In article , Patrick Turner
wrote:
John Byrns wrote:
In article , Patrick Turner
wrote:
John Byrns wrote:
One thing that I haven't seen
mentioned is how the IFT plays into this since it has a sort of
impedance
inverting characteristic, as we discussed earlier, if the secondary is
shorted, the impedance of the primary increases, just the opposite
of what
we would expect.
I found that loading the secondary reduced the gain of the IF amp, ie,
the load is reflected to the primary.
When a double tuned IFT is used, the load reflected in the primary is
opposite to the actual load on the secondary, that is when the load
resistance presented to the secondary of the IFT by the diode goes up, the
tube sees a load resistance from the primary that is going down, just the
opposite of what you might expect.
Nope, when I placed 100k resistors on the IFT secs, the gain went down.
Well we have to be careful how we are defining "gain", and where we are
measuring it. As I said this could be a subject for a Masters Thesis, and
I have only crunched the first few of many numbers that would have to be
crunched if I were writing the complete thesis. In any case when you add
a 100k resistor across the secondary of a double tuned IFT you will see an
increase in primary voltage if it is being driven from a constant current
source like a pentode valve. The increase in primary voltage directly
implies an increase in the voltage gain at that point, and since the
voltage increased, and the current is constant, the power delivered to the
IFT must also have increased and hence the power gain at the primary also
increased.
Now as far as what happens at the secondary, that is affected by many
factors, among them where the losses in the circuit are, and what the
value of "k" is. That is a major part of the thesis and one that I have
only taken a small peak at. In any case I think it is probably safe to
say that the voltage gain to the secondary, that is including the IFT,
goes down when you add the 100k resistor.
The inescapable fact is that when you put a resistor across the secondary
of an IFT, the gain as measured at the plate of the IF amplifier tube goes
up, and at least the voltage gain at the secondary of the IFT goes down.
If by loading the secondary you meant
decreasing the load resistance, then you should have found that the power
gain actually went up, at least from the perspective of the primary. See
my discussion above for more detail.
If a 100k resistance is connected across the secondary of the last IFT LC,
then the the voltage ratio of the IFT increases. Less output IF voltage
comes out,
so less AVC voltage, to the IF amp is biased more positively, so there is more
voltage produced
at the primary or anode connection of the IFT.
Let's leave the AGC voltage out of this and make it constant, AGC is a
separate issue and should be a separate discussion.
That's the only mechanism I know by which gain will increase on the IF
amp tube,
if R loading is applied to the IFT sec.
This simply indicates to me that you haven't done the lab work I assigned
you earlier, and this is the second time I have had to remind you about
this.
I will remind you of the assignment again, it is to take a properly tuned
IFT and measure the primary impedance at resonance, and then to short the
secondary and again measure the primary impedance. Extra credit will be
given for also measuring the primary impedance with a 100 k resistor
connected across the secondary, and for repeating all the above
measurements with several different values of "k".
Once you have all this data logged in your notebook, you can sit down in
your lounge and contemplate what it all means. That the world isn't as
simple as it first seems is one conclusion, and as an old associate often
said, you may reach the next level of consciousness.
Regards,
John Byrns
Surf my web pages at,
http://users.rcn.com/jbyrns/