[Patrick Turner]

On Jul 6, 10:07*am, John Byrns wrote:
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? *

I didn't speak meaningless giberish. But, I do understand that
whatever I do say, and whatever I have posted at my website as
examples of my craftmanship will always be misunderstood, belitlled,
rubbished, dismissed, and ****ed upon shat upon from a great height. I
don't care because whoever is downloading the regular 100MB a day from
my site might not believe in you gibberish theory, and when they try
some of my circuits they find the sound is dramatically better than
has been served up by an industry mainly concerned with the cost-of-
production rather than audio fidelity.

As many of the readers here are approaching nearer to their 90th
birthday closer than I am, of course its likely they might find my
answers to questions and discussions of 101 things related to audio or
the world to be gibberish well before I conceed to myself that I don't
understand my position any better, because of increasing possible
senility. When blokes here say they were apprentices in some
electronics company in 1952, but of course try to hide their age, I
have to allow for the inevitable fact that a percentage of them have
gone soft up top, and I'd be utterly bamboozling, no matter how simply
I explained anything.

I suggest you read RDH4 and visit my website more.

I just re-wired a horrible Howard radio set made in about 1935. The
last IFT secondary is biased at +50V, and it feeds a 1/2 12AU7 CF,
with Ge diode, with 680k + 250pF cap to 0V, then 200k plus 100pF LPF
then direct coupled to another 1/2 12AU7 CF which poweres the 500k
volume control. The sound is HEAD and SHOULDERS above any other crappy
radio.

I suggest you try my methods to learn why I use the methods, for
therin lies the answers to all you testy questions.

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!

Resistance loads the IFTs with resistance, and the effect retains
pentode linearity. I don't always use R damping to reduce Q, but its
handy when the set has been changed to work with all mains operated 7
pin tubes in lieu of the 1.4V filament types which are fragile, have
low gm, and gain, and are becoming hard to obtain, and which ppl don't
want to use any more with batteries. The IFTs in such sets usually
have very high Q, and very low and thus poor sounding AF BW so R
damping is fine. With sets that have an RF stage ahead of the mixer so
hence have a total of 6 tuned circuits, side band cutting sometimes
limits AF to 2kHz, ie, bloody awful, OK if you are listening to long
distance talk back, but useless for music. So with so many stages the
IF Q may be reduced quite a bit and there is still plenty of gain for
the set overall for what is most listened to, ie, strong local AM
stations. Some reduction of Q is best if spread over all 4 coils in 2
IFTs. One must remember that the each LC circuit has only 6dB/octave
attenuation away from the Fo. Let's say each IFT LC has Q = 45. The
pass band = +/-5kHz, or 10kHz. Maybe you get -15dB at +/- 20kHz so
that with 4 such tuned circuits the pass band is 5kHz or less, and at
+/- 20kHz the attenuation = -60dB, considered OK, because strong
locals are usually separated by 45kHz at least. The nose shape of well
done IFTs won't be as sharp as just one one coil because the mutual
coupling is supposed to be arranged to flatten the top of the pass
band somewhat to give wide BW. Anyway, the nose shape won't much
affect the skirt attenuation at more than +/- 20kHz away from Fo. One
has to take all these considerations into account before adding R
dampers.

*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? *

Better sound. Try it sometime. Then you'll know.


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?

Ge diodes have reverse lekage currents. Try using one in a typical
radio circuit and get back to me. Be very careful when observing the
whole depressing performance using a high impedance non loading type
of probe to your CRO.

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.

Its far simpler for me to to humbly suggest you try my methods rather
than expect a full explanation. If I give an explanation its read,
perhaps called gibberish, and forgotten. The reader does not solder
the idea into reality. Nothing is learnt. I have wasted my precious
time. I've put all my answers at my website in the form of schematics.
I've explained all this at rec.audio.tubes several times in the last
11 years and I'm getting tired of repeating myself.

There are far better ways to build an AM radio than by slavishly,
blindly and unquestioningly following RDH4. Its a great book, but many
of the best practices with tubes are omitted because manufacturers
could not afford to implement them.

Patrick Turner.