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Old 11th December 2012, 04:42 PM   #1
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Join Date: Aug 2012
Default Tube Rectifier Limiter

Hi! I have another power supply related question that deserved its own topic.

I hear a lot about anode treatments to protect the rectifier but I am not entirely sure what is the best safe to longevity to ear ratio. The power supply design I'm using recommends on each plate to put a 100ohm resistor in series. The yellow page recommends not resistors, but diodes in series with each plate. The Valve wizard brings up every scenario ever made including the hybrid of the two; A limiting resistor with diodes.

Im using a Sovtek 5AR4… and has been noted to be not the most robust of all designs. Diodes recommended?

Im also trying to figure out if me putting resistors and diodes is gonna limit my B+ voltage much.

I have a 210V-0-210V transformer and I'm roughly shooting for 300…

I know I'm gonna be under a bit, but not a show stopper.

Anyone got any entry level tips for not blowing myself up?
I really mean limiter tips..
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Last edited by woodrough; 11th December 2012 at 07:57 PM.
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Old 11th December 2012, 09:40 PM   #2
FoMoCo is offline FoMoCo  United States
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I'd go for the resistors only and probably lower than 100 ohms.

If the datasheet has a certain recommended secondary resistance, use only enough resistance to increase the total to what the datasheet recommends.
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Old 11th December 2012, 10:15 PM   #3
DF96 is offline DF96  England
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Resistors and diodes do different things here. Resistors limit peak forward current. Diodes limit peak reverse voltage. Which is needed depends on how close to max rating you are running the vacuum rectifier.
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Old 12th December 2012, 01:27 PM   #4
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I can't seem to find Sovteks 5ar4 to find recomended anode limit resistor values... But it looks like from the tube cad site that 100ohms per leg looks like a general rule of thumb for dealing with inrush current. First off... Does it really work for those starting spikes? Or does it just drop the overall voyage more than anything else.

Even though I won't be pushing the rectifier to its limits, I do feel that putting some safety diodes in there won't hurt correct? It won't destroy the sag attributes of the tube and it definitely will prolong the life of the tube yes?

A hybrid of the two is wrong? Or is it too safe
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Old 12th December 2012, 06:34 PM   #5
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Also in addition:

ok, found sovteks 5ar4 datsheet. Looks like I dont have a capacitor or choke input. Being that this is off of a transformer... do I treat this as a capacitor input?
If so, my RMS is about 210Vac which is below their charted resistor values. But judging by what comes next, looks like I would need a 30ohm limiting resistor?
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Old 12th December 2012, 06:46 PM   #6
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Originally Posted by woodrough View Post
Looks like I dont have a capacitor or choke input. Being that this is off of a transformer... do I treat this as a capacitor input?
Uhm, you definitely have a capacitor-input circuit, if the first diagram you posted is the one you're using.

The term xxx-input refers to the first filter element that follows after the rectifier. Is it a cap, as in your case, the PS is capacitor-input, if it is a choke - then it's choke-input.

The datasheet lists the recommended values for the resistors in the anode leads, just choose the value that fits your operating conditions. And don't forget to take into account the secondary resistance of the transformer. If you need as low as 30 ohms, maybe you get away with the secondary resistance (along with the reflected primary resistance, which has to be taken into account as well). Look for 5AR4 datasheets of older 'real' tube manufacturers, they often give more detailed information on the calculation of that resistance.


Last edited by Rundmaus; 12th December 2012 at 06:48 PM.
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Old 12th December 2012, 06:50 PM   #7
DF96 is offline DF96  England
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The 'input' referred to is the first component after the rectifier, so you have capacitor input. I would say you don't need a resistor or diode, as you are running well inside the rectifier spec. The transformer secondary resistance will be much bigger than 30 ohms anyway.

That 'sovtek' sheet is actually just a photocopy of a standard US datasheet - RCA or Sylvania?
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Old 12th December 2012, 07:09 PM   #8
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Ahh! That makes more sense... misleading terminology though. This is good!
How do I measure the transformers secondary resistance? Just across the leads? Its a Center Tap (center out?). Or is it as vague as measuring speaker impedance.

But looks like I dont have to do anything... but it cant hurt to throw in a safety diode just in case ya?
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Old 12th December 2012, 08:26 PM   #9
DF96 is offline DF96  England
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Measure secondary from CT to ends and take the average, as they are likely to be slightly different. Then add on transformed primary resistance too.

The cap or choke input terminology is not misleading if you regard it as describing what the input of the PSU smoothing stages looks like when seen from the rectifier.

A safety diode, being unnecessary, merely adds one more component which could fail.
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Old 12th December 2012, 10:20 PM   #10
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Cool, screw diodes anyway. That's why I chose the rectifire in the first place. So I measured the resistance of the center tap and read 90ohms averaged from both legs. And read from the primary side 8ohms per 120v section. Being that I'm using 120v, I will be running the primary in paralell making it 4 ohms.

So 90 + 4 = 94 ohms! I think that's a healthy enough resistance for the rectifier ya? Does this sound right? Seems like it to me.

Also bonus side question; has anyone used a power transformer as an output transformer? I would like to say that you could do impedance matching etc. could you run something through a power transformer that is not 50 or 60 Hz? They are all made out of similar meat and potatoes right?
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