Help with 6SN7 preamp

[tubedude63]

help with 6sn7 preamp

http://www.diyaudio.com/forums/attachment.php?s=&postid=816820&stamp=1137349686

in the above circuit would you use both halves of one 6sn7 for one channel or one half of two different tubes
thanks
charlie

 

[Tom Bavis]

You can do either, but using one tube per channel will lay out better. The first cathode is at 1V , second one is at 80V. Bias the heater winding positive to reduce the heater-cathode voltage - 6SN7 isn't rated especially high...

 

[Brian Beck]

I agree with Tom’s advice on layout ease. But if the two channels are side-by-side, you might entertain using one section in each tube for each channel. Channel separation will suffer, but if the rest of your layout is decent, not by much. The second tube would then have the two channels’ CFs, and its heater could be biased separately from the first tube’s heater, to the same voltage as the cathode or just above.

Unsolicited comment on this oft-posted circuit: Whenever you have a cathode follower “following” a common cathode stage, try to connect the CF grid directly to the plate. The voltages usually can be worked out. You may have to diddle with the resistor value in the CF cathode to keep quiescent current the same. This avoids passing the signal through the second coupling cap. Obviously you won’t then need the 220K, and the 820 and the 10K become one new resistor (or maybe a CCS). The note on the bottom about a stepped attenuator makes no sense to me. If you can be reasonably sure that you have no source component with DC on the output, then leave out the first coupling cap, but keep the 1M resistor to ground on the first grid as a safety resistor, no matter whether you use a pot or a stepped attenuator as a volume control.

 

[jean-paul]

Again unsollicited comment on this oft-posted circuit: you could use slightly bigger caps instead of the 47 nF. 100 nF or 220 nF are OK.

 

[SY]

Since the grid resistor is bootstrapped, do you really want to go bigger? What advantage do you see?

edit: Never mind, I see that you meant the first cap. And your point is a good one.

 

[tubedude63]

An externally hosted image should be here but it was not working when we last tested it.

i would like to use this ps .i would leave out r1 r2 and c1 c2 and connect r19 to the 250v b+ .what bias voltage do you recomend.
i want to use 5y3 instead of 6ca4 so i will have to learn how to use duncuns psu program to get right part values,this ps is from ax84_m35 filaments will be the 6sn7 not tubes shown

 

[aletheian]

...Whenever you have a cathode follower “following” a common cathode stage, try to connect the CF grid directly to the plate. The voltages usually can be worked out. You may have to diddle with the resistor value in the CF cathode to keep quiescent current the same. This avoids passing the signal through the second coupling cap. Obviously you won’t then need the 220K, and the 820 and the 10K become one new resistor (or maybe a CCS).

So you are suggesting to direct couple the stages and do away with the bootstrapping of the second stage while possibly replaceing the cathode resistor with a CCS? I have never CCS'ed a direct coupled stage before. With a CCS, how would you go about insuring the correct DC bias reference voltage for the secong stage considering the high DC coming off the plate from the first stage?

Would LEDs in the CF cathode be a viable option here?

 

[SY]

With a CCS, how would you go about insuring the correct DC bias reference voltage for the second stage considering the high DC coming off the plate from the first stage?

The CCS insures it. But you do have to design it to withstand the voltage (easy).