Marshall Amplifiers FAQ

[Bacchus]

I thought if there was only one valve in the power section, then it couldn't be biased. I thought it was a matter of biasing tubes to each other.

[Mike]

No, biasing is setting the operating voltage of the tube to be in a "good operating zone" between cut-off and saturation. This is based upon the Plate Power Dissipation of the valve and the plate voltage and is used to set the appropriate cathode current to an appropriate value to put the valve into the operating zone to dissipate betwen 70% (hot) and 50% (cold) of the Plate Voltage (Class AB pushpull amplifiers) or 90% in Class A single ended amplifiers.

Your amp is Class A.

[Bacchus]

Your amp is Class A.

MOJO!

I really fucking hate electronics. I have like a mental block to it or something. So, should I get my amp biased?

It' sounds fine to my ears, which I suppose is good enough.

[Mike]

No, you'll be fine. They'll have designed the circuit to accept a power tube and cope with the differences in tolerances of the manufacturing process - this is simple to do with one tube in a single-ended configuration.

Two or more tubes arranged in a push-pull (one tube amplifies top half of the wave, the other tube does the bottom) is non-trivial, but it can be done (as it is in Fender Twins). The rule of thumb is that if you amplifier has a bias pot that when you change the power tubes you should check the bias and adjust if necessary, and for safety's sake also after a month or so of breaking them in.

[JJLipton]

I think fixed-bias is a pretty good idea, not too much hassle when changing tubes and all. I know mesa/boogie does this with some of their rectifier amps. But i suppose if you're good with electronics, or know how to set an amp bais then changing tubes wouldnt be any problem at all because you can avoid tech fees.

[kim]

ok, i'm not a big marshall fan, and this ain't even about a guitar amp, but i'm gonna mention it here anyway, i tried out this all tube mofo at a shop when i was in brussels thursday for an isis gig;



over an ampeg 610.

<3 <3 <3

it was... fantastic, even with a mex. p bass, i want a toob head, but my back doesn't (the thing is huuuge and a bitch to lift i'm sure.)

so from now on; marshallvba400 (drool))

i'm not even sure wether that standard ampeg classic is number one on my list anymore.

[Mike]

Awesome! Glad you liked it.. I didn't even know Marshall made tube bass heads.

you're not wrong about the weight, my JVM and 6100 heads are heavier than a 4x12"

[mattyb]

i'm sure it's been said a million times but as you've noticed JCM800's are the standard rock band amp, along with ales paul being the standard guitar. I have a Marshall Bi Chorus 200. All i know about it is that it's old, at least 100w and can get really loud. Alot of the bands around here use whatever they can afford, so there's alot of Crate's and whatnot. My opinion crates are shit, the jacks always break which is anoyign and they have decent tone but nothing special.

[Justin]

Remember when the ads about the 65 watt combo Marshall said "with 65 watts you can keep up with the drummer!" I didn't know the drums moved wtf. I'm not dragging an amp behind me chasing after some dummy that can't stay in one place.

[BobArsecake]

wtf? I've just re-read this thread and I missed Paul's amp?! I'm retarded D: That's ace! Weren't there more pages by the way that got deleted?

[FLICKOFLASH]

Great Marshall info here:
http://www.drtube.com/marshall.htm

I think you want this:
2205 JCM800 Split Channel Reverb, 50W head

Marshall Info

[Mike]

Stop WHORING YOUR WEBSITES.

I checked out that forum - it's not great.

[NickS]

Remember when the ads about the 65 watt combo Marshall said "with 65 watts you can keep up with the drummer!" I didn't know the drums moved wtf. I'm not dragging an amp behind me chasing after some dummy that can't stay in one place.

It could be a marching band, but I think you misunderstood. Drummers speed up, remember the old joke:

Q. How do you know a drummer's at the door?
A. The knocking speeds up and he doesn't know when to come in!

The Marshall combo allows you to speed up with the drummer, where ordinary amps would leave you fixed to the original tempo.

On topic. Hello, 118? Can you tell me what's wrong with this Marshall Valvestate? The light comes on but there's no sound.

[Mike]

1. Replace Preamp valve
2. Inject signal into FX return to check power amp
3. Inject Preamp out into another amp to check preamp.
4. Check Speaker by connecting to another amp.

[NickS]

1. Replace Preamp valve

All 3 channels use the valve? OK.
Thanks, Mike, I'll get around to trying these things when I get it home. Or maybe tomorrow, as we're probably eating out tonight.

[Mike]

I don't know about the topology unfortunately mate.

On top of that I would check all fuses when you have the chassis dropped and also check continuity after the input jack etc. All the simple stuff.

[NickS]

Don't forget the bleedin' obvious - plug in the speaker lead. Works fine.

Surely my colleague didn't overlook that?

[Mike]

ROLF

[NickS]

Yup, he overlooked it. When I mentioned the speaker lead he got that "Oh shit, was I supposed to pick up my daughter from playgroup an hour ago?" look. Then he was embarrassed.

[Justin J]

what's the difference between the jmp 2203 and the jcm800 2203? and what would be a reasonable price to pay for either one?
i played the jmp at guitar center and really liked it, but is $1100 too much?