PP output stages - different OPT primary Z
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 18th June 2003, 02:29 AM #1 Guydot diyAudio Member   Join Date: Oct 2002 Location: Israel PP output stages - different OPT primary Z Hi all, I just want to know if i got something right: Since the OPT effectively acts as the plate resistor in PP output stages it means that i can chose any input Z for the transformer as long as the PS can supply the needed voltage and current. For example, my amp has 10K on the OPT primaries and 4ohm secondaries, while the amp is spec'd at 36WPC - that means that the output stage draws I=(36/4)^0.5 = 3A then the output stage consumes 3/25=120mA with the 25 being the turn ratio. So if i want to use an OPT with a 5K primary i can as long as the PS can supply the needed 170mA? (same goes for voltage of course). Cheers, Guy.
 18th June 2003, 08:27 AM #2 analog_sa   diyAudio Member     Join Date: Aug 2002 Location: Cascais All these calcs are pretty meaningless without taking the o/p tubes into account - they determine the power/current through the o/p transformer. You should start everything by looking at the output curves of the tubes and determining the power for your particular output transformer/load cheers peter
 18th June 2003, 01:45 PM #3 Joel   diyAudio Member     Join Date: Oct 2002 Location: Brooklyn, NY No tube output stage I know draws 3 amps!!! Your formula is incorrect. What tubes are you running?
 18th June 2003, 01:49 PM #4 James D.   diyAudio Member   Join Date: Nov 2002 Location: North Herts, UK Hi Joel, I think he meant 3 amps in the secondary... or about 125mA in the primary... I asume both are RMS??? But as Peter says, Guy needs to work from his output tubes and topology first. I recommend some reading on Steve Bench's site about Load Lines as a good place to start. ciao James
Guydot
diyAudio Member

Join Date: Oct 2002
Location: Israel
Hi all,

Sorry about the 3A... the output stage draws about 120mA as James said

I am attaching the output stage of my current amp (WAD KAT88), i'm trying to understand what would happen if i use a 5K primary traffo instead of the 10K one supplied with it originally. I am trying to learn how to conclude correct things from loadlines, but it is a bit confusing.

For example with this output stage - i am looking at the pentode charactaristics of the KT88 and trying to understand - WAD used
-10V of bias (point E), 255V (point D) fed to the grid and the CT has 510V (point A) - how do i know what's the loadline? if i understood correctly (which is not so certain ) i calculate the loadline in the following way:
510/10K=51mA and then draw a straight line from the 51mA to 510V? If so where is the operating point with -10V bias? I'm totally confused, totally.

Can someone try and explain these graphs and loadlines or refer me to a source which does (i'll check out Steve's pages later today).

I want to note that gaining the knowledge is what i'm after, not the specific mod's i need to perform in order to change the OPT from 10K to 5K although it can function as an interesting example.... i think....

Cheers,
Guy.
Attached Images
 outputstage.jpg (25.5 KB, 193 views)

 18th June 2003, 04:11 PM #6 Joel   diyAudio Member     Join Date: Oct 2002 Location: Brooklyn, NY Well, graphically, all loads have a corresponding angle on the plate curve charts. For example, drawing a line with a 22 degree slope on the 6080 charts equals a load of 2500 ohms. Anywhere on the chart that you draw a line with that angle will be showing the behavior of a 2500 ohm load. And then you can see exactly what the tube's plate current and voltage will be as it moves across that line as the signal input gets higher or lower. I don't know the angles for a KT88, but you can figure them out by plotting some load lines. Any RCA tube manual has a tutorial on plotting them. Joel
 18th June 2003, 04:36 PM #7 James D.   diyAudio Member   Join Date: Nov 2002 Location: North Herts, UK ...That's a load off my mind... Guy, Steve Bench's pages are best. His site is Steve Bench Site and the load line tutorial starts at Loadlines Page 1 There are five pages they start with triodes and work through all the uses and theory. Then Steve does push pull load lines and then Pentodes. They are the clearest explaination I have found and easy to work through. Good luck! ciao James
 18th June 2003, 04:45 PM #8 Guydot diyAudio Member   Join Date: Oct 2002 Location: Israel Thanks people - i'm printing the article parts as i write this Cheers, Guy.
 18th June 2003, 09:36 PM #9 Positron   diyAudio Member   Join Date: May 2003 Location: Midwest Impedance... Hi Guydot. I hope I am not butting in. There are some things to consider in PP operation. 1) If the output tubes are run strickly class A, triode, basically no worries. You can run almost any impedance tranny. (I cover myself here). Tetrode, pentode operation may be affected more as the screen may be subject to increased dissapation when clipping. I haven't dealt with tetrode operation in a long time. 2) If you are running the output tubes AB1, or AB2, you have to worry about the area under the curve as far as plate dissapation, and screen dissapation. Clipping is another consideration as this increases the heat dissapation on the plate and screen. Distortion also changes with load. I don't have the figures etc at hand on this. Hope this helps.

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