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Cap size for Aikido heater PS?

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I need some help. I've attached the drawing for the power supply I'm using in my Aikido preamp. My question is do I need to increase the voltage rating of the caps in the heater supply due to the bias voltage from the B+. The bias voltage will be 75 Volts and the caps are only rated at 25 volts, I'm using a transfomer with 290V and running the output of the B+ which should be 340V though a regulator which should drop the B+ voltage down to 300V. In the heater supply to obtain the bias voltage I replaced the value of R6 with a 100K resistor and paralleled a small cap with it and then bridged the two with a 300K resistor and attached the 300K resistor to the regulated B+ 300V. I want to make sure I'm on the right track before I fire this thing up today.

Thanks
Steve
 

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Respect for HT is a good thing. In this case you are wanting 12V across the caps and that is what the (properly working) circuit will present. Floating the heater by connecting it at just one place to the HT will not put a higher voltage across the capacitor. If it did, your heaters would complain!

Two things you may want to keep in mind. Firstly it makes sense to me to keep the cans physically isolated from other parts of the circuit. Secondly, except that you are using regulators which may make this moot, it is better to use larger caps to handle the ripple from a heater supply.
 
If it smoked, you could pull it and measure it but I'd preferrably replace it.

In order to dissipate 3W, R1 would have needed more than 50mA. In your case it could have been much more. You say you don't have the pre connected? Perhaps you need to go back and check your board, especially for solder bridges.

There don't seem to be any components that would draw that much current if a fault arose unless you were supplied the wrong values or they have failed short.
 
Steve Fay said:
I measured R1 in place is that the wrong way to do it? The value seemed ok?
In this case it should be OK. You could lift the choke to see that you have 440VDC across the first caps (discharge them through a resistance before proceeding). Anything less might indicate a problem with the caps, wiring or other.

The only devices that would load the circuit, apart from an accidental short circuit, are the caps (if faulty), the zeners and the 150k resistor.

The next thing I'd check is whether the zeners are in backward as they'd then be close to a short to ground and this could have stressed the transistors.


Edit: Vis-a-vis my first suggestion, I just noticed C2 is rated at 400V in the schematic. Though this should be OK under normal use (when loaded), my personal preference would be to increase the voltage rating if for no other reason than that it would be exceeded under a no load condition, should something go wrong. Small caps in HT circuits tend to sound like a .22 calibre round when pushed too far.
 
jnb,
Thanks again for your help! I did increase C2 to a 600v cap before I assembeled the board. I'm going to replace the 150K resistor because it measures 0.642k. I did use Cree CSDO4060 for D1, D2 and D3. I've used them before with good results. I have them in a TO-220 pkg and pin 1 should be the blocking pin. If I'm correct the orinetation is correct. The caps check out ok. I have attached a drawing of the regulator that this PS board feeds. I've attached the B+ and the ground of my power supply to this regulator which I got off John Broski's site. Then attached the ground coming out of the regulator to the case. The only part of the regulator that I question is the pin connections on the IXCY10M45S. I attached the + pin to the + rail and the - pin to the - rail and the gate to the 0.1 cap and the1M resistor as shown in the drawing. I'm not 100% sure if this is correct? I also concerned that this could be the source of the feedback into the power supply? HELP???

Thank you
Steve
 

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I was thinking you may be able to do away with some of the added complexity. The Aikido has been designed to reject power supply noise, making regulators less necessary.

If you really do need to reduce the supply voltage (as a shunt reg would do), you could simply use a series resistor followed by another capacitor placed after your main reg. This would also further filter the supply (which should now be pretty quiet), and as a circuit it would behave itself and would prove more reliable than the shunt regs, I feel. You'd probably find you'd need to spend more time dealing with noise from your wiring layout and maybe your heating setup.
 
In order to select a series resistor, you need to know the current the preamp will pass. Just as with R1, it has no voltage across it until there is some current draw. The current multiplied by the resistance equals the voltage, so 40(Volts) divided by the current will give you the appropriate resistance.

Changing the diodes will also give you the voltage reduction. Be sure your transistors are rated to handle the extra voltage and are heatsinked appropriately for the extra heat.
 
Steve Fay said:
I'm sorry for all the questions,
Not necessary, Steve Fay. We all need to ask questions :)

I divide 40V by 30mA
Yes. Of course you can't get a 1,333 ohm resistor. Choose something close, perhaps larger to start with.

As far as the capacitor after this resistor is concerned, according to my rule of thumb you would want at least 10uF. This would supply adequate reserve for one stage and would reduce any 60Hz hum from the output of the regulator by about 12dB. If you want to use something larger, feel free within reason.

If you want to power two stages, the simple answer is to halve the resistance and double the minimum capacitance. The better answer IMO is to use a second resistor and capacitor of the same size, side by side with the first to create a second branch of the supply.
 
Steve Fay said:
I've attached a crude drawing of what my understanding is of what your describing to me.
OK. Leave the resistors where they are. Connect the rightmost side of each resistor to one capacitor positive terminal, and also one preamp positive. Connect the other side of the caps to ground.

is it ok to use flim caps for the 10uF
It's better, IMO.

My pleasure, Steve.
 

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