Refer to ABPR & ABSE

In this one both AC & DC component of the detected signal are
passed thru the VC. As the VC is advanced the negative bias on
the 6R7 grid will increase. The AGC is developed by the other diode.

Taken from M N Beitman, Supreme publications.

Cheers, John Stewart

John Stewart wrote:

Refer to ABPR & ABSE

In this one both AC & DC component of the detected signal are
passed thru the VC. As the VC is advanced the negative bias on
the 6R7 grid will increase. The AGC is developed by the other diode.

Taken from M N Beitman, Supreme publications.

Cheers, John Stewart

The detector shown is very common in many old radios, and also a real
pain.
There is a -ve DV developed by the audio detector, similar in magnitude
to the
AGC voltage developed by the AGC diode + RC circuit.
The -DV developed by the AF detector is fed to the 6R7 grid and as the
volume
is increased, so does the tube bias, and the distortion!

The DC flow in the pot often causes a crackly volume control.

When done right, its better to use both diodes strapped together to make
both
AF and AGC voltage, and have a cap coupled volume control.
Despite the AC/DC load being perhaps 1:1 ratio, the first few volts of
the
output with a strong
local signal are not very distorted if the R discharging the peak hold
cap is
chosen correctly for the least thd, which it rarely ever is, because
radio designers
were a conservative lot who were mainly unable to think originally,
or were constrained by the bean counters in the next office along the
corridor.

I have nearly got the bugs out of the complete revamp of a Stromberg
Carlson.
I was forced to use input and oscillator coils from other radios, and
now
forced to wind a new oscillator coil to get the tuning range from 550
to1,700 kHz.

Maybe I will have to wind an input coil as well to make the set track
properly, so that the 455 kHz IF signal is always produced when the the
input RF coil
is tuned to within a couple of kHz of the wanted station.

This is never an easy or quick thing to achieve.

I always get trapped when completely rebuild a radio.


Patrick Turner.

In article , Patrick Turner
wrote:

The DC flow in the pot often causes a crackly volume control.

I agree it would be nice to get rid of the DC on the volume control, but
that is difficult to do in the context of a traditional 5 tube radio,
without causing other problems, bean counters or no bean counters.

When done right, its better to use both diodes strapped together to make
both
AF and AGC voltage, and have a cap coupled volume control.
Despite the AC/DC load being perhaps 1:1 ratio,

A 1:1 AC/DC load ratio is the ideal!

the first few volts of
the
output with a strong
local signal are not very distorted if the R discharging the peak hold
cap is
chosen correctly for the least thd, which it rarely ever is, because
radio designers
were a conservative lot who were mainly unable to think originally,
or were constrained by the bean counters in the next office along the
corridor.

Could you explain what you are proposing in more detail? I seem to
remember this was discussed here several years ago, and I think the
conclusion was that the traditional circuit, with DC on the volume
control, and a separate AGC rectifier, was the best you could do within
the context of a traditional 5 tube radio, even if R's and C's were free.
IIRC it was the general feeling that the traditional circuit was the
result of years of refinement, and there was little more that could be
done to improve it. It seems to me 5 tubes was the limiting factor, not
the bean counters.


Regards,

John Byrns


Surf my web pages at, http://users.rcn.com/jbyrns/

Patrick Turner wrote:

John Stewart wrote:


Refer to ABPR & ABSE

In this one both AC & DC component of the detected signal are
passed thru the VC. As the VC is advanced the negative bias on
the 6R7 grid will increase. The AGC is developed by the other diode.

Taken from M N Beitman, Supreme publications.

Cheers, John Stewart


The detector shown is very common in many old radios, and also a real
pain.
There is a -ve DV developed by the audio detector, similar in magnitude
to the
AGC voltage developed by the AGC diode + RC circuit.
The -DV developed by the AF detector is fed to the 6R7 grid and as the
volume
is increased, so does the tube bias, and the distortion!

The user would adjust the volume control for a reasonable amount of
volume anyway. Thus you'd get a fairly normal bias level. You
would have to select a reasonable sized plate resistor to avoid
"saturation" though.


The DC flow in the pot often causes a crackly volume control.

Is that an issue only when current is drawn across the carbon
track/ wiper interface? The tube grid draws no (or insignificant)
current.

Despite the AC/DC load being perhaps 1:1 ratio, the first few volts of
the
output with a strong
local signal are not very distorted if the R discharging the peak hold
cap is
chosen correctly for the least thd, which it rarely ever is, because
radio designers
were a conservative lot who were mainly unable to think originally,
or were constrained by the bean counters in the next office along the
corridor.

