A (slightly) technical question

[Cack]

Right just tell me if this is right or wrong.
I have a JCM900 100w combo. As far as I know taking out the outer 2 tubes cuts it down to 50 watt. On the back of the amp there is also a half power switch. Would this make the amp 25 watt or am I just crazy? 25 watt for home practice would be pretty nice.

[Hotrod]

Re: A (slightly) technical question

I could be wrong, but I don't think it will work like that. I believe the half power switch will just effectively "remove" two of the power tubes. If you've already done that, I don't think it will do anything. I think Oklastrat has dissected a JCM 900 and probably knows exactly what it would do though.

[toejam]

Re: A (slightly) technical question

I thought it was the two inner tubes to remove, but I could very well be wrong.
What do I know? I still use an old Marshall Valvestate that I haven't even needed to change the preamp tube in yet! [img]images/icons/grin.gif[/img]

[jacksonaxes]

Re: A (slightly) technical question

You can remove the two inner or outer tubes to cut power in half.

[Chuckracer]

Re: A (slightly) technical question

It doesnt matter which pair you pull, either the two inner or the two outer will do the same thing.

The half power switch may put the tubes in triad mode as opposed to pentode mode. Pentode means all five of the elements within the tube are working to give full output, where as triad cuts two of the elements putting the tubes in roughly half power. If this is the case, and I think it is on the JCM900s, then two tubes in triad mode would indeed give roughly half of the 50 watt output.

When running two tubes of the four, remember to set the speaker impedence to half of normal.

Try it!

[Hotrod]

Re: A (slightly) technical question

The half power switch may put the tubes in triad mode as opposed to pentode mode. Pentode means all five of the elements within the tube are working to give full output, where as triad cuts two of the elements putting the tubes in roughly half power. If this is the case, and I think it is on the JCM900s, then two tubes in triad mode would indeed give roughly half of the 50 watt output.

<font size="2" face="Verdana, Helvetica, sans-serif">I knew someone would come along and blow my theory out of the water. [img]graemlins/laugh.gif[/img] Chuck probably knows a lot better on this than me. Thanks Chuck [img]images/icons/tongue.gif[/img]

[Chuckracer]

Re: A (slightly) technical question

Well, I'm just running off of memory here. Its been a long time since I owned a Marshall head. Pete would indeed know the answer better than I, as would many others here.

[kunkler99]

Re: A (slightly) technical question

Chuckracer is right, it puts the tubes in triad mode on the jcm900. I am not 100% sure about taking two tubes out and then flipping the switch to the 50 watt setting. It should by all means make it 25 watts.

The only thing I question is which speaker jack you would hook to. I believe this could only work on a 16 ohm speaker cabinet. You have to cut the ohms in half to run 50 watts so then you would have to half that again and use a 4 ohm output. Correct me if I am wrong please.