[R-390] RE: Help with R-392

[Tom Norris]

>----- Original Message -----
>  >
>  > If I recall correctly, the R-392 has 2 input pins for the 28 volt source;
>>  one supplies the heaters and t'other supplies B+. (If I recall
>incorrectly,
>>  maybe it could be made so with minimumodification).
>>
>>  Some have reported improved performance by upping B+ to 35v or so while
>>  leaving filaments at 24v.  If one were to go that dual power supply route,
>a
>>  low quality 24v supply (maybe even AC?) could heat the filaments and a
>well
>>  filtered supply at higher voltage could provide B+.
>>
>
>The filament current is about 1.5 A and the plate current is about .5 A.
>They don't need to be regulated, but any 60 Hz hum will go right into the
>audio. At startup, the filaments will pull up to 5A or so for a few tens of
>milliseconds - long enough to blow any normal fuse.
>
>I fiddled around with separating the filament and plate voltages. My
>conclusion is that you really shouldn't set the plate voltage over about 32
>volts. It starts sounding rough and distorted above that. I pretty much
>ended up setting it at 30 volts and leaving it.

I hit upon nearly the same combination of voltages as Andy. Since I had the
bright idea of separating my plate and filament back in '79 or so, I have not
ever had to replace a tube. I have ran my filaments at 24 volts and plate at
either 30 or 32 depending on the incarnation of the power supply. Always
regulated both on mine, with my filaments a little "on edge" current wise
so they start up slow. Time now to get those linear power supply 
bricks I bought
a while back into use for a R-392 power supply - will set them to the same
voltage as I have been running over the years with no problem.

I tell you, though, the higher plate voltage *works wonders* and doesn't seem
to add any odd mixing or distortion products. Darned thing acts like 
a full grown
R-390 with around 30 volts on plate. Never saw much in the way of a penalty for
running my fils low though, probably good for a nice long life. I'm pretty sure
I've not changed *any* tubes in this Western Electric unit since I 
got it in '79.
Came with the SS audio module too. My others I have had have given me good
service too, it's just a darned good rugged little RX.


73

Tom KA4RKT