Bleeding caps safely

[EFK]

Can someone offer a definitive yes or no answer on the following methods of bleeding voltage off the amp to ensure safety when working inside? I've read of many methods but tough to find definitive, 'yes this is safe, or 'no you're going to get zapped.'

(1) Amp powered on and standby off - ready to play in other words - pull power cord out of wall.

(2) Jumper from pin 1 of preamp tube (lets say V1 unless it doesn't matter which) to ground. Any sparks or fireworks here?

[Diablo]

Can someone offer a definitive yes or no answer on the following methods of bleeding voltage off the amp to ensure safety when working inside? I've read of many methods but tough to find definitive, 'yes this is safe, or 'no you're going to get zapped.'

(1) Amp powered on and standby off - ready to play in other words - pull power cord out of wall.

(2) Jumper from pin 1 of preamp tube (lets say V1 unless it doesn't matter which) to ground. Any sparks or fireworks here?

Method 1 does not necessarily work on all amps because the large filter capacitors can still hold a large charge (high voltage). If there are bleeder resistors built into the amp, then this method can work if you allow enough time for the charge on the capacitors to bleed off through the resistors to ground.

Method 2 will work, but I would add a large capacity ( high wattage and low resistance) resistor in the middle of your jumper cable so you don't get any big sparks. I use a 50W-300 ohm resistor to dissipate the energy. Make sure your jumper cable is well insulated so you don't accidently become part of the grounding path. I like to ground out pin 3 on my power tubes because I can hear when the cap charge is gone by listening through the speakers. You will be surprised how long the charge takes to discharge completely. Then wait a minute and go back and discharge again through pin 3, and you will find out there is still some charge.

[Enzo]

I'll agree and offer a different outlook.

In many amps, the standby switch does nothing to the discharge process. MAny Marshalls for example switch off the high voltage AC behind the rectifier for a standby. So the bridge rectifier diodes will block any discharging there. You are at the mercy of any bleeder resistors. You can just pull the plug from the wall, and it won;t matter which way teh standby is set.

On other amps, the standby is between the first filter stage and the second. SO lacking bleeders, the amp can discharge all except the main filter stage. You don;t want that, so there you SHOULD leave the standby switch in the run position.

The message so far then is to always leave the standby in RUN whether it matters or not.

If the amp has bleeder resistors then, the amp will discharge itself. If it doesn;t then it sits there charged. I don't consider the above to be a reliable method of discharing the filter caps. I always check with a meter. If they are discharged, great. If they still hold a charge, then some sort of active discharging is required, such as below.

CLipping a ground wire to pin 1 of a 12AX7 will slowly discharge the power supply. SInce the typical 12AX7 has a 100k or larger plate resistor, I don;t see the need for an additional resistaor for discharge. If you had 400v on the tube's B+, then that 100k resistor would limit discharge current to 4ma. I would not concern myself with any tiny spark that 4ma might generate.

That works reliably, but slowly. Watch with your meter and see.

Diablo's special jumper is a good way to discharge faster. Pin 3 of a power tube - assuming 6L6 type socket wiring - is convenient. I usually try to find the main filter itself. Ground a clip wire witha resistor in the middle of it, and use it to ground off B+ at pin 3 or wherever. He uses 50w 300 ohms, I usually grab a 1000 ohm 10 watt. It isn't a critical thing. The resistor doesn;t have to be as heavy as it would if the circuit were remaining powered. It might start at 500v, but it drops a ton almost instantly, so the resistor doesn;t have a chance to heat up much.

[Albert Kreuzer]

The resistor doesn;t have to be as heavy as it would if the circuit were remaining powered. It might start at 500v, but it drops a ton almost instantly, so the resistor doesn;t have a chance to heat up much.

Except when you forget to take it out before powering up (don't ask...).

Cheers,
Albert

[Enzo]

Oh, believe me ALbert, I have done my share of bone headed things.

[EFK]

Thanks very much for the clarifying responses guys. I appreciate it.

