[Heathkit] Screen dropping resistors (was DX-100B on Phone - bad audio distortion /oscillation)

[Ron Youvan]

   Rick Poole wrote:

> In the DX-100B, the PA screens get their voltage from the 800-VDC PA
> plate supply, through two 10K ohm 10 watt resistors in
> series.  (Nothing connects to the junction of the two 10K resistors
> so why they couldn't have used a single 20K... but, I digress...)

   Every resistor has a Voltage rating this screen resistor would need to be rated at 800 Volts.
The 20 Watt resistors that they could get may not have been rated to that Voltage.

> The hV measures 840 volts on my Simpson 260 (a little higher than
> shown on the schematic) and the PA screen voltage is 187 volts,
> almost exactly what is shown on the schematic.

> The power dissipation for that voltage drop calculates out to be 21.3
> watts, or 10.65 watts per 10-watt resistor.

> Just a tad too much.

   If you were operating it from 117 Volts (the spec) it probably would only dissipate 20 Watts.
This resistor has two tube screens and a 6AQ5 clamp tube (in my DX-100) drawing current are you 
making this measurement with the tube conducting or not?  Is the clamp tube controlling this 
Voltage?  (there is a "clamp control" - in my DX-100)
   The drive changes the screen Voltage and therefore the screen Voltage, lowering it.
(increasing resistor dissipation)
   A Lack of drive causes the clamp tube to conduct, dropping the screen Voltage significantly.
-- 
    Ron  KA4INM -    Ron  KA4INM - Did you know ...
                     ... that no-one ever reads these things?