I suspect that AA5 radio design was relegated to junior engineers and
technicians once more interesting stuff for the senior staff was to
be done. Like FM radio design, television, and such. AA5s would
be boring as the bean counters and marketing would say that the
existing low cost design was "good enough" for the market, and doing
improvements that require even cheap caps and resistors would not
pay to produce. Much like today's AM section of AM/FM stereo receivers
and tuners.


I have nearly got the bugs out of the complete revamp of a Stromberg
Carlson.
I was forced to use input and oscillator coils from other radios, and
now
forced to wind a new oscillator coil to get the tuning range from 550
to1,700 kHz.

Maybe I will have to wind an input coil as well to make the set track
properly, so that the 455 kHz IF signal is always produced when the the
input RF coil
is tuned to within a couple of kHz of the wanted station.

This is never an easy or quick thing to achieve.



I suppose that the dial will need new calibrations too. Or
use a larger pulley wheel on the tuning cap to make the needle
(I's guessing a slide rule dial) travel farther above 1600.

John Byrns wrote:

In article , Patrick Turner
wrote:

The DC flow in the pot often causes a crackly volume control.

I agree it would be nice to get rid of the DC on the volume control, but
that is difficult to do in the context of a traditional 5 tube radio,
without causing other problems, bean counters or no bean counters.


Using a 1M volume pot is often a very poor value for the discharge R from the
50 pF cap in the detector.

I routinely rewire old sets to remove the type of crummy DC affected circuitry
with glee,
and also to reduce thd caused in such simpleton designed sets.


When done right, its better to use both diodes strapped together to make
both
AF and AGC voltage, and have a cap coupled volume control.
Despite the AC/DC load being perhaps 1:1 ratio,

A 1:1 AC/DC load ratio is the ideal!

Nope. Not in all cases.



the first few volts of
the
output with a strong
local signal are not very distorted if the R discharging the peak hold
cap is
chosen correctly for the least thd, which it rarely ever is, because
radio designers
were a conservative lot who were mainly unable to think originally,
or were constrained by the bean counters in the next office along the
corridor.

Could you explain what you are proposing in more detail? I seem to
remember this was discussed here several years ago, and I think the
conclusion was that the traditional circuit, with DC on the volume
control, and a separate AGC rectifier, was the best you could do within
the context of a traditional 5 tube radio, even if R's and C's were free.

Trouble is that many old blokes have lost any idea they ought to
check their belief systems....." Heck, I has grown ma corn thisaway
for nigh on 60 years, and no two bit city slicker with a fancy suit and car
issa gonna tell me how I issa gonna grow ma pharkin corn, OK?"

Same scene with old guys and the precious old radios.
It must be right, we did it 60 years sgo!

I am so proud when I can proove what a right git I was when I believed some
line of BS about 30 yrs ago, or 3 years ago, or 3 minutes ago.



IIRC it was the general feeling that the traditional circuit was the
result of years of refinement, and there was little more that could be
done to improve it. It seems to me 5 tubes was the limiting factor, not
the bean counters.

Don't ask me to say all I could about why the status quo in design of old
radios stayed so
backward and moribund for so long.

Luckily some dudes simply invented FM radio with carriers at 100 MHz,
and they thus allowed us to escape the mediocrity of the AM world, its horrid
receivers
falsely touted as being god's gift to listening pleasure, its low brow
programmes,
its noise, its distortions....

Then when stereo came along, the FM was the ideal way to deliver it with
multiplexing, but how many here have ever designed and built a complete
FM receiver and MPX decoder worth listening too?

When I mentioned I wanted to build an RF gene with FM to a group of old radio
hams at a
swap meet, they had no idea, and smiled, and one said, "Aw heck, waddya wanna
change the frequency for,
youse really wanna keep it stable."

I gave up on that lot, and headed for the local Australian National
University,
which has all these reference libraries, and in basements and ancient archives
I found
out how reactance tube modulators worked, so I copied reams of info,
and some of the theory I read to build my own gear.

I suggest all you guys do the same.

Patrick Turner






Regards,

John Byrns

Surf my web pages at, http://users.rcn.com/jbyrns/

Robert Casey wrote:

Patrick Turner wrote:

John Stewart wrote:


Refer to ABPR & ABSE

In this one both AC & DC component of the detected signal are
passed thru the VC. As the VC is advanced the negative bias on
the 6R7 grid will increase. The AGC is developed by the other diode.

Taken from M N Beitman, Supreme publications.

Cheers, John Stewart


The detector shown is very common in many old radios, and also a real
pain.
There is a -ve DV developed by the audio detector, similar in magnitude
to the
AGC voltage developed by the AGC diode + RC circuit.
The -DV developed by the AF detector is fed to the 6R7 grid and as the
volume
is increased, so does the tube bias, and the distortion!

