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Old 19th June 2009, 07:56 PM   #1
flysig is offline flysig  United States
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Default Heater to Cathode max voltage specs, including signal swings?

In a cathode follower circuit the cathode can swing over a very large voltage range. Does this need to be considered when elevating the AC heater voltage, or is the heater to cathode voltage rating only at zero signal conditions?

For example, my circuit has a cathode follower biased with the cathode at 140v and an input signal of +-47v. So the cathode swings from 93v to 187v. The cathodyne phase inverter has the cathode biased at 60v and an input of +-20v, so the cathode voltage swings from 40v to 80v. Both tubes have a max heater to cathode spec of 100v.

What voltage should the heaters be raised to? If signals are considered, these tubes need different heater elevations. Without considering signals, they could use the same heater supply. The rest of the tubes can all run on the same heater supply raised to about 50v.
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Old 19th June 2009, 10:19 PM   #2
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A lot of designs elevate the heater volts offset to around 50 volts.
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Old 28th June 2009, 09:22 PM   #3
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For most tubes the signal is important in one direction but not the other. Take a 12AT7 for example. The heater can only have 100V dc +ve max with respect to the cathode but the dc + peak can be 200V max. So you can raise the heater to 100v dc wrt the cathode and the cathode signal can then go 100V -ve so the dc + peak is 200V. The only circuits where a heater is commonly dc +ve wrt a cathode is in the bottom half of an SRPP for example.

A 12AT7 can also have its heater up to 200V -ve wrt its cathode (dc + peak). So if you have a cathode follower with the cathode at 140V say you could actually have its heater at ground. The cathode could swing up from 140V to 200V and down to 0V no problem. However that does not give the maximum swing so you might elevate the heaters to 70V. The cathode can now swing up to 70 + 200 = 270V up and down to 0V.

HTH

Cheers

Ian
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