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Old 24th December 2003, 08:17 PM   #1
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Question inrush current

Hello, I'm restoring a Leak stereo 20 tube amp, I plan to change all the caps plus a couple of valves.
My question concerns the power supply caps 32+32 uf 350v,
I have some mallory 125+125 uf 350v caps to hand but was worried at overstressing the rectifier on switch on, could I use a thermistor to smooth the switch on by limiting the inrush current, if so does anyone know what value thermistor to use?
Sorry the question goes on a bit but tubes are not my forte!
Thanks, Jules
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Old 24th December 2003, 09:37 PM   #2
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You can use the same device that Nelson Pass uses in his Aleph's and such to limit inrush Current. They are sold by Digi-Key are dirt cheap and work well. They also come in differeing values and be careful as they do run hot as they pass the full curent of the amiplifier. They are very similar to a thermistor.

You can find them here in the catalog in the lower right hand corner.
http://dkc3.digikey.com/pdf/T033/0821.pdf

Mark
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Old 24th December 2003, 09:44 PM   #3
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If you're keeping the tube rectifier, it will itself limit inrush current. But a 125uF input cap may overstress it during the charging part of each cycle- I'd stick to the original vaue, them build up the second cap.
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Old 25th December 2003, 01:53 AM   #4
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Hi,

Or use a hybrid FWB and stop worrying...

Cheers,
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Old 25th December 2003, 05:19 AM   #5
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Quote:
Originally posted by fdegrove
Hi,

Or use a hybrid FWB and stop worrying...

Cheers,
Erm... how's that allow more peak current in the tube diodes?

Tim
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Old 25th December 2003, 11:04 PM   #6
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Hi,

Quote:
Erm... how's that allow more peak current in the tube diodes?
That's not the problem in the first place...

The hybrid FWB will allow a bigger cap after the tube rectifier, I don't it'll will arc over.

Cheers,
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Old 26th December 2003, 12:03 AM   #7
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Quote:
Originally posted by fdegrove
That's not the problem in the first place...

The hybrid FWB will allow a bigger cap after the tube rectifier, I don't it'll will arc over.
I'm not sure what you typo'd there, but I assume a "hybrid FWB" is one with the tube for positive, and SS diodes for negative. This turns out electrically the same as a FWCT with just the tube and an extra diode drop (and whatever negligible switching characteristics that entails), and does not change the peak current it sees. Thus 125uF will still put it over the limits.

If it is in fact a topology where the tube acts as a series resistor handling just the DC current plus a little ripple, then it will be safe.

In any case, if you're having trouble with power-on surges, a larger fuse or a slow-blow should fix it. I would suspect excessively sized filter caps (like, >1000uF) if SS rectification (this obviously isn't the case), or heaters. Power-on surges shouldn't matter for equipment under say, 300W power consumption.

Tim
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Old 26th December 2003, 12:32 AM   #8
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Hi,

There is NO problem with the original caps and rectifier until they go up in size considerably.

Obviously the hybrid rectifier relieves the tube rectifier from most of this strain as the conduction pattern isn't the same and 125F behind the rectifier won't make the tube arc over.

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Old 26th December 2003, 05:38 AM   #9
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Whatever. You are the one with bags of money for buying rectifier tubes, for instance, after all.

Tim
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