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Thanks to Everyone . . . And Another Question or Two

Thanks to everyone for your posts here! Sometimes the debate on such issues can become a little contentious on these boards (to say the least) and your civil discussion of the relative merits of these circuits has made it MUCH easier for someone like myself to learn something.

For instance, Dave mentioned above that the use of unequal plate resistors in a LTP is an effort to compensate for a lower than optimum value on the tail resistor. I was wondering about that because I noticed in the Eico circuit I posted that one plate resistor was 33k and the other was 28.75k.

I've also read that, optimally, the plate resistor should be about 2x the plate resistance. In the case of the 6SN7, that would be 13.4k, so a 15k (or 18k or 22k) would be better than a 33k as long as the supply voltage could be adjusted to get the proper plate voltage. Correct?

The power transformer I'm using has several secondary taps so I should be able to adjust the supply voltage somewhat. I may try to compare the paraphase with the differential (LTP) by building each channel differently. I don't have a scope so the comparison will be by ear only.

So, one question is: What size cathode tail resistor would I need in order to use equal value plate resistors? I'm thinking that, because of the versatility of the PT, I may be able to come up with enough of a negative supply voltage to use a 'longer tail'. Any suggestions?

The other question: On the Eico schematic, the grid of the AF Amp section of the 6SN7 is 120v which is 10v higher than the (110v) grid of the Inverter section. Why is this?

On the LTP channel I'd like to take mikeyb's advice and adjust the voltages downward so I can use a single 6SN7 per channel with no input stage preceding it. Then, to preserve the operating points from the Eico, I'd have 0v on the grid, 180v on the plate and 5v on the cathode (by using whatever combination of negative supply voltage and cathode resistor that would preserve the 7ma of total current).

If I could accomplish this, how would I keep the grid of the inverter portion 10v higher than the other grid? Or would this be necessary/important?

Or would an entirely different set of 6SN7 operating points be better? Suggestions?

I've got a few more holes to drill on the chassis and then, if it quits raining long enough, I'll put some paint on the top of the chassis and start putting it all together.

Thanks . . . Charlie

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