Here is more fat for the fire in order to add some fuel to the debate.
As usual, goto ABSE or ABPR & refer to those circuits which are
diode biased. This information from RCA RC-11.

This one completely solves the AC load problem AFA the diodes
are concerned thru the use of DC coupling. We now have to control
the signal level with the AGC so that the low mu 85 triode can operate
in a linear region. We are helped in this respect by an extremely linear

plate family as you can see from the curves. The problem now becomes
one of avoiding overdrive of the 85 grid.

As usual, RC coupling of the 85 output to a following stage will have
it's own distortion producing issues, but these will not be as serious
as
those found in AC coupled diode detectors. Looks to me that you
could solve part of that problem with a CC triode (mu follower) in
place of the 85 plate load resistor.

Then we might have a truly low distortion AM detector.

With a bit of fiddling, one could probably get a 6BF6, 6R7 or 6SR7
to work in these circuits, altho their mu might be a little on the high
side.
Or you could use seperate diodes (6AL5, Ge, Schottky), Etc & say a
triode connected 6K6 ( mu = 6.8).

These circuits enjoyed some popularity in 30's radios. Just got a flyer
from Antique Electronics & they have 85's on sale for $3.80.

Hoping everyone will continue to keep open minds, but that
may be asking a lot!!!!!!!

Cheers, John Stewart

John Stewart wrote:

Here is more fat for the fire in order to add some fuel to the debate.
As usual, goto ABSE or ABPR & refer to those circuits which are
diode biased. This information from RCA RC-11.

I can't see the posted schematics at abse or abpr.

Patrick Turner.



This one completely solves the AC load problem AFA the diodes
are concerned thru the use of DC coupling. We now have to control
the signal level with the AGC so that the low mu 85 triode can operate
in a linear region. We are helped in this respect by an extremely linear

plate family as you can see from the curves. The problem now becomes
one of avoiding overdrive of the 85 grid.

As usual, RC coupling of the 85 output to a following stage will have
it's own distortion producing issues, but these will not be as serious
as
those found in AC coupled diode detectors. Looks to me that you
could solve part of that problem with a CC triode (mu follower) in
place of the 85 plate load resistor.

Then we might have a truly low distortion AM detector.

With a bit of fiddling, one could probably get a 6BF6, 6R7 or 6SR7
to work in these circuits, altho their mu might be a little on the high
side.
Or you could use seperate diodes (6AL5, Ge, Schottky), Etc & say a
triode connected 6K6 ( mu = 6.8).

These circuits enjoyed some popularity in 30's radios. Just got a flyer
from Antique Electronics & they have 85's on sale for $3.80.

Hoping everyone will continue to keep open minds, but that
may be asking a lot!!!!!!!

Cheers, John Stewart

In article , wrote:

Here is more fat for the fire in order to add some fuel to the debate.
As usual, goto ABSE or ABPR & refer to those circuits which are
diode biased. This information from RCA RC-11.

This one completely solves the AC load problem AFA the diodes
are concerned thru the use of DC coupling. We now have to control
the signal level with the AGC so that the low mu 85 triode can operate
in a linear region. We are helped in this respect by an extremely linear

plate family as you can see from the curves. The problem now becomes
one of avoiding overdrive of the 85 grid.

As usual, RC coupling of the 85 output to a following stage will have
it's own distortion producing issues, but these will not be as serious
as
those found in AC coupled diode detectors. Looks to me that you
could solve part of that problem with a CC triode (mu follower) in
place of the 85 plate load resistor.

Then we might have a truly low distortion AM detector.

Hi John,

How are you going to generate an AGC voltage from that circuit without
adding some AC load to the detector? Do you propose using one diode as a
separate AGC rectifier? The gain of this tube is a little low, that is
going to impact the design of the radio to some extent.


Regards,

John Byrns


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

Plenty of gain in using an 85 or 55 to a pentode output tube. I have
a converted battery radio (Majestic 105) with a diode biased first audio
stage using an 85. The twin-secondary second IFT allowed me to
seperately construct the AVC and audio detector.

