16.5 v AC to 12 v AC for Shaka ??

[MartyMart]

OK,
I've found a nice 16.5 volt AC adaptor rated at 600ma
Can I just increase the 10 Ohm/ 1 watt resistor to knock this down to 12v?
HELP ?
Its reading 18.5 volts without any load

Thanks,
Marty.

[Paul Perry (Frostwave)]

I don't really like using a dropping resistor to take down the output from a wart, because
1. voltages move around as caps in equipment charge up, and heaters warm up,and you might find for a fraction of a second, that an excess voltage appears somewhere (unlikely, but possible).
2. If that wart fails, you might not find exactly the same one again, and another wart will need a different dropping resistor, most likely.

But having said that, you probably will get away with it.

[Paul Marossy]

I would rather try getting a wall wart that will get you closer without having to monkey around with voltage dropping resistors. I used a 13VAC 800mA wall wart with mine and it works perfect with a 10 ohm dropping resistor before hitting the power supply.

The other thing to be aware of is that you don't exceed the maximum voltage for the tube heaters - I wouldn't go any higher than about 13.5 volts. The other reason that you don't want to go higher than 13.5 volts is because your voltage doubler will be sending too high of a voltage to the opamp. The data sheet for the TL072 chip indicates an "absolute maximum" supply voltage for Vcc+ & Vcc- as being +/-18V...

[MartyMart]

Perhaps I should re-think this one, or go and find that 12v AC PSU !!
My calculation says that I need around 50 Ohms 1 watt before the power
board to get 12 from 16.5 and around 165 ma for the tube/op-amp section .

Paul :
Does the power board need to connect via ground to the op amp board also ?
I only see the 12v and GND going to the tubes heaters.. ??

Marty.

[Paul Marossy]

Perhaps I should re-think this one, or go and find that 12v AC PSU !!
My calculation says that I need around 50 Ohms 1 watt before the power
board to get 12 from 16.5 and around 165 ma for the tube/op-amp section .

Paul :
Does the power board need to connect via ground to the op amp board also ?
I only see the 12v and GND going to the tubes heaters.. ??

Marty.

There are other things going to ground, like those caps in the voltage doubler circuit and a few things in the opamp/tube circuit.