bleed resistor on B+
- Thread starter kambo
- Start date
Help Support GroupDIY Audio Forum:
CJ
Well-known member
you want that bleeder resistor strapped across the HV all the time, because it
1) safely bleeds caps so you can work on amp,
2)provides a bit of voltage regulation for the audio circuit,
3) keeps HV from soaring to high on start up which saves the filter caps,
1) safely bleeds caps so you can work on amp,
2)provides a bit of voltage regulation for the audio circuit,
3) keeps HV from soaring to high on start up which saves the filter caps,
CJ
Well-known member
you can work backwards from the wattage rating of the resistor,
a lot of old tube gear used a 2 watt bleeder, figure a 2 watt R will run at 1 watt all day without bakeage,
maybe your B+ happens to be in the 250 volt area,
Power = Volts times Amps
so set our 2 Watt resistor to 1 watt to be conservative, P = Volts times Amps,
one equation, two unknowns, impossible to solve with algebra,so just pick a value for either current or resistance,
let's try 220K, a common value for bleeders in tube gear, and adjust our answer up or down to get 1 watt dissipation,
250 volts/220K = 250/220 milliamps, (when you see a K, you can drop the extra zeroes and make your answer in milliamps, since milli is a -3 exponent and K ohms is + 3 so it magically works out,
so 250V-dc/220K = 1.136 ma, now we can get power, VI,
250 volts times .001136 amps = 0.284 watts,
so we can drop that 220K down to maybe 100K,
250V/100K = 2.5ma, power = 250V * .0025 = 0.625 watts, seems like a 2 watt bleeder would last forever if it runs at 0.625 watts so lets use it, power line volts might go up late at night during recording when everybody is asleep, so we have a margin of safety,
how long does a 100K resistor take to discharge a 20 uf cap? (20 uf picked at random)
RC Time Constant = 100,000 * 20 ^ -6, so 100,000 * .000020 = 2 seconds,
that is for 1 time constant, which means that only about 70 percent of the voltage will be gone, so you still have 30 percent left, so 0.30 * 250 = 75 volts,
after another 2 seconds, you will have 0.3 * 75 = 22.5, plus whatever needs to bleed thru the other 20 uf in the pi filter that might be linked by a 10K resistor which adds a few milliseconds, but if some lineman can work on a 480 volt power line in the rain, then you can probably feel ok about 20 volts DC,
maybe you have 300 V-dc,
300/100K=3 ma, * 300 = 0.9 watt, we still ok with a 2 watter,
RC Time? does not change, but voltage does, .3 * 300 = 90, then .3 * 90 = 27, so wait 4 seconds and listen to the change in the preamp with that constant 3 mil drag brake, maybe a bit tighter bass if you use 2 EF804s which draw about the same current as the bleeder,
maybe you have a crazy Musicman amp circuit with a crazy 750 V-dc pwr supply? better be careful with that, better up the resistor value big time, try 470K,
750/470K= 1.595 ma, .001595 * 750 = 1 .2 watts, time constant = 470K * 70 uf (bigger value for git amp) = 470,000 * .000070 = 33 seconds, better grab a cup of coffee while you wait for this amp to bleed off, and use a 3 watter of 5 watt sand block,
or put the bleeder on the preamp section of the pwr supply which might have a more reasonable 400 V-dc or lower, which is usually the practice seen in old gear, then you can go back to the 2 watt model,
a lot of old tube gear used a 2 watt bleeder, figure a 2 watt R will run at 1 watt all day without bakeage,
maybe your B+ happens to be in the 250 volt area,
Power = Volts times Amps
so set our 2 Watt resistor to 1 watt to be conservative, P = Volts times Amps,
one equation, two unknowns, impossible to solve with algebra,so just pick a value for either current or resistance,
let's try 220K, a common value for bleeders in tube gear, and adjust our answer up or down to get 1 watt dissipation,
250 volts/220K = 250/220 milliamps, (when you see a K, you can drop the extra zeroes and make your answer in milliamps, since milli is a -3 exponent and K ohms is + 3 so it magically works out,
so 250V-dc/220K = 1.136 ma, now we can get power, VI,
250 volts times .001136 amps = 0.284 watts,
