Modulation Bias Adj Pot value on Valiant

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N1BCG:
Suffering from low drive on 20M?

While we're on the topic of "Keeping up with the Johnsons", here's another before & after schematic for an E.F. Johnson update to L6A in the driver tank circuit. It seems as though the stock multi-tap tank coil didn't provide enough inductance on 20M so the 15M turns were dragged in to help and a coil added (L6C: 12T, 5/8" dia) to the bandswitch to take on the 15M tank role.

Band label colors indicate lead colors from L6A...

WZ8J:
Okay back to report on the bias pot issue with the Val-Aint.
I switched the outside leads on the pot and this seem to help...for a short while, but now it is again acting as before. I believe the "pot is shot" to make a rhyme.

I am able after careful fiddling to adjust the mod bias current to about 45 ma, but if I try to go higher it jumps way up and pegs the meter! That can't be good for the modulator transformer.

There are 25 watt 5K pots available from Mouser for $2 per watt - ouch! These look awfully large. I am wondering if the builder or  a previous owner added smaller ones for mod and PA bias at some point to replace the resistors with the taps? Maybe the pots that are in there now are lower power rated and that's why at least one is failing.

I suppose I would be asking for trouble if I pulled a 5K pot out of an old PA amp I have laying around and put it in for the bias mod adjustment? I doubt that pot is good for more than 2 watts.

Finally - I determined that the reason the mod current kept rising during transmissions was due to a weak 6146 mod tube. I swapped in two good matched Taylor tubes and at least the mod current doesn't constantly climb now.

KA2DZT:
Either get two 25W 5K pots from Ohmite or two 5K 25W resistors with the adjustable slider.  Look on ebay for these.

Fred

N1BCG:
For convenience, spring for the potentiometer solution. It's a hassle to have to test, turn off, adjust, turn on, test...etc.

Some possibilities:

https://www.ebay.com/p/NOS-IRC-5000-Ohm-25-Watt-Potentiometer-Rheostat-Wirewound/1541302449

http://www.surplussales.com/potentiometers/rheostats/PotsRheost-4.html

$9.95 + $6.50 shipping :-)  Just gotta cut the shaft a bit:

https://www.ebay.com/itm/MEMCOR-5000-OHM-25-WATT-RHEOSTAT-2-SHAFT-/400682271238

KA2DZT:
The Memcor 5K 25W pots on ebay are perfect and cheap enough.  Get two of them before they sell out.  I have some Memcor power pots and the quality is just as good as Ohmites.

Fred

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