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In Reply to: Re: Problem when large volume posted by waterman on April 11, 2007 at 19:03:11:
Two things. A 1.2K tail resistor in your LTP won't be very effective. You want a very high resistance in the tail to minimize current fluctuations. Second, the 12AX7 isn't a suitable choice of LTP driver. It doesn't have the current capability to drive effectively 6V6-equivalent tubes.What you do with the second grid of the 12AX7 confuses me. You should connect it to ground. I don't know why you'd put it where you do, and I don't know why you use 1M resistors.
The very small size of your coupling capacitors will also affect low-frequency response. I haven't calculated it but generally .1uF is on the smaller end of sizes you'd see, and .47 or even 1uF is a much better idea. You should also either ditch the .05uF/270K RC filter at the input, or also increase its size for frequency response. Also, the less caps in the signal path, the better. Phase shift or something, I haven't learned all of it yet :)
Finally, you might want to increase the cathode bypass cap for your AC/PEN tubes, by at least 4 times or so. 220uF to 470uF is generally a better size, because low frequency response is limited with lower values.
What does your power supply look like?
I hope that some of the advice I give you might help out the sound. Good luck! got any construction pictures?
Follow Ups:
"A 1.2K tail resistor in your LTP won't be very effective."Except that it's not an LTP! It's a paraphase splitter, difficult to get linear. An LTP would give nicer result but probably wouldn't have enough gain with a 12AU7; however, it might be OK with a 12AT7. A 12AX7 could work so long as it can tolerate the input impedance of the output stage. In any case, the plate resistors should be reduced to 100k.
Hi sorenj07,Thank you for your advice. I would try what you suggested this week end.
The bias of ac/pen is only 8V. In my first version, it is 12au7 instead of 12ax7.
I get the LPT circuit from the web site - http://tdsl.duncanamps.com/link.php?target=0925972A, it is the phase inverter from the specification of Philips in 1970. By the way, could you tell me what is different between connecting the grid of the 2nd half 12ax7 to ground and connection like this?
Sorry, I meant "12AT7" instead of 12AY7. The 12AY7 is a good tube but only amplifies 40x, not 60x. Plus it's rarer.
About the LTP, try using an LM334 with a current set resistor of 15 or so. This should net you around 4-5mA to be shared between the tubes. This is too much current for a 12AX7 so I suggest a 12AY7. It has 60% of the gain and is a LOT more robust. What is the bias point for your output tubes? Also, you're dropping A LOT of voltage with those 270K plate loads on the 12AX7, try reducing them to around 82K. I don't know the current draw of your 12AX7's, but even if each is only drawing 1mA, you're still dropping 270V, which YOU DON'T HAVE. Fix this!
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