so we can drop that 220K down to maybe 100K,
250V/100K = 2.5ma, power = 250V * .0025 = 0.625 watts, seems like a 2 watt bleeder would last forever if it runs at 0.625 watts so lets use it, power line volts might go up late at night during recording when everybody is asleep, so we have a margin of safety,
how long does a 100K resistor take to discharge a 20 uf cap? (20 uf picked at random)
RC Time Constant = 100,000 * 20 ^ -6, so 100,000 * .000020 = 2 seconds,
that is for 1 time constant, which means that only about 70 percent of the voltage will be gone, so you still have 30 percent left, so 0.30 * 250 = 75 volts,
after another 2 seconds, you will have 0.3 * 75 = 22.5, plus whatever needs to bleed thru the other 20 uf in the pi filter that might be linked by a 10K resistor which adds a few milliseconds, but if some lineman can work on a 480 volt power line in the rain, then you can probably feel ok about 20 volts DC,
maybe you have 300 V-dc,
300/100K=3 ma, * 300 = 0.9 watt, we still ok with a 2 watter,
RC Time? does not change, but voltage does, .3 * 300 = 90, then .3 * 90 = 27, so wait 4 seconds and listen to the change in the preamp with that constant 3 mil drag brake, maybe a bit tighter bass if you use 2 EF804s which draw about the same current as the bleeder,
maybe you have a crazy Musicman amp circuit with a crazy 750 V-dc pwr supply? better be careful with that, better up the resistor value big time, try 470K,
750/470K= 1.595 ma, .001595 * 750 = 1 .2 watts, time constant = 470K * 70 uf (bigger value for git amp) = 470,000 * .000070 = 33 seconds, better grab a cup of coffee while you wait for this amp to bleed off, and use a 3 watter of 5 watt sand block,
or put the bleeder on the preamp section of the pwr supply which might have a more reasonable 400 V-dc or lower, which is usually the practice seen in old gear, then you can go back to the 2 watt model,
CJ said:
The other equation is called ohms law, it ends up being P=V^2/R or what we need, V^2/P=R
For your first example
250V^2/1W=62k5
Of course it would take more thinking than needed if you need to grab a pice of paper to solve those two equations, but once you solved it once it should remain in your head since is useful in many other cases. Other than missing Ohms Law really nice post and you surely have more info in your head about tube amps than I do.
Regards
JS
![Electronics Soldering Iron Kit, [Upgraded] Soldering Iron 110V 90W LCD Digital Portable Soldering Kit 180-480℃(356-896℉), Welding Tool with ON/OFF Switch, Auto-sleep, Thermostatic Design](https://m.media-amazon.com/images/I/41gRDnlyfJS._SL500_.jpg)
CJ
Well-known member
thanks JS, that is the one i was looking for, derived by algebra substitution,
so now we have two methods, brute force math and a engineers method,
and never trust those bleeders, stick a voltmeter on the caps just to make sure you don't have a bad solder joint or an open resistor,
most of the time the tubes will pull down the pwr supply when you turn off the power, but sometimes we pull the tubes to troubleshoot or work on the chassis, that is when the bleeders are nice to have,
so now we have two methods, brute force math and a engineers method,
and never trust those bleeders, stick a voltmeter on the caps just to make sure you don't have a bad solder joint or an open resistor,
most of the time the tubes will pull down the pwr supply when you turn off the power, but sometimes we pull the tubes to troubleshoot or work on the chassis, that is when the bleeders are nice to have,
Landins
Well-known member
Maths, engineering and physical reality. As we write im overhauling a Philips el6435 140w tube pa "found" in an abandoned tuberculosis sanatorium up here in the grim north. Bleeder have been idling at 5w for probably near half a decade. Resistor measures fine but the legs broke like twigs when inspecting filter caps. So its best to be on the safe side wattwise. This one rated at 10w wirewound.
Tried to find a UL/IEC call out, something about dropping to x-voltage in maybe 5 seconds. IEC 61010-1 but can't find it.
