In article ,
Patrick Turner wrote:
On Jul 1, 1:58*am, John Byrns wrote:
In article
,
*Patrick Turner wrote:
I agree with everything Alex says below except for a few things.
In most old radios with DC flow across the volume control pot track
the adjustment of the volume is often very noisy after the pot has
aged a few years.
So instead of the conventional arrangements put forward by RDH and
most others to avoid parts costs I will have the last IFT coil biased
at say +50V at one end, and the live end goes to a triode grid of 1/2
12AU7 CF buffer to remove any loading effect of diode detection on the
last IFT LC.
What is the purpose and or advantage of using a cathode follower between
the IFT
and detector diode, especially if you are going to add 100k resistors to
widen
the IF response as you discuss below?
The last IFT coil is a high impedance tuned circuit signal source. If
one is going to load it slightly to slightly reduce the Q thus
widening the pass band and AF response then using a pure resistance is
benign. The CF converts the high Z source to low Z source and all the
crapological behaviour of the Ge diode dissappears.
One could still use a 6AL5 if one wanted to.
You haven't really answered the question, just spouted a bunch of meaningless
gibberish. My question was what is the advantage of using a cathode follower
between the secondary of the IFT and the detector diode? The cathode follower
is going to reduce the loading on the IFT, which is going in exactly the
opposite direction you seem to want to go when you recommend loading the IFTs
with 100k resistors! What do you gain by adding the cathode follower to the
circuit? What is wrong with simply choosing the diode load to reflect the
desired load to the secondary of the IFT, 100k or whatever? Also the coupling
coefficient between the primary and secondary of each IFT should be properly
coordinated with the selected loading on the IFTs to achieve the desired
bandwidth with maximum response flatness. A cathode follower after the diode
can be useful as one way to eliminate negative peak clipping due to a poor AC/DC
load ratio. What is this "carpological" behavior of the Ge diode that you are
talking about? Can you define the nature of this behavior?
Then I use Ge diode feeding RC circuit, and this can
directly feed second 1/2 12AU7 CF buffer and then usual CR coupling to
any a volume control and while employing time passive poles to give
say -3dB at 30Hz before any power amp which has NFB. I often add in
another 12AU7 gain stage for tone control to boost/cut treble; bass in
AM is usually OK.
To slightly widen AF response the Q of all IFTs may be reduced by
strapping 100k across each coil. It doesn't work in all sets, but may
be tried.
The coupling between the two coils in an IFT often needs to be increased
somewhat when adding resistors, that may explain why it doesn't work in all
sets, if the IFTs are over coupled adding resistors may ne counter
productive.
Try things. Predjudice don't belong anywhere when you want to make old
junk meet modern expectations.
What and whose prejudice are you talking about here? Are you talking about
people who have a "prejudice" against "improving" old radios? I have no such
prejudice. I do wonder about the design basis and effectiveness of
modifications, whether the modifications are being done in the best and most
effective manner.
--
Regards,
John Byrns
Surf my web pages at,
http://fmamradios.com/