How to spec a power tranformer

[greiswig]

Hi, all,

I'm thinking about upgrading my PT, because I pretty much landed on using a pair of KT88s in my D'Lite build. The plate voltage runs at around 420V, bias voltage is 45V.

I have heard that it will sound a fair amount better if I can run the plates closer to 500V. Heyboer will build me one that can do it, but I'm puzzled by how one calculates the "proper" bias voltage to spec the PT for per given plate voltage? And then how does one go about getting the right bias resistor values to start with?

Thank you!

[Phil_S]

Probably the short journey to the answer is to read it off the graph in the spec sheet: http://tubedata.itchurch.org/sheetsK.html

Use Ohm's law to approximate the correct resistor value.

[greiswig]

Probably the short journey to the answer is to read it off the graph in the spec sheet: http://tubedata.itchurch.org/sheetsK.html

Use Ohm's law to approximate the correct resistor value.

Thanks, Phil. Sadly, my problem is even more fundamental than that. I don't know how to read the charts.

If I look at the attached, it looks like if I want to run the plates at 500V and 45ma, I'd need about -34V at Eg, but I'm not sure what Eg is or whether that's the right thing to be looking at. Pathetic, I know.

[passfan]

This should explain everything you need to know about loadlines.

http://www.freewebs.com/valvewizard1/se.html

[Structo]

Eg is grid voltage which is the negative voltage in a fixed bias circuit.

The tail resistor is what sets the range of the bias pot.

If your current pot position is in the middle of the range then I would try it with the higher voltage.
Of course always turn the bias all the way down when trying something new in the power circuit.

Then if it will adjust to what you want you're good to go.

If not then you need to change the 10K resistor which is what the stock D'lite uses.

[tubeswell]

If you go for a PT with a bias tap of 50VAC or more, you will be fine for just about any type of build (all other things being equal - i.e.: all other windings being suitable for the particular purpose etc). Then you can drop it to the required DC voltage for your bias supply with an appropriate (preferably adjustable) voltage divider.