> maybe you have a crazy Musicman amp circuit with a crazy 750 V-dc pwr supply? better be careful with that, better up the resistor value big time, try 470K,
Note that most resistors also have a Maximum Voltage rating. Often around 300V. Less for small jobs. Read the spec-sheet!
Short-term violation is usually harmless, or a slight change in value.
Long-term violation *may* kill the resistor.
If you are "depending" on a resistor to bleed, slow silent death is a bad idea. (So is *depending* on a resistor for safety.)
As Landin says, even half-rated power can slow-cook parts.
Those 750 Volt supplies, even 450V systems, will often want several resistors parallel.
There is no way to get "rapid bleed-down" without HUGE waste of power. 470K will bleed 40uFd from 750V to 472V in 18 seconds; 74V (<100V) in 95 Seconds. Ninety-Five Seconds is a very long time when you are itching to put your hands in the box. To get down to 9 seconds you need like 45K, which is 13 Watts of steady dissipation. Even in a MusicMan Twin, that's a bunch of heat. And it calls for 26 Watts of resistor rating, so thirteen 2-Watt parts. We might be thinking three per series-string (to keep the voltage down to 250V each) and five such strings parallel to get 30W of parts. So some math gives us fifteen 52K resistors on 20 terminals on a burn-proof board plus a finger-guard.
Or fifteen 2.1K 2W resistors series--- but that's a weakest-link chain (if one resistor fails, the bleeder fails).
> UL/IEC call out, something about dropping to x-voltage in maybe 5 seconds.
Then even more bleeder and heat.
If you find such a spec I would like to see it. However I think there is no spec for the inside of electronic toys such as we play with. They should *only* be opened by Trained Personnel who are aware of the dangers. There probably is a spec for across-line motor and power-factor capacitors which might hold a charge on a "dead line".
Note that most resistors also have a Maximum Voltage rating. Often around 300V. Less for small jobs. Read the spec-sheet!
Short-term violation is usually harmless, or a slight change in value.
Long-term violation *may* kill the resistor.
If you are "depending" on a resistor to bleed, slow silent death is a bad idea. (So is *depending* on a resistor for safety.)
As Landin says, even half-rated power can slow-cook parts.
Those 750 Volt supplies, even 450V systems, will often want several resistors parallel.
There is no way to get "rapid bleed-down" without HUGE waste of power. 470K will bleed 40uFd from 750V to 472V in 18 seconds; 74V (<100V) in 95 Seconds. Ninety-Five Seconds is a very long time when you are itching to put your hands in the box. To get down to 9 seconds you need like 45K, which is 13 Watts of steady dissipation. Even in a MusicMan Twin, that's a bunch of heat. And it calls for 26 Watts of resistor rating, so thirteen 2-Watt parts. We might be thinking three per series-string (to keep the voltage down to 250V each) and five such strings parallel to get 30W of parts. So some math gives us fifteen 52K resistors on 20 terminals on a burn-proof board plus a finger-guard.
Or fifteen 2.1K 2W resistors series--- but that's a weakest-link chain (if one resistor fails, the bleeder fails).
> UL/IEC call out, something about dropping to x-voltage in maybe 5 seconds.
Then even more bleeder and heat.
If you find such a spec I would like to see it. However I think there is no spec for the inside of electronic toys such as we play with. They should *only* be opened by Trained Personnel who are aware of the dangers. There probably is a spec for across-line motor and power-factor capacitors which might hold a charge on a "dead line".
Why not a relay which connects all bleeders when the power is off, that way you could use much faster time constant without steady power dissipation, and maybe just for safety a second bleeder in the usual way but much slower than this, maybe a few minutes. An LED inside may indicate if the caps are charged or not and also work as a third discharging path.
PRR, it doesn't make any sense a 47k 30W resistor rated for 300V, does it?
JS
PRR, it doesn't make any sense a 47k 30W resistor rated for 300V, does it?
JS
kambo said:
That could go fine, if anti flame resistor and power rating just as needed, it only will get hot when amp turns off, if it fails and lives connected it should die quite fast, so if not flammable, you just miss the fast discharging, change a relay and the resistor together instead of just the relay.
