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Old 17th August 2010, 10:51 AM   #1
M Gregg is offline M Gregg  United Kingdom
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Default B+ discharge times!

Just a thought,

Has anyone ever found out if the discharge times of B+ supply is any different with tube or semiconductor rectification?

Just a thought after a previous post!

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M. gregg
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Old 17th August 2010, 11:15 AM   #2
Arnulf is offline Arnulf  Europe
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Huh ? If you're referring to B+ subsiding after power-off this is a matter of energy storage, not rectification method. Capacitors store charge and until this charge is depleted B+ is non-zero.

You can deplete it sooner by adding load across the capacitors (to "bleed" the charge off) but too large a capacitor will affect filtering ability of the capaictors adversely, not to mention dissipation on bleeder resistor. A sane value will be in the range of 10+K or even 100+K, possibly with an indicator (LED) in series.

This however has nothing to do with either rectification method (once you cut the power there is no further source of energy ahead of the rectifier to feed the B+ rail from).
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Old 17th August 2010, 11:23 AM   #3
M Gregg is offline M Gregg  United Kingdom
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I was thinking about reverse leakage if there is any difference in the discharge time after switch off using the tube or the semiconductor.
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Old 17th August 2010, 11:31 AM   #4
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Perhaps after switch off, B+ caps might find a way to discharge through the still hot cathode of the rectifier valve?
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Old 17th August 2010, 11:48 AM   #5
M Gregg is offline M Gregg  United Kingdom
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Quote:
Originally Posted by Soonerorlater View Post
Perhaps after switch off, B+ caps might find a way to discharge through the still hot cathode of the rectifier valve?
There is no perfect diode as far as I know and tubes cool and semiconductors remain in the same state.

Perhaps one for wavebourne...LOL

I must admit I have never checked because the loading of circuits will change the time!

I suppose the only way to know would be to charge a Cap with both and time the B+ volt drop at power off!

Just to make it more interesting if the heaters cool before B+ is Low do we not have the same as B+ on with no heaters?

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M. Gregg

Last edited by M Gregg; 17th August 2010 at 11:57 AM.
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Old 17th August 2010, 12:02 PM   #6
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I once changed a leaky B+ cap in my one and only valve rectified amp (EZ81) with a new modern cap. It still lost it's charge after switch off. After much scratching of the wooden stuff I decided that valves just do that and until now never looked back.


Perhaps I need to go looking for another leaky cap
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Old 17th August 2010, 12:10 PM   #7
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Quote:
Originally Posted by Soonerorlater View Post
I once changed a leaky B+ cap in my one and only valve rectified amp (EZ81) with a new modern cap. It still lost it's charge after switch off. After much scratching of the wooden stuff I decided that valves just do that and until now never looked back.
It's not anything inherent about valve amplifiers. It may indicate the designer who made the circuit included a "bleeder" resistor from supply to ground - a safety device intended to discharge the B+ supply after the amp is turned off. It might be around 150K ohm. It will only draw a few milliamps while the amp is running, but it will ensure the caps don't stay charged up when the amp is off.

Leaving capacitors charged with hundreds of volts is dangerous. Someone might turn the amp off (and maybe even unplug it) and think it is safe to go troubleshooting inside.
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Old 17th August 2010, 12:12 PM   #8
M Gregg is offline M Gregg  United Kingdom
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B+ still up with no heaters or warm heaters makes me wonder about people using expensive 211's etc.
There are a lot of amps out there with no discharge resistors.

Its interesting we put B+ delay on (important) B+ still up on power off with warm or no heaters (It's O.K.)

Last edited by M Gregg; 17th August 2010 at 12:28 PM.
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Old 17th August 2010, 12:35 PM   #9
SY is offline SY  United States
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Two things:

Discharge through any reverse leakage in the rectifiers is a looooooong process. Bleeders totally dominate.

If you use a temporary bleeder for discharge, beware the DA voltage. You can have 20V or more come back on an electrolytic after you thought you discharged it. That can be a major hazard if you brush those terminals accidentally with something conductive (screwdriver, wedding ring, scope ground clip...).
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Old 17th August 2010, 12:37 PM   #10
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Quote:
Originally Posted by Arnulf View Post
A sane value will be in the range of 10+K or even 100+K, possibly with an indicator (LED) in series.
That's a jolly good idea - one can check the resistor is working when the set is on - as the led will be lit!!

The spookiest live cap I found was on a Pioneer sx838 (transistor) that leaves the capacitors charged at 40V per rail for weeks, just a strange effect leaving no drain path through the two connected amplifiers when off!
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