#1
How do I re bias my JCM 800 after changing the power tubes? Is this hard to do because there isn't anybody with in 100 miles of me that works on amp

Thanks
#5
http://www.dreamtone.org/JCM.htm#14

http://www.webervst.com/tubes/calcbias.htm

There's nothing unique about the 4210/2205 (pretty sure that's what you have), it's the same procedure as nearly any tube amp.

If you're sticking to the same power tubes (6550 or EL34), then a re-bias really isn't necessary. I know many people who have never re-biased their tubes and have no issues at all.

But if you're comfortable enough inside an amp it can't hurt to do it.
Last edited by earthwormjim at Jul 19, 2008,
#6
I use a Weber Bias-Rite. I've found it to be absolutely indispensable. Go to webervst.com and look into the products- find the one that suits your needs the best (I use the BR4, I think). I've even spoken with Ted on the phone on a couple of occassions- Very helpful, very polite, very professional.

I bought mine a couple of years ago, when I had just acquired and needed to re-tube a 1979 Marshall MKII Master Volume Lead series 100 watt (Model 2203)- which is the amp that became the JCM800 in the 80's.

Even if you are sticking with your current class of tube (6550 or EL34) it's always a good idea to make sure that your tubes are biased appropriately. Too low (cold) and your amp will sound lifeless. Too hot, and your tubes will wear out quicker- but the amp will sound pretty hot. Too hot, and your just asking for trouble, IMO.

When I'm using the amp only for hi gain (distorted) output (ie, no distortion but what the amp has to offer) I prefer to bias my tubes to proper tolerance, and then go just a bit hotter. In that case, I don't mind re-tubing a couple of times a year.

Currently, I'm using the low-gain input only (using a Rocktron G300 Utopia processor for distortion), so I've biased my current tubes to proper tolerance.
#7
do you guys think it would be benificial to replace anything inside the amp? Capacitors or anything like that or should I just leave it alone?
#8
Capacitors don't really wear out. About the only wear items are the tubes and possibly the transformers.

If you're amp is functioning fine, other than the tubes needing replacing, leave it alone.

However, if you do gig this amp, I'd suggest unsoldering the input jack from the pcb and hooking it up with wires. It's just a safety measure in case the input cord gets jerked or yanked on, and could potentially crack the board. The same can be done for all the control pots.
#9
Quote by earthwormjim
Capacitors don't really wear out. About the only wear items are the tubes and possibly the transformers.

If you're amp is functioning fine, other than the tubes needing replacing, leave it alone.

However, if you do gig this amp, I'd suggest unsoldering the input jack from the pcb and hooking it up with wires. It's just a safety measure in case the input cord gets jerked or yanked on, and could potentially crack the board. The same can be done for all the control pots.

Caps DEFINATLEY wear out, they are the first things to wear out usually. Same with resistors, jacks, pots and more.

Preamp tubes and transformer rarely wear out. Eventually they will.