JCM900 load question

[tribi9]

I got a JCM900 that keeps on blowing the valve fuses. I'm going to replace the tubes but I want to make sure my impedance is right.

At the back there's a switch for high and low power and a switch for impedance that says 8 on one side and 16 (4) on the other side.

I got 2 16 ohm speakers wired in parallel so my total load would be 8 ohms, right?

My question is if I run the amp on low power (2 power tubes) What does the load requirement change to?

[breakfastbuddy]

load with 2 tubes in should be double , so then you set it on 4 ohm out to 8 ohm speaker load , with all 4 tubes it will be on 8 ohm out to 8 ohm speakers . check the bias voltage , it seems like you have a missmatch there .

[paulster]

I can't remember whether the JCM900 series lift the cathodes for their half power mode or whether they have a pentode/triode switch like the Jubilees do, that are a similar vintage.

If it's a pentode/triode switch then you don't need to change the impedance, but if it's lifting the cathodes then you want to do exactly what breakfastbuddy suggested and run the amp set to half the impedance of your cab.

[tribi9]

I believe it has a pentode/triode switch. So I guess I can leave it the same.

The amp had the improper fuses it had a 1A and a 500mA fuse in it. Both were blown.

I just got 4 replacement fuses and got the tubes tested on a friend's tester.

The tubes tested strong so I'm thinking there's got to be something wrong with the speaker load.

Also the amp has matched tubes but 2 of them say 35 and the other 2 say 43. Could that be a problem?

[tribi9]

Ok, so I did a test...

I removed the tubes and put the 2 new fuses. The fuses didn't blow up.

So I put back the old tubes, this time the fuses didn't blow and the amp seems to be fine. I put the switch on the high power and left the impedance speaker on 8 ohms.

The only thing that seems weird and I don't know if it was a hallucination brought in by all the stuff I smoked back in the day... I didn't get a voltage reading on the cathode??? Cathode voltage in relation to ground read "0"

[jjman]

Yes, cathodes are grounded since it's a fixed(grid) bias setup. The power level switch is pentode/triode.

[tribi9]

Thx guys. It's been working fine since the new fuses.

Cheers