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In Reply to: Re: Can you comment on this schematic ? posted by KevinC on October 01, 2003 at 10:57:09:
I disagree. The 5687 has a plate resistance of from 2 to 3k. Triodes like to see 3 times or more their plate resistance as the plate load. The 10k plate resistor would fit in well if the cathode resistor is bypassed. If you remove the bypass capacitor the plate resistance will be 6 to 9k. Then you would need a plate load of from 18 to 27k.The driver stage is loaded with the LL1660 and it's imp. is very high, no harm will come from removing the bypass cap from this stage (except it will introduce neg. feedback) Some don't like neg. feedback in any form.
Follow Ups:
Thanks for the correction. You are right; the plate resistor value should go up. I neglected to mention that. In my experience with similar amps, but not with that tube, it sounds better that way, theory aside. I won't budge either on my recommendation regarding the cathode bypass cap on the driver stage. Again with a different tube, I find that it makes the amplifier more graceful sounding at high volume levels. I rationalize this by hypothesizing that the output tube grids need the lowest drive impedance that we can reasonably give them to hang with the music signal. Amps with low impedance drivers (like 2A3s, 6BX7GTs, 5687s) and no transformer step-up (meaning an interstage transformer ratio of 1:1 or less) just seem to sound better.One of the beauties of audio as judged by perception is that we can disagree and still both be happy. :=)) Thanks for your input.
Kevin Carter
K&K Audio
www.kandkaudio.com
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