Marshall 1960 AX Cabs with 25W Greenbacks

D28boy

I seen a lot of good reviews for these but I'm concerned that using the cab with a 100 watt head might be a bit too much.. Some ads suggest using a power brake ...should I be worried. Btw I'm never anywhere near playing at 11  lucky to get to 5 ! 

ICBM

If you're not driving the power stage into distortion there's no risk really.

What 100W head is it? If it's a DSL/TSL or JVM they put out much less maximum power than an old Super Lead or 2203.

You can always drop the amp to 50W by removing two power valves and setting the impedance to half that of the cab, if you're worried.

D28boy

Thanx ICBM I thought it would be ok... It's a JCM 2000 DSL btw from 1998

Have to admit I'm no tech with amps but I'm really interested about what you said about running on two valves.  I've always worried about blowing a valve at a gig and so the band carries a spare head. I have heard people talking about running a 100 w as a 50 by removing two tubes but thought this must surely damage the amp...So you're saying if I was using my 1960A which is a 16 ohm cab if I lost a tube I could (switch off first obviously)  remove the dud tube plus any other (?) then re-set the ohm on the amp to 8 and that would be ok?

If so that's very handy to know

ICBM

Yes, it's safe to run a fixed-bias amp with less than the full number of power valves. (Not safe with a cathode-biased amp with a shared cathode resistor, which is most of them, eg AC30.)

On a DSL, the best two valves to remove are the one closest to the power transformer (call it "4") and the second one from the other end (call it "2"), if that makes sense - these are the valve positions that most often cause the arcing which can damage the board on these amps.

Then set the amp to 8 ohms to run the 16-ohm cab. Unfortunately for maximum reliability like this you need to bypass the ground switch in the 16-ohm jack and parallel the impedance selector as well, which are jobs for a tech - and worth doing anyway, if you're having the amp worked on for any other reason.

If you don't reset the impedance it will still work OK but you may shorten the valve life slightly and the tone will be more compressed. Impedance matching is only really important if the power stage is fully cranked up, in fact - at lower power levels it doesn't matter much at all.