This worked well for me with three AVC controlled amplifier stages
(#39/44), with the screen grids operated at 75 volts (B+ of 225). The
AVC is always 18 volts or less, the 85 triode never gets cut off.
Excess LF response is trimmed by an 0.0068uF inter-stage coupling cap
and my table radio sounds great, with no sign of excess low frequency
drive to the speaker.
The AVC circuit is very similar to the RCA T7-5 with the delay created
by directly biasing the controlled stages negative instead of using the
more usual cathode resistor drop (and the parallel-fed grids). The AVC
changes a bit with modulation - compressing the audio slightly - because
of the AVC delay. An unforseen consequence of the circuit, but it
allowed me to ground every tube's cathode and use Majestic spray
shielded tubes without worry of shorting the cathode bias.


The reasoning for the high Mu first audio stage as I've read over time
is to shape the audio high frequency rolloff and so the volume control
can be operated closer to the bottom end of the resistance so it
minimilly upsets the AC to DC load ratio. After all, if the volume
control is turned completely down, only the R of the volume control
remains to the detector.. No first audio coupling cap, no need to keep
the volume control down for load ratio's sake. Plus, the low-Mu first
audio stage certainly has better frequency response than the 70 and 100
Mu triodes do.


Jeff Goldsmith





John Byrns wrote:

In article , wrote:

Here is more fat for the fire in order to add some fuel to the debate.
As usual, goto ABSE or ABPR & refer to those circuits which are
diode biased. This information from RCA RC-11.

This one completely solves the AC load problem AFA the diodes
are concerned thru the use of DC coupling. We now have to control
the signal level with the AGC so that the low mu 85 triode can operate
in a linear region. We are helped in this respect by an extremely linear

plate family as you can see from the curves. The problem now becomes
one of avoiding overdrive of the 85 grid.

As usual, RC coupling of the 85 output to a following stage will have
it's own distortion producing issues, but these will not be as serious
as
those found in AC coupled diode detectors. Looks to me that you
could solve part of that problem with a CC triode (mu follower) in
place of the 85 plate load resistor.

Then we might have a truly low distortion AM detector.

Hi John,

How are you going to generate an AGC voltage from that circuit without
adding some AC load to the detector? Do you propose using one diode as a
separate AGC rectifier? The gain of this tube is a little low, that is
going to impact the design of the radio to some extent.

Regards,

John Byrns

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

In article , wrote:

Here is more fat for the fire in order to add some fuel to the debate.
As usual, goto ABSE or ABPR & refer to those circuits which are
diode biased. This information from RCA RC-11.

John, do you know when the RC-11 was published? The Figure 4 on page 128
that you posted from the RCA RC-11 is interesting, as I have seen it used
in a couple of high performance MW AM Broadcast Receivers and am wondering
if it was another RCA invention? It doesn't do anything for the AC/DC
load ratio problem, but it includes a DC amplifier for the AGC system,
providing additional gain in the AGC system. The only thing it is missing
is using one of the diodes to provide a sort of delay function for the
AGC, or at least to prevent the AGC voltage from going positive, but that
may have been a later refinement of the circuit.


Regards,

John Byrns


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

In article , wrote:

Here is more fat for the fire in order to add some fuel to the debate.
As usual, goto ABSE or ABPR & refer to those circuits which are
diode biased. This information from RCA RC-11.

This one completely solves the AC load problem AFA the diodes
are concerned thru the use of DC coupling. We now have to control
the signal level with the AGC so that the low mu 85 triode can operate
in a linear region. We are helped in this respect by an extremely linear

Another possibility for a detector/audio tube is the 6AG11 which could be
used to replace the 6AV6 commonly used in five tube radios. The 6AG11
contains two completely independent diodes, and two completely independent
triodes which could be configured into many different detector circuits,
the only limitation being that the triodes are both High mu, which
probably rules out some types of detectors.

