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Old 24th September 2008, 09:43 PM   #1
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Default PS choke voltage/current quetsion

Hi,

Designing a power supply for an EL34 SE amp, have some questions around the voltage and current rating of the choke in a CLC supply.

What DC voltage rating is required? I had thought B+ plus some margin but using PSUD to model the supply it indicates 20-30v accross the choke which is obviously the voltage I am droping accross it. So do I need full B+ or is lower voltage OK?

Current rating. I am assuming I need a rating that covers total current draw of the amp - around 160ma I estimate. But I also read here http://www.aikenamps.com/Chokes.html that with a capacitor/choke filter you do not need the full current rating of the whole amp - confused.

have searched the forum to no avail!

Any help appreciated, thanks.
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Old 24th September 2008, 09:54 PM   #2
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Um, just ocured to me on the voltage - the frame is earthed so voltage between the winding and frame is full B+ right?

Cheers
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Old 24th September 2008, 10:11 PM   #3
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Default Re: PS choke voltage/current quetsion

Quote:
Originally posted by ChrisMmm
Hi,

Current rating. I am assuming I need a rating that covers total current draw of the amp - around 160ma I estimate. But I also read here http://www.aikenamps.com/Chokes.html that with a capacitor/choke filter you do not need the full current rating of the whole amp - confused.
Quote:
Originally posted by ChrisMmm
Um, just ocured to me on the voltage - the frame is earthed so voltage between the winding and frame is full B+ right?

Cheers
That article linked assumes that you aren't drawing the plate current through the inductor, which seems strange to me. You are making a single ended amp, so you definitely want that filtering for the output stage. I would say that a 100ma choke would work fine for one channel. I wouldn't worry too much about voltage, I don't think you can get high enough to stress out any of the chokes available.

You are correct about your earthing statement. I'm always shocked when I see people make tube amps with chokes screwed into wood frames. Leaving the channel frames ungrounded is asking for noise/issues. Also, grab some Keystone shoulder washers that go from a 1/4" hole to a #8 screw hole and insulate your iron from the chassis.

-Paul
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Old 24th September 2008, 10:25 PM   #4
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Thanks for that Paul.

Yes, the article confused me too!

Re voltage - the B+ in the amp I am building is 450v (its a Tubelabe Simple SE). I was going to use a Hammond 156R (well 2 actually - CLCLC configuration gives better ripple control according to PSUD) but noticed they are 400v rating - are you saying thats ok or should I move up to the 500v hammond range?

I am happy building lower voltage power supplies fo SS amps but tubes are a different kettle of fish!

Thanks again
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Old 24th September 2008, 11:14 PM   #5
cerrem is offline cerrem  United States
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Keep in mind to use the DC current draw you would get at FULL POWER OUTPUT..... Not the DC current at idle......
That is common mistake I see, where the inductor is saturated at full power output and then the intermods goes way up.....
The big offender to sucking up current is the screen grids.....El34's at idle figure anywhere from 4 to 8 mA ...... Then at full power output those things can get up to 24mA each right before clipping occurs....
ALso keep in mind Worst-Case-Analysis, meaning that many inductors you buy are +/- 20% .... So take that into account...
You are better off getting an inductor that is over-ratted....
When I figure the inductor I need.... I assume the MAX DC current at full ratted power output + AC flux density......
The Peak to Peak ripple current can be quite high depending on the value of Henries you choose.... Half of this value need to be converted to AC flux density to be added to the DC flux desnity to see how far the total excursion is pushed on the BH loop....

Chris
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Old 25th September 2008, 07:52 AM   #6
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If you're going to use a CLC filter, the choke will have a fairly easy time and if it's selected to carry the maximum current +50%. I would think it should be OK.

Maximum voltage rating is an indication of the strength of the insulation. If you think it's likely to be a problem, you can use the choke in the ground side of the power supply, which is what I do.
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