[R-390] Tube questions

[James A. (Andy) Moorer]

The 7788 is a SHARP cutoff pentode. I can see why you like it - the 
transconductance is quite high. In fact, I don't think I have seen 
transconductance so high. But, that notwithstanding, it shuts off at a grid 
voltage of -2 to -5 volts. The 6BA6 shuts off at -40 volts. The 6DC6 shuts 
off at somewhere in between. If you substitute the 7788 for either of those, 
it will mess up the entire AGC system. Every time I have ever tried this 
kind of thing, I have regretted it - it generally results in distortion on 
strong signals (because the tube is running very close to shutoff, and with 
a strong signal, it will go into shutoff for the negative-going part of the 
input waveform, resulting is audio distortion).

As for the 6BY6, it looks similar enough to the 6BE6 that I don't think you 
will notice any difference. Again, they have somewhat different grid 1 
voltage ranges which could produce some saturation problems on the SP-600 
receivers that inject the AGC voltage into the first grids of the second 
mixer tubes - some models do this, such as the JX21 and JX26. The tubes are 
similar enough so that I don't know that there will actually be any audible 
difference one way or the other. I don't see that it could do anything 
particularly good (unless you happen to be out of 6BE6 tubes).

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



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

> Hi Gang,
>
> Has anyone used the 7788 tube in a front end of a R390 or SP600 or other 
> HF receiver?  (Yeah, I
> know it's a nine pin.)  If so, how did it work out??  Does anyone want to 
> sell some at less than
> epay prices?  I know they are good tubes as Tek used them in their scopes.
>
> Has anyone subbed the 6BY6 for the 6BE6 mixer?  If so, hod did it work 
> out?
>
> Regards, Perrier
>