Nick wrote: ↑Sat Feb 16, 2019 12:07 pm
1) with a DC heated DHT using Andrews DC board, is there a benefit to using virtual cathode resistors
When this question was asked, my answer was "as there is no disadvantage to doing so, and some theoretical justification, then may as well"
Now there is a downside, that justification is removed.
2) with a pair of DHT with a shared cathode, is there any reason not to wire the heaters in series
Is there any reason to expect two heaters to require the exact same current to run at the ideal temperature?
Thanks Nick. Let's explore both the downside and the upside to using VC resistors when a DHT is DC heated.
Downside is that they appear in parallel to the filament to the DC heater board.
When the filament is working they lower the overall impedance and take DC current. Nick was a great help to me in understanding the different currents and I put a spreadsheet together to calculate it. I settled on using 200R VC because it minimised the heater current through them.
When the filament fails, the VC resistors provide a current path for the DC heater board, which sees a big increase in impedance and reduces the output current accordingly to maintain the voltage across the onboard sense resistor. Andrew may want to correct my description here. The existence of the VCs makes filament failure detection more complex, though it can still be done (ala Mike).
Upside of the VC resistors is something I don't understand well enough to document.
I recall reading articles saying that DC heating meant that one side of the filament is at a higher voltage than the other and thus that the filament coating was likely to become depleted, implying shortened tube life. I don't know if that's true or not, or whether the charge space surrounding the cathode negates or minimises that problem. Nor do I know if as one end of the filament becomes depleted whether the other end naturally compensates, nor if the problem can be reduced by occasionally switching the polarity of the DC voltage.
I also don't understand how the Virtual Cathode resistors would help in this regard. Even with them in place they don't change the voltages at either end of the filament.
I'm glad you re-opened the question Andrew. I suspect there is little evidence one way or the other. I'll do a bit more digging and see if I can collect references to anything useful, I'm sure others can weigh in also. At the end of the day though, one of the big advantages of DIY is that I can prove it to myself by trying it.
I may well try changing the 26 valve pairs (which are easily accessed at the front) to series heaters, something like this.
There will be a voltage difference and thus a bias difference between the two valves, which could be minimised by taking the cathode connection to the joined filaments from the centre pins.