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Anode series resistor

 

Dear Members

I am trying to design a PSU for a simple ECC88 line stage with a broken PSU. The original one has SS rectification but I'd like to rebuild it completely using EZ81 followed by CLCLC filtering. The reason for the EZ81 is that I have a lot of them! I used the super-cool PSUD2 software but I have some problems because it says that the input current at start is too much. I need to feed a 40mA load at B+ = 250V?

The max input cap suggested for the EZ81 is 50uF so I played around with values like 30uF and 25uF but the PSUD2 still says that there are problems. I should keepp the first cap down to 10-15uF to solve the problem... but I've never seen such a small input cap in a CLCLC filter.??

Now, searching the web I found people saying that with the EZ81 it is important to reach the series resistance stated in the datasheet. My transformer has a secondary rated at 70 Ohms so probably I need something around 100 Ohm. This resistor should go between the secondary and the EZ81's anode, but it seems it is not possible to add it on PSDU2. I am almost sure that this is the same setting? 170 Ohm in the secondary in PSUD2, am I?right?

The other thing is about experiences with the EZ81. If somebody worked with this tube and he's willing to share some experiences, it would be great to get any suggestion!

Regards
Pierre?
 

Hi Pierre,
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PSUD2 should be able to deal with this for you. If you measure the leg of the transformer at 70 ohms, simply add the required resistor value (100 ohms) to give your secondary of 170 ohms.
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Double click on the transformer icon to access the transformer properties box:
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The press the [...] button shown by the arrow. This will bring up the Source Impedance Calculator:
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Enter your off load voltage on one leg and the winding resistance, again for one leg. In reality, you may find the two secondary legs have a different resistance so just average two. Note that I've used 170 which is 70 plus the 100 extra. PSUD (in this example) calculates the source impedance as 230.6 ohms, however it is taking the effect of the primary into account too.
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Click OK and it will feed the transformer parameters back to PSUD2 and you're good to go.
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Having a small input cap on CLC filters is OK, it helps to increase the conduction angle and reduce the peak current in the rectifier. Hope this is helpful.
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Regards,
Duncan
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I think PSUD assumes the full load is there at startup unless you set a delay. Normally, the cold valve does not conduct for a few seconds. The anode resistors are calculated for full voltage and current, so the transformer winding is normally enough on its own. If a cathode follower is involved and you are using NOS valves, watch the cathode-heater voltage. The modern 6922 equivalents have a much higher rating.?

Richard


On 24/09/2024 08:48, Duncan Munro wrote:

Hi Pierre,
?
PSUD2 should be able to deal with this for you. If you measure the leg of the transformer at 70 ohms, simply add the required resistor value (100 ohms) to give your secondary of 170 ohms.
?
Double click on the transformer icon to access the transformer properties box:
?
?
The press the [...] button shown by the arrow. This will bring up the Source Impedance Calculator:
?
?
Enter your off load voltage on one leg and the winding resistance, again for one leg. In reality, you may find the two secondary legs have a different resistance so just average two. Note that I've used 170 which is 70 plus the 100 extra. PSUD (in this example) calculates the source impedance as 230.6 ohms, however it is taking the effect of the primary into account too.
?
Click OK and it will feed the transformer parameters back to PSUD2 and you're good to go.
?
Having a small input cap on CLC filters is OK, it helps to increase the conduction angle and reduce the peak current in the rectifier. Hope this is helpful.
?
Regards,
Duncan
?
 

Thank you so much!??
?
Pierre

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