still good. They only ran these amps with a bit over 400V B+
and lean bias 6550 last forever in there. Originally Marshall shipped
EL34's in everything - the distributor started sticking in 6550 in
self defense, too many returns. Marshall adapted and altered a few
things to suit those tubes, but the moment they got their distribution
deal back they went straight back to EL34. A lot of amps were
converted back to EL34 the day they were sold, others at their first
tube change.
For mods, I like a post Phase Inverter Master V, and for my tastes
prefer to warm up the lo gain channel and take something off the high
gain. This might not do you any good either. These amps were
just-right for an awful lot of people and styles. A lot of guys play
truly brutal with distortion pedals plugged into that super clean
Lo-Gain input.
These are remarkably versatile amps that respond well to mods, so it's
really a flexible tool for you to create your tone.
Ron
PS: I really like the Svet EL34's, fine quality great price.