The Itch along the 1:10 SUT should suffice for gain even if your cart is 0.2mV to 0.3mV. Already having a line preamp with some gain presumed.
I had a hard time finding any info on it. All I know it is close to a Audioquest AQB100MH, which is listed as 1.6mv (was built by Scantech and rebranded and labeled for the US). I do think that the rated output is correct for the Onyx. It's no more that 2mv. Yes I do have a line stage, a 26 5:1 xfmr coupled.
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 ?
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 ?
I think so, that's the way I have it loaded through my SUT.
BTW, I did a search through all my schematics I've archived over the years, and found a very similar circuit to the itch. I was by the late great N. Shishido, published in MJ mag in the 80's and later republished in Sound Practices. Dual 6DJ8 with a passive Tango LCR riaa between stages and 1:10/20/40 Tango SUT input. The 6N1p input and the SSHV should really make this itch circuit sing (very clever Salas).
BTW, I did a search through all my schematics I've archived over the years, and found a very similar circuit to the itch. I was by the late great N. Shishido, published in MJ mag in the 80's and later republished in Sound Practices. Dual 6DJ8 with a passive Tango LCR riaa between stages and 1:10/20/40 Tango SUT input. The 6N1p input and the SSHV should really make this itch circuit sing (very clever Salas).
What you may learn...Shishido had done alike is pretty nice. I would never use same amount gain stages, the BGs, the high bias commanded for driving the LCR, and the stoppers though. I am confident the Itch is 10dB better for noise by implementing the head amp concept and possibly has more accurate EQ.
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.
The upper half would violate its cathode to heater max spec if done otherwise.
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 ?
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 ?
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 ?