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Old 10th December 2012, 03:45 PM   #31
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Also yes! what about the fact that the heater is connected to the cathode? That means the heater floats on top of the 300B+ voltage? or is it below. Does this mean that if one were to use capacitors, would it include the attached B+?

Also along this note… its ok to use DC in the heater circuit with this attached cathode layout?
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Old 10th December 2012, 03:56 PM   #32
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AC / DC doesn't matter. In the rectifiers you MUST ensure that the heater PSU is fully isolated from the reaminder of the circuitry. It might only be workong at 12V or so but it could well be at 700+ Volts above GND.
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Old 10th December 2012, 03:59 PM   #33
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by woodrough View Post
So correct me if I am wrong with the V=IR. I want to take 12vac to 5vac. that is a 7 volt drop. When the heater to the rectifier draws 1.9 Amps in the circuit, that 7volts divided by 1.9Amps, I would get a Resistor value of 3.7ohms.
So power dissipated? 7 Volts x 1.9Amps = 13.3 Watts. Not the worst right? I can source some sturdy power resistors with a 20+ watt rating. I would call this a medium-hot worst case scenario.

Could some sort of X rated capacitor help this scenario out? Or is the amp rating the capacitor killer. Any way to bias more load onto the resistor than the feeble capacitor?
The calculations are fine but 13.3 watts is a lot to lose.

The Class X (and Y) capacitor is a totally different thing and used in totally different applications, normally such as across live and neutral and..... this explains it quicker than I can type it They are small value only.
Line-Filter

If the heater floats or is tied to some given voltage then the cap assumes that voltage too but it changes nothing in the way it all works. It just means the cap has (say) 300 or whatever volts DC on both sides of it.
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Old 10th December 2012, 04:00 PM   #34
DF96 is offline DF96  England
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Does your 12V winding have good enough insulation to sit at the rectifier cathode voltage? Not all low voltage secondaries have high voltage insulation. Some do, some don't.

Why not use an EZ81 - that has an insulated heater.
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Old 10th December 2012, 04:01 PM   #35
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Mooly, I think you would struggle to get 2A out of a reactive PSU.
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Old 10th December 2012, 04:04 PM   #36
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by KatieandDad View Post
Mooly, I think you would struggle to get 2A out of a reactive PSU.
You mean a simple capacitor, using its reactance ?

I've done it this morning with a 30 Vac tranny, a 270uf/400vdc cap (a small type used as a reservoir cap in a TV/VCR SMPS) and feeding a 5 ohm. That gave 10 volts RMS across the resistor. The cap ran cool.
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Old 10th December 2012, 04:08 PM   #37
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The transformer I am using is a X-Class Edcor transformer. It looks like the cable is a beefy 18 Gauge with thick pvc insulation. It should be plenty fine to be elevated that many volts being that the legs coming out of the same transformer are 210-0-210.
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Old 10th December 2012, 04:26 PM   #38
DF96 is offline DF96  England
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The winding won't have the same insulation as the leadout wires. However, if the transformer was intended for valve use then it is possible that low-voltage secondaries have sufficient insulation for elevated heater use. A dedicated 5v 2A secondary would be a better guarantee, as transformer winders know that this will be used with an octal rectifier.
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Old 10th December 2012, 04:31 PM   #39
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by DF96 View Post
The transformer should be OK, as the primary current will balance the secondary current. You will be drawing a little DC from your mains supply, but people do that anyway.
That is a complete impossibility: no DC can go through a transformer.
The primary current waveform will be distorted, but will not have any DC component.

Anyway, drawing such a high imbalanced current from a transformer, even a big one, is certainly not a good idea (remember: that is more than 6.5A peak, not negligible at all).

Phase control would be less nasty: at least, the current is balanced.
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Old 10th December 2012, 04:38 PM   #40
kevinkr is offline kevinkr  United States
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Instead of speculating contact Brian over at Edcor about the insulation rating of the 12V winding, it may even be included in the specs. Not a hard question to ask.

The dropping resistor is the simplest, safest, and long term most reliable solution you are going to find other than a separate transformer, and a 25W 3.6 ohm power resistor will be fine. Use a sand cast wirewound in free air and there would be no concern about its insulation to chassis.
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