#1
Hey, I have a quick question about tubes.

I have a Marshall JCM 2000 DSL 401. Right now I only have two tubes in it that are biased. There are supposed to be 4. I've heard it's ok to play with just two tubes though. As long as they are the inner two or outer two. Is this right? I'd only be doing it until I could replace my tubes.
Gear:
Gibson SG Special Faded
Marshall JCM2000 DSL 401
#2
If you halve the impedance for the speaker. Itll then be running at halve the wattage.

Its not safe though, especially with cheaper made amps, i blew out my Laney VC30s output transformer doing that.

Its much cheaper to shell out 40 quid for a matched quad of JJs than 100 to get your OT replaced.
#3
Pulling two tubes out demands doubling the impedance. If you have a 8Ohm cab - plug it in a 4Ohm inlet. or 16 into 8.
#4
I have a matched set of tubes. I just don't have a way to bias them right now. Would it be ok to use them for a bit unbaised until I'm able to bias them?
Gear:
Gibson SG Special Faded
Marshall JCM2000 DSL 401
#5
If there were tubes in the amp before and the amp was operational - you can get away pretty well if you just don't touch the bias control and put the new valves in.

When you first turn on your unbiased amp with new tubes - watch the tubes themselves as you switch from "standby" to "on". If you notice one or both of them slowly (or not so slowly) go red-plated - turn off the amp immediately.
If there was only one overheating valve - you can swap them and try again.
If The amp works stable for a couple of minutes - you're quite safe.
Last edited by Ace Carson at Aug 20, 2011,
#6
Ive never bothered biasing an amp.

As long as they dont redplate theyre fine. Usually most companies bias a bit cold from the factory, so if you havent biased it since it should be okay (FYI. Im not sure, but i think the DSL might be cathode biased). The amp wont sound its best this way but you get a longer life out of them and less gain.
#9
Quote by Ace Carson
DSLs are fixed biased.

Fixed bias just means that a fixed DC voltage is applied to the grid. That voltage is normally adjustable. The DSL401 is adjustable. Here's how you do it:
http://www.drtube.com/schematics/marshall/dl40-68-00-iss2.pdf
It requires pulling the chassis out and working on a live circuit carrying hundreds of volts so don't do it unless you know how to work with high voltages.
Gilchrist custom
Yamaha SBG500
Telecasters
Randall RM100 & RM20
Marshall JTM45 clone
Marshall JCM900 4102 (modded)
Marshall 18W clone
Fender 5F1 Champ clone
Atomic Amplifire
Marshall 1960A
Boss GT-100


Cathbard Amplification
My band
#11
I didn't say it isn't - I just said it was fixed. Actually only fixed biased amps are adjustable. :P
#12
Quote by Ace Carson
I didn't say it isn't - I just said it was fixed. Actually only fixed biased amps are adjustable. :P

To a noob your statement would have been very misleading. Next time try to be more comprehensive with your replies. You and i may know that fixed bias amps are the adjustable ones but the term "fixed" tends to confuse.
Gilchrist custom
Yamaha SBG500
Telecasters
Randall RM100 & RM20
Marshall JTM45 clone
Marshall JCM900 4102 (modded)
Marshall 18W clone
Fender 5F1 Champ clone
Atomic Amplifire
Marshall 1960A
Boss GT-100


Cathbard Amplification
My band