Diagram of radio using the 12BA7 socket adapter:
Try replacing a 6SA7 with a 6SB7-Y, twice the conversion
transconductance, same pinout.
No tuned circuit tweaking should be needed.
Using a 6BS7-Y in an AA5 to
replace the 12SA7.
Using a 717A doorknob tube in place of a 12SK7
Try replacing the 12SK7 with a
12SG7, similar
pinout, twice the gain.
May not work or be stable in all radios.
Also replace the 12BA6 with a 19HR6 for about 4dB more IF gain
Use a longer loopstick or larger loop antenna for some more signal gain
Using a 6AS6 dual control pentode
for more AVC action in the IF strip.
And
another 6AS6 as a mixer.
Use a 12BE6 or 12SA7 as an AVC gain controlled audio driver in place of a 12AV6 or 12SQ7
Lower heater voltage on a diode detector tube for better fidelity
Modify the IF transformers and squeeze a few more dBs of signal strength from your AA5. For experts only, practice on spare transformers first!
I tested this 12BA7 converter tube mod in four different AA5 AM tube radios, and had no problem with unstable oscillations or tweets or birdies. And signal gain went up in all those cases. It was not really effective in an AA6 (an AA5 with RF stage) though. Because the RF stage provides enough gain to cause the AVC always to cut in. But the 12BA7 works well in the standard AA5 radio.
I built an adapter socket going from the 7 pin 12BE6 basing to the 12BA7 9 pin basing. This makes the radio easily restorable back to the 12BE6 if desired. The following cross-connections were done: (7 pin socket pin) to (9 pin socket pin(s)), 1 to 2; 2 to 3, 6; 3 to 4; 4 to 5; 5 to 9; 6 to 1, 8; 7 to 7. Keep the lead lengths as short as possible. Several of the lugs on the 7 pin socket will line up and directly connect with several of the 9 pin socket, to make a short and tight mechanical structure. This connects all the similar types of electrodes the same way as in the original AA5 circuit. The 12BA7 has an extra function, an internal shield (pin 8). I connected this to the B+ line, 7 pin socket pin 6, as this line is at RF ground potential. It's just a small piece of metal interspaced between the plate and grid 3 (the signal input) leads inside the bottom of the tube. So, the DC potential has no effect. I also connected a short length of insulated wire to this RF "ground" and placed this wire into the center space holes of both the male and female sockets, to act as additional interlead shielding. This is similar, with respect to RF, to the grounded center of tube sockets. If you decide to replace the 7 pin tube socket on the chassis or circuit board with a 9 pin socket, you should connect pins 6 (screen grid) and pin 8 (internal shield) to ground. See here. Other connections are as above. But do this only after you are satisfied that the adapter scheme yields good results.
Finding a male 7 pin tube type socket will probably be difficult. I
took a molded female socket and removed the contacts. And then I soldered
an inch or so length of solid 18 gauge (AWG) copper wire to the
part of the contact that usually grabs the tube's pin. This thickness of
wire is very close to the same size as tube pins. Did 7 of these
and reassembled the socket, now a male.
Now, time to test the mod. Be sure you have physical room for the adapter and 12BA7 next to the tuning cap (and you can tune), and inside the radio cabinet. The 12BA7 without adapter is as tall as a 50C5. The low end of the AM MW broadcast band should tune as it did before. The high end of the band may be off calibration something like 50KHz, the top (1600) now not reachable. Tweak the tuning cap's oscillator section trimmer to bring 1600 back again (backing the screw looser should do it). Once you get the stations about where they belong on the dial, tweak up the antenna section trimmer of the tuning cap. Best tracking is usually had if you tweak up on a weak station near 1400KHz.
One could instead use a 12BZ6 for the 12BA6 but the cathode and suppressor (G3) grid pins are swapped. And the 12BZ6 will need a tube shield (grounded with respect to RF) to work. I have had success with the 12BZ6, but the 19HR6 is easier and less expensive.
You may need to retune the IF
some to achieve good sensitivity and stability. One thing to
check for is good sound quality on strong stations. To be
sure that the IF stage isn't overloading, and is driving the
detector stage properly. This
19HR6 mod shouldn't be too hard for beginners, try it on a radio
you don't care too much about first. This mod is less likely to
yield much benefit in receivers with RF stages and multiple IF
stages, though. These sets already have lots of gain, and the
AVC will just assert itself more often with a mod.
.
