I am building a 810 modulated by a pair of 811's...
How do I go about testing the input side? Using the notes from the 813 Tesla to build it. Do I simulate the grid impedance with a resistor and drive it with my mfj-269 tuning for lowest input SWR? If not, how?
also can I reverse the process on the output tank in a similar manner.
Also, been searching unsuccessfully for how to do a first power up on a new rig. Are there any pointers to follow? Not there yet, but will be soon.
John
Hi John,
Yes, it's called "cold" tuning - using an MFJ-259B with a fixed carbon resistor to simulate a load will work for BOTH tuning the input tank circuit and also tuning the pi-net output. IE, getting it in the ballpark. On the output pi-net circuit put in a resistor to simulate the 4K (or whatever value) across the tube plate load (R=E/I) and look for 50 ohms, 1:1 swr on the pi-network output. For the input use grid current and grid voltage for the same resistor calculation, loaded across the grid to ground. R=E/I. Be sure to calculate the Q for the pi-net so that you will be close to optimum when seeing that desired 1:1 swr.
I think you may mean the pair of 813s X 813s, Tesla 300 on Steve's AM window?
http://www.amwindow.org/tech/htm/813/813.htmBTW, there's an omission on that schematic. The 2.5 mH RF choke connected in the grid circuit - it should have a .001 disc bypass cap from the choke's left side to ground. And, all meters need a .001 across their terminals.
Testing: Firing it up is always done in stages. Be sure the metering is working, fils work, relays work, all tanks are close to tuned... all voltages are OK, drive is there using a scope on the grid pin, etc. Then slowly add some RF drive and see if the power output comes up little by little. Starting with 1/3 plate voltage or whatever you can manage is a good idea. A temporay plate HV Variac is always a good idea to reduce voltage. Keep an eye out for parasitics on the scope and jumps on the metering. Smooth meter movement is desired and any jumping is a bad sign when tuning.
Get the neutralizing cap adjusted (cold neut) with the HV off, just fil voltage. Put a little drive in and tune the neut cap for minimum RF (a dip) when watching the 50 ohm pi-net output on a scope. Later do a hot neut by watching plate current dip as maximum output power occurs.
Modulation testing is basically the same with small amounts of audio input and watching the output RF waveform for proper response. Careful not to blow the mod iron by going below 60 Hz or too much audio drive.
This should get ya started.
T