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Title: 813 vs 2 x 811A Post by: K9ACT on May 05, 2008, 12:42:57 AM Now that I have my 2 811's in parallel on the air and working, I am stuck having to use a rice box to drive it because my HB driver only puts out about 15W.
I notice that an 813 only requires a watt or so of drive so it would be fun to swap the 811's for an 813 but it seems like I would not accomplish anything buy doing this, assuming I use the same 1KV power supply. Two 811's pull 300 ma and put out about 200W. Can I somehow force an 813 to pull 300 ma at 1kv? Any other ideas? There is no simple way to increase the output of my exciter and I want about 200 W carrier out of the rig. js Title: Re: 813 vs 2 x 811A Post by: k4kyv on May 05, 2008, 02:41:21 AM You might get it to draw that much, but the modulation linearity might go to pot. I have actually had to lighten the load on my pair of HF-300's by decreasing the antenna coupling, because at full rated plate current for plate modulated AM, I was getting poor modulation linearity on the positive half of the audio cycle. I increased the modulating impedance from 4000 ohms to 5000 ohms and now the trapezoid pattern shows almost perfectly straight sides.
What do you use in your HB driver stage? Title: Re: 813 vs 2 x 811A Post by: K9ACT on May 05, 2008, 08:35:38 AM What do you use in your HB driver stage? 6AG7 VFO, 6CL6 Buffer and 807 PA. The problem is the light weight power transformer. It's only rated at 70 ma. There is a description of it at http://schmidling.com/radio.htm js Title: Re: 813 vs 2 x 811A Post by: WA1GFZ on May 05, 2008, 10:41:25 AM 813 at 1 KV might be good for 100 watts.
Title: Re: 813 vs 2 x 811A Post by: The Slab Bacon on May 05, 2008, 10:54:28 AM 813 at 1 KV might be good for 100 watts. OY Vey............you never heard of an ART-13?? ::) ::) Title: Re: 813 vs 2 x 811A Post by: Opcom on May 05, 2008, 11:10:30 AM What do you use in your HB driver stage? 6AG7 VFO, 6CL6 Buffer and 807 PA. The problem is the light weight power transformer. It's only rated at 70 ma. There is a description of it at http://schmidling.com/radio.htm js That is a very nice transmitter! I noticed the neutralizing capacitor's knob in this picture: http://schmidling.com/2x8000.jpg Do you actually reach down in there with it on, or is this done with the HV turned off? Title: Re: 813 vs 2 x 811A Post by: K9ACT on May 05, 2008, 12:43:48 PM That is a very nice transmitter! I noticed the neutralizing capacitor's knob in this picture: http://schmidling.com/2x8000.jpg Do you actually reach down in there with it on, or is this done with the HV turned off? Thanks for the kind words. The knob is just a "touch of class" that I happened to have. If you will note, there is a window (square hole) on the front panel for viewing pleasure. I have a long polystyrene tube with a soft rubber tip on the end. I can reach through the hole and turn either the knob or the cap plate directly for adjustment. This is more just to be able to do it from the front as there is no need to adjust with the HV on so I never do it. I can get it very close with a scope and link and then tweak it in a bit at a time with the HV off and check for grid current change with the HV on. If you are wondering what those other NC's are for, they are a bandaid for when I added the second tube. The spacing got too small on the main NC for safety so I put them in parallel and set them once to about their minimum and just adust the home made one. As a point of interest, I have no trouble driving this rig with the HB driver so this problem of not enough drive really doesn't make much sense. On second thought, maybe it does as I do not use the 8000 rig on 40 meters and the 811's do work on 80 with it. js |