JS
I'll tag on a bit about an early 1930's PSU and preamp system I was just restoring. Very interesting brute force approach.
The PSU is designed to feed 1-4 of the same type preamp, each draws 26mA.
There is a vented cage off the front of the PSU with two 100W wirewound resistors connected in series, one resistor being tapped, so 3 taps possible.
For four preamps, these resistors are not connected, and there's a 1M bleeder across the output.
For three preamps, the power resistors are connected at the largest resistance, and they burn off 26 mA power.
You can see where this is going....
Get down to one preamp, and the PSU is shunted by a single 100W 2500Ω (!) resistor, burning off 78mA!
This is dropping an unloaded PSU B+ of 568VDC to a state of 300VDC (no preamp load) and 275VDC (preamp load).
I was lucky to find the paperwork for guidance, the 100W R's had gone open with age and poor storage and I didn't have R values, just the expected operating conditions. The current calculations matched perfectly in the real world, I had some 8K 50W power rheostats that delivered expected no load voltage when set to 2500Ω, and they got too hot to touch very quickly. Lacking the correct power resistor, I did further work with a pair of the 50W in parallel, and they still got pretty hot, so these things would heat a room, no problem. If I did the calc correctly, it’s burning 20W, so 1/5 of the resistor rating and lots of heat.
You have to put yourself in the mindset of early 1930's AC, probably with high fluctuations across the day. I suppose this sort of sunk load helped stabilize voltages to a degree. This probably made more financial and service sense than a series of power transformer secondary voltage taps. Interesting side note, the PT primary has 110 and 120 taps, with a panel switch for changing between them.
In this case capacitance in the supply was very low, paper/foil 2 and 4 mfd caps and an 80 (5Y3) rectifier. I rebuilt as 10/15/15 (CLCLC), and with 2500Ω, it bleeds down pretty fast! Listening confirmed it was plenty of capacitance to reduce system hum well below tube hiss, and given the current demands are close to max for that rectifier, increasing C beyond that seemed like an invitation to eat rectifiers quickly.
The PSU is designed to feed 1-4 of the same type preamp, each draws 26mA.
There is a vented cage off the front of the PSU with two 100W wirewound resistors connected in series, one resistor being tapped, so 3 taps possible.
For four preamps, these resistors are not connected, and there's a 1M bleeder across the output.
For three preamps, the power resistors are connected at the largest resistance, and they burn off 26 mA power.
You can see where this is going....
Get down to one preamp, and the PSU is shunted by a single 100W 2500Ω (!) resistor, burning off 78mA!
This is dropping an unloaded PSU B+ of 568VDC to a state of 300VDC (no preamp load) and 275VDC (preamp load).
I was lucky to find the paperwork for guidance, the 100W R's had gone open with age and poor storage and I didn't have R values, just the expected operating conditions. The current calculations matched perfectly in the real world, I had some 8K 50W power rheostats that delivered expected no load voltage when set to 2500Ω, and they got too hot to touch very quickly. Lacking the correct power resistor, I did further work with a pair of the 50W in parallel, and they still got pretty hot, so these things would heat a room, no problem. If I did the calc correctly, it’s burning 20W, so 1/5 of the resistor rating and lots of heat.
You have to put yourself in the mindset of early 1930's AC, probably with high fluctuations across the day. I suppose this sort of sunk load helped stabilize voltages to a degree. This probably made more financial and service sense than a series of power transformer secondary voltage taps. Interesting side note, the PT primary has 110 and 120 taps, with a panel switch for changing between them.
In this case capacitance in the supply was very low, paper/foil 2 and 4 mfd caps and an 80 (5Y3) rectifier. I rebuilt as 10/15/15 (CLCLC), and with 2500Ω, it bleeds down pretty fast! Listening confirmed it was plenty of capacitance to reduce system hum well below tube hiss, and given the current demands are close to max for that rectifier, increasing C beyond that seemed like an invitation to eat rectifiers quickly.
radardoug
Well-known member
Put a neon on it, then you can see when its got down to the neon voltage. Or put a voltmeter on it.
Similar threads