The 6AG11 could replace the common 6AV6 using one diode as a separate AGC
rectifier, and the second as a normal diode envelope detector. The first
triode could be configured as an AC coupled cathode follower to present a
very high impedance load to the detector diode, thereby eliminating the
AC/DC load problem, and the second triode section could be used as the 1st
audio amplifier stage, analogous to the triode in the 6AV6.


Regards,

John Byrns


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

John Byrns wrote:

In article , wrote:

Here is more fat for the fire in order to add some fuel to the debate.
As usual, goto ABSE or ABPR & refer to those circuits which are
diode biased. This information from RCA RC-11.

John, do you know when the RC-11 was published?

My copy of RCA HB RC-11 is in good shape & shows a Copyright date of 1933.
I bought it for 10 Bucks from a guy out on the country here about 10 years ago.

At the time he was in the radio restoration business & seemed to be doing well.

The Figure 4 on page 128
that you posted from the RCA RC-11 is interesting, as I have seen it used
in a couple of high performance MW AM Broadcast Receivers and am wondering
if it was another RCA invention?

AFAIK, RCA didn't invent much but bought or stole whatever they could from
others,
altho not sure of that. Somewhere in my pile I have a circuit which shows
simple AGC
in a 30's radio using a DC coupled 6R7. I will dig it out.

It doesn't do anything for the AC/DC
load ratio problem, but it includes a DC amplifier for the AGC system,
providing additional gain in the AGC system. The only thing it is missing
is using one of the diodes to provide a sort of delay function for the
AGC, or at least to prevent the AGC voltage from going positive, but that
may have been a later refinement of the circuit.

I think one could use any of a variety of signal diodes separate from the
triode
to get all the other functions. We are no longer limited to a single,
multipurpose
cathode.

Cheers, John

Regards,

John Byrns

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

The 6AG11 could replace the common 6AV6 using one diode as a separate AGC
rectifier, and the second as a normal diode envelope detector. The first
triode could be configured as an AC coupled cathode follower to present a
very high impedance load to the detector diode, thereby eliminating the
AC/DC load problem, and the second triode section could be used as the 1st
audio amplifier stage, analogous to the triode in the 6AV6.

I think it would have to be DC coupled to eliminate the AC/DC problems
from a coupling cap, yes?

In article , Robert Casey
wrote:


The 6AG11 could replace the common 6AV6 using one diode as a separate AGC
rectifier, and the second as a normal diode envelope detector. The first
triode could be configured as an AC coupled cathode follower to present a
very high impedance load to the detector diode, thereby eliminating the
AC/DC load problem, and the second triode section could be used as the 1st
audio amplifier stage, analogous to the triode in the 6AV6.

I think it would have to be DC coupled to eliminate the AC/DC problems
from a coupling cap, yes?

No, as long as the added AC load resistance is very high, I don't think AC
coupling makes any practical difference. The grid resistor in a cathode
follower is bootstrapped, which raises its apparent value to a very high
level.

My ideal detector uses direct coupling to the cathode follower, but this
requires adding an expensive negative supply to the radio, something that
the bean counters would certainly want to avoid. Once you have the
negative supply though, the need for a seperate AGC rectifier disappears,
and you can take the AGC voltage from the cathode of the cathode follower
tube. There is one problem with this that must be overcome though, and
that is the fact that the cathode of the cathode follower will have a
significant positive voltage on it with weak signals. This problem can be
avoided in either of two ways. One way would be to connect the second
diode in the 6AG11 in series with the AGC feed from the cathode follower,
creating a delayed AGC effect. This approach is a little unsettling
because we are connecting a nonlinear device, the second diode directly to
the audio output of the detector, although the delayed AGC action it would
provide is attractive. The second way would be to return the main diode
detector circuit and the diode load resistor to a small negative voltage
source, rather than to ground, to counteract the positive offset voltage
of the cathode follower cathode from the grid.


Regards,

John Byrns


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