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godfrey
diyAudio Member

Join Date: Nov 2009
Location: Cape Town
Quote:
 Originally Posted by hummel The output transformer is a toroid 115/115-7/7 that calculates to a 8640 ohm primary with 8 ohm load on the secondary.
Yes, if you're using a single 7 volt secondary (or 2 in parallel).
Quote:
 Originally Posted by hummel One question is regarding the load line. Is that correct for this circuit? The slope is based on the anode load of 2160 ohm (8640/4) and the line's position on the anode to cathode voltage of 170V and the quescient current of 36mA. The bias is effectively -80V. (If I got that right, I think I have a handle on load-lines, maybe.)
The way I figure it is: If you removed one output tube, then the remaining one would see a load of 2160 ohms, as you say. However with both tubes installed, each one only has to deliver half the signal current for a given output voltage, so the effective load seen by each tube is 4320 ohms. I'd expect the load line to be something like what I've drawn in green. Looks OK.

I wouldn't panic too much about the transformer inductance. They're intended to work at full voltage at 50 or 60 Hz, so the bass can't be that bad. If I was building a valve amp, I'd be using toroidal mains transformers for the output, mostly because they're obtainable and affordable.
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costis_n
diyAudio Member

Join Date: Aug 2008
Quote:
 Originally Posted by godfrey Yes, if you're using a single 7 volt secondary (or 2 in parallel). The way I figure it is: If you removed one output tube, then the remaining one would see a load of 2160 ohms, as you say. However with both tubes installed, each one only has to deliver half the signal current for a given output voltage, so the effective load seen by each tube is 4320 ohms. I'd expect the load line to be something like what I've drawn in green. Looks OK.
Quite the opposite. When they are both in the circuit, each sees the 2 windings in AC parallel. When one is cut-off, the other sees only one side of the transformer primary.

 31st December 2011, 07:58 AM #5 godfrey   diyAudio Member   Join Date: Nov 2009 Location: Cape Town ...which is why he divided by 4 in the first place.
 31st December 2011, 02:56 PM #8 hummel   diyAudio Member     Join Date: Jun 2011 Location: Hamburg Mantra, thanks again! OK, I see how fixed bias could be a problem without some kind of feedback (servo) to keep things in control. BTW, apropo drift, is this why I see the voltage moving around some when I look at, say, the top of the cathode resistor (not 50/100Hz ripple but some mV of very low frequency, seeming random, well, drift I guess)? I did run across an argument (and circuit) for a splitter with CCS here Practical Phase Inverters.. (This site helped a lot.) FYI, my background is in IC design, i.e., logic and peripherally, some fabrication, so math, a bit of physics and programming. I have built stuff from schematics (e.g., The Belfry In the Glow) but wanted to learn more. And I understand what you mean about separating the "wheat from the chaff". But my HiFi system really does sound better with those zigtausend Euro palladium interconnects. (nicht ernst!) Noch mal, guten Rutsch! Tschüß

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