The user would adjust the volume control for a reasonable amount of
volume anyway. Thus you'd get a fairly normal bias level. You
would have to select a reasonable sized plate resistor to avoid
"saturation" though.

AAAARRRGGHHHH!




The DC flow in the pot often causes a crackly volume control.

Is that an issue only when current is drawn across the carbon
track/ wiper interface? The tube grid draws no (or insignificant)
current.

Cracks in the tracks and dust don't wreck the AC signal too much,
but they cause huge bothers with DC turning on/off.



Despite the AC/DC load being perhaps 1:1 ratio, the first few volts of
the
output with a strong
local signal are not very distorted if the R discharging the peak hold
cap is
chosen correctly for the least thd, which it rarely ever is, because
radio designers
were a conservative lot who were mainly unable to think originally,
or were constrained by the bean counters in the next office along the
corridor.

I suspect that AA5 radio design was relegated to junior engineers and
technicians once more interesting stuff for the senior staff was to
be done. Like FM radio design, television, and such. AA5s would
be boring as the bean counters and marketing would say that the
existing low cost design was "good enough" for the market, and doing
improvements that require even cheap caps and resistors would not
pay to produce. Much like today's AM section of AM/FM stereo receivers
and tuners.


I have nearly got the bugs out of the complete revamp of a Stromberg
Carlson.
I was forced to use input and oscillator coils from other radios, and
now
forced to wind a new oscillator coil to get the tuning range from 550
to1,700 kHz.

Maybe I will have to wind an input coil as well to make the set track
properly, so that the 455 kHz IF signal is always produced when the the
input RF coil
is tuned to within a couple of kHz of the wanted station.

This is never an easy or quick thing to achieve.



I suppose that the dial will need new calibrations too. Or
use a larger pulley wheel on the tuning cap to make the needle
(I's guessing a slide rule dial) travel farther above 1600.

The dial is a truly horrid little circular thing that sits on the tuning gang
shaft
fitted with a lousy concentric ball bearing reduction drive that feels
slightly lumpy,
and sometimes slips.
I had to replace the tuning gang, which had busted ceramic supports.
Lucky I had one exactly the same out of about 50 I have collected.
There is a window about an inch square to read where you are.
The previous owners tried to pencil in where the stations were.
I won't bother to remake the dial or recalibrate it, but leave it as it is,
and let the owner tune by guess work, which is easy with the AM stations
here;
you don't need to read a dial to tune the radio, or know what station you are
on.
You'd need a microscope to read the numbers anyway.
It'll be a case of "nice sound, shame about the dial".

Patrick Turner.

Patrick Turner wrote:

John Stewart wrote:

Refer to ABPR & ABSE

In this one both AC & DC component of the detected signal are
passed thru the VC. As the VC is advanced the negative bias on
the 6R7 grid will increase. The AGC is developed by the other diode.

Taken from M N Beitman, Supreme publications.

Cheers, John Stewart

The detector shown is very common in many old radios, and also a real
pain.

Of the 240 receiver circuits (1926-1938) shown in the Beitman
compilation only one uses this rather odd hookup, so may not have
been common practice here in North America. Early circuits for the
most part seem to be anode bend using triodes or the 24A while
the later circuits are pretty much the AC coupled diode already
well discussed recently in this forum.

Cheers, John Stewart

I have two 38-93 Philcos that use this detection method. I have no test
equipment to measure distortion, but these two radios sound as good or
better than any radio I have restored so far.
"John Stewart" wrote in message
...


Patrick Turner wrote:

John Stewart wrote:

Refer to ABPR & ABSE

In this one both AC & DC component of the detected signal are
passed thru the VC. As the VC is advanced the negative bias on
the 6R7 grid will increase. The AGC is developed by the other diode.

Taken from M N Beitman, Supreme publications.

Cheers, John Stewart

The detector shown is very common in many old radios, and also a real
pain.

Of the 240 receiver circuits (1926-1938) shown in the Beitman
compilation only one uses this rather odd hookup, so may not have
been common practice here in North America. Early circuits for the
most part seem to be anode bend using triodes or the 24A while
the later circuits are pretty much the AC coupled diode already
well discussed recently in this forum.

Cheers, John Stewart



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The user would adjust the volume control for a reasonable amount of
volume anyway. Thus you'd get a fairly normal bias level. You
would have to select a reasonable sized plate resistor to avoid
"saturation" though.


AAAARRRGGHHHH!


