[Brinkman]

Hi there. Couple questions about the Cornet2:

I am wondering if the specified inter-electrode capacitance of the 12AX7 is integral to the RIAA equalization of the Cornet2. In other words, would a low-capacitance 12AX7 variant like the recent Techtube 813CC not perform properly in the circuit?

Along the same lines, if one were to substitute current sources in place of the the plate resistors on the first and second tube positions, would the output capacitance of the CCSs disrupt the EQ?

Thanks.

[Brinkman]

I found an answer to my second question...

Putting current sources as plate loads will change the EQ.  Presently EQ is dependent on the rp of the tube (running without cathode bypass), the plate resistor, and the grid bias of the following stage in parallel.  Removing the plate resistance may make the result too high requiring a different capacitor value.  It will also be noiser (more hiss).

Unfortunately, this is one of those cases where things work against each other.  Grid bias current wants to see a low impedance, the EQ likes to have well defined impedance, noise likes low impedance, and the tube load should be as high as possible.  So we want both very high and very low resistance at the same time.  It is best not to go to either extreme. 

The best solution is actually both.  High and low impedance at the same time.  As far as I know, the only way to do this is with a plate choke or transformer.

The Cornet is a very carefully tuned circuit that puts tube bias, plate load, etc. all in harmony for optimal results in that configuration.  Yes, you can do better with plate chokes.  Going to a balanced design will also help (Trumpet).  Interstage transformers may work out better than coupling caps.  Higher B+ helps, too.  There are always compromises in any design.  The goal of the Cornet was super low cost all tube design with minimal performance loss.  I could have made it without tube rectification or a cathode follower, but that would have been a performance degradation.  Besides, the cost of a time delay relay circuit for high voltage isn't that much cheaper than a 5Y3.  For me, it's all about elegance and simplicity.

jh

[hagtech]

would a low-capacitance 12AX7 variant

No.  The EQ capacitors are relatively large in value and will completely swamp put any parasitic from the tubes.

jh