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PICTURES - 813s X 813s wired and ready for testing - "Hollywood"




 
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Author Topic: PICTURES - 813s X 813s wired and ready for testing - "Hollywood"  (Read 1389 times)
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K1JJ
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"Let's go sailing, Tommy!" - Yaz


« Reply #25 on: May 16, 2020, 08:45:59 PM »

Chuck,

77% efficiency on 75M is hi-hi FB on your new 813 rig!


http://people.physics.anu.edu.au/~dxt103/calculators/pi_tank.php

Yep, that's the calculator I used. I just ran 75M to be sure - and it shows the same 140 pF, 848 pF and 12.8 uH as I posted above.

Probably the difference you see is my assumption of a K constant = 2 for a class C stage. This was from another website.   I came up with 3500 ohms for the plate impedance using K=2, 2800V and 400mA IIRC.  I've heard to use K=1.5 too, so maybe you can advise.



The site says: "3. The Plate Current (Ip), is calculated by the formula: a. Ip = Pin Ep 1. Plate Voltage (Ep) 'maximum', for the Tube's class of operation, is obtained from the Tube Manufactures Data Sheet.
 4. The Class of Operation uses a 'K' factor for a specific Amplifier design which is: a. For Class A Operation it is (1.3 ~ 1.4)  b. For Class AB Operation it is (1.5 ~ 1.7)  1. For Class AB1 Operation it is (1.5) 2. For Class AB2 Operation it is (1.6 ~ 1.7) c. For Class B Operation it is (1.8 ~ 1.9) d. For Class C Operation it is (2.0)
5. The Plate Load Resistance (RL), is calculated by the formula: a. RL = Ep K x Ip 1. Plate Voltage (Ep) 'maximum' is obtained from the Tube Manufactures Data Sheet."


http://rfcec.com/RFCEC/Section-3%20-%20Fundamentals%20of%20RF%20Communication-Electronics/04%20-%20AMPLIFIERS%20-%20RF%20POWER%20AMPLIFIER%20BASICS/RFPA%20-%20Design%20Formulas%20(By%20Larry%20E.%20Gugle%20K4RFE).pdf

I didn't account for stray from the circuit and tubes which will change things too.

But the bottom line is I planned on getting the taps in the general ballpark and then run tone peak and power tests to see where the fine tuning lies.  Notice I didn't solder the tank straps on and will use a strong clip to hold them tight during initial testing.   I'm finishing up the new 3KV supply now and then the mod iron and GFZ MOSFET audio driver next.

10M?  I'd probably be at the end of that coil - 20M is probably it ... it wud need a smaller 10-15M stout coil switched in and C1 at min to pull it off on 10.

Glad you took the time to look over the new rig, Chuck!

T
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K1JJ
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« Reply #26 on: May 21, 2020, 10:31:42 PM »

I will be interested to see if you have any audio distortion with this transmitter.  The reason being: I have built the 813x813 transmitter and cannot for the life of me
get rid of the audio distortion.  I have tried 3 different modulation transformers but all give the same results, audio so distorted as to be unreadable.  Maybe it's like
Einstein said, "making the same mistake each time but expecting different results."  Anyway, good luck with yours and please keep us posted.


Just an update...


Roy tells me via email he has solved the distortion problem and all is well with his 813 rig.  Turns out it was a wiring error on his schematic involving the modulation reactor coupling cap. He is now using the correct wiring from the AMFone schematic and the audio is now FB.


Roy, I just finished my HV supply and all that is left is to wire up the WA1GFZ MOSFET audio driver.  Then it will be testing time and real on-air trials.  You might consider Frank's audio driver with audio negative feedback. (NFB).  I now use one on my 4X1 rig and can't wait to see how the second one performs with this new 813 rig. I'll let ya know when I am ready and we can have a 813 to 813 QSO on 75M at night.  I am making it a goal to get this 813 rig sounding absolutely perfect to set a high bar for the guys who've already built them - but still need some more audio work to get that "sound."

T
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Chuck...K1KW
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« Reply #27 on: May 22, 2020, 06:08:17 PM »

My mistake...I didn't see the Q=12.  I run a Q of 20 to 25 on all rigs and have these values almost memorized.

The reason I do this is that at a Q of 12, a mismatch could lower your Q quite a bit!  A 2:1 SWR with a load impedance high and reactive, it could go as low as 6 with values selected for a Q of 12 into 50 ohms.  Ever run into a situation where the rig "just won't load up"?  The resulting low Q at an attempt to match something well off 50 ohms load probably did it.  Raising the Q by lowering L will fix it.  Changing the feedline length is another hack to fix it.

So....I use a Q of at least 20 to avoid this and still get decent second harmonic reduction.

The reduction in efficiency at Q=20 vs Q=12 is not measureable on a Bird watt meter.  Perhaps it's a needle width but likely less.  If the Q of the inductor is above 200 or so, the loss should be minimal.  A decent inductor will often hit 300 but when you start shorting turns on a tank inductor to change bands, it will drop quite a bit.  That's where the loss comes from.  Dedicated single coils for each band is the way to go.

Chuck
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73, Chuck...K1KW
K1JJ
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« Reply #28 on: May 22, 2020, 10:26:26 PM »

The reason I do this is that at a Q of 12, a mismatch could lower your Q quite a bit!  A 2:1 SWR with a load impedance high and reactive, it could go as low as 6 with values selected for a Q of 12 into 50 ohms.  Ever run into a situation where the rig "just won't load up"?  The resulting low Q at an attempt to match something well off 50 ohms load probably did it.  Raising the Q by lowering L will fix it.  Changing the feedline length is another hack to fix it.

So....I use a Q of at least 20 to avoid this and still get decent second harmonic reduction.

The reduction in efficiency at Q=20 vs Q=12 is not measurable on a Bird watt meter.  Perhaps it's a needle width but likely less.  If the Q of the inductor is above 200 or so, the loss should be minimal.  A decent inductor will often hit 300 but when you start shorting turns on a tank inductor to change bands, it will drop quite a bit.  That's where the loss comes from.  Dedicated single coils for each band is the way to go.

Chuck

Very interesting.   I still have the chance to move my tank switch inductor taps to give a Q=20... nothing is soldered on the coil yet.

I can see that if an antenna matcher is used, then there could be a 1:1 all the time. But in my case (and yours and many others) we are using coax or hardline so that the swr will vary.  I will try the higher Q taps on this rig and see how it works.  I have always liked using higher Q tanks better anyway. The sharp tuning gives me a charge....  Cool

T

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K1JJ
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« Reply #29 on: Yesterday at 01:22:28 PM »

The reduction in efficiency at Q=20 vs Q=12 is not measureable on a Bird watt meter.  Perhaps it's a needle width but likely less.  If the Q of the inductor is above 200 or so, the loss should be minimal.  A decent inductor will often hit 300 but when you start shorting turns on a tank inductor to change bands, it will drop quite a bit.  That's where the loss comes from.  Dedicated single coils for each band is the way to go.
Chuck

Instead of shorting the coil, what if we simply tapped the coil and left the extra turns open?  I realize we would have Tesla coil action, especially on the higher bands and possibly arc over the band switch or coil itself.  But what if we had a 160M coil that was tapped 1/2 way for 75M with no arcing? Would this help preserve the Q? Or would the auto-transformer circulating  current effect still take its toll on Q even when not arcing?   Tapping a coil rather than shorting it always looked cleaner to me, assuming we didn't do it on a big coil above 40M or so.

T
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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. 

There's nothing like an old dog... or a puppy, or a dog in his prime!  Help a good dog - give him a "Forever Home!"
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