resrvoir cap not draining

[Smokebreak]

Amp is typical AB763 type PS.
SS rectifier>reservoir cap>standby>choke.
I'm usingle a single cap(in multicapcan) for the reservoir, not series caps, and so no bleeder resistor.
I've always read "Caps can hold their charge for days...", but never found that to be true, even without a bleeder. Well this amp has been unplugged for days and I've got 150VDC at the res cap.
Is this going to shorten the life of the cap? I've built other amps with this setup, and the caps always drain. Any ideas why this one won't drain "on its own"?
Another way to ask this...why does the Express have series bleeder resistors for the res caps that are in parallel?

[NickC]

Bleeder resistors drain the charge from caps. Otherwise, those PS caps can hold onto enough energy to potentially kill you, even after the amp has been off and unplugged from wall AC for weeks (or possibly longer). Your meter is telling you the truth of it.

Even multi-section can caps can be wired with bleeder resistors.

Another strategy is to leave the amp in play mode (standby not active) when powering it down. That can also serve to drain the PS reservoir caps. But for my money, I always build in the bleeder resistors. And I always meter the caps before working on a amp. Better safe than sorry.

[Reeltarded]

I always gank a big hard chord and flip the amp off. It sustains to extinction. (about 20v)

THEN I put a drain on the plate of V1.

[Leo_Gnardo]

What NickC said ^^^. Also, holding a charge for days weeks months will not have any adverse effect on the cap. Only you, if you get zapped.

An early form of capacitor was called the Leyden Jar. An early scientist who caught a jolt from one wrote "I thought is was all up with me." Try not to go there. I've been hit with 6000 yes six thousand volts and it kicked me across the room about 25 feet. Not recommended. I was poking a tube plate with a pencil - big DUH! Learned not to do that anymore. A friend who's a lineman took a 10 kV hit from a power line & burnt the skin off his hands. Lucky to be alive. When in doubt, discharge. I like hearing that pop noise when I do - because I know I did the right thing.

Nobody likes to get a shock, except - - -

[Smokebreak]

Bleeder resistors drain the charge from caps. Otherwise, those PS caps can hold onto enough energy to potentially kill you, even after the amp has been off and unplugged from wall AC for weeks (or possibly longer). Your meter is telling you the truth of it.

Even multi-section can caps can be wired with bleeder resistors.

Another strategy is to leave the amp in play mode (standby not active) when powering it down. That can also serve to drain the PS reservoir caps. But for my money, I always build in the bleeder resistors. And I always meter the caps before working on a amp. Better safe than sorry.

Oh for sure, but there are plenty of designs without them, which leads me to believe there is a design element involved(besides just keeping us safe), as I understand the bleeding effect to discharge the amps is merely a side effect of their main purpose : to keep voltage somewhat equal between series caps. So when I see them in amps without series caps, it makes me go hmmmm...are they really spending the money to save a tech the time it takes to manually bleed it off?
I guess I've just never actually witnessed caps holding charge for that long, either

[NickC]

.........
I guess I've just never actually witnessed caps holding charge for that long, either

You don't want to be the path to ground that makes you the witness. Trust me on this.

[Reeltarded]

I have, but the real scary is when the drain falls off with no bleeder, and the cap recharges to 90v.

[Smokebreak]

.........
I guess I've just never actually witnessed caps holding charge for that long, either

You don't want to be the path to ground that makes you the witness. Trust me on this.

Right on, and this is the bigger point here.

[Leo_Gnardo]

So when I see them in amps without series caps, it makes me go hmmmm...are they really spending the money to save a tech the time it takes to manually bleed it off?

I don't think so. Sometimes the designers just want to have a litlle more control over power supply voltages. The safety factor is a fortunate side effect. I've seen some older amps - RCA PA amps for instance - where the bleeder follows the lowest-voltage HT node, instead of across the highest voltage.

[NickC]

I have, but the real scary is when the drain falls off with no bleeder, and the cap recharges to 90v.

Exactly. Makes me wonder if PS caps are sadistic little bastuds. After one of them volts you, they send out a call to their neighbor caps "Hey! Loan me some charge, will ya, this guy is still alive." I hate that.

[Smokebreak]

So when I see them in amps without series caps, it makes me go hmmmm...are they really spending the money to save a tech the time it takes to manually bleed it off?

I don't think so. Sometimes the designers just want to have a litlle more control over power supply voltages. The safety factor is a fortunate side effect. I've seen some older amps - RCA PA amps for instance - where the bleeder follows the lowest-voltage HT node, instead of across the highest voltage.

Yes I seem to recall a D-type that did that at the last node. So what can I expect if I hang a 220K to ground? I would imagine less B+, but a negligable amount, sonically speaking

[Cantplay]

You could use a double pole toggle for power, and in off position have it switch in a drain.

john

[Leo_Gnardo]

Yes I seem to recall a D-type that did that at the last node. So what can I expect if I hang a 220K to ground? I would imagine less B+, but a negligable amount, sonically speaking

At the high end of the B+ string, correct, hardly a difference. At the low end, it will pull down the voltage noticeably and most especially on the lowest voltage node. Think of all those power supply resistors in a string - it's a voltage divider made of what, 3, 4, 5, 6 resistors. Depending on what your ears want to hear, having a lower plate voltage on your first stage of preamplification could be a good thing. Try a bleeder at one end then the other, listen and choose.

[boots]

Something that I think some guys do that CAN hurt the caps is discharging them with a direct short. We've all heard that you should short across the cap with a screwdriver to discharge it. Don't do that!

Caps don't like to discharge instantaneously through a dead short. At least not if they have very much charge left in them. And the snap and spark is scary too. Always discharge through a resistor.

[tubeswell]

On what Leo said, you can use a higher-resistance (470k-1M) bleeder on the pre-amp supply node without it pulling the HT down too much. It will just take longer to discharge when the amp is off.

I use bleeders on all my amps, and I do this in a way that doubles as a voltage divider for heater elevation.

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