In article ,
Patrick Turner wrote:
snip for brevity.
John Byrns
Surf my web pages at, *http://fmamradios.com/
You grasped the idea perfectly well John. This frequency compensation is
useful to prevent a lousy OPT and 6AQ5 overloading in a lousy boring audio
amp found in those boring AA5 style radios.
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?
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.
Related to this is the fact that the instruction manual for Heathkit's first AM
tuner kit recommended placing a resistor(s) across the first IFT if a narrower
bandwidth was needed, so things don't always work as expected. The reason the
resistor(s) narrowed the bandwidth in the Heathkit is because it was a wideband
tuner with an over coupled 1st IFT, so adding the resistor(s) takes the
transformers close to critical coupling, giving a narrower response. I don't
remember if the Heathkit mod for reducing the bandwidth involved adding
resistors on both the primary and secondary, or on only one winding, I will have
to see if I can find the manual, IIRC they also used a resistor or resistors
during alignment to eliminate the over coupling, allowing the tuner to be
aligned by peaking the IFTs.
There is a simple method explained in RDH4 to add a few
turns of fine wire around the primary of IFT1 and switch it to being
in series with the sec of IFT1 which will give a doble peak to IFT
response which effectively increases IF channel bandwidth from a
typical 4kHz to 8 kHz. Quad used it.
QUAD's first superheterodyne AM tuner, the Acoustical AM tuner, used a different
scheme to vary the bandwidth. Rather than using an IFT they used two separate
IF coils with low side capacitive coupling, the low side capacitive coupling
could be switched to vary the bandwidth.
I've used it - finally there is the the treble which most sets refuse
to give me.
I like to use paralleled 12AX7 as V1 and EL34 in triode as V2 for the
audio amp and with 12dB global NFB.
This works far better than anything with 6V6 or EL84 etc.
The Ge diode in the detector is biased on with a low current in the R.
Its works best with high level signals and up to 100% mod, with 10Vrms
available if you want it.
But anyone could use the normal arrangement of IFT plus tube
rectifiers found in IF amp tubes like 6N8 and thus get say -2Vdc
generated by a carrier with 2pk volts of signal. at 100% mod the Vpk-
pk of audio = 4V, so max audio = 1.4V, no?
The audio signal with negative vdc and some 455kHz ripple could be
direct coupled to a CF triode buffer stage but you'd need a -150Vdc
rail for the cathode resistor, while the anode would go to a +100V
supply. The CF buffer and negative rail was viewed as a waste of money
in 1950, so it was never done. But there's no reason why it can't be
done NOW. And the screen voltage applied to mixer and IF amp is best
regulated at 100Vdc, so that's a good point of supply with CF anodes .
I don't much like tubes like the 6AV6 with grounded cathodes and 10M
grid leak biasing.
The very low F signals generated in AM sets don't seem to cause any
problems in anything I have made.
--
Regards,
John Byrns
Surf my web pages at,
http://fmamradios.com/