Cascode tube phono preamp

[Walter66]

Hello,
please comment this attempt on a cathode biased cascode phono preamp. Could the negative feedback be applied like in this schemo on the cascode? Will it be necessary to linearize the amps output? Most transformer output amps had such a negative feedback network, is it because the transformer have non-linear transmission characteristic?

 

[ruffrecords]

You don't need the grid stopper at the input but you might need one close t the grid of the 6AWA8A. I don't know if 100nF is enough to decouple the top grid of the cascode. What are you doing with the triode in the 6AW8A?

Cheers

Ian

 

[Walter66]

Thanks, Ian. Do you think the first stage will have the same mu=100 like a 12AX7? That would be my goal for this circuit.
Only the pentode section is in service, triode out of use. Wanted to use a small pentode, triode wired. It doesn't have high amplification but it can run 10mA @200V.

 

[ruffrecords]

Thanks, Ian. Do you think the first stage will have the same mu=100 like a 12AX7? That would be my goal for this circuit.

I am not sure. It is difficult to work out the gain of a mu stage by inspection. However with identical tubes is is basically gm x Ra where Ra is 51K in your case and gm is the defined by the operating point. Your schematic says 1,5mA plate current which according to the 6072 datasheet implies a gm of 1.2mA/V which gives you a stage gain of about 60.

Cheers

Ian

 

[Walter66]

Thank you. Gain of 60 will be sufficient in combination with the second stage. I can fiddle around with the anode resistor value or the neg. feedback loop to make it work. Will the elevated heater cathode voltage of 77V do harm the 6072 tube in the long run? They can tolerate up to 90V.

 

[ruffrecords]

Thank you. Gain of 60 will be sufficient in combination with the second stage. I can fiddle around with the anode resistor value or the neg. feedback loop to make it work. Will the elevated heater cathode voltage of 77V do harm the 6072 tube in the long run? They can tolerate up to 90V.

Remember that Vhk is a peak so it is the sum of the dc and any ac components. In this case the ac component is very small so it can be ignored. A 6072 should last forever with a 77V Vhk.

Cheers

Ian

 

[Walter66]

Thanks Ian, you made my day! I'm running a preamp with 12BH7A on 115V elevated heater voltage for now nine years without problems. It's the Teletronix LA2A output stage and I don't wanted to run into problems with the 6072 either.