Pass B1 pre with BF862

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Slightly higher gm i.e. lower Z_out mostly. Noise performance might be considered irrelevant since B1 is a line level buffer (no voltage gain). Sonical differences are on the level of nuances - I prefer BF862 but you might like k170 more...
 
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You can drop the BF862 right into the original design, although you might
do a little better to put some resistance on the Source to Ground connection
of the current source transistor, connecting its Gate to Ground so as to
take the current down a bit, say to 15 mA. This will lower the dissipation
and (hopefully) bias the upper gain transistor to slightly less than its Idss.
 
Thanks Juma. Do you have any designs with buffer stage and attenuator ?
Also is it necessary to use ferrite bead at the gate to reduce noise when using Bf862?
Is there any need to change other components when using BF862 instead of K170?
Kinku,
no ferrite beads are necessary, gate stopper (220R - 1k) is all you need.
What do you mean by "designs with buffer stage" ? B1 is a buffer stage.
Attenuator of your choice (10k - 25k) is best put before the B1.

Source resistors, mentioned above by Mr. Pass, should be 5R - 10R, depending on Idss of your particular BF862 JFETs (the higher the Idss, the higher value of source resistors) in order to keep the Id at 80 - 90% of Idss
 
Attached the new schematic:confused:. I guess grid stop resistors only needs to be at the gates of Q1 and Q3?
 

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  • B1 BF862 V.png
    B1 BF862 V.png
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Grid stoppers are used at the grids of tubes - only gate stoppers here :D
Add them at the gates of Q2/Q4 too.
R102/R202 are superfluous. Value of R104/204 should be in range of 15R - 22R (just to isolate capacitance of the next stage/cable and not to dramatically increase the Z_out of the buffer).
I didn't check the input switch contraption - it's drawn confusingly. R201/202 and R100/101 can be left out - just make sure that input signal comes undisturbed at pin 1 of the P100/P200.
 
Juma , why the value of R104/R204 is significantly different from original B1 design?they are 1K in this schematic
It's a commercial design with short-circuit protection at the output applied that way (at the price of raising output resistance to more than 1k). It is not a big deal, but still, I find it hard to call it a buffer if it has Z_out of 1k or more.

Also wondering the reason behind removing the R102 and 202? Did you mean to say R100 and R101?
You added GSR into schematic so you don't need two sets of gate stoppers.
 
Got it.Thank you Juma.
Why there are reistors between input and ground 100/101 and 200/201? and are you recommending not to use them?
Is it better to leave the R102 /R202 before the C100/C200 cap in that case,rather than introducing new GSR in the circuit after the C100/200? Or either places are ok for a gate stopper?
Original B1 design does not have gate stoppers at Q2 and Q4 and I thought their gates are tied to ground. Why do you recommend using Gate stoppers for them?
 
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Juma ,the problem with reading part is usually information is conflicting and usually lead to more questions. I did the reading before and could not find answer to specific questions. I got offers on exotically matched components when I do that . But not answers to my questions. Usually people walk away saying do more reading. It reminds me of my way of answering students when they ask me difficult questions. my response " why don't you read about it and tell me tomorrow!":D
 
Juma, if I was an expert should I be asking any question ?The real experts are one who can explain facts in laymans language. It is just ego that is an issue here.
How am I going to learn all the basic you recommend in few days ,that took years for you to learn?
You can either help me out by explaining a bit more or direct me to some specific articles. Go and read yourself is not a polite way to respond, just saying.
 
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...How am I going to learn all the basic you recommend in few days ,that took years for you to learn?...
Exactly ! You got to understand basic stuff first (with internet at your disposal the sources are abundant).
For starters, just build the B1 the way it's explained in author's article and after that you can try to improve it by experimenting, measuring and listening to results of your work - no shortcuts, sorry, but homework has to be done... It would be useless to explain in detail function of every single resistor and other parts if you are not familiar with basic concepts.
 
Got it.Thank you Juma.
Why there are reistors between input and ground 100/101 and 200/201? and are you recommending not to use them?
Is it better to leave the R102 /R202 before the C100/C200 cap in that case,rather than introducing new GSR in the circuit after the C100/200? Or either places are ok for a gate stopper?
Original B1 design does not have gate stoppers at Q2 and Q4 and I thought their gates are tied to ground. Why do you recommend using Gate stoppers for them?

Please read starting here after post #12
 
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