Forgot to mention that the triode and plate circuit need to be designed
to operate properly with the given signal amplidudes and bias levels
it will see in this scheme. A half 12AX7 (aka 12AV6) seem to work
reasonably well here. Maybe use 220K instead of 470K for the plate
resistor, or use higher B+ if you want. But it's still only mid-fi.
For hifi I'd use the CF buffer.

In article , Patrick Turner
wrote:

John Byrns wrote:

In article , Patrick Turner
wrote:

When done right, its better to use both diodes strapped together to make
both
AF and AGC voltage, and have a cap coupled volume control.
Despite the AC/DC load being perhaps 1:1 ratio,

A 1:1 AC/DC load ratio is the ideal!

Nope. Not in all cases.

When isn't a 1:1 AC/DC load ratio ideal?

the first few volts of
the
output with a strong
local signal are not very distorted if the R discharging the peak hold
cap is
chosen correctly for the least thd, which it rarely ever is, because
radio designers
were a conservative lot who were mainly unable to think originally,
or were constrained by the bean counters in the next office along the
corridor.

Could you explain what you are proposing in more detail? I seem to
remember this was discussed here several years ago, and I think the
conclusion was that the traditional circuit, with DC on the volume
control, and a separate AGC rectifier, was the best you could do within
the context of a traditional 5 tube radio, even if R's and C's were free.

Trouble is that many old blokes have lost any idea they ought to
check their belief systems....." Heck, I has grown ma corn thisaway
for nigh on 60 years, and no two bit city slicker with a fancy suit and car
issa gonna tell me how I issa gonna grow ma pharkin corn, OK?"

Same scene with old guys and the precious old radios.
It must be right, we did it 60 years sgo!

I am so proud when I can proove what a right git I was when I believed some
line of BS about 30 yrs ago, or 3 years ago, or 3 minutes ago.

It sounds like you are admitting that you haven't a clue how to design a 5
tube radio any better than the old blokes did!


Regards,

John Byrns


Surf my web pages at, http://users.rcn.com/jbyrns/

John Byrns wrote:

In article , Patrick Turner
wrote:

John Byrns wrote:

In article , Patrick Turner
wrote:

When done right, its better to use both diodes strapped together to make
both
AF and AGC voltage, and have a cap coupled volume control.
Despite the AC/DC load being perhaps 1:1 ratio,

A 1:1 AC/DC load ratio is the ideal!

Nope. Not in all cases.

When isn't a 1:1 AC/DC load ratio ideal?

the first few volts of
the
output with a strong
local signal are not very distorted if the R discharging the peak hold
cap is
chosen correctly for the least thd, which it rarely ever is, because
radio designers
were a conservative lot who were mainly unable to think originally,
or were constrained by the bean counters in the next office along the
corridor.

Could you explain what you are proposing in more detail? I seem to
remember this was discussed here several years ago, and I think the
conclusion was that the traditional circuit, with DC on the volume
control, and a separate AGC rectifier, was the best you could do within
the context of a traditional 5 tube radio, even if R's and C's were free.

Trouble is that many old blokes have lost any idea they ought to
check their belief systems....." Heck, I has grown ma corn thisaway
for nigh on 60 years, and no two bit city slicker with a fancy suit and car
issa gonna tell me how I issa gonna grow ma pharkin corn, OK?"

Same scene with old guys and the precious old radios.
It must be right, we did it 60 years sgo!

I am so proud when I can proove what a right git I was when I believed some
line of BS about 30 yrs ago, or 3 years ago, or 3 minutes ago.

It sounds like you are admitting that you haven't a clue how to design a 5
tube radio any better than the old blokes did!

Initially I was led astray by the "Old Blokes" mentality used to produce appalling

audio quality in most old radios.

I had to unlearn their legacies of BS.

I will always seek the truth, and never give blind loyalty to my own peers, or
my predecessors.

That is what I admit to.

There simply was a complete absense of any real desire for fidelity from 90 % of
AM receivers
in 1955.

I heard strong rumors that there were some Mythical Receivers built by AWA for
monitoring
and re-transmission, supposedly really good synchrodyne sets,
but I have never seen one, and I think they all were chucked into the bin, the
whole 3 of them,
in 1960 when stations ungraded to solid state.

At some point, I had to go right back to first principles, and examine each design

premise, and build up from them, and make up a couple of ideas myself along the
way.

Some of the AM radios in the electronics magazines in the last 30 years show that
this process of mine
has been taken far further.

But I found that with careful design one could make a superb sounding superhet
using 1955 hardware
instead of chips and transistors, or amazingly complex synchrodynes.

Patrick Turner.





Regards,

John Byrns

Surf my web pages at, http://users.rcn.com/jbyrns/