Re: JCM800 2204 Marshall Clone

ed s wrote:

On Nov 8, 5:29 pm, Jim <as...@xxxxxxxxxxxxxxxxx> wrote:

Jim wrote:

BobF wrote:

"Jim" <as...@xxxxxxxxxxxxxxxxx> wrote in message
news:13j70s46qvki9b7@xxxxxxxxxxxxxxxxxxxxx

Charmed Snark wrote:

A friend of mine, emailed me with this link. The guy's done a
wonderful job of building this amp:

http://www.joepopp.net/jcm8002.htm

Snark.

My question: Did the guy know about the JMP 2204 circuit? It uses
lower voltages for a browner sounding distortion. All that effort,
he should've built the more desirable amp.

Maybe he built the one most desirable to him

Maybe. I E-mailed him, and I share his response if I get one.

In my experience, a lot of guys just don't know about the JMP. If they
do know about it, they tend to think it's only a cosmetic difference.
It's not.

Also, let me add that until I posted the factory voltage chart that came
with my JMP 2204, I had a couple of online techs telling me that the
correct voltage was 450-460 range. I scanned my chart to prove that the
JMP *is* different. So, if techs are confused, there's no shame in
guitarists not knowing the difference.

I'm told that the JMP 2203 (100W version with four output tubes) does
have the higher voltages, just like both the 50 and 100W JCM 800. Same
with the non-master 50W four input Marshalls. The 50W JMP master volume
2204 was the exception.

Now, taste is highly individual, and there is no "right or


wrong." But of the people that I've talked with that owned both
versions? They preferred the JMP.


Jim - I have a JMP MKII 50w master volume head. What is the "Exception
you mention" .It has the EL34's - do you know the bias specs? And any
other tips I should know ? - Thanks Ed S.


The exception is the lower voltage power transformer.

Re-cap:

The JMP MKII 50W 2204 Master Volume head has different power transformer specs than the JCM 800 2204. I believe that the early JCM 800 has the same circuit, but different transformer. Then there were some changes in the power supply filtering (which I doubt you can hear) along with the circuit board mounted pots and jacks.

The amp came with resistor sets to bias the GE 6550A. There are a couple of ways to modify the bias circuit to work with the EL34 tubes. IIRC, I ended up putting a resistor "piggy back" on top of another one, and got a bias range that allows me to dial in 6550 -or- EL34. But this is trial and error, and a tech needs to do it.

When is the last time you changed power tubes? The next time you do it, take it to a decent tech and ask him to check the bias. If it's not right, it will be inexpensive for him to get it right. Most of the time is pulling the chassis. Then he'll adjust a trim pot for bias. If he can't get in the right range, he'll swap (or add) a resistor or two, and you'll be dialed in.

Also, if you have the original filter caps and bias cap, I'd change them. Electrolytic capacitors have a finite life span. If it's on the bench, might as well do them at the same time.

On power tube choice: "Some day" I'll install Mullard power tubes. But they are very expensive, and I honestly end up relying more on preamp distortion than I'd like to (power tube crunch is pretty loud in the home environment). I currently have JJ E34L installed (different than JJ EL34). They sound fine to me. They also now make a "reissue" Mullard EL34 that gets mixed reviews. It's made in Russia, supposedly to sound like a real Mullard. But those who compare seem to have comments from "no way" to "close, but no cigar."
.

Relevant Pages

  • Re: JCM800 2204 Marshall Clone
    ... haven't decided about servicing or power tubes at this point. ... JMP *is* different. ... The amp came with resistor sets to bias the GE 6550A. ... couple of ways to modify the bias circuit to work with the EL34 tubes. ...
    (alt.guitar.beginner)
  • Re: JCM800 2204 Marshall Clone
    ... haven't decided about servicing or power tubes at this point. ... JMP *is* different. ... The amp came with resistor sets to bias the GE 6550A. ... couple of ways to modify the bias circuit to work with the EL34 tubes. ...
    (alt.guitar.beginner)
  • Re: Can I leave the resistors in after biasing my amp?
    ... biased AB1 amp: ... you shouldn't be adjusting bias if you aren't familiar with Ohm's ... tube distortion" by biasing hotter (i.e., less negative bias voltage), ... IOW, it is a "GIGO" scenario, and the power tubes will be putting out ...
    (alt.guitar.amps)
  • Re: Question about grid behavior with triodes
    ... The bias of 2.4V suits having a healthy ... has a gain of around 17. ... coupled to the power tubes which are cathode biased now at about 70mA. ... But would there be alot of grid current driving the second stage? ...
    (rec.audio.tubes)
  • Re: Marshall TSL 602
    ... Amp runs cooler to touch; sounds like good old Jethro Tull ... But even if you rewire the circuit to that of a JMP 2204, you still need correct voltage on the plates. ... The JMP 2204 only had about 395 volts on the power tubes. ... Early Marshalls can be way higher, and later ones still in the 450V range. ...
    (alt.guitar)