Just be sure to never operate the radio with a tube missing from one
or both parallel heater circuits. A short time in this condition
won't really hurt the tubes, as tubes have about twice their heater
voltage applied for a while during manufacture. But the radio
won't work like this, so you know to check it out.
.
The military designated as VT 269. It is a 6AK5 laid on its side
It is used to improve receivers. It took the place of the 6SK7, 6SJ7.and the
other pentides on the receivers at that time. As it's a sharp cutoff pentode,
it has less AVC action, but you can get less signal distortion in the final IF stage
on AM stations. It changed the transconductence
from 675 to 1,440 uohms. Note that the 12SK7 screen grid pin and cathode pin in the radio's tube socket
will be shorted together when using the 717A. This may jumper out a cathode resistor if one
is present. The IF stage stayed stable like this for me.
You may want to touch up the IF alignment. Try it you will be amazed, and
besides, it looks
cool on the radio chassis!.
This mod involves using a 12BE6 with the audio input from the
pot injected via cap with resistor to ground into
the G1 (grid that usually does the local oscillator, it's
sharp cutoff) and AVC applied to G3 to vary the gain.
Thus this tube is being used as a dual control device.
The audio input waveform will not suffer distortion because
it doesn't see a remote cutoff grid curve over its
voltage range. Grid 1 amplifies linearly the audio
and it is for grid 3 that determines how many of the electrons
representing this
amplified audio signal get to the plate. What amount of audio electrons that don't
reach the plate are diverted by grid 3 to grid 2. This
diversion of the audio doesn't introduce distortion into
the audio. Grids 4 and 5 just look like the screen
and suppressor grids respectively in an ordinary pentode.
Audio out on plate is coupled to the 50C5. Note that this audio stage resides
outside the AVC feedback loop. "Forward AGC" it is called.
The secret to getting enough drive and clarity to
drive the 50C5 is how to bias grids
2 and 4 (screens). Used a 100K from B+ to G2 and G4,
bypassed to ground with a 0.1uF cap. The voltage there
is around 12 to 20V. Plenty of gain now. B+ is 95V.
Same plate load as the 12AV6 had, 470K. G1 sees the
same 6 to 8 megs as the 12AV6 did, develops about the
same "contact" bias voltage of about -1V.
Only problem is distortion if the AVC goes beyond about
-4V. You could use a voltage divider and another filter cap
here. As seen in the above curves for the 12BE6, the screen
voltages I have actually make sense. The plate is running
around 100uA, similar to what the 12AV6 used to. The screens
are running at about 700uA at around 10V. Grid 1 has the
curves of an ordinary triode with a sharp cutoff
characteristic, and that the -1v grid bias is about right.
Here the "triode" would see grid 2 as its plate.
And you can see the plate current
diversion to the screens caused by grid 3 (the amount of current
would be scaled back by the lower screen voltage). Conversion
transconductance is not meaningful here.
For comparison of the 12BE6 grid 1 "triode" with the 12AV6, here's its curves with
the usual operating area in yellow: If you have 12BE6's coming out of your ears, you can use one in the
IF amp stage. Disconnect pin 7, and pin 7 now goes to the AVC line.
The other pins will likely need no changes.
The new tube will probably need a shield. This tube is acting similarly to
the audio 12BE6, except G1 also gets some AVC action here like that seen in
a sharp cutoff pentode. With enough switching, one might be able to
make use of the AM 12BE6 found in many AM/FM radios as an FM IF stage
like the AM IF stage described here.
Of course you'll need a detector of some sort
in the radio now that the 12AV6 is gone.
Maybe
the infinite impedance detector.
Or use a separate diode, like an EA76 or 5647. You can reduce the
contact potential by reducing the heater voltage down from 6.3 to
4 volts. In a series string, that can be done by paralleling a
100 ohm resistor with the heater. This should increase the
fidelity of AM detection. The 5896 below is a dual diode version. A larger loop antenna can also be done.
Expect to loose 6dB of signal, and hopefully more than 6dB of
noise. You'll need to tweak the antenna trimmer to compensate
for whatever stray capacitance was added by the shield.
With the other tricks on this page, that signal loss shouldn't
be a problem.
