Easy mod for improving AA5 AM detector circuits.

[Robert Casey]

Make use of the second diode in the 12AV6 or 12SQ7. The AVC capacitor
can back bias the detector diode so it fails to detect correctly the low
levels
of the station's RF. Which happens at high audio level, you get large peaks
and large valleys of RF level. The peaks do fine, it's the valleys that
can get
clipped. See http://n2.net/k6sti/ma.htm for more discussion.

So the mod is to separate the AVC from the audio detector. It
requires two cheap parts: 30pF cap and a 470K resistor. See
http://pw2.netcom.com/~wa2ise/radios/feedback.html#modacc
for more discussion and a diagram. With this, you could get around
40 to 45dB down with distortion demod for medium level and strong
signals. DX will not come in as good, but the background noise may
help mask it anyway... (But DX won't sound any worse then before
the mod). The last IF transformer might need a little tweaking after
you add this stuff.

This "use the 2nd diode" scheme seems to perform better than more
exotic stuff like infinite impedance detectors, grid leak detectors and
plate detectors. All these seem to have less dynamic range than the
simple diode detector without the AVC hanging off it.

Larger AA5s, consoles, and AM sections of AM/FM stereo receivers
would benefit from this mod.

2 possible reasons why manufacturers didn't do this would be the cost of
the resistor and cap and the labor of installing it. Also the second reason
was that radio stations didn't limit and compress the program material audio
as much back then as they do today. So occasional clipping of the detected
audio would be less noticed.

[Brian]

Post by Robert Casey
So the mod is to separate the AVC from the audio detector. It
requires two cheap parts: 30pF cap and a 470K resistor.

Bob, I've gone back and installed this mod on nearly all of my radios.
I found that a 1-meg load resistor works well. This value reduces
loading on the IF amp.

Even though I originally did this mod to improve modulation acceptance
and reduce distortion, what I like about it most is that the radio no
longer boosts its gain as you tune off a station. This eliminates the
raucous sound typically emitted as you approach the passband edge of a
strong signal.

Brian

[Patrick Turner]

Post by Brian

Post by Robert Casey
So the mod is to separate the AVC from the audio detector. It
requires two cheap parts: 30pF cap and a 470K resistor.

Bob, I've gone back and installed this mod on nearly all of my radios.
I found that a 1-meg load resistor works well. This value reduces
loading on the IF amp.
Even though I originally did this mod to improve modulation acceptance
and reduce distortion, what I like about it most is that the radio no
longer boosts its gain as you tune off a station. This eliminates the
raucous sound typically emitted as you approach the passband edge of a
strong signal.
Brian

How about posting a schematic of your circuit idea?

Patrick Turner.

[Robert Casey]

Post by Patrick Turner
How about posting a schematic of your circuit idea?
Patrick Turner.

See it at: Loading Image...

[John Stewart]

Post by Robert Casey

Post by Patrick Turner
How about posting a schematic of your circuit idea?
Patrick Turner.

See it at: http://home.earthlink.net/~wa2ise/images/AA5-2diodedet.jpg

Unless I'm missing something there is nothing new in the detector circuit

you have posted. As shown, it still has the AC loading problem on the
diode detector that results in distortion at a high modulation index.

AGC taken from the IF2 primary is not new & shows up in many
30's receivers.

Please point out the differences in this circuit where they exist.

Cheers, John Stewart

[Robert Casey]

Post by John Stewart
Unless I'm missing something there is nothing new in the detector circuit
you have posted. As shown, it still has the AC loading problem on the
diode detector that results in distortion at a high modulation index.

True that there is still the coupling cap from the volume control wiper
and the 12AV6 grid,
but that is a minor contributer to the problem compared to the AVC cap.
And it could
be mitigated further by connecting across the volume control top to
ground a 150K
resistor.

Post by John Stewart
AGC taken from the IF2 primary is not new & shows up in many
30's receivers.

I'm not saying that I invented that, but that it's something not usually
done in AA5's and
consoles based on the AA5.

[John Byrns]

Post by Brian

Post by Robert Casey
So the mod is to separate the AVC from the audio detector. It
requires two cheap parts: 30pF cap and a 470K resistor.

Bob, I've gone back and installed this mod on nearly all of my radios.
I found that a 1-meg load resistor works well. This value reduces
loading on the IF amp.
Even though I originally did this mod to improve modulation acceptance
and reduce distortion, what I like about it most is that the radio no
longer boosts its gain as you tune off a station. This eliminates the
raucous sound typically emitted as you approach the passband edge of a
strong signal.

Brian, any thoughts on what causes this change in the tuning
characteristic when the AGC rectifier is feed from the primary of the last
IFT? I have speculated on this several times over the years, most
recently with respect to the general practice of feeding the tuning eye
tube from the audio detector, rather than from the AGC rectifier, when the
AGC rectifier is feed from the IFT primary. My speculation is that the
different tuning characteristic results from the broader double humped
response characteristic seen at the primary of the IFT, which results in
an apparently sharper tuning characteristic as a result of the modified
AGC action. My suggestion that the primary response may actually be
double humped with a dip in the center has been generally scoffed at, so I
am curious what other explanations there might be?


