Questions about the bleeders

[nee]

Greetings!

1. Is the purpose of the bleeder resistors purely to discharge the PS caps quickly? If so, how do they discharge the caps after the standby switch?

2. Why are they rated at 3W or (2 in series) 6W? Applying Ohms law and assuming a B+ of (585/2)*1.414 = 413VDC, the current across them is only 413/200000 = 2mA, and .002*413=0.85W.

Thanks!

[sliberty]

If so, how do they discharge the caps after the standby switch?

Just flip the standby switch to the play position.

[billyz]

If you run two resistor in parallel the wattage rating is increased the resistance is divided. In Series I don't believe increases the wattage rating , the resistance is additive of the two resistors.

I have been using a single 220K 2 watt bleeder resistor with no problems. It does take several minutes to bleed down.

[paulster]

If you run two resistor in parallel the wattage rating is increased the resistance is divided. In Series I don't believe increases the wattage rating , the resistance is additive of the two resistors.

In series or parallel the wattage rating of the network is doubled since each resistor can dissipate the maximum power rating they are specified for.

If you take a 220K 3W resistor then it'll be able to handle 812V before it reaches maximum power dissipation.

Put two in parallel and you've got an overall 110K network, and that can handle 812V also, since each resistor will have the same voltage dropped across it, and therefore the maximum power dissipated will be 6W.

Put two in series and you've got an overall 440K network, with the voltage dropped across each resistor being half of the overall voltage. Therefore your supply voltage can be 1624V, at which point each resistor is seeing a drop of 812V and therefore dissipating 3W, yielding an overall network rating of 6W total dissipation.

Is the purpose of the bleeder resistors purely to discharge the PS caps quickly? If so, how do they discharge the caps after the standby switch?

The bleeders don't discharge the caps after the standby switch unless the amp is in play mode, which is a good reason for always flicking an amp into play mode before working on it (with the mains lead disconnected). However, with tubes in the tube side of the standby switch will be pretty well bled down by the current drawn by the tubes themselves. Not worth chancing your life on though, of course.

[gearhead]

Greetings!

2. Why are they rated at 3W or (2 in series) 6W? Applying Ohms law and assuming a B+ of (585/2)*1.414 = 413VDC, the current across them is only 413/200000 = 2mA, and .002*413=0.85W.

Thanks!

Have always assumed it's because of cap discharge surge (non-DC); the current component of VI=Power.

Go further down the power chain to that 25w 1K resistor. Same situation if you look at it from a DC standpoint. It feeds the screens, which pull a lot of ac current. Some have tried lower watt values and found it got way too hot.