An interesting aspect of dual control pentodes is that the plate
resistance increases in value as grid 3 is made more negative. This will
load the output IF transformer's LC circuit less, causing its
bandwidth to narrow. This will make for wider bandwidth on
weak signals (and thus higher demodulated audio frequency response),
as these signals will drive the AVC (and thus grid 3)
less negative than strong stations would. But narrower bandwidth on strong signals. This is
not a feature, as weak signals will be nosier, and lower audio frequency
response would be desirable. Backwards to what I was hoping for.
One could add a triode plate to the primary of the IF transformer
to variably load the transformer and thus vary its bandwidth.
This mod assumes that the IF transformers are not overcoupled, as usually the
case in AA5 radios. One
would need the inverse of the usual AVC voltage to feed its grid.
When one increases the current thru the triode, its plate resistance
decreases. We want
a triode that has a rated plate resistance around 20 to 40K,
like a 12AT7. We want that to happen on strong stations to get better
high frequency audio fidelity. I used a zener to provide a
constant bias for the cathode. Use a quiescent current resistor to
keep the zener functioning when the triode passes very low current.
So one needs a DC signal that gets
more positive with more signal strength, so we can control the triode
with it.
One could get that using an infinite impedance detector
instead of the usual detector diode. An interesting side effect of
this triode circuit is that it also attenuates strong signals more
than weak signals, thus helping the usual AVC circuit function. One
has to be careful to balance things so it doesn't hog too much of
this function. The diagram shows it hanging on the first IF
primary, but an additional triode on the primary of the 2nd IF
should also work if the detector is an infinite impedance type.
The usual diode loads the 2nd IF enough such that a triode on that
one wouldn't do much. That might even do enough AVC action so that
the regular AVC circuit could be eliminated, but I haven't yet
tried this. There may be unwanted feedback when using both triodes on a 12AT7
as both plates are not shielded from each other.
This scheme would be too expensive to build into consumer AA5's
back during AA5 production.
Now back to the 6AS6 circuits....
The heater specs out 6.3V @ 175ma, but the
tube didn't seem to mind at all being in a 150ma heater string.
The cathode lighted up about as bright as cathodes should be,
and the voltage measured about 5.9V.
6AS6 mixer
If you want to, you can use those 12V B+ space charge tubes intended for
auto radios for the RF converter and IF sections of an AA5.
No real advantage to, other than for the fun
of doing it. The 12AD6
is a pentagrid converter, the 12AC6 a
reasonably sharp cutoff pentode. The 12EK6
and 12AF6 are somewhat remote cutoff pentodes.
The 'EK is higher gain. All these pentodes can be used in the IF amplifier, though
for better AVC action, the remote cutoff ones are better.
All these are pin compatible with their usual AA5
counterparts. There appears to be no problem using
the series string AC current to heat these tubes, even though
these tubes were intended for use in 12V DC car radios.
Note that one needs to add a 39K 1/2 watt
resistor to drop the B+ fed to the 12AD6 and 12AC6 tubes.
And a bypass cap of around 30uF. An alternative, shown in
the diagram below, shows how to get the low voltage B+
from the audio output tube's cathode. Normally, the 50C5
has about 6 to 7 volts on its cathode (cathode resistor
bias). This can be doubled by attaching a resistor the
same ohmage as the grid to ground resistor (about 500K) from
the control grid (G1) to the cathode. This creates a
voltage divider with the existing grid to ground resistor
so the tube will pull enough current thru the cathode
resistor to make the grid 6 to 7 volts lower than the
cathode. And the cathode will find itself twice the
grid voltage to ground. To keep the cathode current to be similar to what it
was before, I increased the cathode resistor from 150 to
330 Ohms. Be sure to use the 30uf cap mentioned before to
bypass the cathode. Also, I found that I needed to change
the AVC filter cap from 0.05uf to 0.22uf to reduce hum
pickup. If the radio's ground happens to be connected to
the hot side of the powerline, hum would come from being
coupled from the outside world capacitively.
A 60Hz hum signal that would modulate the
RF and IF stages, which would cause variable hum on the
detected audio signal. I tested this on an
otherwise boring common AA5, and it did work.
A variation of the above radio. This radio uses low voltage car radio
tubes for the RF, IF, detector and first audio driver, and output
stage.
The 1st audio driver stage uses a bit of negative bias voltage
developed by the converter local oscillator circuit. This bias
voltage is about -1. 3V DC. In this set, this voltage didn't
vary with tuning from one end of the band to the other.
A 4.7 meg resistor is mounted next to the
oscillator circuit to avoid detuning the oscillator LC tank. Small
amounts of stray capacitance on the audio side of this resistor
filter out the RF of the local oscillator.