Regards,

John Byrns


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

[Ken Scharf]

Post by Robert Casey
Make use of the second diode in the 12AV6 or 12SQ7. The AVC capacitor
can back bias the detector diode so it fails to detect correctly the low
levels
of the station's RF. Which happens at high audio level, you get large peaks
and large valleys of RF level. The peaks do fine, it's the valleys that
can get
clipped. See http://n2.net/k6sti/ma.htm for more discussion.
So the mod is to separate the AVC from the audio detector. It
requires two cheap parts: 30pF cap and a 470K resistor. See
http://pw2.netcom.com/~wa2ise/radios/feedback.html#modacc
for more discussion and a diagram. With this, you could get around
40 to 45dB down with distortion demod for medium level and strong
signals. DX will not come in as good, but the background noise may
help mask it anyway... (But DX won't sound any worse then before
the mod). The last IF transformer might need a little tweaking after
you add this stuff.
This "use the 2nd diode" scheme seems to perform better than more
exotic stuff like infinite impedance detectors, grid leak detectors and
plate detectors. All these seem to have less dynamic range than the
simple diode detector without the AVC hanging off it.
Larger AA5s, consoles, and AM sections of AM/FM stereo receivers
would benefit from this mod.
2 possible reasons why manufacturers didn't do this would be the cost of
the resistor and cap and the labor of installing it. Also the second reason
was that radio stations didn't limit and compress the program material audio
as much back then as they do today. So occasional clipping of the detected
audio would be less noticed.

I've seen this circuit used in some AA5's, IIRC it was refered to as
DAVC, or delayed avc.
On an aside, I wonder why the 12Sq7/a2AV6 always used a 5-10m ohm
grid resistor to obtain 'contact bias' instead of using a cathode
resistor for self bias.

[John Byrns]

Post by Ken Scharf

Post by Robert Casey
Make use of the second diode in the 12AV6 or 12SQ7. The AVC capacitor
can back bias the detector diode so it fails to detect correctly the low
levels
of the station's RF. Which happens at high audio level, you get large peaks
and large valleys of RF level. The peaks do fine, it's the valleys that
can get
clipped. See http://n2.net/k6sti/ma.htm for more discussion.
So the mod is to separate the AVC from the audio detector. It
requires two cheap parts: 30pF cap and a 470K resistor. See
http://pw2.netcom.com/~wa2ise/radios/feedback.html#modacc
for more discussion and a diagram. With this, you could get around
40 to 45dB down with distortion demod for medium level and strong
signals. DX will not come in as good, but the background noise may
help mask it anyway... (But DX won't sound any worse then before
the mod). The last IF transformer might need a little tweaking after
you add this stuff.
This "use the 2nd diode" scheme seems to perform better than more
exotic stuff like infinite impedance detectors, grid leak detectors and
plate detectors. All these seem to have less dynamic range than the
simple diode detector without the AVC hanging off it.
Larger AA5s, consoles, and AM sections of AM/FM stereo receivers
would benefit from this mod.
2 possible reasons why manufacturers didn't do this would be the cost of
the resistor and cap and the labor of installing it. Also the second reason
was that radio stations didn't limit and compress the program material audio
as much back then as they do today. So occasional clipping of the detected
audio would be less noticed.

I've seen this circuit used in some AA5's, IIRC it was refered to as
DAVC, or delayed avc.

The basic circuit, as Robert shows it, is not Delayed AGC. Delayed AGC
requires adding bias to the AGC rectifier so that it doesn't start
conducting until a certain signal level is reached. Delay voltage is
sometimes generated by using cathode bias on the triode which shares its
cathode with the diodes. The load for the main detector diode must be
returned to the cathode, not ground, in this case to prevent serious audio
distortion. Delayed AGC of this sort does place a non linear load on the
IF amplifier tube that increases the audio distortion over a certain range
of signal levels. Another way to get delayed AGC is to add a third diode
in a clamping circuit to delay the AGC.

Post by Ken Scharf
On an aside, I wonder why the 12Sq7/a2AV6 always used a 5-10m ohm
grid resistor to obtain 'contact bias' instead of using a cathode
resistor for self bias.

I think this is done to minimize the AC loading on the detector and it
doesn't hurt that it also saves a few parts.


