The small 12-0-12 vac xfmr provides 24 vac, that is fed to the doubler....(CT not used). The resulting 68-70 vdc is used to speed up all 4 of the 26.5vdc vac relays. The 120 ohm @ 50 watt metal finned resistor is used as the series drop resistor. The 200 uf lytic in parallel with the resistor is used to further speed up the 4 x vac relays. It does this by extending the time the overvoltage is applied, by a split second. This is called a ....'RC hotshot circuit' and is described by Jennings, Kilovac, and also gigavac.
Be careful though, the resistor and 200 uf cap form a RC time constant.... = 24 msecs. (.0002 X 120 = .024 seconds, or 24 msecs). When amp is un-keyed, it takes 24 msecs for the 200 uf cap to fully discharge through the paralleled 120 ohm resistor. We want the cap fully discharged before the VOX / PTT is activated again. You don't want an RC constant much more than 30 msecs for VOX / PTT use.
For QSK CW, 24 msecs is way too long. 1-2 msecs is about the max you can use for QSK CW.
The way the over voltage works is..... when T=0, the relay coil is inductive, and impedes current. Most of the relay's operate time, (80%) is used just to develop enough magnetism in it's coil. The remaining 20% of the operate time is actually the time it takes the COM contact, to physically move from the NC contact...over to the NO contact. The initial overvoltage speeds up the magnetism buildup. Once full current flows, you then get a V drop across the series resistor. Just ohms law to size the resistor. 70 vdc over voltage - 26.5 vdc coil voltage = 43.5 vdc required drop across the resistor. 43.5 vdc / total loop current (sum of all 4 x relay coil currents) = drop resistor value. Total Loop current squared X resistor value = resistor dissipation.
With the lytic in parallel with the drop resistor, when T=0, ALL the current flows into charging up the lytic. IE: the discharged cap initially looks like a dead short.... which also shunts out the resistor. The ENTIRE overvoltage is applied to ALL the relay coils. Once the cap is charged up, current no longer flows into the cap, (caps block DC), and THEN the current flows through the resistor....resulting in the required V drop across the resistor.
On buddy's YC-156 amp, he used a 120 to 120 vac isolation xfmr, and ran the 120 vac from the secondary winding to a FWB, = 170 vdc. The 170 vdc overvoltage is wired through an even bigger value drop resistor, to speed up his paralleled Gigavac G2 relays on the output of the amp. (26.5 vdc coils in parallel). Contacts also in parallel...and done such that the RF current splits 50-50. He's a QSK CW operator, so the max value of cap in parallel with the drop resistor could only be .5 uf. The G2 (same as a RJ2B) is rated for 15 msecs operate and 9 msecs release. With the above RC hotshot setup, we managed 2.4 msecs operate and release..which is plenty good enough for QSK use.
Note, Jennings / kilovac/ gigavac's op/rls times are actually for the pickup voltage, not the rated operating voltage. Any Jennings's RJ1A, although rated for 8 msecs operate, is actually just 2-3 msecs with 26.5 vdc applied... which can be sped up with an overvoltage, resulting in 1 msec op/rls...including contact bounce.
When lower overvoltage's used, bigger value caps can be used, and vice versa. (Assuming the RC time constant is the same in both cases). IF you are voltage limited, the 12 vdc version of these vac relays can be used. IE: 18-30 vdc can easily be used to speed up a 12 vdc vac relay coil.
