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HB Linear Amp Mods




 
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K1JJ
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"Let's go kayaking, Tommy!" - Yaz


« Reply #50 on: March 07, 2009, 11:52:49 PM »

Rich,

If you look carefully where the "roller tap" lead comes out to its connection, you may find it passes very close to one end of the coil. If so, there can be huge voltage from one end to the other and you're getting that arcing.  Maybe you can find a way to isolate the roller wire, dope it with goo or something else.

Though, the best way will probably be to wind a new coil using 1/4" copper tubing and tap it.   Use some kind of 3"-4" form to wind it on and then slip it off and support it as an air coil.  Don't worry about taps until you install it and look for the sweet spots by trial and error. Use the tank calculator to get close and go from there.

Do a search on Google for "Pi Network Calculator."   Plug in 50 ohms for the output, the band frequency, a Q=12 and the output impedance of the pair of 813s. It will tell you the inductance for L and capacitance for C1 and C2.  Then search for an inductance calculator and plug in your coil dimensions and find how many turns you need and tap it.  Tune C1 and C2 for max using this L. 

The best coils for highest Q are made with the physical length to diameter ratio held between 1:1 and 2:1 over the various bands. This will require more than one coil for 5 bands, but don't sweat it if you want to use one coil. 

There are calculators on this BB also.



T
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« Reply #51 on: March 09, 2009, 03:55:38 PM »

Hi Tom,
     I like the 1/4" tubing idea but that would make the coil too big/long.  I think I can squeeze in a 3" coil but will have to use a smaller diameter inductor to get the turns/inch I need.

This is a nice doc showing some homebrew inductors:

http://www.arrl.org/tis/info/pdf/9708033.pdf

I am researching how to calculate the plate resistance of the 813's.  There is enough mixed messages out there that I have no found a clear answer.  Maybe it is as simple as the plate voltage/current but then I have to make an assumption about the current....

Rich


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K1JJ
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« Reply #52 on: March 10, 2009, 12:55:38 AM »

Hi Tom,
    I am researching how to calculate the plate resistance of the 813's.  There is enough mixed messages out there that I have no found a clear answer.  Maybe it is as simple as the plate voltage/current but then I have to make an assumption about the current....

Rich


Rich,

For operating points other than class C, the load impedance isn't the simple resistive calculation it is for Class C due to the dynamic impedance variation throughout the conduction cycle, even for Class A where the observed DC plate current is constant.  The plate impedance is usually much less, for example, for Class AB single ended RF service, it is typically half the average resistive value.  For Class A RF service, it can be about a third. 

I'd go with about 4K for two 813's as the class AB linear load impedance for your pi-network calculations.  If the actual impedance is a bit higher, this just means the Q of the tank will be higher which is fine. 
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Use an "AM Courtesy Filter" to limit transmit audio bandwidth  +-4.5 KHz, +-6.0 KHz or +-8.0 KHz when needed. 

Nothing like a new homebrew rig. Come into the shack, flip on the switches and everything works perfectly.

And, nothing like an old dog.
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