Regards,

John Byrns


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

[John Stewart]

Post by Ken Scharf

Post by Robert Casey
Make use of the second diode in the 12AV6 or 12SQ7. The AVC capacitor
can back bias the detector diode so it fails to detect correctly the low
levels
of the station's RF. Which happens at high audio level, you get large peaks
and large valleys of RF level. The peaks do fine, it's the valleys that
can get
clipped. See http://n2.net/k6sti/ma.htm for more discussion.
So the mod is to separate the AVC from the audio detector. It
requires two cheap parts: 30pF cap and a 470K resistor. See
http://pw2.netcom.com/~wa2ise/radios/feedback.html#modacc
for more discussion and a diagram. With this, you could get around
40 to 45dB down with distortion demod for medium level and strong
signals. DX will not come in as good, but the background noise may
help mask it anyway... (But DX won't sound any worse then before
the mod). The last IF transformer might need a little tweaking after
you add this stuff.
This "use the 2nd diode" scheme seems to perform better than more
exotic stuff like infinite impedance detectors, grid leak detectors and
plate detectors. All these seem to have less dynamic range than the
simple diode detector without the AVC hanging off it.
Larger AA5s, consoles, and AM sections of AM/FM stereo receivers
would benefit from this mod.
2 possible reasons why manufacturers didn't do this would be the cost of
the resistor and cap and the labor of installing it. Also the second reason
was that radio stations didn't limit and compress the program material audio
as much back then as they do today. So occasional clipping of the detected
audio would be less noticed.

I've seen this circuit used in some AA5's, IIRC it was refered to as
DAVC, or delayed avc.
On an aside, I wonder why the 12Sq7/a2AV6 always used a 5-10m ohm
grid resistor to obtain 'contact bias' instead of using a cathode
resistor for self bias.

If an unbypassed cathode resistor was used the diode detector would
be biased at an audio rate. Alternatively, if a bypassed cathode resistor
was used, the detector diode plate would then be required to exceed
that potential before any current could flow. Either way, you get more
distortion than in the grounded cathode version of the circuit.

The 5-10 M solution is not the best either. Although the high R reduces
loading on the detector diode, there is distortion produced by the contact
potential itself. I actually managed to measure this in some audio
amp testing a few moths ago.

Cheers, John Stewart

[Ken Scharf]

Post by John Stewart

Post by Ken Scharf

Post by Robert Casey
Make use of the second diode in the 12AV6 or 12SQ7. The AVC capacitor
can back bias the detector diode so it fails to detect correctly the low
levels
of the station's RF. Which happens at high audio level, you get large peaks
and large valleys of RF level. The peaks do fine, it's the valleys that
can get
clipped. See http://n2.net/k6sti/ma.htm for more discussion.
So the mod is to separate the AVC from the audio detector. It
requires two cheap parts: 30pF cap and a 470K resistor. See
http://pw2.netcom.com/~wa2ise/radios/feedback.html#modacc
for more discussion and a diagram. With this, you could get around
40 to 45dB down with distortion demod for medium level and strong
signals. DX will not come in as good, but the background noise may
help mask it anyway... (But DX won't sound any worse then before
the mod). The last IF transformer might need a little tweaking after
you add this stuff.
This "use the 2nd diode" scheme seems to perform better than more
exotic stuff like infinite impedance detectors, grid leak detectors and
plate detectors. All these seem to have less dynamic range than the
simple diode detector without the AVC hanging off it.
Larger AA5s, consoles, and AM sections of AM/FM stereo receivers
would benefit from this mod.
2 possible reasons why manufacturers didn't do this would be the cost of
the resistor and cap and the labor of installing it. Also the second reason
was that radio stations didn't limit and compress the program material audio
as much back then as they do today. So occasional clipping of the detected
audio would be less noticed.

I've seen this circuit used in some AA5's, IIRC it was refered to as
DAVC, or delayed avc.
On an aside, I wonder why the 12Sq7/a2AV6 always used a 5-10m ohm
grid resistor to obtain 'contact bias' instead of using a cathode
resistor for self bias.

If an unbypassed cathode resistor was used the diode detector would
be biased at an audio rate. Alternatively, if a bypassed cathode resistor
was used, the detector diode plate would then be required to exceed
that potential before any current could flow. Either way, you get more
distortion than in the grounded cathode version of the circuit.
The 5-10 M solution is not the best either. Although the high R reduces
loading on the detector diode, there is distortion produced by the contact
potential itself. I actually managed to measure this in some audio
amp testing a few moths ago.
Cheers, John Stewart

The saving grace with the 12SQ7 and 12AV6 is their very high mu.
They don't need much bias (esp with the low plate voltage used
in an AA5 circuit and in a resistance coupled amplifier) to
be in class A. The input signal must remain less than the bias
though or grid current will flow, resulting in distortion
(some low mu power tubes will remain linear in class A2, but
generally not high mu small signal types).

[Brian]

Post by Robert Casey
So the mod is to separate the AVC from the audio detector. It
requires two cheap parts: 30pF cap and a 470K resistor. See

30 pF is a bit on the low side. I measured the diode-cathode
capacitance of a socketed but unwired 6SQ7 as 4.8 pF. This
capacitance, acting as a capacitive voltage divider with 30 pF, should
reduce the AVC loop gain by about 1.3 dB. The drop is 0.4 dB with 100
pF.

Brian