#1
When I remove two tubes from my JCM800, I pull out the two outside or two inside tubes. Do I then have to change the output load? Its on 16 ohm as I have two 8 ohm speakers. Anything I should know?
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#2
Yes, double the impedance.
Or, in your case, just switch the output to 8 ohms.

EDIT: any particular reason you're trying this?

It's NOT going to make much difference in volume...
Last edited by Rutch at Nov 20, 2009,
#3
Well it makes the power tubes break up lower doesn't it?
Epiphone Les Paul Custom
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Ibanez Gio

JCM 800 4211

Vox 847
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Need more amps!
#4
Slightly.

In my experience, it's more useful to pull them so you don't go through as many tubes.

IE, buy a matched quad, run two, when they die, pop in your other two, which probably won't even need biasing (because they're matched).
It just doubles your mileage, so to speak.

There is a reduced level of volume at break-up, yes, but it's certainly not half!
On a 100+ watt amp, it's barely noticable.

Important note: JCM800s are tough little f$%kers, so try it out, but not all OTs like this set-up.
JCM2000s like to fry OTs to begin with, this only hurts...

Either way, you ought to re-bias, since plate voltage can spike a bit.
#5
Lol got that wrong, I have two 16 ohm speakers so the load is on 8 ohms atm. Do I change it to 4 ohm? How do I rebias?
Epiphone Les Paul Custom
Les Paul Copy
AX Copy
Ibanez Gio

JCM 800 4211

Vox 847
Marshall Guv'nor 2

Need more amps!
#6
Ok, make sure you've got that speaker load right!

2 x 16 ohm speakers wired in parallel = 8 ohms.

If you pull two tubes, set the amp to 4 ohms.

^^^this is probably what you have (it's been a while since I've seen a JCM800 combo, sorry!)


This gives you an 8 ohm load into the 4 ohm tap, doubling the resistance, which is halved by the yanking of half the tubes, setting things straight.

(Note for you tech-sticklers: I know, that's not really what's going on, but it's an easy way to keep track for someone who's not thinking about the how & whys...)

As for re-biasing, well, that would be a tutorial in and of itself!
Put it this way:
Try pulling the tubes, and go nuts with it.
If it sounds GREAT, and you KNOW you're going to keep it that way, then have a tech check it to make sure everything's kosher.
Otherwise, just keep swapping back and forth, it won't really do any harm, it just might not be functioning at it's absolute best.
#7
If I can't rebias should I just put the tubes back in and leave it alone untill I take it to a tech?
Epiphone Les Paul Custom
Les Paul Copy
AX Copy
Ibanez Gio

JCM 800 4211

Vox 847
Marshall Guv'nor 2

Need more amps!
#8
Quote by Rutch
Ok, make sure you've got that speaker load right!

2 x 16 ohm speakers wired in parallel = 8 ohms.

If you pull two tubes, set the amp to 4 ohms.

^^^this is probably what you have (it's been a while since I've seen a JCM800 combo, sorry!)


This gives you an 8 ohm load into the 4 ohm tap, doubling the resistance, which is halved by the yanking of half the tubes, setting things straight.

(Note for you tech-sticklers: I know, that's not really what's going on, but it's an easy way to keep track for someone who's not thinking about the how & whys...)

As for re-biasing, well, that would be a tutorial in and of itself!
Put it this way:
Try pulling the tubes, and go nuts with it.
If it sounds GREAT, and you KNOW you're going to keep it that way, then have a tech check it to make sure everything's kosher.
Otherwise, just keep swapping back and forth, it won't really do any harm, it just might not be functioning at it's absolute best.


Ok cool. I'm fine with most physics, just not the intense workings of a tube amp lol. Cheers
Epiphone Les Paul Custom
Les Paul Copy
AX Copy
Ibanez Gio

JCM 800 4211

Vox 847
Marshall Guv'nor 2

Need more amps!
#9
Quote by GuitarGetsMeSex
If I can't rebias should I just put the tubes back in and leave it alone untill I take it to a tech?


No, you'll be alright if you just want to check it out and see what kind of difference it makes.

In the long-term, though, if you decide that you want to keep it that way, then yeah, have the bias checked (but not necessarily re-biased, it very well may need no adjustment at all, but it's good to play it safe in the long run...)