Back-to-back PT current question

[Ripthorn]

Okay, quick question, but I couldn't find the answer. I am going to be using back to back transformers in my amp. I will have a 115/12.6V transformer and then a 115/16V transformer for my plate voltages (I'm using submini's). My question is that I have calculated out that I need about 300 ma at B+ to be able to accomodate the absolute current max for all tubes. My question is about the current supplied by the 115/16V transformer (wired backward). I am looking at a 375 ma 115/16V transformer, but then it donned on me that if it is drawing 375 ma at 12.6V (the 16V winding), then it might not produce enough current on the HV side. Does anyone have a definitive answer here? Thanks.

[xk49w]

I am looking at a 375 ma 115/16V transformer, but then it donned on me that if it is drawing 375 ma at 12.6V (the 16V winding), then it might not produce enough current on the HV side. Does anyone have a definitive answer here? Thanks.

Just remember the approximation that the power should be the same in both the primary and secondary. 375mA @ 12.6V is 4.725W. Reflected to 115V would give about 41 mA.

[skyboltone]

Use one of Hammond's torroid transformers. 234/117 in 117/234 out. Bridge supply. Perfect for this application and pretty cheap. 300 ma with mini tubes seems like a lot though. That's like 40 watts isn't it. Or am I missing the point?

http://www.mouser.com/Search/Refine.asp ... 4294608396

[Ripthorn]

I was looking at the absolute ratings, but this is my first tube project, so I think that maybe there is something that I am missing. If I can go with lower value transformers, the better. I was looking at plate and cathode currents, summing them for all the tubes and using that value as the current I need my B+ to provide. Am I off here? I couldn't really find anything about it.

[skyboltone]

I was looking at the absolute ratings, but this is my first tube project, so I think that maybe there is something that I am missing. If I can go with lower value transformers, the better. I was looking at plate and cathode currents, summing them for all the tubes and using that value as the current I need my B+ to provide. Am I off here? I couldn't really find anything about it.

Plate (+screen) and cathode current are the same current. In on the plate, out on the cathode. (although that can cause a fight, either way it flows, it's the same current) so you always count just plate (and screen) current at full output. Pre amp tubes do not use much current, they are voltage amplifiers so you give them about 1 ma per stage. You can pretty much add up your plate and screen currents, add 10% and you're done. Filament current is not part of that equation. Add up the filament currents and that winding must be rated for that current.

[skyboltone]

Post your schematic.

[Ripthorn]

This schematic is the basis of my amp. I don't have the tonestack after the first gain stage and some component values are different. The power amp will be two 5902's in push pull. There is also a 6021 cathodyne phas inverter just before the power amp, so a total of three 6021's and two 5902's.

[img:800:582]http://i652.photobucket.com/albums/uu24 ... in6021.jpg[/img]

[Ears]

Well, that cct will draw micro amps.
How have you decided to run the 5902's?

[Ripthorn]

I don't have the 5902's yet, but they will be in there. So could I go with a small transformer (in terms of current rating)? How do you determine the current draw, anyway? I think that would come in right handy. Though I will be running the plates at a higher voltage (about 100V, I am thinking).

[Ears]

I don't have the 5902's yet, but they will be in there. So could I go with a small transformer (in terms of current rating)? How do you determine the current draw, anyway? I think that would come in right handy. Though I will be running the plates at a higher voltage (about 100V, I am thinking).

You have 18V on plates. Even ignoring the tube's resistance, plate and cathode resistance is approx 230k for V1. Ohm' s law means plate draw is 0.000078A, add the tube itself and it will probably be half that. Not a typical operating point even if the bias is about -0.4 to -0.8v
If you are running plates at 100V then you should put that in the schematic! People quickly stop answering questions if feed dodgy data.
Preamp is running in class A or A1 , so for your expected draw just use the quiescent current figure (assume max power [edit: dissipation] at idle) from the curves i.e. where load line intersects the Vg = chosen bias curve. I've not got the full data for the tube but for typical operation at 100v on plates and cathode resistor of 150ohms you'd expect about 6.5mA per triode (RC-28 manual, page 641 ) that condition has a bias voltage of about 0.98volt.
Where did you get your cct inspiration and values from?
Expect most of your draw from the power tubes and that will be dependent on the load and class of operation.

[Ripthorn]

I apologize for the inaccurate plate voltage. This schematic was actually drawn as an idea by someone else on a stompbox forum (thus the 18V), but I have made a few tweaks on the breadboard to get a sound that I like, but I am going to probably still make more modifications. My most up to date schematic is just on a piece of notebook paper at the moment. Thank you for all your help, I really do appreciate it.