Tube Amp Question

[gf]

Hi,

I have a Marshall JTM-60 guitar amp which has two EL34 and four 12AX7.

Today after playing for a while I checked the tubes and one of the EL34 was not lighten up (I think that's the term). The amp, however, is working fine right now (with just one tube). It is possible for the amp to run fine with just one power amp tube ?

Thanks

[brett]

Probably not.  It'll risk damage your amp.  That's why you should NEVER pull a power tube from a live amp (I think the output transformer will be at risk, but the experts will know more).
Some checks:
The valve should have 5 to 10 ohms resistance in the heater element,
there should be a couple of megohms betweent he element and all other connections (especially the heater-cathode resistance, where the insulation is most likely to fail).  
If you have a standby switch, turn it on, but leave the amp OFF, pull the valve and check for 6.3V in the tube socket where the heater connections are.

cheers

[gf]

Thanks brett for the help, I've also found this page, which has info about a similar topic:

http://www.londonpower.com/faq.htm

[Stevo]

I think according to Mesa I can run on one power tube F-30 ...But put another in there and see if that one works...Mesa I think says it is a way to see which power tube is bad with micro phonics... I may be wrong but it is safer to put one in there maybe time for a re-tube :?