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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 15:04:27
How does the square wave heating affect the bias? With DC, you can shift the bias by the filament voltage depending which end of the filament you reference to ground. With AC, you simply take 1/2 the filament voltage and add it to the bias. I would guess you would do the same with the HFac but then making it a square wave makes me scratch my head a bit.
The reason I bring this up is I have a µTracer that operates off of a 20V laptop supply. In order to do the filaments it uses PWM off the 20V supply and alters the duty cycle to give the desired heating. What is interesting about this is that it samples each data point for 1ms and times it to sample when the PWM is in an off cycle. This means that the tube is actually traced with 0v on the filament and no filament gradient whatsoever which is very different that the gradient given from DC or AC sine heating. Moving this to HF square waves makes me see the bias voltage abruptly shifting the full filament voltage and wondering if that is a good thing.
thoughts?
dave
Follow Ups:
The technique I'm using applies ultrasonic AC to the filament in a manner that is both differential and balanced WRT ground. It does not affect bias. The circuit below is a simplified version. I've only used this with DHT output stages, so I apply bias to the grid. Bias could alternately be applied to the cathode, for instance by inserting a resistor or parallel resistor/cap combination in series with the wiper of the hum pot. The ultrasonic heater supply does not cause bias to vary in any case.
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Buy Chinese. Bury freedom.
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
"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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Buy Chinese. Bury freedom.
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
In Dave's cathode supply where one leg is tied to ground without a humpot, I can see where that may be an issue. I might experiment with this on a GM70 once I get the 120w units. Attached a picture of typical hum pot circuit.
Edits: 12/08/16
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