This resistor, in combination with the resistance to ground,
provides the biasing to the grid of the 1st audio tube. You
may have to select a somewhat different value to get good audio.
The tube used here is another 12V tube, the
12AE6.
The amount of audio output power isn't much, about 50mW. Similar
to that of a transistor radio. The output tube is a space charge
tetrode, a 12K5.
Except for pin 5, it will fit the same
socket connections as a 50C5. Pin 2 is the control grid, and
pins 5 and 6 of the 12K5 are the space charge grid, which connects to B+.
This tube was intended for output stage transistor driver circuits in car
radios. Output transformer T4 has about a 7:1 turns ratio with
an 8 ohm speaker as load. Thus the primary impedance is about 500
ohms. An output transformer from a pocket transistor radio is like this, and could
be used here. Use the entire primary winding.
This is similar to the load impedance this tube saw in car radios.
This radio isn't exactly a boom-box :-).
Maybe if you use headphones, this radio could be a vacuum tube
Walkman! If you don't mind the use of a very big battery to power it
(heater current is around 1 Amp.).
Here's another version with a pair of 12K5's in push pull. The interstage
is a 7K to 15K CT transformer
and the output transformer, 500 ohm CT to 3.2 ohm, is from a transistor
radio. The interstage
and output transformers are from transistor radios. The output audio
level is higher, about that of a typical larger transistor radio.
This is more of a "What if" type project than a practical
mod. So "What if it is desired to build an instant on
AM radio using only tubes and no selenium rectifier
(because selenium rectifiers were never
invented)?"
"And no using any power on heaters while the set is "off".
Center city and factory areas: 10- 50 mv/m
mv/m is the field strength in Milli Volts per Meter. An approximate
formula for the Voltage induced in a "small" air loop is as follows:
E = 2 * pi * e * N * A / L
where
E = Resultant Voltage acting around the loop
Note that when the field strength is given in mv/m, it must be multiplied
by 1000 to convert it to Volts/Meter as used by the loop formula.
(from a post by John Byrns)
Using a 6SB7Y in an AA5 with octal tubes, to replace a 12SA7.
The 6SB7Y pentagrid converter tube has about twice the conversion
transconductance of the 12SA7, so using the 6SB7Y will yield 6dB
more signal gain (assuming no AVC action).
The signal to noise ratio is also
better with the tube with the higher conversion transconductance.
Not a serious issue in the MW band (atmospheric noise is more significant
in the MW band) , but converter tube noise performance can be important in
the higher SW bands. This 6SB7Y tube is pin compatible with the
12SA7,
but the 6SB7Y uses 300ma heater current, instead of the 150ma the 12SA7
uses. There was no 12SB7 made, BTW. One can modify the radio
to accept the 6SB7Y by rewiring the heater circuits. See diagram
above. The 12SK7 and 12SQ7 tube heaters are wired in parallel to
draw 300ma heater current. Of course, you could substitute a
6SK7 and a 6SQ7 respectively if you have them on hand, thus avoiding
this parallel wiring for these tubes.
Then that is wired in series with
the 6SB7Y. And the 35Z5 and the audio output tube
are wired in parallel. To do this, the old 50L6 needs to be changed to
a 35L6. A capacitance of 7.5uF @ 250VAC
is used to drop the excess voltage in the new 300ma heater string,
between the 6SB7Y and the 35L6/35Z5 tube heaters. The other
end of the 35L6/35Z5 paralleled heaters connects to the top
side of the powerline. This allows the continued use of a pilot
light in the rectifier and heater circuit.
In a set without a pilot light, another method of modifying the radio
can be done. Replace the 12SA7, 12SK7, 12SQ7 and 50L6 with the
6SB7Y, 6SK7, 6SQ7, and 25L6 respectively. And the 35Z5
is replaced with a 25W4 in an adapter socket (as the pinout is
different). The adapter socket also allows easy insertion of the
heater voltage dropping capacitor, here the 7.5uF @ 250VAC cap.
Connect this cap between the high side of the powerline
(usually pin 3 of the 35Z5 socket), and to one of the heater pins of
the 25W4 tube. The other heater pin connects via the 35Z5 socket
and the chassis wiring to the now 25L6 tube heater.
Also be sure to insert in the adapter a 15ohm
(not critical) 1/2 W resistor in series with the cathode or plate
of the 25W4 rectifier. This will limit the peak filter capacitor
charging current to an amount the tube can handle.
