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Building HF Amp
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Hi all,
My b-i-l suggested I join this group and seek comments.? My age is 75 & I've built electronic stuff for the past 60+ years.? My "ham Elmer" had a Globe Tx with a 4-250 modulated by 2 x 811's and powered by 2 x 866A's.? I've wanted to build something like this ever since.
The FCC changed the rules to 1500W PEP output so I will use my 3 new Penta Labs 4-400C tubes in a parallel, grid driven class AB1 configuration.? The basic circuit I'm using is a modified version of the one in the '66 ARRL Handbook.? ?Yes, I know a grounded grid circuit is a lot less complex & has fewer stability issues but I want a really "old school" project.? I will use a passive, resistor loaded, low impedance input rather than a tuned grid input to help with stability.? ?I also plan to use a band-switched PI-L output tank to reduce harmonics.? ?The amp bias and screen voltages will be switched in order to run class C so it can be plate modulated by 2 x 572B's.?
I plan to use a modified version of the power supply circuit, a resonant choke input, that Henry Radio used in its amps.? Hammond sold me a P. Dahl heavy duty filter choke designed as a Henry 2K replacement :)? I will use a bridge rectifier circuit with 4 x 866A/3B28 tubes.? Heyboer Transformer wound me a tapped primary 240 VAC plate transformer so my supply can be switched to provide either 1175, 1750,? 2350, or 3500 VDC at up to 1 A.
I have most of the parts I will need except for the plate output PI-L coils, but I have not yet started the build because I need to finalize certain elements of the design.
Comments are welcome!!
Larry - KF7EYF
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开云体育Hello. I'm 76 years old. I are in my L7 ! ? I will use a bridge rectifier circuit with 4 x 866A/3B28 tubes. I think you're making things a lot more complicated on this point, even though it's nice to see this type of tube working. ? 3 new Penta Labs 4-400C tubes in a parallel Are you considering neutrodynizing the PA? This is where it gets more complicated compared to a ground grid. The big advantage is the gain, but you'll lose it in the aperiodic input circuit. ? 73 -- F1AMM Fran?ois De la part de Lawrence A Lobdell Jr KF7EYF |
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Francois, Yes, the 3B28's and a resonant choke filter makes things complicated.? The HV power supplies as far back as the '66 Handbook use silicon diodes in a voltage doubler, capacitor input filter, with a much lower voltage plate transformer secondary (and cheaper).? But I wanted real "old school" before solid state.? Exception - my bias and screen supplies will use silicon diode rectifiers, but they will also have VR and other tubes. I thought about neutralizing the PA tubes, but most of the stuff I read says it is not necessary with a passive input circuit.? And yes, the passive input negates the low driving power needed by grid driven amps.? Ohms Law indicates I will need a peak driving power of 90 watts into a 450 ohms grid input in Class C, plate modulated use.? I will use 4 x 1800 ohm, 100 watt non-inductive resistors in parallel and a 1:9 ferrite transformer input.? ?Again, more complicated than G-G. Thanks for commenting.
On Wednesday, April 9, 2025 at 01:31:58 AM EDT, Fran?ois via groups.io <18471@...> wrote:
Hello. I'm 76 years old. I are in my L7 ! ? I will use a bridge rectifier circuit with 4 x 866A/3B28 tubes. I think you're making things a lot more complicated on this point, even though it's nice to see this type of tube working. ? 3 new Penta Labs 4-400C tubes in a parallel Are you considering neutrodynizing the PA? This is where it gets more complicated compared to a ground grid. The big advantage is the gain, but you'll lose it in the aperiodic input circuit. ? 73 -- F1AMM Fran?ois De la part de Lawrence A Lobdell Jr KF7EYF |
开云体育Hello Lawrence. ? I understand your "old-school" project. ? My Radio Amateur's Handbook, the only one I have, is from 1965. I bought it in London when I was a kid. It really inspired me. ? You don't indicate which frequencies you're considering for your project. You're starting with 3 x 4-400C. You're considering a push-pull of 2 x 4-400C, which is very easy to neutrodynize using two cross-connected capacitors. This would improve stability and gain, even in multiband. At that time, it was common to install the LC output circuit directly in the tube plate circuit. This avoided problems with the choke coil. Of course, with a push-pull, output coupling can become a problem in multiband. ? Good luck with your project. -- F1AMM Fran?ois De la part de Lawrence A Lobdell Jr via groups.io |