[Amps] SB-200 Problem

[Joe Subich, K4IK]

> Both tubes light...I have an "old" set of 572B's that also 
> exhibit the same symptom but they were old and I was getting 
> suspect of how much life they had left which is why I replaced 
> them.  They went out of service working fine,
> though.  I'll check the tubes with an ohmmeter to be sure.

What brand of tubes do you have?  Some brands (Svetlana) required 
much higher cut-off bias and were unreliable in certain amplifiers 
unless the bias circuit was modified to provide a higher voltage.  

You might check your bias circuit to make certain the tubes are 
properly cut-off unless the amplifier is in transmit and connected 
to proper input/output loads. 

73, 

   ... Joe, K4IK