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Old 22nd April 2004, 07:43 PM   #1
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Post 6n1p pre with active load

Hii all!

I'm trying to build a 811-3 SE amp, and here is "my" preamp.
It has a gain of about 25 with a load of 1Meg (the grid resistor I'm planning on using).

As I'm a complete newbie I'd appreciate the help from all you gurus here...

Do you guys think it will be enough, or will I need an extra cathode follower stage (or something else I haven't thought about...)?

Is it possible to use a IT with this design ?



Thanks
-Miguel
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Old 22nd April 2004, 11:16 PM   #2
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Hi,

Is that a 500V B+ you're using?

Quote:
It has a gain of about 25 with a load of 1Meg (the grid resistor I'm planning on using).
If that's the gridleak resistor of the first stage of your amp then it's way too high.

Cheers,
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Old 23rd April 2004, 12:14 PM   #3
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Hi

what is the purpose of the unbypassed 15k R18 resistor?

Federico
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Old 23rd April 2004, 01:25 PM   #4
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Quote:
Is that a 500V B+ you're using
yap ... I'm thinking about using half of 500-0-500 supply needed on the power stage. It seemed simpler... Is it a bad idea ?



Quote:
If that's the gridleak resistor of the first stage of your amp then it's way too high.
Not quite sure I understand what you mean... The 1Meg is the resistor from the grid of the 811-3 of the power stage to B-. Perhaps I need an extra 100ohm resistor before the grid: ?

100r
from pre >-----####-----(Grid
|
#
# 1Meg
#
|
v
B-

Quote:
what is the purpose of the unbypassed 15k R18 resistor?
Just to make about 150V drop so I dont fry the 6n1p


-Miguel
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Old 23rd April 2004, 01:46 PM   #5
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Quote:
Just to make about 150V drop so I dont fry the 6n1p
In that case you should bypass it with a cap from the upper anode to gnd.

As Frank hinted 1M is too much, especially if you intend using fixed bias. Try 200k as max value. I also don't see how are you going to get enough gain - you need something like 90v at the 811 grid if i am not mistaken.
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Old 23rd April 2004, 02:59 PM   #6
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Quote:
In that case you should bypass it with a cap from the upper anode to gnd.
But if I do I loose about half the gain ... Why do you say that I must bypass it ?

Quote:
you need something like 90v at the 811 grid if i am not mistaken
The svetlana datasheet typical usage section has 3 scenarios:
130Vpp grid drive with -65V fixed bias and 450V plate
30Vpp grid drive with -165V fixed bias and 800V plate
280Vpp grid drive with -65V fixed bias and 450V plate

I was aiming for the second one, but now that I think of it it may be a typo... The grid drive voltage is quite low compared to the others Gotta work out the maths of this one
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Old 23rd April 2004, 04:17 PM   #7
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Feel free to read about SRPP at TubeCad
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Old 24th April 2004, 01:03 AM   #8
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If you want the upper tube to act like a follower, eliminate the plate resistor. Then, since you're taking output from the cathode of this tube, you're basically getting CF behavior. You should have fairly low Zo ( not as low as a true follower). Also, depending on what you're driving, I think the 5u could become a 1u.
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