Two G9 power supply design questions

Jakob, I'm using the G9 power supply for another project and ran across a couple things I didn't understand. When I built my G9's, I was more-or-less following a "recipie", but now I am getting a little deeper into the design aspect.

1) You suggest using 35VA transformers for both values. Is there any reason that the second step up transformer needs to be this large, since the HT plate currents wil be relatively small?

2) How did you choose the resistor values for the TL783 regulator? I see from the TL783 data sheet that the ratio of the parallel and shunt resistors determines the Vout. R1=517 and R2=100K (2W). How did you choose these values and not a different multiple of the same ratio? How can you determine the current draw in advance in order to specify the resistor wattage?

The reason that I am asking is because I am changing the design slighlty. I want HT=220V, so I plan on changing the paralleled resistor pair on the TL783 to 570 ohms from 517. I am also replacing the 12V regulator with an LM317 for 6.3V heaters. I plan on doing 330 ohms and ~1330 ohms (with a 1.25W trimmer), but these values are chosen nearly arbitrarily. Does this sound OK? The data sheet shows and example using R1=240 ohms, R2=5K trimmer, which is where based my starting point.

Any design guidance would be appreciated.

-Chris

I'm not Jacob but I can answer one question for you.

You can just calculate the current through the two resistors with ohms law and the output voltage from the TL783. The ref leg of the 783 will not add significant current through the resistor to ground. So with R1=517ohm and R2=100K and an output voltage of 340V you'll get about 3.3 mA of current. 340V * 3.3mA = about 1.12W most of it is dissipated in R2 because R1 drops only 1.75V so you'll need a big resistor there.

The 783 needs some current through it to regulate properly. The examples in the datasheet all have R1 specified as 82ohm to make sure this happens even when nothing is connected to your psu. Based on that you can choose R2.
But for high voltages like this you dont want to draw a lot of current through these resistors because this will generate a lot of heat. So jacobs design just assumes your preamp will always be connected to draw enough current so you can make these resistors higher to dissipate less watts in R2

Spot on, Mendelt :thumb:

Started with the standard-value 100K, and added the 470+47R to get to the desired 245V.

Chris,

The reason for using same-size transformers for both power transformers is that we want relatively predictable voltage translation - so even though we could use a quite small secondary transformer, it's higher copper loss could make results unpredictable. But you could experiment a bit to see what fits your locally available transformers..

Jakob E.

[quote author="gyraf"]Spot on, Mendelt :thumb:
[/quote]

Good to know I learned something in all those hours spent reading this forum :grin:

Thanks guys. This helps alot. I'm really starting to understand this stuff a little. So running the same calculations on my changes, everything looks good. Increasing the 517 ohm resistor to 570 works, and my LM317 resistors seem fine. ~4mA and ~.03 watts. :thumb: I allready have a small transformer for HT (7VA), so I'll let you all know how it works.

-Chris

For LM317, you need to limit the in/out voltage to a much lower voltage (39V?) instead of the 120V on the TL783, or it will probably die at start-up. Use only one 39V Zener in stead of the three-in-a-row..

This will also limit the max. allowable input voltage to some 39V over input voltage

Jakob E.

Ahhh, so the zeners are for protection in order to keep the max deltaV across the regulator below 120V. I assume they are for bypassing any voltage peaks during power-up around the regulator? Are they serving some other purpose?

I currently don't have any zeners on the LM317. I am running it off of the 15VAC input, which I swapped for 9VAC. This gives ~12.7VDC after rectification, which gets regulated down to 6.3VDC. Would I still need a zener here? Max deltaV is 40V, so I guess the 39V zener is just right, but what are the real chances of the peak Vin exceding 46V?

-Chris

[quote author="Emperor-TK"]Ahhh, so the zeners are for protection in order to keep the max deltaV across the regulator below 120V. I assume they are for bypassing any voltage peaks during power-up around the regulator? Are they serving some other purpose?

I currently don't have any zeners on the LM317. I am running it off of the 15VAC input, which I swapped for 9VAC. This gives ~12.7VDC after rectification, which gets regulated down to 6.3VDC. Would I still need a zener here? Max deltaV is 40V, so I guess the 39V zener is just right, but what are the real chances of the peak Vin exceding 46V?

-Chris[/quote]

No zener needed with that setup.

with a variable heater PSU you can underheat the tubes for a different tones, may be good, may be bad, but at least it's easy to try out

in general try to stay a bit under 6,3V and the tubes will last longer

what are the real chances of the peak Vin exceding 46V?

Click to expand...

Aah - I misunderstood - thought you were trying the 317 for HT voltage.. Someone talked about that lately..

Jakob E.

> don't have any zeners on the LM317. ... ~12.7VDC after rectification, ... what are the real chances of the peak Vin exceding 46V?

For that to happen, not counting workbench accidents, your "120VAC" power outlet would have to rise to 434VAC. That would kill most of the appliances in the house before it hurt the LM317. Don't fret.

The 250VDC regulator has to live much closer to the limits, so it does need protection.