Hello !
How much swing can I expect with a 6au6 driving a 126c interstage driving a 6b4g or 2a3?
I got only 90v full wave and it seems short.
R cathode is 330 ohms and v anode is 175 v
Gilles.
How much swing can I expect with a 6au6 driving a 126c interstage driving a 6b4g or 2a3?
I got only 90v full wave and it seems short.
R cathode is 330 ohms and v anode is 175 v
Gilles.
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A 6AU6 pentode should probably be connected in Triode Wired Mode to drive an interstage.
The u of a 6AU6 in triode wired mode is 36, and the plate impedance is 7500 Ohms.
At mid frequencies, if the transformer impedance is high enough, there will be a fair amount of gain, perhaps near to u (36).
Suppose you have 1.25V cathode self bias, and a bypass cap across the self bias resistor.
Then 36 x 1.25 = 45V, and you get 90V peak to peak.
But . . .
Suppose that at low frequencies the primary inductive reactance is 7500 Ohms.
And Suppose that at high frequencies, the distributed capacitive reactance of the primary is 7500 Ohms.
Then . . .
At the low and high frequency extremes, because of the inductive and capacitive reactances, the gain will be u x ((7500 Ohms)/(7500 + 7500)) = 18.
That would be - 6dB there, versus the mid frequencies.
In that case, you would get 22.5V peak (45V peak to peak), at low frequencies, and at high frequencies.
This may or may not be exactly what you got, but you can see the things you have to know, to calculate, and to measure.
The u of a 6AU6 in triode wired mode is 36, and the plate impedance is 7500 Ohms.
At mid frequencies, if the transformer impedance is high enough, there will be a fair amount of gain, perhaps near to u (36).
Suppose you have 1.25V cathode self bias, and a bypass cap across the self bias resistor.
Then 36 x 1.25 = 45V, and you get 90V peak to peak.
But . . .
Suppose that at low frequencies the primary inductive reactance is 7500 Ohms.
And Suppose that at high frequencies, the distributed capacitive reactance of the primary is 7500 Ohms.
Then . . .
At the low and high frequency extremes, because of the inductive and capacitive reactances, the gain will be u x ((7500 Ohms)/(7500 + 7500)) = 18.
That would be - 6dB there, versus the mid frequencies.
In that case, you would get 22.5V peak (45V peak to peak), at low frequencies, and at high frequencies.
This may or may not be exactly what you got, but you can see the things you have to know, to calculate, and to measure.
Assuming you have triode wired the 6AU6 and the transformer is terminated with a ~10K resistor, you can swing up from 170 to 260 and down from 170 to 110V. The 10K load line at that operating point doesn't look great, nor does it really look all that great anywhere on the 6AU6 triode curves. Rp with a 6AU6 triode strapped is pretty high for a 10K transformer.
From your post, you could maybe get 90V if you do not have the secondary of the transformer terminated with a resistor and you are measuring at say 1kHz.
From your post, you could maybe get 90V if you do not have the secondary of the transformer terminated with a resistor and you are measuring at say 1kHz.
And without the 10k termination on the secondary, you will have the frequency response dropping off of both the low frequencies and the high frequencies.
I used to use interstage transformers.
Really good interstage transformers are Expensive (Expen$ive).
I use RC coupling now.
I used to use interstage transformers.
Really good interstage transformers are Expensive (Expen$ive).
I use RC coupling now.
An interesting fact :
I have another amplifier with the same schematic and opérations points but with expensive transformers.
6sh7 instead 6au6 but they are the same
The interstage is a big C core custom and I got 160 volts full wave at 1 kHz.
Twice with another interstage.
Price is more 10*.
I have another amplifier with the same schematic and opérations points but with expensive transformers.
6sh7 instead 6au6 but they are the same
The interstage is a big C core custom and I got 160 volts full wave at 1 kHz.
Twice with another interstage.
Price is more 10*.
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