24-05-2012, 11:43 PM
Hi Lawrence,
This is all getting a bit complicated, isn't it? "Complicated", that is, when only the written word is used to explain these things: hence, the value of a drawing, then with a few supplementary comments thrown in.
So let me try to consolidate what you have there now and where you want to take this project; this also gives me the chance to verify my understanding of those two things.
At the moment, you have an R.F. amp. with an L/C tuned grid (transformer coupled from the aerial). Its anode cct. has an untuned transformer primary. Its secondary comprises the 'L' of an L/C tuned cct. feeding the grid of the mixer. Good: all very conventional. The two variable 'C's are not ganged: debatably unconventional.
If you wish to replace the R.F. amp. valve with a triode cascode pair, I would choose the series cascode connection, as opposed to the shunt cascode (former is easier to implement). Then the anode of the 'top' triode can be connected to the primary of the original anode load transformer as before. The secondary winding is isolated from the HT rail: no need for HT isolating capacitors.
And all of that is effectively what the 'front end' of my modified Eddystone 888A receiver looks like - except for the ganging of the variable 'C's, the application of A.G.C. and an R.F. gain control. The valve is an ECC85, incidentally.
You're making good progress: perhaps it's time for an updated drawing!
Keep at it,
Al.
This is all getting a bit complicated, isn't it? "Complicated", that is, when only the written word is used to explain these things: hence, the value of a drawing, then with a few supplementary comments thrown in.
So let me try to consolidate what you have there now and where you want to take this project; this also gives me the chance to verify my understanding of those two things.
At the moment, you have an R.F. amp. with an L/C tuned grid (transformer coupled from the aerial). Its anode cct. has an untuned transformer primary. Its secondary comprises the 'L' of an L/C tuned cct. feeding the grid of the mixer. Good: all very conventional. The two variable 'C's are not ganged: debatably unconventional.
If you wish to replace the R.F. amp. valve with a triode cascode pair, I would choose the series cascode connection, as opposed to the shunt cascode (former is easier to implement). Then the anode of the 'top' triode can be connected to the primary of the original anode load transformer as before. The secondary winding is isolated from the HT rail: no need for HT isolating capacitors.
And all of that is effectively what the 'front end' of my modified Eddystone 888A receiver looks like - except for the ganging of the variable 'C's, the application of A.G.C. and an R.F. gain control. The valve is an ECC85, incidentally.
You're making good progress: perhaps it's time for an updated drawing!
Keep at it,
Al.
