[AMRadio] Push-Pull Parallel Tubes In Modulator

[Merz Donald S]

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