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Author Topic: Testing input and output of a plate modulated transmitter  (Read 2138 times)
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w9jsw
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« on: December 09, 2018, 11:16:13 AM »

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
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WBear2GCR
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« Reply #1 on: December 09, 2018, 07:54:50 PM »


Sorry what is an 813 "Tesla"?

And drive what with an MFJ tuna?

By the input side do you mean the RF drive circuitry??
Or do you want to test the 810 and the output tank?
Is it built yet?
Schematic?
Pix?
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K1JJ
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« Reply #2 on: December 09, 2018, 08:34:21 PM »

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.htm
BTW, 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

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w9jsw
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« Reply #3 on: December 10, 2018, 07:07:45 AM »

Thanks for the replies.

Yes, 300 Tesla for the grid input only. Other designs for the rest of the circuit based on K9ACT and W8ACR single tube triode designs posted over the last few years.

MFJ-269 is a MFJ-259 Antenna Analyzer with UHF also included. Sorry about not being more clear.

Attached is the schematic.

Concerns

My supply is a 4200VCT S-49 into a FW with a 10H choke and 2 10mf Oil Filled caps. Trans is rated for 1500VDC at 300ma. I worry that that is not enough to drive 2 811's and an 810 if I look at maximum plate current. I could go for a split supply and run the Mod at 1000V at 300ma but then worry about blowing up the mod trans if something goes wrong on one of the HV supplies. Or I could drop the 810 and go to an 812 to better balance the max current.

John

* 810 Transmitter-1.pdf (116.65 KB - downloaded 154 times.)
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KA2DZT
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« Reply #4 on: December 10, 2018, 10:33:28 AM »

UTC S-49 will run both the 811s and the final PA like one 813 with out any problems.  Remember, the 811 class B modulators don't draw steady current.  The PA will draw about 200 ma and the modulators 50-150 ma.  The slight intermittent overload on the S-49 won't matter much.

Fred
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