I was checking B+ and had a meter clipped to pin 3 when I pulled the plug from the wall - I watched the voltage drop down over the course of @ 5 seconds to 2 volts (from 481). Is this indicating the presence of bleeder resistors?

[Diablo]

Thanks very much for the clarifying responses guys. I appreciate it.

I was checking B+ and had a meter clipped to pin 3 when I pulled the plug from the wall - I watched the voltage drop down over the course of @ 5 seconds to 2 volts (from 481). Is this indicating the presence of bleeder resistors?

Yes, or a short in the wiring....
Do you have a schematic for this amp?
Verify on the schematic that there are bleeder resistors.
Also, while you're in the amp, you can measure for voltage on all the electrolytic caps just to make sure it's safe inside.

[Chuck H]

I don't think that indicates the presence of bleeder resistors. I've noticed that most new electrolytics simply don't hold a charge very long at all. Older amps could store a charge. But with new electrolytics even without bleeders I've measured only a few volts on the main cap in the time it takes to pull a chassis. And this is with new Sprague Atoms. I have an amp opened up right now with Atoms and no bleeders. I'll monitor voltage from the time of shutoff and report back.

As a peripheral discussion (not hijacking) what difference is there between the old caps that do hold a charge and newer caps that don't? And how does this effect tone and amplifier performance?

Chuck

[Chuck H]

Ok. I just checked my findings. I turned the amp on and even played it for a bit to get the caps "working". Then with my meter in place and reading 356 volts on top of the main filter I switched the amp off. In about five seconds the voltage dropped to 10 and hovered there. This amp has Sprague Atoms and no bleeders. It's about a year old and exhibits no ghosting hum or any other problems. I've seen similar behavior on other amps with new electrolytics.

Chuck

[Diablo]

Boy that's wierd Chuck. Caps are supposed to store charge, that's their only purpose. I have no idea if electrolytic caps are of different composition today and yesteryear.

[martin manning]

I'm thinking that since the cathodes are at working temperature, the tubes will continue to conduct idle current for a few seconds (until they cool down) after you cut the power. That might be long enough to drain most of the charge.

MPM

[bnwitt]

method 1 (leaving power and standby switches on and unplugging the amp from the wall while strumming your guitar) will take the amp down to about 20 volts.

Method 2 will drain the entire power supply through the 100k plate resistor safely and take it down to 0 volts if you keep the jumper in place for about 20seconds.

Method two is my method of choice.

[Chuck H]

Boy that's wierd Chuck. Caps are supposed to store charge, that's their only purpose. I have no idea if electrolytic caps are of different composition today and yesteryear.

I guess it's not that wierd. I've seen this behavior repetedly with new caps. I suppose they do store a charge long enough to be effective as a reserve for audio power amps but don't hold a charge long term like a battery...Like the older caps did. In fact, if you research it you'll find that you have to get a specialty cap now if you intend to use it as a battery or power fault backup.

Has anyone noticed a tonal difference between current production caps that are prone to this behavior and older caps that hold a charge longer than a half a second?

Chuck

[Chris / CMW amps]

method 1 (leaving power and standby switches on and unplugging the amp from the wall while strumming your guitar) will take the amp down to about 20 volts.

I know certain amps that will still have enough juice to shock you with this method.

Once a year I do forget and it reminds me to always use this method:

find main filter cap or a B+ point (standby on, power OFF, mainscable not connected to mains) and then putting a 1k/>5W from that to ground.

[Diablo]

I guess it's not that wierd. I've seen this behavior repetedly with new caps. I suppose they do store a charge long enough to be effective as a reserve for audio power amps but don't hold a charge long term like a battery...Like the older caps did. In fact, if you research it you'll find that you have to get a specialty cap now if you intend to use it as a battery or power fault backup.

Has anyone noticed a tonal difference between current production caps that are prone to this behavior and older caps that hold a charge longer than a half a second?

Chuck

A little googling showed that some capacitors are built with an internal bleed resistor so that the cap discharges within a few seconds of powering off. It's a safety feature. Not sure if this is universal in modern electrolytic cap production.