How do you calculate voltages from a PT

[Ed_Chambley]

Lets say you have a 310-0-310 PT and you want to build a tube rectified amp.  How can you determine the initial voltage from the rectifier without wiring it up.  I assume there is a formula?

[sluckey]

Assuming conventional full wave rectifier...

You must know the load current and the PT internal resistance/losses. Then you can use tube data charts to get in the ballpark. I've seen lots of people bounce around numbers like this tube will deliver a B+ that's equal to 1.2 times the AC voltage input, or that tube will give 1.3 times the AC voltage input. And while those numbers will give you an indication of how this and that tube will behave in a particular amp, you just can't say that a 5Y3 will yield 1.2 X AC input for any amp. You must know the load current and PT losses.

One thing you can count on though... When you have no load current the B+ from any rectifier tube will be the same as a SS diode, ie, 1.414 X AC input.

For an example, say you have a PT that's rated for 70ma, and you have another PT thats rated for 200ma. Both put out the same no load voltage. Put the little PT in a test jig and increase the load current. The PT will soon reach it's rated current and when you increase the load current beyond that 70ma point, the voltage will begin to drop. The more you increase the load current, the more the voltage will drop. It's called sag. And it's caused by the PT internal resistance. Now, without changing the load current of your jig, replace the 70ma PT with the 200ma PT. The voltage will jump back up because you haven't reached the current rating of the big PT. The big PT has a lower internal resistance and can put out more current before beginning to sag.

It's not quite that cut and dried. The no load voltage from a PT will begin to drop as soon as you put any load on it and the voltage will continue as the load increases. It's not like the PT just puts out a continuous steady voltage until the max current is reached and then starts to sag. This stuff ain't voltage regulated. However, a big PT will sag less that a small PT.
 

[tubenit]

Here is a tube data cheat sheet that hopefully will help you.  Thjs is something that LooseChange helped me with yrs ago.

Those forumulas of 1.2 x 300 = 360v (5V4 = 1.2 as example)  .............. has been a reasonable starting place for me and usually has ended up in the ballpark

With respect, Tubenit

[HotBluePlates]

Lets say you have a 310-0-310 PT and you want to build a tube rectified amp.  How can you determine the initial voltage from the rectifier without wiring it up.  I assume there is a formula?

Radiotron Designer's Handbook, 4th Edition, Chapter 30, pages 1170-1179. The link goes to Hoffman's Library of Information, in which there is a copy of RDH4 hosted for you to download.

Sluckey told you the straight dope.

But answer this question for me: "How much gas will it take me to drive a vehicle, from where I'm at to somewhere else?"

Well, are we talking a tractor trailer or a Prius (gas mileage/PT internal losses)? Will I drive around the corner or across the country (distance/load current)?

Until we define the question with some more information, we can only reasonably guess the worst case scenario, which is the maximum possible output voltage with a given transformer secondary voltage. But I can't tell you if my guess on how much gas I'll need will get me around the corner or across the country, and I can't say for sure what B+ voltage you'll wind up having.

[jjasilli]

Transformers have poor voltage regulation.  This means that the voltage they supply does not remain constant, but drops as the amount of current draw (load) increases.  Running the amp produces a load.  This causes current to be pulled from the AC wall outlet, through the PT, then into the amp.  The more current that flows through the PT, the larger the voltage drop caused by the PT.  Per the above posts, this is why a PT must be rated not only for voltage, but also for current.  A PT cannot be rated for 310V in a "vacuum".  It produces 310V only at one particular current draw.  The current rating stated by the manufacturer (if accurate) tells us 2 things:  1. Exceed that current draw @ your own risk; and 2. The stated voltage will be produced at that amount of current draw.  If your load is smaller, the voltage will be higher, and vice-versa.  

Note also that many manufacturers persist in rating their PT's at 115 VAC on the primary.  But today's wall voltages are usually 120VAC or higher.  For a 310V rated PT:  310/115 = 2.7; 2.7 X 120 = 324, not 310 any more.  

[tubeswell]

Here is a tube data cheat sheet that hopefully will help you.  Thjs is something that LooseChange helped me with yrs ago.

Those forumulas of 1.2 x 300 = 360v (5V4 = 1.2 as example)  .............. has been a reasonable starting place for me and usually has ended up in the ballpark

With respect, Tubenit

Hi tubenit

FWIW the heater current draw for an EL34 is 1.5A. Not sure if there are any other mistakes in there - just noticed that.

[Ed_Chambley]

Lets say you have a 310-0-310 PT and you want to build a tube rectified amp.  How can you determine the initial voltage from the rectifier without wiring it up.  I assume there is a formula?

Radiotron Designer's Handbook, 4th Edition, Chapter 30, pages 1170-1179. The link goes to Hoffman's Library of Information, in which there is a copy of RDH4 hosted for you to download.

Sluckey told you the straight dope.

But answer this question for me: "How much gas will it take me to drive a vehicle, from where I'm at to somewhere else?"

Well, are we talking a tractor trailer or a Prius (gas mileage/PT internal losses)? Will I drive around the corner or across the country (distance/load current)?

Until we define the question with some more information, we can only reasonably guess the worst case scenario, which is the maximum possible output voltage with a given transformer secondary voltage. But I can't tell you if my guess on how much gas I'll need will get me around the corner or across the country, and I can't say for sure what B+ voltage you'll wind up having.

I understand your comments.  I was looking for a starting place.  So consider 1/2 of my voltage is 275vac  (550c.t.  173ma) and I use solid state rectification I will have somewhere in the neighborhood of 388vdc.  Typical voltage of a El-34 is 385-475vdc with ma of 120-160 respectively.  In class AB with a load resistance of 6500 ohm I should have somewhere around 34 watts.  Is this anywhere near correct or have I missed the boat completely?

[Ed_Chambley]

Radiotron Designer's Handbook, 4th Edition, Chapter 30, pages 1170-1179. The link goes to Hoffman's Library of Information, in which there is a copy of RDH4 hosted for you to download.

[/quote]
I have downloaded and read what you suggested.  Thanks.  I am now attempting to apply some understanding.  Please bear with my ignorant questions.  Thanks you guys like you I am learning a lot.

[RicharD]

Another way is to download Duncan's PSUD II.  You can either key in manufacturer specs or measure the off load voltages and DC resistances, then apply your estimated loads and it will return very accurate results.