I want to drop 30V on the B+, help with math!

[kricke40]

I'm shooting for original B+ volts on a "2pc El84 Pool" with a Hammond PT (275 -275 VAC) I calculate that will render me rectified about 275 x 1.4 x0.95 = 366 VDC B+1 .
That is about 30 volts over target from what I read here.
The original use 260 VAC taps as I know and end up at about 335VDC B+1. I dont want to use a Zener on CT but instead add a resistor to drop 30 volts after the rec. bridge. Can anyone help me with a Ohms law calc for the value.PT is 150 ma.
THANKS!

[John_P_WI]

kricke40,

The math is the easy part, but first you need to know the current that will be seen through the resistor.

V = I x R

or 30 Volts / current = resistance needed.

A pure guess that will probably get fairly close is to use the same value that you will for the power tube cathode resistor. Of course you will have a little more drop (10 - 15%) due to the screen current and pre amp tubes.

Don't forget to size the resistor for power accordingly, p=VI or p = 30v x 150ma = 4.5 watts, so use a 10 watt flameproof resistor.

The resistor used to drop B+ will also add sag into the power supply.

John

[kricke40]

Ok and the easiest way to calculate the current will be?
Or will have to be measured?

[John_P_WI]

Without knowing the bias condition and tube operating point any values mentioned for current would be a guess at best. Looking at the tube data sheets will give some a clue to the current. Multiply by two since you have two tubes.

So yes, measuring current will give the most accurate values.

John

[LOUDthud]

The problem with what has been discussed here so far is that a resistor placed between the rectifier and filter cap will see pulsed DC. The current pulses will be at least 3 times higher than the DC current drawn from the filter cap. That means three times the expected voltage drop. The multiplier is a function of the size of the filter cap.

Duncan's PSU designer is a surpurb tool for looking at this. You'll need accurate numbers from your power transformer for resistance and turns ratio. You get the turns ratio from measuring the no load secondary voltage, not the "advertised" voltage which is measured under load. I believe you can place an RC filter right after the rectifier.

I would lean towards the zener unless this is a class A power amp.

[Firestorm]

Does a 'pool run Class A or close? If not, the current through that resistor will vary pretty widely. If you set the B+ for the correct voltage at idle, as soon as you apply signal it will drop more. The Zener approach is better for Class AB. If you object to Zeners, your only real alternative is adding a bucking transformer. Sort of a pain. Or the equally ugly Fender trick of using high power shunt resistors to drop some voltage. The current across a shunt resistor is more or less constant compared to a series resistor.

[kricke40]

I don´t object to zeners per se, maybe thats the better route to take as you say. I have read somewhere that you could use a power resistor in series with CT to ground as well? Is this correct?
If I wanna order a zener, what do I need? Cathode in the base to ground right?

[Doug H]

Wire up a MOSFET like in this schematic: http://www.aronnelson.com/gallery/main. ... alNumber=1

In place of the 220k, put a 1M pot wired as a variable resistor (use one outer lug and the wiper). Use long wires so you don't have to reach inside the chassis to adjust it. Clip your voltmeter to B+ and gnd and fire up the amp. Adjust the pot until you get B+ where you want it. Turn it off and install the tubes and fine tune it. When you get B+ where you want it, turn it off, remove the pot, measure and install a fixed resistor in its place. Done.

This is the simplest way I know to get B+ exactly where you want. I've done it on 2 amps now and it works great. Since the Drain is grounded, you don't need to insulate the MOSFET when you attach it to the chassis. The chassis acts as a heat sink, although it may not get that hot, depending on how much voltage you are knocking off. MOSFETs are a lot cheaper than Zeners. I actually used IRF840's, which are a little more "heavy duty" than IRF820's, since they were plentiful at my local surplus place. Check the datasheet if you want to sub something for the IRF820.

[mat]

Wire up a MOSFET like in this schematic: http://www.aronnelson.com/gallery/main. ... alNumber=1

In place of the 220k, put a 1M pot wired as a variable resistor (use one outer lug and the wiper). Use long wires so you don't have to reach inside the chassis to adjust it. Clip your voltmeter to B+ and gnd and fire up the amp. Adjust the pot until you get B+ where you want it. Turn it off and install the tubes and fine tune it. When you get B+ where you want it, turn it off, remove the pot, measure and install a fixed resistor in its place. Done.

This is the simplest way I know to get B+ exactly where you want. I've done it on 2 amps now and it works great. Since the Drain is grounded, you don't need to insulate the MOSFET when you attach it to the chassis. The chassis acts as a heat sink, although it may not get that hot, depending on how much voltage you are knocking off. MOSFETs are a lot cheaper than Zeners. I actually used IRF840's, which are a little more "heavy duty" than IRF820's, since they were plentiful at my local surplus place. Check the datasheet if you want to sub something for the IRF820.

Sorry kricke, dont want to hijack the thread but Dough, would this work also in ODS50 (2x6L6) style amp ? I have B+1 488V and would like to knock it down a bit. What is the source, drain and gate on the schem when looking from top to downwards of the IRF840 ?

[Doug H]

I don't remember the pinouts, you'll have to google the datasheet. I think the IRF820 and 840 pinouts are the same, though.

[mat]

I don't remember the pinouts, you'll have to google the datasheet. I think the IRF820 and 840 pinouts are the same, though.

I have the datasheet. I just dont know how to relate the pins of the datasheet symbol to the schem symbol which is different looking.

[Doug H]

The drain goes to ground, gate goes to the 220k/39k, and the source goes to the center tap of transformer. HTH

[mat]

The drain goes to ground, gate goes to the 220k/39k, and the source goes to the center tap of transformer. HTH

Big thanks Doug ! I will try this with my express build

[kricke40]

Thanks Doug, very neat solution! Does the resistors need to be able to handle any effect?Or just plain 0.5 w?

[Doug H]

1/2 Watts will do.