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In Reply to: RE: Why is Square / Rectangular Wave Deal of the Day? posted by Triode_Kingdom on December 07, 2016 at 21:38:47
Let me try to put this in a different manner. What you describe is the AC nulling out to zero but I am talking about the "static" DC bias position.
To take it a bit to the extreme for my illustration lets take an 841 and assume a 500V P-k and -10V of bias for 4ma of current.
Everything is all nice and tidy until we realize that there is a voltage gradient of 7.5V across the tube which means one end of the filament is at -10V and the other end at -17.5V What happens if we suddenly feed the filament with a -7.5V which is done by simply referencing the + side of the filament to ground? Suddenly the tube will draw in the realm of 13ma of current and if we switch to 7.5Vac the bias reference goes to the midpoint of the filament and we get 8ma. What has always interested me about AC heating is that the voltage gradient covers the entire range of + and - DC options plus a factor of 1.4 and at some point has a voltage gradient of 0 across the filament.
When we look at DC square wave heating from a DC bias viewpoint we are establishing an average bias point at 1/2 the heating voltage but is it any different conceptually that switching the bias reference point from the + to the - side of the filament at a given rate of time?
dave
Follow Ups:
"Suddenly the tube will draw in the realm of 13ma of current and if we switch to 7.5Vac the bias reference goes to the midpoint of the filament and we get 8ma."Maybe we have a miscommunication, but I think you're not grasping the concept of a floating, differential heater supply. The only ground reference is the tap on the hum pot. If that's electrically centered, bias voltage and current are unaffected by the AC heater supply, even when analyzed for instantaneous changes. Put this into SPICE, insert a 0.1 ohm resistor in series with the hum pot wiper connection, and you'll see no HF current flowing to ground. Yes, there's a constantly changing gradient across the filament. However, it's balanced with respect to ground, always sums to zero, and has no effect relative to bias voltage or the common mode signal path.
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Edits: 12/08/16
I see what you are saying, so if you place a similar reference for a DC supply filament polarity will not matter so reversing it at a high rate of speed is not an issue. I was caught up on one end or the other being a reference for DC and not considering what happens if you reference the middle.
dave
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