Don, When running tubes in push pull parallel, you can add a small amount of resistance to the plate, value depends on plate voltage and current.
On screen grid tubes, you can add resistors in the screen and/or plate circuits. To much resistance will cause problems, as will tubes that are much different. In my ppp 100TH mod deck, I have something like 25 to 50 ohm 2 watt resistors in line with each plate, I don't know the exact value as they have heat shrink tubing over them. All the tubes glow with resting current, and light up more under modulation, and all are equal. The modulator runs 2000 volts, 120 ma (30ma per tube) resting current. You can likely discount peak current, as its short, and does not add much to the resistor dissipation. Push pull parallel using triodes is not a great way to go. It works, but is never as clean as something running AB1. Even driving the grids with a very low impedance source presents such a variable load on the driver, very hard to keep clean. With AB1, its only voltage, so you can drive lots of grids without problems. I don't think you gain much with triodes, substantial driving power is needed, at a low impedance, harder to do than a stiff screen voltage supply in AB1. With high power solid state driver devices, the triode method likely works very well. Brett N2DTS > > I have had this modulator chassis sitting around here that I > was fiddling with over the weekend. It came to me with no > tubes. It uses 4 tubes in push-pull parallel. The mod > transformer is a UTC VM-3 rated at 125 watts. The VM or > "Varimatch" series of UTC mod transformers was sold pre-war > and evolved into the later "CVM" series that is more commonly > seen today. This chassis may have come from a commercial > amateur rig because the construction standard is strictly > professional. > > Through some artful guesswork using only the finest > assumptions, I have concluded that the modulator tubes should > be Taylor TZ-20's. The TZ-20 is (quoting Taylor) "a high-mu > triode designed for zero bias class B audio operation". The > Taylor catalog even suggests this modulator design under the > TZ-20 listing, saying "4 of them in push-pull parallel > operation will form a most economical 140 watt modulator", > though no actual design for such a unit is given. > > Fortunately I have some TZ-20's (though I can use a few > spares if anyone wants to part with a couple). But I know > nothing about running tubes in PP-parallel. Are there any > special considerations? Should the tubes be matched in any > sense? And lastly, I suspect this should be obvious but I'll > ask anyway, how critical is the resistor between the 2 plate > caps of the tubes in parallel? > > Comments? > > 73, Don Merz, N3RHT > > _______________________________________________ > AMRadio mailing list > [email protected] > http://mailman.qth.net/mailman/listinfo/amradio
