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813 modulators
My Ranger driven 813 modulator is working very well but now I want to
move on to a self-contained mod. I have ordered a 15 W solid state amp but need some help picking a transformer for between it's 4 ohm output and the 813's running as triodes. I am sure this has been done many times so perhaps someone can save me the pain of trying to determine the impedance needed for the output of the trans. Thanks, JACK K9ACT -- PHOTO OF THE WEEK: http://schmidling.com/pow.htm Astronomy, Beer, Cheese, Fiber,Gems, Sausage,Silver http://schmidling.com |
813 modulators
On Mar 9, 8:02 am, Jack Schmidling wrote:
My Ranger driven 813 modulator is working very well but now I want to move on to a self-contained mod. I have ordered a 15 W solid state amp but need some help picking a transformer for between it's 4 ohm output and the 813's running as triodes. I am sure this has been done many times so perhaps someone can save me the pain of trying to determine the impedance needed for the output of the trans. Thanks, JACK K9ACT Wild guess: in pentode mode, a PP pair of 813's in AB1 needs approx 160 v peak-to-peak to develop 250 watts of power. 15 watts at 4 ohms is about 20 volts peak-to-peak. So you need a voltage step-up of 8 or more, round up to 16 for a little headroom. Don't round up too much or you'll lose some current ability. If you're looking for impedance ratios they are voltage ratios squared, so 1:64 (250 ohms, sounds low) to 1:256 (1K ohms, about right). Note that 600 ohms is a commonish value, you're in luck if it has a center tap! As a sanity check, typical 811A (high mu triode) class B pairs take about 10 watts at 1000 ohm impedance for driving too. Generally the voltage ratio is more relevant than any impedances printed on the transformer. If you find a hi-fi or 70.7V PA transformer that's great, but lots of people have pressed filament transformers into use too. A center-tap on the output side (even if it's really the input side!) is really nice, you'll note that "international" filament transformers have two 120V windings that can be put in series for 240V or in your case to make a center tap. My question for you: what kind of modulation transformer do you use to drive the plate for the 8000? I'm jealous that you apparently had one just sitting around :-). Tim. |
813 modulators
On Mar 9, 8:02?am, Jack Schmidling wrote:
My Ranger driven 813 modulator is working very well but now I want to move on to a self-contained mod. I have ordered a 15 W solid state amp but need some help picking a transformer for between it's 4 ohm output and the 813's running as triodes. I am sure this has been done many times so perhaps someone can save me the pain of trying to determine the impedance needed for the output of the trans. Jack, I agree with Tim's comments, and will add just a few words. One concern I have is that the 15 watt amp will see a load of varying impedance from the 813 grids. If you are running them Class AB, their input Z is near infinite until grid current flows. That reflects back to the 15W amp, which may or may not affect it. I suggest putting a resistor across the secondary of the driver transformer, so the amp always sees a load. The resistor then sets the impedance of the transformer. Some back-of-the napkin numbers: As Tim says, 15 W at 4 ohms is about 20 volts peak-to-peak. To give some headroom, let's use 10 volts. Again using Tim's numbers, figure about 200 volts peak-to-peak at the 813 grids. That's a 1:20 voltage step up, or a 1 to 400 impedance step up. The output Z is therefore 1600 ohms or so. The resistor should be 1600 to 2000 ohms, 20 W. Could be a bunch of smaller resistors in series- parallel. The 20 W rating is very conservative, too - 10 W may be enough. Besides the transformers Tim mentions, keep an eye out for Stancor "rectifier transformers". These have been around forever, and have the numbers ranging from RT-201 to RT-208 or so. They are all the same windings, the difference is in the power rating. An RT-201 is more than enough VA for this application. The RT series are well-documented, common and not too expensive, and have the desired dual primaries (which you use in series to drive the 813 grids). They also have lots of secondary taps so you can try all sorts of ratios. One request for your website: MORE PICTURES! 73 es GL de Jim, N2EY ....that 8000 is just beautiful, btw... |
813 modulators
Tim Shoppa wrote:
If you find a hi-fi or 70.7V PA transformer that's great.... What does 70.7v mean? My question for you: what kind of modulation transformer do you use to drive the plate for the 8000? I'm jealous that you apparently had one just sitting around :-). No such luck but I did find someone else by trolling who did. It's a UTC SS22 and about right for the job but I would feel more comfortable with something a little bigger. I only need about 200W of audio but I would like to run it at 2000 volts to get legal limit out of the final but I am a little reluctant to push the mod trans. I am currently running at 1600V and everything is happy. JACK K9ACT -- PHOTO OF THE WEEK: http://schmidling.com/pow.htm Astronomy, Beer, Cheese, Fiber,Gems, Sausage,Silver http://schmidling.com |
813 modulators
Jack Schmidling wrote:
Tim Shoppa wrote: If you find a hi-fi or 70.7V PA transformer that's great.... What does 70.7v mean? It is a standard method of distributing multiple PA or music speakers from an audio amplifier. If you start out with a 100 watt amp with a 100 watt/70 V audio output transformer you can run small gauge wire to the remote speakers. At each remote speaker a 70 V transformer steps down to the speaker impedance by way of a tap corresponding to the power you want delivered to the speaker. If you have 10 speakers, use the 10 watt taps etc. -- Joe Leikhim K4SAT "The RFI-EMI-GUY"© "Treason doth never prosper: what's the reason? For if it prosper, none dare call it treason." "Follow The Money" ;-P |
813 modulators
On Sat, 10 Mar 2007 00:46:14 -0500, **THE-RFI-EMI-GUY**
wrote: Jack Schmidling wrote: Tim Shoppa wrote: If you find a hi-fi or 70.7V PA transformer that's great.... What does 70.7v mean? It is a standard method of distributing multiple PA or music speakers from an audio amplifier. If you start out with a 100 watt amp with a 100 watt/70 V audio output transformer you can run small gauge wire to the remote speakers. At each remote speaker a 70 V transformer steps down to the speaker impedance by way of a tap corresponding to the power you want delivered to the speaker. If you have 10 speakers, use the 10 watt taps etc. That must be some kind if americanism, since I have only heard about 100 V lines, for distributing audio power at a reasonable level around a large house, Paul OH3LWR |
813 modulators
On Mar 9, 11:52�pm, Jack Schmidling wrote:
wrote: One concern I have is that the 15 watt amp will see a load of varying impedance from the 813 grids. If you are running them Class AB, their input Z is near infinite until grid current flows. That reflects back to the 15W amp, which may or may not affect it. There is some idling grid current flowing.. about 30 ma if that helps any. *This is a zero bias set up with screens and grids tied together. Then you don't need the resistor I mentioned. I suggest putting a resistor across the secondary of the driver transformer, so the amp always sees a load. The resistor then sets the impedance of the transformer. Someone else suggested a resistor in series but I forgot why. *Are you addressing the same problem? No. I was suggesting a resistor across the transformer secondary - from grid to grid. But since your setup is zero-bias Class B, my concerns don't apply. The 813 grid/screen connection you are using provides a load for the driver. I think the series-resistor idea meant a small (~100 ohm) noninductive resistor in series with each grid lead, installed right at each 813 socket, in case of instability. You obviously don't have that problem. One request for your website: MORE PICTURES! Thanks but I have been doing a Photo of the Week this for 12 years and that's about 624 to date. *If you just want to fall asleep watching them, try the slide show. They're GREAT! ...that 8000 is just beautiful, btw... Very satisfying to turn a bookend into the heart of a real transmitter. Yep, I know the feeling. 73 de Jim, N2EY |
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