Determining optimal OT turns ratio - diyAudio
 Determining optimal OT turns ratio
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 29th June 2007, 06:27 PM #1 diyAudio Member   Join Date: Mar 2004 Location: Budapest, Hungary Determining optimal OT turns ratio Hi tube DIY-ers, I built a push-pull amp with 807s (triode connected) some time ago. Althogh it sounds great, I am thinking if it could be done more optimal. I have a theoretical question: Is there any way to determine the optimal turns ratio of the output trasformer? I consider optimal which gives the lowest distortion at a given output power in open-loop (NFB disconnected). Or which gives the maximum output power at a fixed (say 3%) distortion, at fixed anode voltage and cathode current. There must be some method to determine the best turns ratio from the anode voltage/anode current vs. g1 voltage curves. Here are the design values of my amplifier: Vb = 425 V anode voltage Ia = 30 mA cathode current T = 25 : 1 output transformer turns ratio Vg1 = -45 V grid bias Pout = 32 W output power RL = 8 ohms load Any suggestions welcome.
 29th June 2007, 07:13 PM #2 diyAudio Member     Join Date: Oct 2003 Location: Finger Lakes, NY Go to Steve Bench's website: http://members.aol.com/sbench101/. Study the sections on loadlines. __________________ January 20, 2009: Bush's last day
 29th June 2007, 08:50 PM #3 diyAudio Member     Join Date: Jul 2004 Location: Ardeche Hi Here is my suggestion. http://www.dissident-audio.com/PP_6L6/DA112.gif I use it with 2 x 6L6G (not GC) at 400V anode, screen regulated at 270V. It will be difficult to reach 32 Watts in triode mode, expect 15, at most. http://www.dissident-audio.com/PP_6L6/Page.html The 807 IS a 6L6 with a top cap. Yves
 1st July 2007, 12:42 PM #5 diyAudio Member   Join Date: Dec 2003 Location: earth assuming core area in cm squared = 4x root power out that relation will hold for m6 silicon steel scrapless IE core. and no I didn't pick it out of the hat, I have spent days deriving transformer relations for optimal parameters. some of the info you are after is in the old books, its well understood, but I am only just scratching the surface of that atm, I don't know if I can derive any simpler relations based upon it, but the transformer relations are coming out well. http://216.239.59.104/search?q=cache...lnk&cd=4&gl=uk
 1st July 2007, 04:52 PM #6 diyAudio Member   Join Date: Dec 2003 Location: earth http://www.turneraudio.com.au/loadma...p-triodes.html more good info here
 1st July 2007, 05:18 PM #7 diyAudio Member   Join Date: Mar 2004 Location: Budapest, Hungary Thanks for the explanation and useful info (especially the last site has some good practical guidelines). However, I think it is easier to determine the optimal turns ratio experimentally. I have an OT with 20-40-60-80% taps, originally intended for UL. I will play with the various turns ratios, using a dummy load at the secondary. Results will come...
 1st July 2007, 05:37 PM #8 diyAudio Member   Join Date: Dec 2003 Location: earth The approximate class AB1 minimum PP load for triodes = 5 x Ra, Where Ra is measured for the working point. RL for PP class A triodes = ( 2Ea / Ia ) - 4Ra. where ea and ia are the dc values of the operating point, and ra is the tubes ac resistance, measured at the operating point. from turner audio if your dummy load is fixed, then the turns ratio is fixed and you will be looking for the optimal primary load, Z. for your 807 at about 1k5 ra, that gives 7500 ohms for class AB1, and 22k for class A at your figures. anywere between those, assuming 8 ohm load, turns ratio between 937 and 2590, sorry that's impedance ratio, turns ratio is between 30 and 51 with my formula for primary turns, that makes about 2919 primary turns, and around 4894 for 22k make core area 25 cm squared, meaning 5cm x 5cm, or around 2.5 inch each way.
 2nd July 2007, 01:09 PM #9 diyAudio Member   Join Date: Mar 2004 Location: Budapest, Hungary Thanks for the suggestion. Based on all the above recommendations, I will set the following values: Vp = 425 V Vg2 = 300 V Vg1 = -40 V (adjustable) Ip = 60 mA N = 30 : 1 Npri = 3000 Nsec = 100 A = 5 cm2 I will use two primary windings with the same number of turns. One will drive the anodes, with the center tap tied to 425 V. The other will drive the screen grids, with the center tap tied to 300 V. This way I get true triode mode, whilst keeping the g2 voltage within the safe area.
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Join Date: Jul 2004
Location: Ardeche
Quote:
 Originally posted by oshifis Thanks for the suggestion. Based on all the above recommendations, I will set the following values: Vp = 425 V Vg2 = 300 V Vg1 = -40 V (adjustable) Ip = 60 mA N = 30 : 1 Npri = 3000 Nsec = 100 A = 5 cm2 I will use two primary windings with the same number of turns. One will drive the anodes, with the center tap tied to 425 V. The other will drive the screen grids, with the center tap tied to 300 V. This way I get true triode mode, whilst keeping the g2 voltage within the safe area.
Mmmmh !

I fear it won't be possible to roll all that wire on a such small core.
Unless you make a typo, 5cm2 is by far too small for this power.
As shown above, I suggest some 13cm2.
You will need less turns (2200) to acheive both reasonable induction (1.3 Tesla at 25Hz, full power) and a 300H primary inductance.

If you use the same turns number for screen and plate but with a lower screen voltage they will go negative !

Yves.

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