For many tubes, it does not seem to be a problem. 813's seem to modulate great with a choke in the screen circuit to allow self modulation. The 4-125/250/400 tubes do also.
My 813 pair can hit incredible positive peaks, 2500 watts plus. I did not do the trapezoid test, but as far as audible differences between the push pull 812 rig, there are none that I can tell. In truth, the 813 rig sounds a little cleaner because the mod deck is AB1 and has lots of extra power. The class B 811a mod deck does not sound quite as clean, even though I drive it with a solid state amp. I don't think you will ever see a class B push pull triode hi fi amp... But for ham work, that's nit picking... I retired the push pull parallel 100th mod deck. It looks way cool, 4 glowing tubes behind glass, but sounds quite poor in comparison to the 4x150a deck. I have not had the heart to remove it from the audio rack and take it apart. Two 100th tubes in class B is not bad, but four is impossible, the stiff and erratic loads to the audio driver cause all sorts of distortion. People don't report it sounds bad, but it does to me in the mod monitor. Best bet for a transmitter seems to be push pull triodes and a hi fi AB1 modulator. When I get the quad of KT90's mod deck built, I can try it on the 812a RF deck. On a side note, the 4x150a/4cx250b tube makes a great modulator. The great things about those tubes are: Low screen voltage, 350 volts that is easy to electronically regulate, AB1, NO driving power needed, just voltage, You can variac the plate voltage between 1000 volts to 2000 volts, changing the impedance and power a lot, without changing the screen voltage or bias voltage a bit! They will put out 600 watts of clean audio. Brett N2DTS -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of John Coleman Sent: Wednesday, September 29, 2004 1:03 PM To: AMRadio Subject: [AMRadio] HOMEBREW I prefer triodes because they tend to have linear plate modulation characteristics with out a lot of other circuitry. Tetrodes, on the other hand, not only require screen circuitry, but require a lot of audio level screen compensation in order to reach the same characteristics of the triode. But I'm very critical on passing the trapezoid pattern test. I wrote an article similar to this point at http://www.qsl.net/wa5bxo/pptriodes/pptriodes.htm It is some what under construction still. So I have not made a link from the home page http://www.qsl.net/wa5bxo I was trying to clear up some things that or either not always clear in other books or not mentioned at all. Hope this helps someone. 73 John, WA5BXO -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Brett gazdzinski Sent: Wednesday, September 29, 2004 8:15 AM To: 'Discussion of AM Radio' Subject: RE: [AMRadio] Home Brew It sounds simple, and the RF stuff is simple, but it's the control and metering that adds complexity. Step start, push to talk, grid current and voltage metering, plate or cathode current and plate voltage meters, plate voltage control (variac or power trans primary taps),filiment voltage control and metering, protective bias plus grid leak bias, etc. But overall, it is quite simple, one step at a time... I like building a small deck just for the rf, and do the control on a separate deck, with lots of meters and knobs to twist. Big wire wound pots work great for grid leak resistors. Add in control grids and it gets more complex, with sequence starting, more power supplies and metering, overload circuits, etc. Its loads of fun to figure out what will work with what parts you have or can get. On the subject of meters (I use a lot of them), I use the cheap radio shack meters, they used to make 0-15 volts, 0-500 ma, and 0 to 1ma meters that took a very small hole in the panel. I just make the scales indicate whatever I want, 0-15 volts can be 0 to 150 volts, 0 to 1500 volts, 1.5 amps, 150 ma, etc. The scales come off, they are held on with glue, and you can make up your own numbers or transfer them from one meter to another, as I think they only sell the 0-15 volt meter now. I should use a computer program to make up real scales. So you can have a load of meters, all the same, cheap cost, small panel holes, and modern looking. Shunts will give any current you want, voltage dividers give whatever voltage you want. Black painted panels with P touch labels looks nice, and don't count on remembering what meter does what, or where its supposed to read after 10 years or more, or the next guy who gets the equipment! You can mark the correct voltage/current with a red felt tip marker. A pair of 813's takes how much grid drive? Screen current? Where did you run the plate current on the 4d32 rig? Are the 4x150a filaments 6 volts, or 6.3? Is this stuff fun or what? Brett N2DTS ______________________________________________________________ AMRadio mailing list Home: http://mailman.qth.net/mailman/listinfo/amradio Help: http://mailman.qth.net/mmfaq.html Post: mailto:[email protected]
