Do bleeder resistors waste power?

[Gaz]

Never thought of this really until I read the Wikipedia on bleeder resistors. Does this matter in guitar amps? If so, how could one determine the resistance and wattage that wouldn't eat power while the amp was on? Thanks for any thoughts on this.

-Alex

[txstrat]

I've always used 220k at 2w parallel to the filter caps. Never had an issue with it.

[Enzo]

Think for a moment about your car. The electrical system runs off the engine, the battery is charged mostly to be there for starting, but any load on the electrical system drags on the motor. Some night sit ther in the dark and listen to the engine, then turn on your brights and see if the thing slows down a few RPM. That means that your sound system is wasting horsepower, same thing with the heater fan. And anything in the windstream costs power as well. Outside mirrors waste engine power. And weight costs power too. The extra couple of pounds the seat belts weigh cost power.

But as far as the car is concerned all that isn;t much.

Bleeder resistors are like seat belts, they help make the amp safe. Whatever small power they use is not coming away from the speaker anyway.

What value to use? The easy way is to look on some commercial circuit and copy them.

Some high value like 220k comes to mind.

The wattage? You know your voltage, I don;t. Take that and the resistance you select and use Ohm's Law to determine current and wattage dissipated. Then select a resistor of at least twice that wattage.

[Robert M. Martinelli]

Hi,
I'd like to elaborate a little, because I don't feel bleeder resistors WASTE power, they USE ( DISSIPATE ) power to do something useful, so that cannot be called a "waste", but rather a conscious way to add a feature at the cost of a little expense.

If your amp's +B is 450 V and the bleeder resistor is 220K, then the current bleeding through it is 2,045 mAmps. The "wasted" power is less than 1W, which is nothing compared to the number of W any tube amp draws when working. ( some 135 W for an AC30 draws , some 375 W for a Marshall 100 W head ).

If you're still concerned about this tiny "wasted" power, you could use a "Three Pole Double Throw" switch as the mains switch, using the third pole to insert the bleeder resistor only when switching off the amp.

Cheers

Bob

[Steve Conner]

But what if that third pole's contacts got dirty and stopped working?

Also, in amps with series connected electrolytics, the bleeder resistors have a second function as balancing resistors to equalize voltage across the caps. So they must be left in circuit all the time. The amp doesn't require any other bleeders besides these, since all the other capacitors can drain back into the main ones.

[Robert M. Martinelli]

But what if that third pole's contacts got dirty and stopped working?

Also, in amps with series connected electrolytics, the bleeder resistors have a second function as balancing resistors to equalize voltage across the caps. So they must be left in circuit all the time. The amp doesn't require any other bleeders besides these, since all the other capacitors can drain back into the main ones.

Hi Steve,
well, designing automation systems for industrial plants I deal with the "what if..." question several times a day, always trying to figure out the "worst case" to make things safer, so I understand and share your concerns.

Having said that, the proposal of a bleeder "switch" to save power was sort of "provocative", because I made it very clear bleeders are useful and they are by no means a "waste of power", ( we're talking about saving less than one Watt while consuming hundreds of them.... ) - when it comes to safety nothing can be considered a "waste" especially when it raises the safety level of a design.

Cheers

Bob

[octal]

Yeah, if you're worried about wasting power, you'd be better off putting a switch to disable the B+ to the reverb drive tube when you're not using reverb. And be sure to bias those output tubes cold!
Going from a tube rectifier to SS would probably be a bigger saving yet.

Honestly, if someone is really THAT concerned with being green, an SS amp and a amp emulator box would be the way to go. (Or solar panels, a battery and an inverter!) Or, for the ultimate- an acoustic guitar...

[Enzo]

When i read it , I assumed he was afraid the bleeders would sap power from the output.

[mooreamps]

Never thought of this really until I read the Wikipedia on bleeder resistors. Does this matter in guitar amps? If so, how could one determine the resistance and wattage that wouldn't eat power while the amp was on? Thanks for any thoughts on this.

-Alex

They are not in line with the output power of the amplifier. They are in parallel with the filter caps in the high voltage power supply. They subtract "nothing" from the loud speaker. I use them in my high voltage power supplies to discharge the filter caps after I turn the amp off ; mostly because I would still have my fingers inside the amp when I am working on it.




-g

[Gaz]

Thanks for clearing that up everyone!

[Old Tele man]

"Waste Power?" No!

They just generate HEAT while they (a) help regulate the B+ and (b) discharge the caps upon power OFF...so you don't get "bitten" by the residual high voltage(s).

(wink,wink)

[Robert M. Martinelli]

"Waste Power?" No!

They just generate HEAT

(wink,wink)

Hi OTM,
Is that a DRY heat as well ?

( Sorry, I couldn't hold myself back....)

Cheers

Bob

[Old Tele man]

Hi OTM,
Is that a DRY heat as well ?

( Sorry, I couldn't hold myself back....)

Cheers

Bob

...nah, it's a DAMp heat, especially when you pee yourself when you get japped(wink,wink).

[Robert M. Martinelli]

Ouch!

[bob p]

Also, in amps with series connected electrolytics, the bleeder resistors have a second function as balancing resistors to equalize voltage across the caps. So they must be left in circuit all the time. The amp doesn't require any other bleeders besides these, since all the other capacitors can drain back into the main ones.

i guess that what i call them depends on whether i'm thinking about charging or discharging the caps; i like to think of them as "balancing resistors" when i'm thinking about energizing the circuit, and as "bleeder resistors" if i'm thinking about de-energizing the circuit.

i was recently drawn into a similar conversation on one of the HiFi audio forums when we were discussing the repair of an amp for which the right parts were unobtainable. it seems that a lot of the HiFi crowd aren't as familiar with the concept of stacking caps to make high voltage caps when suitable caps aren't available. we all know that this is more of a problem today than it used to be, because there isn't as great a selection of high voltage caps for tube applications as there used to be.

when the subject turned to bleeders, i made the same point that Steve made -- that the bleeder resistors can be thought of as a simple voltage divider that balances the potential across the caps so that no cap is subjected to a voltage that's beyond its rated specs. of course, some know it all made the point that external voltage balancing isn't necessary if you're only dealing with a 2 capacitor stack, and that its only really necessary if you're stacking 3 or more caps.

although this is indeed true, i still think that external balancing falls into the realm of "best engineering practice." instead of relying on the caps to police themselves and equalize the voltage across them, external balancing with bleeders ensures that there won't be any problem. in many respects using them reminds me of that old adage that "an ounce of prevention is worth a pound of cure."