The AM Forum

Greetings
I have a curious issue with my Valiant II I am finishing up from restoring... a "Hot Pot".
R-51 is the drive control, a 25K 4 watt wire wound type and it smoked. Replacement did not fix the problem.
This has several hundred volts on it, as it's tapped from the B+ HV line of the plate supply, and it feeds the
screen grid of the multiplier tube. I have no idea where to begin so I need some advice. Was this a common problem? The wiring is fine and apparently was a factory wired transmitter.
Any clues?
Thanks in advance.
Billy N6YW

Tim, WA1HLR, wrote this a number of years ago on the original Valiant but I suspect it would equally apply to the Valiant II.

"The drive control pot is a weak link in the system. Place a 5k, 5-10 watt wire wound resistor in series with the bottom of the pot as well as another 5k resistor in series with the B+ to the top of the pot. This will reduce the dissipation of power in the pot and improve reliability. One may also go with choke input which will drop the low voltage to 250 volts. This will greatly reduce the load burden on the LV and filament transformer. The downside is that there will not be enough drive on 10 meters without some changes."

The complete article is here: http://www.amwindow.org/tech/htm/valiantbytron.htm

Thank you Pete.
Funny, I had just found that article and was reading it when you answered me.
The SSB adapter works too, which is good seeing how I have too many projects already.
You may pull this thread.
73 de Billy N6YW

Now that I have thought about it, an idea came to me from what I use in one of my guitar amplifier designs.
Power Scaling, or otherwise known as VVR... variable voltage regulation.
This is not a new idea but it works wonderfully and maybe is a more elegant approach to what Timtron wrote about, not disparaging of course.
It's a small package a little larger than a postage stamp and it contains just a few parts including a MOSFET to control the voltage level with a potentiometer.
It takes a fixed voltage and adjusts to the users needs. For example, say we have a plate voltage of 450 volts applied to the power tubes of an amplifier. We want to electronically turn down the volume by turning down the plate voltage. That's exactly what this unit does and it can drop that voltage down to as little as 35 volts to the plate.
So, my reasoning applies to voltage adjustment without the current burden being placed on a pot.
I shall try it and report the results.
73 de Billy N6YW.

What's old is new again....   Lol

http://www.amwindow.org/tech/htm/drivepot.htm

Heard you here in Bakersfield about two weeks ago Billy.  Only have 20 whole watts of carrier,  so couldn't break in,  but you sounded good to my inv v at 32 foot apex!

--Shane
KD6VXI

Thanks Shane!
The BC-610E does it's thing like a Timex watch. :D

You could use a power FET in a source follower configuration as a voltage regulator.  Connect the gate to the slider of a 1meg pot which goes from B+ to ground.  The drain goes to B+ and the source to the screens. Pick a FET with a voltage rating about 1.5 times the B+, one of the 1200V SIC FETs should do nicely. It will require a .1 mfd bypass cap to ground at the source to prevent mild oscillation.  The gate will be about 4 volts higher than the source. Do the math to calculate power dissipation across the FET (E*I) and heat sink accordingly but it may not need any.
I use 2 60 amp FETs this way as a 50V 30A regulator in a home brew FET linear amp and they work FB.  You can add a small SCR from gate to ground which when tripped will instantly drop the output to zero for protection from whatever sensor(s) tripped it.
73  Nigel

Here is the VVR I told you about.
Neat little package that has worked very well as a voltage controller in the amps I build.
I am about to install it in the Valiant II as I described. I will let you know how it works.
73, N6YW

Here is the data sheet for the power MOSFET 21N90K5.
http://www.datasheetlib.com/datasheet/157954/21n90k5_stmicroelectronics.html

Here is the installed device. It works exactly as I had planned. It is advised that one should
install a filter cap after the B+ output, anything between 16 to 47 uf is fine, 450 volts.
73 de Billy N6YW

Another approach is to use the Generic Buffer-Driver circuit included, with which I have had success.

RDpot, CSDf, and RSd, are all new components rated for the LV B+ and drive current.

Whether it be an E.F. Johnson or Heathkit, this circuit is often populated with underrated components for 120VAC input, which is why the Driver pot fails.

Phil - AC0OB

Phil, thank you.
I will compare this to the Valiant schematic. Right now, I have it working but there is a bad hum on the carrier, even after a full recap. Troubleshooting begins, again. The pitfalls of resuming someone else's mods halfway through when I got it.