This method doesn't require any below chassis work. The 25AX4
or 25D4 tubes will also work here.
Yet another method uses a small 12.6V center tapped transformer. See
diagram above. Be sure to phase the primary connections so the
secondary voltage is in phase with the voltage that would be
present across the old 12SA7 heater. That the secondary voltage
"subtracts" from the voltage across the entire heater string.
The 6SB7Y heater is connected to one of the 6.3V AC segments
of the secondary. Be sure to mount the transformer away from
the audio sections, and also away from ferrite core RF and IF
coils. The transformer's stray 60Hz magnetic field can actually
cause small varying changes in the inductance of ferrite core coils, thus
causing hum.
And yet another approach: Place the 6SB7 heater between the set's B- line and heater string, and the
incoming powerline, neutral side (with respect to the radio chassis). This is similar to the
pilot light and 35Z5 rectifier tube tapped heater section circuit, which sees approx 300ma true RMS.
Except we now place the 6SB7 heater at the other end, at the set's ground (B- line and the set's
ground end of the heater string). After everything is warmed up (takes about twice as long as usual,
as B+ current needs to start up and pass thru the 6SB7 heater along with the 150ma heater string current)
the 6SB7 heater sees 10VAC 25% of the time (when both the heater string and rectifier tube conduct to
recharge the first filter cap),
and 3.5V 75% of the time (just the heater string conducting). As seen by a scope.
This works out to underpower the heater a little, if these measurements are accurate.
Adding a 1uF 250VAC cap from the set's B- line to the other side of the line will give us about
another 45ma of current (though 90 degrees phase shifted so it's not a regular addition) to
get the total current thru the 6SB7 heater up some to get the power
closer to spec. The radio will be quiet until the 6SB7
"wakes up", when it starts oscillating and thus converting RF to IF. Figure about
twice the normal heater warm up time.
Replacing the 12SK7 with the 12SG7 pentode in AA5 IF strips.
The 12SG7 is a semi-remote cutoff pentode with almost twice
the transconductance of the 12SK7. 4100 mhos vs 2350 mhos, respectively,
at plate voltage of 100V. And the plate resistance of the 'SG7 is
twice that of the 'SK7, 250K vs 120K, respectively. Up to
6dB additional gain (assuming no AGC action) is possible. Only drawback
is that the 'SG7 has a picofarad more capacitance on the input
grid than the 'SK7, so the input IF transformer may need a little
tweaking. Also, the 'SG7 cathode and suppressor grid pins are
connected together internally. This will cause any resistor for
the 12SK7 cathode to be jumpered out, if the 12SK7 suppressor
grid pin is tied directly to ground. This might cause instabilities
such as "birdies" and oscillations to occur. But this tube
substitution is easily tried, tune in weak stations and see if
you have any problems, before you tweak the IF.
The 12SG7 can also
work well in place of the 12SK7
in the RF stage of six tube AM radios, like the GE model 422.
Again, check for birdies and instabilities on weaker stations, and
also check strong stations for distortion. Tweaking the antenna
trimmer cap when tuning the high end of the band should be
all you need do.
WE 717A doorknob pentode in place of a 12SK7.
Unusual tube substitutions in an AA5. Here is a Western Electric 717A where a
12SK7 would usually be. It's a sharp cutoff pentode. The heater current is spec'ed at
175ma at 6.3V but on 150ma in the heater string it ran at 5.5V. a bit low.. I then added resistors to the
series string to make up the extra current for the 717A beyond the string's 150ma. See diagram.
The values I used ended up being more resistence than I calculated, to get the 717A's heater
to 6.3V. Seems my tube wants about 160ma, not the rated 175ma. In a parallel wired heater system,
this wouldn't matter, so it seems the manufacturer didn't worry about it 
Using a 12BE6, 26D6, 18FX6 or 12SA7 as an AVC gain controlled audio driver in an AA5
The purpose of this is to give more uniform audio
volume out of the speaker when tuning across the
dial from strong to weak stations. This can put those
leftover 12BE6s and 12SA7s from above to use.
A longer ferrite loopstick will capture more signal out of the air.
If you have the space, try making a longer one out of a pair of
shorter sticks. Save the original loopstick with its coil intact
so if you are not
satisfied with the results, you can restore the radio back to it.
