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Old 24th February 2013, 05:54 PM   #1131
Salas is offline Salas  Greece
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Quote:
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Hi Salas

I am now studying the Itch and allready have some basic questions:

In the first stage you use a rather highish plate resistor for the lower triode (R2 = 82k) and I believe you do so because you do not need a large voltage swing on the output of that mu stage.

Anyway, the dynamic impedance of that resistor should be around R2 / (1 - Avcf)

What value should I consider for Avcf (Voltage gain of the CF on top?)

Around 0.9 ?
Acts as a current source the top part. Increases gain and PSRR. The first stage has high mu and high rp so it gets a highish plate load for linearity in low swing even, because it lacks cathode degeneration for absolute minimum en (input noise equivalent) reasons avoiding electrolytic decoupling also.
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Old 24th February 2013, 05:58 PM   #1132
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What about the lesson Ricardo? I'm trying also to learn meanwhile I enjoy my Itch
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Old 24th February 2013, 05:58 PM   #1133
Salas is offline Salas  Greece
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Quote:
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Second question:
I see you ground V1 V2 filament heater supply but you bias V3 V4 filament supply at half B+... why ?
The upper half would violate its cathode to heater max spec if done otherwise.
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Old 24th February 2013, 06:11 PM   #1134
RCruz is offline RCruz  Switzerland
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I understand the top tube acts like a ccs so the lower tube sees a much larger dynamic plate resistance and that makes it more linear ok

Thank you for the filament explaining... I was not aware of the cathode to heater max spec.

Now, please indulge my lack of knowledge.... I am starting to understand tubes now but still do not know how to bias a triode... I did not know oine could make Vgrid = Vcathode =0 and make it work like a fet at Idss ....

Would you please enlighten me ?
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Old 24th February 2013, 06:24 PM   #1135
Salas is offline Salas  Greece
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Its not like that. The current forced through as fixed in the upper part makes it bias. The valve type should be carefully selected here to be linear enough at low current and having minimal grid leakage. Batch quality can be easier to detect in such "naked" bias arrangement in high gain input position. Maybe Fellipe had felt that subjectively when choosing input valves.
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Old 24th February 2013, 06:26 PM   #1136
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Yes Salas big improvement using very good input valves.

N.B. Have you in mind to desgin a tube headphone amp for high Z headphones?
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Old 24th February 2013, 06:30 PM   #1137
Salas is offline Salas  Greece
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Maybe if I get other than Grado that I have.
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Old 24th February 2013, 06:31 PM   #1138
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Erno Borbely likes the Grados and I love my Sennhs HD600

If I owned Grados didn't look for headphones amps: I have Erno Borbely balanced lineamp....
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Old 24th February 2013, 06:39 PM   #1139
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Maybe if I get other than Grado that I have.
You are welcome to Sennhs HD600
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Old 24th February 2013, 06:48 PM   #1140
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Quote:
Originally Posted by Salas View Post
Its not like that. The current forced through as fixed in the upper part makes it bias. The valve type should be carefully selected here to be linear enough at low current and having minimal grid leakage. Batch quality can be easier to detect in such "naked" bias arrangement in high gain input position. Maybe Fellipe had felt that subjectively when choosing input valves.
Thank you again... I was not understanding how a triode without negative grid bias could cope with positive signal overdrive correctly.

Now another question.... It seems penthodes make much better CCS than triodes.... why did you choose a triode for the upper CF ?
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