812 Rig Loading

[KA8WTK]

  The 812 rig has run into a bit of a snag.
  The HV supply is designed with a light bulb that can be switched into series with the HV transformer primary thus limiting current and voltage for tune up. Works pretty good. You can set the grid drive, tune the grid and find the plate dip. The problem comes when you go to full voltage.
  When you apply full voltage, the current goes way up and the fuse blows. The tank in this rig uses a plug-in coil style setup with fixed links for the output. The rig is from the 1947 ARRL handbook and was designed for open line. My thinking is that the output link was designed for 400 to 600 ohms and the rig is "overloaded" when placed into a 50 ohm dummy load. I can find no other reason for the high current.
  Other rigs I have owned required a capacitor in series with the coaxial line to adjust the loading higher if the variable link coil could not be made big enough or was in all the way already. Do I need one acreoss the coax to lower the loading?
  Any other thoughts?

Thanks
   

[KA8WTK]

  Well, I have discovered that a variable cap across the output will lower the loading. However, the current still goes too high on keying full power.
  Not finding anything wrong with the wiring, I think it may just be the layout of this old rig. There is a possibility that the output coils are so close that it is loading with both coils. Below is a picture of the deck. You can see how close the two coils are.
  My next "idea" is to just reove the 20/10 meter coil and jumper the "missing" link.

[The Slab Bacon]

Bill, If you getting too heavy of a load the link is prolly tuned for a fairly High-Z outpoot. A quick and dirty way to lower the outpoot impedance is to put a commercailly made (or home made if you feel like winding it) 4:1 balun backwards on the outpoot of the link. You still need some way of adjusting tha loading. Without a swinging link you MUST have a variable capazitor in series with 1 leg of the outpoot link. I will call you on the telefonium to discuss further as it is too much to type.

                                                                                    The Slab Bacon

[Steve W8TOW]

Hi Bill, ya know all my rigs here (except the Johnson 500) are link output..
lots o experience dealing with 50 ohm matching...
I will stick with examples from my 812A PP tx, tho...
For it, on 80m, I use a 8 turn link in the PA.
To couple it to 50 ohms, install a 470 pF HV TX Cap (5KV rating) in series to ground ( chasis) from one
side of the link.
Connect the ground of the RG58 to the chasis point of connection too.
Take the center condx of the RG58 to the other side of the swinging link.

If you have soaring plate current tho, I bet you also have another problem, like a
bad RF choke, bad plug in coil, corroded jack bar, bad 812A, or even a bad dowkey
relay contact!?
Gl es let me know if I can assist.
73 steve
8tow

[2ZE]

If you're finding resonance at lower voltage, then the rig is not too heavily loaded. Sounds like something Steve pointed out, bad RF choke or bypass cap breaking down.