Hello!
I'm wondering at the best way to ground 300mA heaters powered by a DC current source. I can see 3 options, probably there are many more. I use PL509 as an example, but this could be any P* tube:
I discard A, because the heater is positive against the cathode, and some datasheets only specify negative heater against the cathode.
Option B is fine, but we have both ends of the heater at a different voltage. Probably means nothing and it's a valid approach.
Option C is my preferred one, resulting in the same potential on both ends of the heater. There is, of course, a bit of current lost in the divider, and the resistor values are quite high.
I'm probably overthinking this, especially taking into account that P* tubes are designed for relatively high values of voltage tolerance between the heater and the cathode, but still would like to hear opinions, especially when doing tube measurements for gm, Ia vs Vg1 etc, like typically done in a tube tester.
Regards,
Jose
I'm wondering at the best way to ground 300mA heaters powered by a DC current source. I can see 3 options, probably there are many more. I use PL509 as an example, but this could be any P* tube:
I discard A, because the heater is positive against the cathode, and some datasheets only specify negative heater against the cathode.
Option B is fine, but we have both ends of the heater at a different voltage. Probably means nothing and it's a valid approach.
Option C is my preferred one, resulting in the same potential on both ends of the heater. There is, of course, a bit of current lost in the divider, and the resistor values are quite high.
I'm probably overthinking this, especially taking into account that P* tubes are designed for relatively high values of voltage tolerance between the heater and the cathode, but still would like to hear opinions, especially when doing tube measurements for gm, Ia vs Vg1 etc, like typically done in a tube tester.
Regards,
Jose
Hello Jose,
as a general rule, in serialized heater strings, usually the tubes with the smallest signals should be closest to the grounding point of the heaters string, regardless of AC or DC constant current heating. Power tube and rectifier heaters in the chain usually are situated farest (most positive) from the grounding point, since they usually also have the best heater/cathode maximum voltage ratings.
In your special case of a single, fat power tube it probably doesn´t matter or hurt too much to use figue C), but anyway just standard rules apply if you want to make it "perfect": 1) Elevate the virtual heater middle connection (joint of R1 and R2) to about 40V above gnd to get the least noise from the heather / cathode layer diode and 2) also be save not to exceed maximum heater / cathode rating.
Kind regards, Tom
as a general rule, in serialized heater strings, usually the tubes with the smallest signals should be closest to the grounding point of the heaters string, regardless of AC or DC constant current heating. Power tube and rectifier heaters in the chain usually are situated farest (most positive) from the grounding point, since they usually also have the best heater/cathode maximum voltage ratings.
In your special case of a single, fat power tube it probably doesn´t matter or hurt too much to use figue C), but anyway just standard rules apply if you want to make it "perfect": 1) Elevate the virtual heater middle connection (joint of R1 and R2) to about 40V above gnd to get the least noise from the heather / cathode layer diode and 2) also be save not to exceed maximum heater / cathode rating.
Kind regards, Tom
Agreed, but the voltage is halved.
No free lunch anyway.
My application is static (tube tester), not for audio, therefore this is all probably irrelevant, doe to the fact that 300mA tubes have always quite a decent max Vkh.
Good suggestion, thanks! I'll read a bit about heather to cathode impedance.