Re: Marshall JMP MKII new tube time

On Feb 28, 1:58 pm, Jim <as...@xxxxxxxxxxxxxxxxx> wrote:
ed s wrote:
On Feb 28, 1:31 pm, Jim <as...@xxxxxxxxxxxxxxxxx> wrote:

ed s wrote:

I have a 1978 JMP MKII 50watt master volume head that need new tubes -
suggestions ??? Do not want mega distortion, want warm creamy tones
( currently to shrill and brittle) . Thanks Ed S.

I own a 1980 production JMP 2204.

I *highly* suggest a real Mullard for at least V1. V2 also benefits
from a Mullard, if you can do it. I'm not talking about the relabeled
stuff or the Groove Tubes fake Mullard. Cross your fingers and buy a
used (but not used up) Mullard off of ebay. I've had very good luck
when I did that a few years back. Now prices are higher, but they ain't
ever gonna get cheaper.

These amps were designed around Mullards. There IS a difference in
tone. Mullards have a better sounding midrange distortion that just
works in 2204's. I've verified this with my one ears in blind tests
(another person was swapping tubes, I didn't know what I was playing).
I happened to pick a 1958 long plate as first choice for V1, but the
short plate 60's production tubes also sound very good. They may have a
bit more of a mids emphasis, in fact.

1978 was the transition year for the 2204. Here's a schematic dated
2/28/78 for the cascaded gain version of the 2204.http://tinyurl.com/2cn2e6Prior to that, the preamp looked more like
the old four input type non master, and the amp has less preamp gain.
You might want to verify the build date. I don't know what date the new
design went into production. One way to tell which version you have is
to put two guitar cables into both inputs. Do they both play with
expected loudness? If so, you have the EARLY version of the amp. If
not, you have the cascaded version, with higher gain. I'd be VERY
interested in your build date and which version it turns out to be.

V1 is the "gain hole" where the initial amplification occurs. It's a
critical tube that will affect tone through the rest of the amp.

V2 is a cathode follower, IIRC. That means that some modern production
tubes won't live long, including the Sovtek 12AX7LPS. It also has gain,
and drives the tone stack. I find that a real Mullard does make a
difference. If you only want to spring for one Mullard (for V1), I
found like I liked the OLD version of the Ei Yugo long gray plate in V2.
They tend to be a bit brighter than some tubes, so I think it
compliments the mid heavy Mullard quite well. I am NOT talking about
the more recent production run that included silver plates. I don't
care for those at all. These are earlier versions, and are kind of a
smooth plate telefunken copy. On the down side, many were microphonic.

V3 is the least critical, being the phase inverter. Any strong non
microphonic tube will work. You can even experiment with tubes that
test off match between triode sections, to see if you like the
difference in tone. The better match at the output stage, the more you
cancel even ordered harmonics (but also, the more you cancel hum). So
just experiment with different tubes and use what sounds best to your ears.

At some point in time, Marshall shipped all US 2204's with GE 6550A
power tubes. I do not know what the cutoff date for that was. The vast
majority of 2204's are running EL34, that's what the amp was designed
for. However, if it shipped with 6550, the only proper way to run EL34
is to have the bias circuit modified. It's a very simple mod that any
decent tech can do in a couple of minutes. If you want to get really
picky, you should also move the negative feedback loop from one output
transformer tap to another (can't remember exact details at the moment).
There may also be a difference in screen resistors, I can't remember.
But the bias circuit change is critical.

The GE 6550A gives my 2204 more headroom, more bottom end, and when it
does distort it is LOUDER and more of a metal sounding distortion. The
EL34 is the traditional Marshall breakup.

If it shipped with 6550 and now has EL34, the only way to know if the
bias circuit is right is to measure bias voltage or static tube
dissipation. I have a bias probe that makes it easy. Unless you KNOW
the safety rules, refer this issue to a tech. It has high enough
voltage to KILL YOU. If you do have a tech check it out, please ask the
tech to report the output tube plate voltage back to you. The best
sounding 2204's (IMHO) run output plates at just under 400V. JCM800
versions have higher voltages on the tubes, and don't sound the same.

Your power tubes are most critical if you play LOUD. "One of these
days" I'm going to buy real Mullard EL34. But I don't crunch the power
tubes enough, since I rarely even find the opportunity to jam. I'm
currently using JJ E34L (which are different than EL34). I'm satisfied
with the tone, but I still lust after the real Mullard output tubes.
The problem is, so does everybody else, and they are VERY expensive.

If your filter capacitors and bias cap have not been updated, they are
past due. Have a tech put in new caps (unless you know all of the
safety rules and are competent to do yourself).

You own one of the best amps ever built. Get the right tubes in it, and
ENJOY IT!

Thanks Jim - I'l get the best I can ,,, its sn# 08988L ( if I'm
looking at the correct scrap of paper in my wallet ) - it had new caps
etc. a few years back but I'm gonna take it in for bias and a check up
when I get the new pre and power tubes soon ( my 56th birthday present
to myself ) Ed

Please do the input cables check to see if it's cascaded, and have the
tech write down the output tube plate voltage and QC sign off dates, if
there's still a sticker on the chassis. I'm interested in exactly when
Marshall changed circuits and power transformers.

Ok -ed
.

Relevant Pages

  • Re: JCM800 2204 Marshall Clone
    ... tubes, haven't decided about servicing or power tubes at this point. ... couple of ways to modify the bias circuit to work with the EL34 tubes. ... Mullard EL34 that gets mixed reviews. ...
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  • Re: Marshall JMP MKII new tube time -ping JIM
    ... I own a 1980 production JMP 2204. ... I *highly* suggest a real Mullard for at least V1. ... (another person was swapping tubes, I didn't know what I was playing). ... critical tube that will affect tone through the rest of the amp. ...
    (alt.guitar.amps)
  • Re: Marshall JMP MKII new tube time
    ... I *highly* suggest a real Mullard for at least V1. ... Mullards have a better sounding midrange distortion that just works in 2204's. ... I've verified this with my one ears in blind tests (another person was swapping tubes, I didn't know what I was playing). ... It's a critical tube that will affect tone through the rest of the amp. ...
    (alt.guitar.amps)
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  • Re: Marshall JMP MKII new tube time
    ... I own a 1980 production JMP 2204. ... I *highly* suggest a real Mullard for at least V1. ... (another person was swapping tubes, I didn't know what I was playing). ... critical tube that will affect tone through the rest of the amp. ...
    (alt.guitar.amps)