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 Post subject: Power supply switching with OA2
PostPosted: Dec Sun 08, 2019 3:31 am 
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I am building a stand-alone power supply for a receiver and I want to have 4 switch positions—Off, Standby, Receive, and Cal. I am switching the B+ on (connecting it to the receiver) in the Receive position. Is the switch before the OA2? Or does it matter. I know I need to limit current through the OA2 to less than 30 ma, but still keep the B+ before the OA2 at 185 volts or a little more for starting the tube.

The radio draws about 105 ma on the 150 VDC B+.

Norm

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 Post subject: Re: Power supply switching with OA2
PostPosted: Dec Tue 10, 2019 3:30 pm 
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Hi Norm

If the entire receiver runs on the 150 volt line and draws that much current, you will have
a problem trying to find the correct dropping resistor.

The 0A2 would need to handle over 100 mA when the radio is switched out. I'd put
the regulator after the switch, and I think you will be spending some time trying to find
a resistance value that allows using that tube to supply that much current. You'd be
trying to regulate a current draw of 115 mA with a 30 mA tube and there is little margin
for error in the resistor value. And if the receiver current draw changes greatly (with RF
gain, etc.) the problem becomes ever more sticky.

I'd personally be looking at a solid state pass regulator circuit.

Pete

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 Post subject: Re: Power supply switching with OA2
PostPosted: Dec Tue 10, 2019 5:58 pm 
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Thanks, Peter. I suspected there were problems with using the OA2 even though the design in the Radio Handbook that I started with does show a power supply circuit with an OA2. The tube data sheet has info on parallel OA2 operation to handle more current and I probably need to start there. The receiver current draw in the past has varied from as high as 150ma down to under 100ma. I am now using a regulated power supply that shows current draw, so I will monitor over the next few operating sessions and see what I find. My earlier observations have been over the course of building the radio and I doubt they represent the expected final conditions of use. OA2 tubes can handle higher starting current but that affects regulation. I will place the switch before the tubes. Obviously using a zener diode or other solid state regulation will be an option, but because the radio is a tube radio, I would prefer to have a tube regulated power supply--even though I will be using silicon diode rectification.

Norm

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 Post subject: Re: Power supply switching with OA2
PostPosted: Dec Tue 10, 2019 7:56 pm 
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Hi Norm, if the current varies that much you may have trouble with the 0A2. With that much current variation it may either extinguish or draw too much current. I am not familiar with using gas regulators in parallel. Perhaps using a lower voltage regulator tube for the oscillator
sections in the receiver might be a better solution?

One other consideration: that tube will ignite the instant it sees B Plus, if the current limits
for the tube assume that the receiver is drawing current, then it will be drawing excessive
current until the receiver's tube filaments are fully warmed up.

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 Post subject: Re: Power supply switching with OA2
PostPosted: Dec Mon 16, 2019 4:29 am 
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My thought as well - regulate the B+ to the oscillator, run the rest on unregulated DC. Shunt regulators like a VR tube need to absorb ALL the current when the load goes away - not good for large load changes. A series regulator is more complicated, requiring a reference voltage, error amplifier, and pass device, so three tubes at least.

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 Post subject: Re: Power supply switching with OA2
PostPosted: Dec Tue 17, 2019 9:59 am 
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And don't forget to include line voltage variations in your resistor calculation.

Better to run the critical circuits from the regulated voltage and everything else from the unregulated voltage.

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 Post subject: Re: Power supply switching with OA2
PostPosted: Dec Wed 18, 2019 3:46 am 
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I thought about the power supply a great deal and then I actually looked in the book at the last page of the receiver design--and there was the supply they suggested--so I am using it. I had forgotten it was there. It is a voltage doubler which works with my transformer. I have a drawing below that shows the circuit. I tacked together the components and checked voltages. The output is about 330 VDC with R1 at about 20 ohms--which was their suggested value. I tried the OA2 and it gets hot so I tried two in parallel and they both get hot. I measured the receiver resistance, 15k, and tried a resistor at 20K as the load. I also have tried several resistors at R1 and it looks like something around 300 Ohms to 400 Ohms will drop the voltage to around 185 VDC, which the tube data calls for in order to "start" the OA2. The OA2 will start at lower voltages, but because the data sheet calls for 185 VDC, I will use that as my unregulated B+ value. A single OA2 does fine at that voltage for handling the current and maintaining a steady 150 VDC without heating up at all. The filter choke I am using is a 17 henry choke out of a NC 173.

I will switch the B+ before the OA2. This means bringing the B+ into the radio to the switch on the panel and then returning it to the power supply where it is regulated. Fortunately I have an 8 pin plug and socket for the power cord. I also have some 9 wire cable. This will allow for the B+--3 wires, line AC--2 wires, ground, and 2--6.3 VAC circuits. I received the chassis I ordered this week and now have all the components, so next thing I build will be the power supply.

Norm


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