Clean and epoxy the two new sticks end to end. You could
even use a whole bunch of ferrite beads to make a rod. You
need to use beads that are effective in the MW AM band. Try
the beads out by stringing them on nonconductive string to create
a rod's length as a test. Continue the below
before you epoxy them together.
Using one of the
coilforms from one of the new sticks (remove the extra and toss
into your junk box), connect it to the radio's antenna circuit.
Be sure you can slide it on the stick. Tune a strong station
near the low frequency end of the dial while sliding the coilform
around the end of the stick. The extra amount of ferrite rod will
increase the inductance, and sliding the coilform off the end will
counter this. Once you get a peak, try a weaker station. If you
get oscillation, you may be tuning in the IF frequency, thus too
much inductance. Once you see how far off the end of the rod the
coil now peaks at, you can start stripping turns that extend
beyond the end of the stick off the coilform. And a few more.
Ideally, you just want enough turns so the coil will peak resonate
at around the 2/3 position along its length. The radio should be
more sensitive now. Check that strong stations are not distorted
or causing intermod in the band. 
I just made an antenna for a radio I installed into an old
bookshelf speaker (not a good one, so don't worry).
I made it large to capture more signal out of the air.
What to do: Get a spool of wire about a hundred feet long.
Select wire without solder coating on it. Skin effect
would force most of the RF current thru the solder, which
has a higher resistance than copper, and that will kill the Q.
Better yet if you have a supply of silver coated wire, usually
done with Teflon wire (Teflon does bad things to copper,
so the silver plating). Silver is a better conductor than
copper, but as silver is much more expensive, everyone
uses copper to wire their houses. Or use litz wire for the antenna if
you happen to have some. Anyway, get some
corrugated cardboard from that box your latest ebay
win came in, and make a coil form. Cut an odd number of
notches one every 2 inches or so. You will wind the wire
in a basket weave pattern. This basket wave pattern
needs the odd number of notches or else it won't work out.
That reduces stray interwinding
capacitance. You have seen this done in older AA5
antennas. I made one about 15 inches by 11 inches.
It took about 17 turns.
But be prepared to add or
remove turns or fractions thereof. Smaller size loops will need more turns.
Tune the radio
the antenna is for to a weak station near the bottom
of the band, like 570KHz. Connect the inner wire
of your antenna to the "hot" part of the antenna tuning cap,
and the outer wire to the AVC "RF ground". Use a
high impedance voltmeter to monitor the AVC voltage.
Add or remove turns of wire from your antenna to
peak the signal strength of that station selected above.
A few solder joints in the antenna won't hurt anything
as long as you can't get shorted turns. You can have
a few feet of wire between the antenna and AVC
line as you're tweaking the antenna. You are
actually adjusting the antenna inductance to resonate
it to the tuning cap. After you get the peak, tune
up to a station around 1500KHz and tweak the
antenna trimmer cap. If you did the basket weave
pattern, you should be able to get a peak.
The radio should be
more sensitive now. Check that strong stations are not distorted
or causing intermod in the band.
Now you're done.
A loop antenna will work in a solid state radio, though almost all solid
state AM radios use loopsticks. The input impedance of an AM antenna transistor or IC front end circuit
is too low to feed the top of the antenna LC circuit into it. So a secondary (or a tap) is wound on the loopstick
to get the impedance to match the front end circuits. Discrete transistor front ends appear to
need a tap around 7% from the RF ground end of the loop, and if the AM radio is built using
an application specific IC (ASIC) the tap for these is around 15%. Look at the loopstick of a
similar radio to see where this tap should be. Compare the size of the primary vs the secondary.
Then you can attach a tap to your new loop antenna at a reasonable spot.
This is the modified Admiral AA4 with the solid state front end. 
I changed out the 35W4 with a solid state
rectifier diode, and changed the heater dropping capacitor, to account for less series heater voltage being needed. I used the 50C5 output tube's cathode circuit to get the 4VDC to run the front end chip.
Used a voltage divider to reduce the 50C5 cathode voltage, and filter bypassed the tap so I could feed
the chip with a clean supply.
Cascading all the above tricks will probably give diminishing
returns as the last tricks are done.
Doing just one should give the
greatest increase in gain. Adding another trick will give yet more
gain, but not as much beyond what the first trick did, on average.
There's only so much signal out there to be heard, anyway.
Improving signal to noise of an AM
radio with ferrite rod antennas.
AM radio station signals are half magnetic and half electric fields.
