Voltage doubler questions

martin manning · Post by **martin manning** » Mon Jan 27, 2014 11:02 pm

Napkin sketch.

jon · Post by **jon** » Tue Jan 28, 2014 12:12 am

That did it thanks guys

Invertiguy · Post by **Invertiguy** » Tue Jan 28, 2014 1:45 am

Doesn't that method also drop the B+ by the same amount as the bias voltage? If so, it could prove incredibly useful, presuming I can then trim it...

martin manning · Post by **martin manning** » Tue Jan 28, 2014 11:24 am

Invertiguy wrote:Doesn't that method also drop the B+ by the same amount as the bias voltage? If so, it could prove incredibly useful, presuming I can then trim it...

No, this bias circuit is just tapping some AC from the PT secondary.

jon · Post by **jon** » Tue Jan 28, 2014 11:27 am

BTW I love the toilet paper sketch.

JoeCon · Post by **JoeCon** » Tue Jan 28, 2014 4:55 pm

Martin

Love the napkin sketch! We do a lot of our work around here with those. Eventually a lucky tech, me, gets to log those into the lab notebook. What fun...

martin manning · Post by **martin manning** » Tue Jan 28, 2014 5:16 pm

Thanks! Next time, "back-of-an-envelope." ;^)

NickC · Post by **NickC** » Tue Jan 28, 2014 5:41 pm

martin manning wrote:Thanks! Next time, "back-of-an-envelope." ;^)

The problem with the "back-of-an-envelope" method is, when you run out of space ..... you can't just unroll more.

Jack Kerouac typed "On The Road" on a paper towel roll.

Invertiguy · Post by **Invertiguy** » Wed Jan 29, 2014 12:52 am

No, this bias circuit is just tapping some AC from the PT secondary.

Yeah I just got a little mixed up, I was thinking about the method of using a zener diode between the PT center tap and ground to create a negative voltage source, which does lower the B+ voltage

Aaron · Post by **Aaron** » Tue May 05, 2015 3:02 am

I'm running a similar circuit to the napkin sketch but I can't get my bias low enough. My PT is 270vac each side but I don't have the 10k resistor and second 10uf cap.
When I had the 56k in the bias only went to -25vDC, I changed to a lower resistor and the bias only went to -10v DC. I go up to 120k and bias is +10v DC.
I have replaced the diode and if I disconnect the rest of the bias circuit it doesn't make any difference. The voltages are spot on for the rest of the amp, it's just the negative bias I am struggling with.
Thanks,
Aaron

martin manning · Post by **martin manning** » Tue May 05, 2015 6:53 am

You say 270VAC each side... Does your PT have a center tap? Can you post a schematic?

Aaron · Post by **Aaron** » Wed May 06, 2015 9:50 am

Hi Martin,

I have attached the schem.

Thanks,

Aaron

martin manning · Post by **martin manning** » Wed May 06, 2015 9:59 am

You need to move the diode to the other side of the 56k resistor.

Aaron · Post by **Aaron** » Wed May 06, 2015 10:16 am

Sorry,

Just double checked, it already is. I drew the schem wrong.
Revised schem attached.

Thanks,

Aaron

martin manning · Post by **martin manning** » Wed May 06, 2015 10:21 am

Is the cap shorted? You'll need a 1kV rated part there.

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