That's good to know. I have esssss-loads of 12BY7As, and a possible project for them.
I may switch over to using some duall dissimilar vertical deflection triodes for the fron end of this amplifier, which can possible solve two problems 1) getting sufficient gain for good drive to the trioded 6CB5As (they're voltage-hungry, just like any other low mu triode), and 2) low Rp in the driver tube. Some of the trioded video driver pentodes are ok in that department, but a number of the vertical deflection tubes have high power triodes that kick butt in terms of Rp, and some also have transconductances that are fairly respectable.
Look at the possibility of using a tube that has pin compatible "friends". I am thinking of the 6GF7 - 6KY8 - 6LR8 combination, but there are others. The 6GF7 is a dual dissimilar triode. The 6KY8 is a mid sized triode pentode. The 6LR8 is a larger triode pentode. The socket can be wired so that any one of the 3 can be plugged in. In fact in a little "spud" SE amp with an LED biasing up the input triode, the only thing I had to change when swapping tubes was the B+ voltage.
I'm more concerned with the gain distribution between the two halves of the tube. What I want will depend on how I ultimately structure the drive. If I go common cathode + common cathode with an isolated interstage XFMR/splitter loading the plate of the second triode, I'll want relatively low mu in both triodes - this limits my options. If I put all the gain in the first triode and use the big second triode as a cathode follower driving an autotransformer splitter, the options open up quite a bit. This may be the path I ultimately pursue.
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I think I've made up my mind to try a high gain front end first, with a cathode follower driving an autotransformer phase splitter. The leaves me with a lot of options as to appropriate dissimilar vertical osc/amplifier tubes, including 9-pin miniatures, 9-pin Novars, and a couple of octals. I have a certain 9-pin mini in mind for first trials.
I also think I have the construction details of the autoformer down such that I can try winding a couple of trial bobbins. I'm shooting for low leakge and capacitance, as well as matched resistance between winding halves.
I scored some leather punch and notching tools on E-pay, which should be useful for notching the bobbins where needed. More will be reported when I actually wind up a couple of bobbins.
I also think I have the construction details of the autoformer down such that I can try winding a couple of trial bobbins. I'm shooting for low leakge and capacitance, as well as matched resistance between winding halves.
I scored some leather punch and notching tools on E-pay, which should be useful for notching the bobbins where needed. More will be reported when I actually wind up a couple of bobbins.
hey-Hey!!!,
Try one like this; two bay bobbin, one bay wound reverse of the other. WInd T/4 in each bay, then continue T/2 turns worth in the other bay, and finish T/4 just as you started. Capacitance will not only be low, it will be the same for both halves, and both will have the same DCR.
cheers,
Douglas
Try one like this; two bay bobbin, one bay wound reverse of the other. WInd T/4 in each bay, then continue T/2 turns worth in the other bay, and finish T/4 just as you started. Capacitance will not only be low, it will be the same for both halves, and both will have the same DCR.
cheers,
Douglas
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