A question for Jim, Peter and the other Elmers. I take it you are persuaded by the ARRL book's argument that the 6C4 needs to run a relatively low plate voltage (200V, according to the schematic) in order to avoid oscillator instability and damage to the crystals? I must admit that until I read that bit about damaging the crystals, I was so-so about the idea of messing around with the existing dropper resistor. After all, the TX is working fine, and the tone is very nice -- I am dubious about whether there is any chirp at all, for instance.
Yes. Not that there's a guarantee of crystal damage, but why take a chance?
Note too that the 5763 is probably being overdriven if the 6C4 plate voltage is too high. (I would not change the 5763 grid resistor, either).
The logical first step is to try Jim's voltage divider solution. Reading his excellent explanation, I was able to understand it, although I could never have come up with the detailed current/wattage calculations on my own.
You would, with a little practice.
As for the other solutions:
--Zeners have been used by other modern builders of this circuit, but mainly because they were having serious stability/chirp issues, often caused by using "modern," much thinner quartz crystals.
--There's really not much room on that chassis for a couple of regulator tubes and it would ruin the "museum reproduction" aspect.
Also probably unnecessary overall. BTW, I would never
put VR tubes under a chassis - the glow is too pretty!
Concerning the 5763, as I understand it Peter, the issue is not with the plate voltage, which is currently well within spec, but the screen voltage, which is slightly over-spec at 260V (spec is 250) and greatly above the suggested ARRL voltage -- 195V, to "prevent excessive dissipation." I view this issue as much less important than the oscillator question. The worst thing that might happen, presumably, is that I could blow the 5763 (very unlikely?) and this tube type is still widely available. However, while I'm in there, I might as well go ahead and replace R5 with a somewhat higher value. The current value is 6.8K. We are aiming for more like 200V and we must avoid screen dissipation in excess of the maximum value of 15ma (from the RCA book) and perhaps aim more for the "typical" value of around 7ma. I tried doing the calculations but ended up tying myself in knots. What value should I try, and does it need to be a beefier power rating than the existing 1W? Surely not, screens don't take much current?
The 5763 screen voltage is quite important. Remember that the transmitter was designed for a particular set of operating conditions, which aren't necessarily those in the tube book.
We can figure out what the new resistor should be, easily enough.
In the original design, the supply voltage (key down) was 240 and the screen voltage 195. So the 6800 ohm resistor dropped the voltage 45 volts. Dividing 45 by 6800 we get .0066 amp - 6.6 mA.
In the version you built, the supply voltage is (key down) is 290 and we want the same screen voltage as originally - 195. So the resistor must drop 95 volts at 6.6 mA. 95 divided by .0066 gives is 14,394 ohms - 15 K or 12 K should be fine.
95 time .0066 is .627 watt, so a 1 watt or preferably 2 watt is adequate.
Ironically, if I hadn't measured those voltages, I would have been none the wiser and the TX is probably just fine as it is. Knowledge is a dangerous thing! (1968 Novice Ham probably didn't have a VTVM and maybe not even a VOM, and nowhere in the instructions does ARRL suggest measuring the voltages against the schematic ... that was my idea, silly me.)
1968 ham probably had lower line voltage and such. Plus in 1968 one could get all the FT-243s, custom ground to desired frequency, for $3 each, new. Surplus cost even less.
It never hurts to check.
For TR control I recommend a simple switch or a relay.
73 de Jim, N2EY
Logistically, I'm thinking that once the three 15K bleeders are removed, there will be room to build the V1 voltage divider on the power supply terminal strip -- and then run a wire from the center of the divider network, to V1. This would be much easier than trying to cram the resistors into the area around the tube socket, and would remove a source of heat from that area. Finally, I could "remove" the old R2 simply by cutting it off with wire snippers, which is much easier than un-soldering and re-soldering new resistors.
As you can see, there should be plenty of room on the PS strip once the bleeders are gone:
I need to order a few parts to carry out these modifications (voltage divider on V1 and larger dropper on V2), so there will be a time lag before we find out the result.
73 de Martin, KB1WSY