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In Reply to: RE: More questions regarding voltage regulator tubes posted by email@example.com on August 03, 2017 at 12:36:51
Vs 230VDC drop to 150V = 80V, so your calculation should not based on a 230V drop, but rather the 80V. (good news for you).
Load current is 10mADC
You have 10mA load current. Choose 15mA through the VR shunt, 25mA total.
And if you pull the load the VR tube can sink this 25mA with no problem, OK. We are talking DC current here.
If you want to drop 80V at 25mA = 3k2 (3k3) power dissipated is the DC current squared multiplied by the resistor value. In this case (0.025 x 0.025) x 3k3 = 2W. Use a 10W wire wound resistor and be done with it.
Edits: 08/04/17Follow Ups:
The B+ is 380 volts. The stage after that needs 150 volts @ 10ma. 380 volts less 150 volts =230 volts. I'm sorry if my original post did not make that clear.
I should have read it properly, the other poster has the better idea, something like this perhaps?.
You are stacking two 150volt tubes, what do you need the 10K resistor for? Why not just take the voltage from the bottom tube?
Up to you.. but it reduces the current through the upper tube from running at 83% of Pd max to 33%, probably aids with strike of the lower tube, and costs very little.
If no room for additional socket, can you reduce your B voltage some how?, choke input?, lower value C value for cap input etc. Higher forward volt drop rectifier.. etc etc.
Anyway, all the best with the project.
Do you not have enough room for another socket? You can put two VR tubes in series and decrease the size of that dropping resistor considerably.
150V isn't a lot for a B+ supply for a tube, is it a choke loaded tube stage?
The load is the primary of an interstage transformer. Unfortunately there is not room for another tube. Also after several hours of operation the heat builds up to the point where the top of the chassis is very warm. I've tried drilling ventilation holes but seems to make little difference. I'm concerned I am cooking the other components inside the chassis so I will probably remove the VR and replace with a dropping resistor which by my calculations should only generate about 2 watts of heat.
Just use a much larger (in terms of wattage rating) resistor than the calculations yield and the heat will be nicely spread out. For what you're doing, the regulator is worth it sonically.
After about 3 hours of operation the top plate was very hot. At this stage I was committed to using this chassis so I drilled many holes in the top plate but it did not seem to make any difference with regards to temperature. I used a 5K 25 watt resistor and a 3.5K 25 watt resistor, both chassis mount so no problem in terms of power rating. However heat is still heat and seems to build up. There is also 4 ohm 50 watt filament bias chassis mount resistor that dissipates about 6 watts. No room for a fan really and are noisy and cheap ones fail quickly.
The problem I believe is that the way I attempted to bias the tube fitted the plate curves which do not seem to be very accurate. At -5V bias, 150 plate volts and 10ma the VR tube should not have had to work very hard. At this bias and current the tube would prefer to see 170V. So I think that the VR tube is drawing a lot of current trying to maintain 150V and consequently the series resistor is generating a lot more heat than the "average".
If my understanding is incorrect please let me know. All I know is that having removed the VR tube from the circuit and using appropriately sized resistors to get my plate voltage results in greatly reduced heat buildup.
Chassis mount resistors are garbage in terms of the power they can actually dissipate. I'd recommend reading the entire datasheet on a well documented one like the big Vishay/Dale ones. You'll end up seeing a chart of what they can handle without a chassis, and at some point you should also end up seeing what they consider a "chassis", which may be a 12"x12" piece of aluminum 1/4" thick.
Xicon has some 25W wirewound resistors that aren't too expensive and are easy to find on Mouser.
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