[ARC5] Receiver Voltages.

[Kenneth G. Gordon]

On 29 Apr 2015 at 14:30, Dennis Monticelli wrote:

> FYI.  I have a low noise HP precision power supply that is variable from 0 to
> 100V.  So I did do some experimenting in the past and it agrees with the
> general consensus of the thread.  The performance of a stock (only bad caps
> replaced and no re-biasing) continuously degrades with reductions in B+.

It seemed to me when I did this experimentation, that the audio output 
dropped drastically as voltage was reduced, but sensitivity remained pretty 
constant.

>   I
> noticed minor degradation down to 100V, a pretty significant drop off below 60V
> (including BFO start-up issues)

As it is, the BFO operates on a plate voltage that is between 72 and 85 VDC 
at most when the HV is 250 VDC, and what's even worse, the HFO's normal 
operating plate voltage is between 13 and 29 VDC (!). Therefore, if the HV is 
100 VDC, the BFO is operating with a plate voltage of about 30 VDC while 
the HFO is operating with a plate voltage of 12 VDC

> and what I would consider an unacceptable
> performance dropoff at the 28V filament level.   When running off a clean bench
> supply I like to use 90V and when running off 9V batteries, something like 60 to
> 70V.   Current draw at 60V is only 10mA because the leaky caps had been
> replaced.  Re-biasing would definitely improve the low B+ situation if one were
> to do that.

Well, as it is, with 250 VDC HV, the screen voltage on all tubes is 125 VDC. 
At 100 VDC, the screen voltage would be 50 VDC.

According to the tube manuals, the 12SK7 can "use" a screen voltage as 
high as 300 VDC, as long at the current is minimal at that level. When 
operated as a Class A RF amp, the screen voltage should normally be 100 
VDC.

For the 12A6, again according to the tube manuals, the screen voltage 
should 250 VDC when used as a Class A amplifier at a plate voltage of 250 
VDC.

In my opinion, one can easily run the receivers effectively on 100 VDC as 
long as the screen voltage connection is moved to the HV end of the divider 
resistors.

As I said earlier, if you don't readjust the screen voltage connection, the 
receivers work best at a HV input of about 170 VDC or so, and even then, I 
see nothing at all wrong with moving the screen voltage tap to the high end.

It is a simple, completely reversible mod, which allows our receivers to 
operate very well at much lower voltages.

>  Just rebiasing the screens would make a notable improvement. 

Well, I can assure you from my experiments that doing so makes a very 
significant improvement in the operation of the receivers at low B+ voltages.

Ken W7EKB