[R-390] Tubes glow blue

[Tim Shoppa]

>  I've seen guitar amp output tubes get to glowing a
> nice blue or purple when they are driven pretty hard

Yeah, I've seen 6146's in finals do this too especially when
they break out into a parasitic oscillation.  Not a "nice" glow
as usually some capacitors or chokes die shortly afterwards...

But the 6AK6 in question isn't a beam power tube, is it, it's just a
beefy pentode?  IIRC the tube is getting pushed pretty hard (if you've
solid-stated the power supply and not put in a dropping resistor
you may be pushing right up close to 300V on the B+, which is
the max plate voltage speced for the 6AK6.  I think typical
consumer equipment ran this tube closer to 150V.)

Tim.