[R-390] Sub 6AU6 for 6BA6?

[James A. (Andy) Moorer]

It won't make a lot of difference, but it will make a little difference. The 
amplitude of the oscillation is determined by the characteristics of the 
tube. In general, the oscillator will make as big a signal as it can, until 
the loop gain starts falling below the critical value due to it running into 
saturation. With the 6BA6, that is a "soft" saturation, whereas with the 
6AU6, that is a "hard" saturation. You could expect the 6BA6 to produce a 
slightly more pure sinusoid, whereas the 6AU6 will have somewhat more 
harmonics. This might produce some more imaging or IM distortion. With the 
powerful IF filters in the 390A, it is probably a wash - you probably won't 
be able to measure the image feed-through, but you might on some bands in 
some tuning ranges (how's that for qualified? Think I could be a 
politician?)

James A. (Andy) Moorer
www.jamminpower.com

----- Original Message ----- 

> In several places (notably VFO and BFO oscillators) in a cow-orkers
> R-390A, I notice that there's 6AU6's instead of 6BA6's.  I know that
> the 6AU6 is a sharp cutoff pentode, and the 6BA6 is a remote cutoff
> pentode, and suspect that in an amp (especially AGC-controlled amp) or
> mixer that there'd be some change in dynamic range/AGC characteristics
> through such a substitution which may not be good.  But in a simple
> oscillator, does it make much if any difference?  (I know that probably
> the VFO has to have its endpoint adjusted, but that's true even when
> putting in a different 6BA6 so is "no big deal").
>