Here is an alternative modulator tube for the Ranger...
I've never seen this discussed before but the baby version of the 6550 is the 7591/7868/6GM5 series of tubes. These are much smaller tubes, the 7591 is about the size of a 6V6, but they have a stout 19 watt Pd rating. The high Gm, allows for 45 watts RMS out from a pair push pull class AB1 with a B+ of 450v. This level of output can be had with a mere 20v peak grid drive.
I think of these tubes as about 1/2 a 6550 with differing bases, and MUCH lower inter-electrode capacitances than the big 6550. In fact, I use a pair of 7591'a in my Central electronics 20A as a class Ab2 RF linear amp, where I get ~ 65 watts PEP RF output on 80m with 560 volts on the plate. These make great RF tubes in addition to audio. In the 20a the 7591's did a better job than a single 6550.
The 7591 is currently in production by JJ and EH for ~ $15-20 each. The EH uses are larger bulb. NOS, and used versions are plentiful but cost much more. Look on EPay. The 6GM5 might be around NOS, and the 7868 is also in production (EH), but more expensive.
I've had an old pair of "Fisher" branded 7591's in my 20a where they idle 30ma each at 600v, and have done so since 2003 when I built this rig. These are very stable tubes with long service life. The HV at 600 is above ratings, and I certainly would not go higher.
http://www.tubezone.net/tubedata.htmlJim
WD5JKO
