mark I 60/100 watt switcher

[guy_incognito]

small question, but when using a Mark I switched to the 60-watt position, are all four of the tubes in action, or just two? if the answer is the latter, then should this be visible? (it looks to me like they're all on either way)

(the reason I'm asking is that I'm having a problem with weird noises when the reverb is in use, I suspect it may be a microphonic tube, and shuffling them around hasn't done a thing)

 

[PRS-Ian]

Only two are operating in the 60W mode, however all four will 'glow' as power is still being drawn by the heater elements.
Ian

 

[guy_incognito]

which ones are the active ones in 60 mode, then?

 

[James]

the outer two

 

[guy_incognito]

thanks

and another followup question: since my tubes are old and need replacing, if I only plan on using it on the 60 setting, would it matter if I only replaced the two outer ones rather than all four?

 

[stokes]

If you are going to use the 60 watt only you can get away with only replacing the outer two.

 

[enigma]

Good info! I've just been blown away, again, how awesome this amp is. The other day when I plugged in my Gibson Blueshawk this little, powerful amp just produced awesome, liquid-shimmering tone!!!