Re: OPT equalising circuit

Phil Allison wrote:
> 
> "Alex" <apogosso@[EMAIL PROTECTED]
> 
> > Overall NFB will be about 12dB. It is applied to the cathode of the
first
> > 12AX7 stage.
> 
> ** Absurd thing to do.  It will completely ruin the " tone " of the
radio
> which in part depends on the audio power stage having a high source
> impedance.
> 
> Time you discovered just how the Z of loudspeakers varies with drive
> frequency and that this variation is commonly used to equalise the sound
of
> valve radios and guitar amps.

I know exactly what you are saying, and sure, some radios need to have a 
current source pentode output tube to extend the bass and treble.

There have been plenty that were deliberately engineered 
to have a response from say 100Hz to 5kHz, and the actual 
audio response of the set due to RC audio couplings and very poor IF
bandwidth 
is only 200Hz to 2.5kHz, and the rising Z of the speaker at each end of
the band means you end up
with 100Hz to 5kHz.

Some were a lot better than others to achieve these design aims but most
were 
appalling junk that is only good for speach and the news.   

But usually there is also 10% of acumulated other distortions in the
radio, and above a whisper
they are really horrid things to listen to.

On the other hand, many radios including reflex sets where the IF amp
also 
amplified the audio as well had a shunt FB network between OPT sec and
the audio source 
including the volume control. This saved the cost of an extra tube.
I have one which I repaired which wasn't picked up after I worked two
days on the critter.
Its nowhere nearly as good sounding as my own design of AM radio
which is also in my kitchen.

Time YOU woke up to what the possibilities are with re-engineering 
old radio sets. One day, all the analog free to air broadcast radio will 
be turned off and it'll only be digital, but until then analog AM radio
has a 
big part in my house to bring me Radio National; here its 846kHz, and
obviously 
has 30Hz to 9kHz bandwidth, AND has low distortion, and and my radio
with its special capabilities 
IS BETTER than all the others I have ever heard.

I found most old AM tube radios are bloody old junk.

Then after everyone suffered from 1930 to about 1960,
we got transistor AM radios which were worse.

Finally we got FM and AM was left as is, and makers of receivers and
tuners
mainly treated AM like ****, ie SNAFU.

Patrick Turner.




> 
> ......  Phil