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

[Rick Poole]

At 09:57 AM 12/16/2011, Ron Youvan wrote:

 >>>>>
>If you were operating it from 117 Volts (the spec) it probably would 
>only dissipate 20 Watts.
<<<<<

I'm running it from a measured 119 volts AC so it probably won't make 
any significant difference.

I do notice that the HV is considerably higher than the schematic 
indicates (a measured 840 VDC vs 740 on the schematic), don't really 
know what to do about that.

Anyway, it's usually not a good idea for long-term reliability to run 
a 10-watt resistor at exactly 10 watts.  For one thing, those power 
ratings are in free air, usually at 25 degrees C (77 degrees 
F).  Where these resistors are located is very closed in and near to 
one another so I'm sure they'd have to be seriously derated.

 >>>>>
>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?
<<<<<

I don't know.  I adjusted the clamp control per the manual, and the 
screen voltage isn't greatly different key up to key down (key up = 
830V HV, 189V screen, 20.5W resistor dissipation; key down = 780V HV, 
161V screen, 19.16W resistor dissipation).  Those numbers are 
slightly different from what I measured last night, but the input AC 
voltage is also slightly different (118VAC vs 119 last night).

XYL is in the kitchen baking cookies and cupcakes in the electric 
oven; maybe that's part of the reason :-).

In any case as I said these resistor power ratings are typically in 
free air at 25 degrees C which ain't the environment in which they 
are operating.

 >>>>>
>A Lack of drive causes the clamp tube to conduct, dropping the 
>screen Voltage significantly.
<<<<<

As noted above, this is opposite from what I'm seeing... key-up (no 
drive), the screen voltage is higher than key down.  Maybe the clamp 
circuit isn't working, although as I said, the clamp control works as 
described in the manual.

I'm pretty sure I checked the 6AQ5 clamp tube in the tester and it 
checked good, but I'll do it again after it cools off a bit.  :-)

Rick WA1RKT