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Old 19th October 2020, 05:58 PM   #1311
smoking-amp is offline smoking-amp  United States
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I guess if all the plate outputs are individually AC coupled, then X*X + (-X)*(-X) cross coupling will work. But just using the X*X output alone should be fine here.

(the two BDT tubes over the CCS are used to get the grid 1 functions linearized, otherwise one would get X*(X^3/2) due to non-linearity of grid 1. Some cathode degeneration resistors could also be included for even better linearization here. Or, for extreme linearity, a third BDT could be used in place of the CCS in a split cascode like arrangement, so not using grid 1 control at all then. )

Last edited by smoking-amp; 19th October 2020 at 06:03 PM.
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Old 19th October 2020, 06:50 PM   #1312
Tubelab_com is online now Tubelab_com  United States
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I would also suggest getting some 6197 tubes...
The 6197 is a 6CL6 with a cathode coating formulated for "computer use" where long periods of operation (days) with zero cathode current are possible. I have several hundred of each tube, over a thousand 5686 tubes, and several boxes full of other military surplus and computer tubes from the 50's and early 60's. All are well used and have not seen power since the 1960's.

I did experiment with all of these during the development of my vacuum tube VCA (voltage controlled amplifier). The VCA started with a clone of the famous Fairchild 670 compressor circuit, build with genuine 6386 tubes (real expensive now) from the 1960's as my benchmark. I used the same Edcor transformers in each build to remove that variable from the testing. My vacuum tube VCA works as good as the 670 design, but uses a pair of $1 tubes and a pair of mosfet followers for buffers. The vacuum tube Moog ladder VCF (LPF) is not quite up to Moog specs yet though.

If I were to revisit the squarer circuit with opamps, it would be trivial. We (Motorola) developed a circuit in the early 80's to recover a data stream from the discriminator of an FM radio. We called it a "center slicer" and it used a single stage of an opamp or comparator chip. The raw DC coupled discriminator output fed one input, while the LPF (a few Hz corner) filtered signal fed the other input. The output of the chip is the data stream. The corner frequency of the LPF has to be high enough to track frequency drift between the receiver and the transmitter, but low enough not to follow some pretty slow data (around 100 baud). Max data rate was 2400 to 4800 baud.....it was the 80's and we were using 25 KHz RF channels.
Tubelab, I blow stuff up so that you don't have to.
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