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Old 28th January 2007, 11:42 PM   #1
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Default Choke rating for choke input duty

Continuing the Aikido story.....

I managed to get my B+ down to 260VDC, but removing the 1.5uF cap.

This now means that I am running a 125-0-125 transformer from both ends to give 250V, going into a diode bridge, into a 10H choke; 60uF cap; 10H choke; 60uF cap. My outlet gives 123VAC.

So, I have a couple of questions:

1. Is 123VAC too much for a Hammond transformer rated for 115V? I am using the 269AX.

2. Can my chokes handle being used for a choke input filter? They are Hammond 193G rated for 150mA. I am drawing around 24mA from the circuit.

If I modelled this in PSUDII, what would I look for to see if the chokes can handle being in a choke input filter?

As far as the pesky hum, all I can say is that it's all quiet on the sn front. I lifted the ground by another 10 ohms to 20 ohm. If I put my ear to the speaker, I can hear a little bit of 60Hz, and may end-up with a 30 ohm ground lift.

Regards,
Charlie
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Old 29th January 2007, 04:21 AM   #2
2wo is offline 2wo  United States
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Charlie,

I just wrote a nice long response and the @#%$& computer ate it. The bottom line. Your good to go…John
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Old 29th January 2007, 04:31 AM   #3
Tweeker is offline Tweeker  United States
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If possible use more capacitance, youve got a really really high q filter cascade there. You may see large spikes at start up. The chokes are waay overrated for their duty, a lower current higher inductance model would be more typical. These chokes are wasted here, put them in a power amp. Id use a 159M choke 15H 256DCR and follow it with both caps for 120uF, with a bleeder if need be.

True choke input should give <225V DC (<240V given higher mains here) output. If 24ma is not enough draw to make the system act as choke input, you can add a bleeder resistor. If you must stay with the current chokes, use both in series for 20H and follow them with the 120uF, it should act as intended. If theres noise; youve most likely got a grounding issue, it should give output more than clean enough.
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Old 29th January 2007, 12:00 PM   #4
Merlinb is online now Merlinb  United Kingdom
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Default Re: Choke rating for choke input duty

Quote:
Originally posted by cbutterworth

2. Can my chokes handle being used for a choke input filter? They are Hammond 193G rated for 150mA. I am drawing around 24mA from the circuit.

If I modelled this in PSUDII, what would I look for to see if the chokes can handle being in a choke input filter?
On PSUDII look for the current flowing in the choke input. The choke will need to be rated for the peak ripple current, which I suspect will be about 100mA, so you should be ok.

Why are you using choke input at all?
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Old 29th January 2007, 01:05 PM   #5
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With a solid state rectifier, always use a snubber cap (.05-.22, 1KV or more) or an MOV across the input - the inductor will kick back a voltage s[pike on turn-off, which can take out a rectifier sooner or later. With cap input, you have to consider peak current, with choke input, it's peak voltage.
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Old 29th January 2007, 01:20 PM   #6
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I get an ac ripple current of ~19ma for a 10H choke @ 267VAC RMS input (123VAC mains assumed) for a total of 43ma+ bleeder current seen in the choke. Hammond rates its chokes conservatively.
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Old 29th January 2007, 02:35 PM   #7
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Quote:
Originally posted by Tom Bavis
With a solid state rectifier, always use a snubber cap (.05-.22, 1KV or more) or an MOV across the input - the inductor will kick back a voltage s[pike on turn-off, which can take out a rectifier sooner or later. With cap input, you have to consider peak current, with choke input, it's peak voltage.
should I put caps across the negative UF4007 legs in a 6CA4 hybrid bridge? B+ is around 360V (tubes should draw more current and bring B+ down to 300V)...
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Old 29th January 2007, 03:32 PM   #8
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Great comments and advice. At least for the present, I'll continue with the PSU laid out as it is, I suppose that I could go in search of some low value caps (less than 1uF) to place before the first choke.

I should certainly use a snubber, but where exactly should this be placed? Should I put it in the same position as I would the cap for a cap-input filter? Or should I put it over the on-off switch?

Finally, I opened up my solid-state power amp (AKSA 55N) to remind myself of the grounding work that I did when I made it. The star-earth is connected via a single ground cable to the chassis near the IEC inlet. I may redo this with some beefier cable, and redo the earth bolt.

So....if I am still getting some hum, which I do (in part because of a semi-lifted ground-loop, as well as poor tubes), would I be likely to gain anything (or lose hum) by adding a ground-lift resistor/cap network (say 20 ohms) to my power amp??

Thanks,
Charlie
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Old 29th January 2007, 03:53 PM   #9
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Put the cap from input to ground - it's to protect the rectifier from the reverse voltage if current is suddenly interrupted (as it is every time it's switched off...).

As for hum... if you're lifting the SAFETY ground, a pair of back-to-back 25A diodes (use a bridge) will lift if and maintain a safe path for short circuits.
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Old 29th January 2007, 05:23 PM   #10
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Tom,

Please could you be real specific about the location of the cap. Is it at the switch? I doubt it. Is it across the output of the bridge rectifier? Oh....I already take the +ve output of the bridge rectifier through a schottky prior to entering the first choke.

As for the ground lift, I have implemented the following circuit:

http://sound.westhost.com/earthing.htm

Thanks,
Charlie
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