[Heathkit] blue glow is gassy tube DX-40

[radioman]

your driver tube is gassy.. it should not glow blue.
gassy tubes tend to be wildly erratic.. and may produce more power than
spec..

your plate current surging..  could be due to loss of  grid drive.. or
loss of resonance or loading.. due to bad band switch contacts.

Get some Caig De-Oxit D5.. and use it on every switch contact, tube pin,
and also the coax connector.. do you still have an RCA plug ?  Is is
secure and reliable ?

Visually inspect your bandswitch contacts ... use magnification..and
make sure the fingers are actually contacting the wiper and are not
burned up from hot switching.

A bad dc blocking capacitor ( this will be a mica unit of 1000 V or
more) connected to the plate circuit of the 6146... can cause the
problems you discuss.

I presume you have replaced all the electrolytic capacitors in the power
supply ?
This is a MUST.

Check for any carbon paths which are indicative of HV arcing.
particularly on bakelite.. when it is exposed to moisture and dirt.

Larry W6WUH