The AM Forum

I want to make the Variable Screen adjust mod to my ranger which calls for a 4 watt pot which are difficult to find.  Is it really necessary to use a 4 watt pot?  It would seem to me that at full loading there is probably only 1 or so mA of screen current at 190 volts.  Wouldn't a 2 watt pot be sufficent?  I'm sure I must be missing something......

A few related comments:

1. The screen current in the Ranger's 6CL6 is higher than you think it is. I don't know the exact value... but looking at the data sheet for a 6CL6, my guess is that it is closer to 5 mA, and possibly more. So, with (for example) a 300 volt value of the low B+, and to create a screen voltage supply that is reasonably "stiff" ... you would want the pot to be drawing around 10 mA from the 300 volt supply  (with the screen of the 6CL6 drawing zero current). Thus you end up with 3 watts for the minimum dissipation rating of the potentiometer.

2. Using a few modern devices, you can substitute a low wattage pot. You need to add a transistor (and NPN bipolar transistor or an FET) that (to be safe with respect to some hypothetical transient conditions) can handle the 300 volt low B+ (actually, you should assume that it can be as high as 400 volts). Ref: http://www.amwindow.org/tech/htm/drivepot.htm

In the referenced article, and the associated schematic, there is a 6.8k ohm "existing screen resistor" shown. This resistor does not exist in the Ranger... and you might want to add it to reduce the power that the transistor has to dissipate.
Also, the label:  "to screen of 5763" should be changed to "to screen of 6CL6" in the case of the original Ranger (a.k.a. Ranger 1).

Best regards
Stu

Ah so.  That makes total sense.  I forgot about the screens on the 6CL6's.  I was only thinking about the 6146.   That circuit is simple and can hide under the chassis.  Since you have to remove the crystal socket to install the pot, anyway, that doesn't matter much.  I'm sure I have transistors laying around here to do the job.  I didn't know about that particular article, thanks!  The mod that I had seen was  the TimTron one.

Just so you know, both Allied Electronics and Mouser have 4 watt wirewound pots.  You may have to call tech support to find them.

  ht all:
 DIGI-KEY has 5watt wirewound pots also   
                                     JOHN

I think I looked on Mouser's site (not sure which) and found some 4 watt pots for like 60 bucks each.  I'll look around some more.  Thanks, guys.