Noise tends to be mostly electric fields. Placing some electrostatic
shielding around the antenna can improve signal to noise of
radio station reception. A cardboard tube like the sort
from a roll of toilet paper can be used as the basis of such a
shielded antenna. Wrap aluminum or copper foil around the tube, but
leave a gap as shown above. Otherwise you will have a shorted
turn and that will give both magnetic and electric field shielding.
And thus no reception! Connect the foil to an RF ground. Place
the cardboard tube around the ferrite rod, positioning it over the
area where the antenna coil is. Maintain at least a quarter inch
(7mm) clearance to avoid excessive stray capacitance (which will
impact the upper end of the band the most). Spacers like large rubber
grommets could be used for this.
Using a 6AS6 dual control pentode in the IF strip,
more AVC control.
Here a 6AS6 (aka 5725) tube is used in place of the 12BA6. The
6AS6 is a dual control pentode, which means that grid 1 and
grid 3 are both independent control grids. The pinout is the
same EXCEPT the cathode and grid 3 are swapped. I tied the
cathode to ground via a 100 ohm resistor,
and grid three tied directly to
the AVC line. Grid 1 goes to the IF signal lead, which
also has the AVC voltage superimposed on it (this gives us extra AVC action on
strong signals that yield more than -2V of AVC, see curves just below;
actually this may cause "modulation rise" distortion if we use it in the
IF stage, but would be good for the RF stage tube in an AA6 set, as signals
are far smaller there and suffer little distortion).
As the control actions on both control grids
are essentially multiplied together to yield the final gain
of the plate circuit of the tube, this creates a variable
gain action of the IF signal, thus AVC. *There is a drawback in that if the
signal is distorted because of too much gain from G1, G3 gain reduction
is too late to fix it.*
Data sheet for the 6AS6.
Here the 12BE6 converter tube is replaced by another 6AS6 (5725)
as a mixer, and a 5977 triode as a local oscillator. This triode
has a mu similar to that of the oscillator portion of the 12BE6,
about 18. Its output is applied to the second control grid (G3)
of the mixer 5725. This creates heterodyning action, RF * LO,
which yields with other things the IF frequency. Using the AVC
bias on the RF input causes a small amount of gain change on the
mixer tube. This can be seen by looking at the below diagram.
Sharp cutoff pentodes have a constant slope for much of their
characteristic curve, but make the control grid negative enough,
and you will leave the linear region and get a change in slope.
High values of AVC voltage will
bias the tube far enough into this changed slope area to cause
gain reduction. The RF signal itself is small enough not to
suffer significant distortion.
Even sharp cutoff pentodes have some remoteness.
USING 12V B+ SPACE CHARGE CAR RADIO TUBES IN AN AA5: 
A tube radio with a "power" output stage operating from 12VDC B+
For reference, a standard AA5 radio circuit:
An "instant on" AM radio using all tubes and no selenium
or other solid state devices:
Portable tube radios power up quickly enough to be
considered "instant on" because they use directly heated
cathode tubes. So, using a directly heated rectifier
tube for off line operation would also be "instant on"
(instant being within 5 or so seconds). A 5W4 tube
would be good here, as current demands are not very
high. And has relatively low heater current compared
to other tubes like the 5U4. But how to power the
heater? One solution would be a small 5V transformer
(which could be an extra winding on the clock motor
if this radio is a clock radio). Another solution
would be a series capacitor to the powerline. This
puts a restriction on where in the circuit to install
the rectifier, as cathodes of rectifier circuits are
not usually directly connected to the line. If you
use the rectifier to supply negative voltage instead of
the more usual positive voltage, the problem of how
to supply power to the heater via series capacitor
goes away. Pass heater current from one side of the
line thru the heater and then thru the series cap to
the other side of the powerline. The only place the
heater cathode is not on the "wrong" side of the tube
is using it to rectify a negative supply. The B+ of
the set is directly connected thru a small resistance
to the line, and the
negative "ground" is supplied by the rectifier tube.
Some time ago someone was asking what numbers are on
AM radio signal strengths. Found some ballpark numbers:
residential suburbs: 2- 10 mv/m
Rural areas: 0. 1- 1. 0 mv/m
Sky wave DX from out-of-town: >0. 5mv/m 50% of the time
e = strength of radio wave, Volts per m (normal to plane of loop)
N = number of turns in loop
A = loop area in square meters
L = wavelength of radio wave, m
