I don't think this circuit is any good.
The cathode resistors in the first stage are only 33 Ohm so the cathode voltage will be something like 0.2 V or even less. At a bias voltage like that some grid current will flow.
Worse is the fact that the second stage is missing a grid resistor. Without that resistor it can't function proper.
The amplification factor of the 7308 is 33, so the voltage gain can't be much more than 20. The amplification factor of the 6R-HH8 is 45 so the voltage gain will be something like 20 to 25. But to be able to calculate the total gain you also need to know the loss that the EQ circuit causes. Since this circuit is highly suspect, I wouldn't bother to find out. I wouldn't be surpised if the values of the EQ-section are also wrong.
The cathode resistors in the first stage are only 33 Ohm so the cathode voltage will be something like 0.2 V or even less. At a bias voltage like that some grid current will flow.
Worse is the fact that the second stage is missing a grid resistor. Without that resistor it can't function proper.
The amplification factor of the 7308 is 33, so the voltage gain can't be much more than 20. The amplification factor of the 6R-HH8 is 45 so the voltage gain will be something like 20 to 25. But to be able to calculate the total gain you also need to know the loss that the EQ circuit causes. Since this circuit is highly suspect, I wouldn't bother to find out. I wouldn't be surpised if the values of the EQ-section are also wrong.
Last edited:
About 35dB @1kHz. Edit -- I should have noted the missing grid resistor 🙂
But 6KN8 (6RHH8) has much lower plate resistance and a better choice for the input tube, then you can use a lower buildout resistor which will yield a lower noise RIAA network.
But 6KN8 (6RHH8) has much lower plate resistance and a better choice for the input tube, then you can use a lower buildout resistor which will yield a lower noise RIAA network.
Both these odd tubes can be replaced by any of the 6BK7/6BQ7/6BZ7 family.
The pinouts are the same. And the H-K insulation is better. And more available at a lower cost.
6BQ7 was used as a cascode front end in some low noise preamps of the 1950's that were used
with millivolt magnetic phono vinyl record pickups.
The pinouts are the same. And the H-K insulation is better. And more available at a lower cost.
6BQ7 was used as a cascode front end in some low noise preamps of the 1950's that were used
with millivolt magnetic phono vinyl record pickups.
