Other tubes that work here are the 6550 and the new JJ-EL509 ($40) that was a TV sweep tube but has now been modified for improved use in audio applications with no plate cap. As has been pointed out many of these JB modulators have wiring problems with the bias circuit being reversed for receive. What's often not noticed is they put half the plate voltage on the screen and over dissipate the crap out of it with 400 volts!
Many only have a 2 pole relay that will not allow you to bias the tube into cutoff during receive. You should use a 3 pole relay so bias can be switched. Since the 400 volts on the screen is really way too much, we need to cut that back and shunt regulate the voltage with a 200 volt 5 watt zener fed through a 20K ohm 10 watt resistor.
To keep these parts from dissipating heat in receive, it's better to leave the bias fixed in TX and RX around -47 volts and switch your screen supply through the relay. Take the resistor feeding the zener and tube and connect it to the common relay terminal. The normally open contact goes to the 400 volt feed and ground the normally closed contact so the screen is "pulled" towards ground through the 20K resistor when in RX.
This keeps the tube in cutoff during RX and prevents the screen from melting during TX. Another real cheap tube that works good when you do these mods is the old 6DQ6B. You'll have to drill a hole to bring the plate connection up to the top cap and move 2 wires on the socket but at $8 for NOS on eBay, you can't beat it.
Even if the JB is wired correctly and has a 3 pole relay, it will still eat the tube alive in TX because of the 400 screen volts. Just key the mic and look at the tube. Not the outside plate either but the much more fragile screen wire under the plate. This will turn red almost as soon as you key. At the price of an 8417, you can't underestimate the importance of lowering this screen voltage.
Even with 200 volts on the screen and -40 volts on the control grid, the tube is still conducting because they have over 700 volts on the plate! To get the most modulator effect the tube should barley be conducting any idling current in TX so that the carrier has less amplification than the audio peaks. Since it runs in this non linear region between carrier and swing, it's not a linear at all.
You'll probably get best results keeping a constant -50 to -55 volts on the control grid and switching the new regulated 200 volts on the screen when in transmit. If the output is more than 10 watts dead carrier, adjust the screw on the back for 5 to 10 dead key. Some of the early models of these amps placed the front panel meter directly inline with the B+ high voltage and if the front plastic on the meter is broken, it can kill you if you touch it!
