Regenerative receiver performance

[Circlotron]

Say we are making a regenerative receiver with the choice of two different vacuum tubes, one from your great grandfather's stuff that has a gain of 10, and a much more modern tube that has a gain of 100. Fire up the receiver and check the results. If we advance the regeneration there will be a point of maximum gain just below the point where the circuit begins to oscillate.

The question is, is the maximum gain figure before oscillation the same for both the x10 tube and the x100 tube? And if that is the case, would it be more desirable to use the x10 tube because it would require more regeneration to reach the same maximum gain figure and presumably / consequently have greater selectivity? And is there an optimum gain figure for a tube that is to be used in a  regenerative stage? 

[szoftveres]

A regenerative receiver can be viewed as an oscillator, built with vacuum tube and a feedback ("tickler") coil.
As per the Barkhausen-criteria, you need amplification to make up for the loss in the LC tank. Whether you reach this amplification with a tube with higher mu, closer spacing of the coils or changing the turns ratio doesn't really matter; the input impedance of the tube will likely be very high (let's assume ideal case) and hence all the three methods will get you to oscillation; at (or near) regeneration the total voltage gain of the system will be Av=1 and the LC tank will appear to have infinite Q, irrespective of which device and configuration you have.

Going further in the discussion, you need to specify your system; pick a method for adjusting the regeneration:

• adjusting the plate or screen voltage affects the mu of the tube (are we using triode? pentode?)
• increasing the spacing between the coils unloads the LC tank while simultaneously decreases the impedance seen by the plate and hence the Av of the stage
• bypassing the feedback coil with a variable resistor loads the LC tank and degenerates its Q and simultaneously decreases the impedance on the plate and hence the Av of the stage
• bypassing the feedback coil with a throttle capacitor only decreases the impedance on the plate and hence the Av of the stage
• (tube mu can also be varied by adjusting the filament current via a rheostat)
• any static variation of the above methods combined with one variable method

[Circlotron]

Going further in the discussion, you need to specify your system; pick a method for adjusting the regeneration:

• adjusting the plate or screen voltage affects the mu of the tube (are we using triode? pentode?)

Okay, let's keep it simple. Triode in both cases, and adjust the anode supply voltage. Stone age home made tube with an amplification factor of 10 vs half a 12AX7 with an amplification factor of 100. Lets assume inter electrode capacitances are the same for both tubes.

[radiolistener]

Receiver performance is limited with noise figure and dynamic range of your RF frontend.
So the only way to improve it is to reduce noise figure and improve dynamic range of your RF frontend circuit.

[szoftveres]

Triode in both cases, and adjust the anode supply voltage

I don't think that the question in this context makes any sense (maybe I implied that there was a simple answer to your question in my previous reply, sorry about that).
Let's call the "gain figure" mu for now. Assuming identical LC tanks and identical plate voltage in both cases, the mu=10 tube will need tighter feedback coupling than the mu=100 tube to get to oscillation This by itself makes the two receiver setups different so it's not an apple to apple comparison any more. And if you use identical coupling in both cases and degrade the mu of the "better tube" by lowering its plate voltage to match the "older tube", you'll get two identically performing tubes (and consequently identical performance).

I'll try to respond to the questions:

The question is, is the maximum gain figure before oscillation the same for both the x10 tube and the x100 tube?

It should be clear from the above statement that the answer is yes in the mu degrading case, and the overall gain of the system must be Av=1 in any ways (Barkhausen criteria), whether achieved with a higher mu tube, or by using tighter coupling.

And if that is the case, would it be more desirable to use the x10 tube because it would require more regeneration to reach the same maximum gain figure and presumably / consequently have greater selectivity?

Regeneration will always be equal - Av=1.

And is there an optimum gain figure for a tube that is to be used in a regenerative stage?

The optimum mu range is quite wide. You can build a great performing regenerative receiver with a modern high-mu tube as well as with a ridiculously unsuitable, low mu korg nutube (see below). The end result will be the same: LC tank Q will approach infinity (which means that all losses inside it are recovered).

[Circlotron]

Thank you for your in depth replies. I appreciate that. Back in the early 70s I made numerous small regenerative tube radios. It is indeed amazing how much gain you can get out of a single tube by that method.

One thing I never tried because I didn't think of it at the time - how about regeneration and reflex, that is to say make it amplify the audio as well. Would that be compatible with the regenerative function?