The diagram on the right shows the modifications done to an
otherwise standard AA5 AM radio circuit. How a manufacturer
might have designed an AA5 for 240V AC countries.
5ive methods, just below is the "heater current in series with
the plate current" method. Second
method is the "higher B+ on the 50L6 audio output tube, AC cap
in the heater string" method. A third is to use a
capacitor in
series with the entire radio circuit.
And a 4th radio where the usual tubes
are replaced by those whose
heater currents are 90 (117L7) and 70ma (three 26x6).
Yet another using 100ma heater tubes.
The diagram on the right shows the modifications done to an
otherwise standard AA5 AM radio circuit. How a manufacturer
might have designed an AA5 for 240V AC countries.
If it is desired to convert a normal "All American 5ive" 120V AC AM tube radio design to run off of 240V AC, the following is a relatively low energy consumption way to do it. Especially for the collector living in Europe or other parts of the 240V AC powerline world. Olympic back in '53 made an adaptor cord, which consisted of a 430 ohm power resistor around 40W in series with the entire set, for their AA5 sets. Curtain Burner? Cheap, but burns a lot of power (about 30W in the resistor plus the 30W for the AA5). My following mod saves about 20W. It does require the use of some "sand", though. Not totally out of place, as solid state silicon rectifier diodes did exist in the last decade of AA5 production. Or use a selenium rectifier for earlier sets, to be "authentic" to the technology available when your radio was manufactured. Also need a 500 to 600 ohm power resistor, of about 20 watt rating. What we are going to do is place the heater string in series with the B+ current of the radio, and the diode and resistor (wired in series) is to provide heater current for the opposite direction of the flow of the radio B+ current. Thus, after warm-up, the heater string will see a reasonably balanced AC current. The diode and resistor also provides a critical start-up function to get the tube heaters warmed up enough so the radio B+ current starts pulling more current thru the heater string. Can't do that until the cathodes are reasonably hot first. It takes several minutes before everything gets warmed up and balanced out. Idea is to have the heater string act as the series dropping resistance to operate the radio's B+ circuits from, and also to use the radio's B+ circuits current as a series dropper for the heater string. Rather than burning more power by passing all the heater current AND B+ current thru a single big resistor.
More detail: first step is to make sure the radio works before doing any mods. Then, at the rectifier tube, the rectifier plate needs to be separated from the top of the heater string. The top of the heater string gets connected to one side of the 240V AC line (let's call it "A"), the rectifier tube's plate is connected to the other side of the 240V AC line (call this "R"). The heater string connection to the radio's B minus line stays in place as it was. This places the heater string in series with the radio's B+ supply circuit. Now take the silicon rectifier diode (1N4007 for example) and connect its cathode to the point where the rectifier tube's plate is connected, point "R". The anode of this diode connects to the power resistor, and then the other end of the power resistor connects to the radio's B minus line. Be sure to connect to the spot where the heater string itself connects to the B minus line. This avoids inducing hum into the radio circuits due to ground loop currents.
You may note that the AC current thru the heater string has a DC bias current (in other words, more current in one direction than the other). This is not a problem, heaters are just resistive elements. But it can make taking AC voltage measurements difficult to get meaningful information from. However, you can meaningfully measure the B+ from B minus voltage, I read 114V DC, which is reasonable. As for the heaters, use your experience and look at the color of the heaters and cathodes to see if they "look" the right color and brightness. Or use a 'scope:
Radio now consumes about 40 watts, vs. 30 watts for a standard AA5 set.
To design the above mod, I used a bit of calculus type math. I enjoyed calculus class in college so much, I took it twice! :-) But the following is fairly easy calculus. To determine power consumption over one cycle of the AC powerline, integrate, from 0 to 2p, (150(sine2(X))) dx. Sine squared because power is current squared times resistance of the heaters. This gives a reference number we need to match with the B+ and diode/resistor currents. Number was about 470.
We now figure the power dissipated in the heater string by passing
the radio's B+ current thru it. Figure 60ma. This is effectively
an RMS value, the actual waveform is anything but even. So we do
an integral, from 0 to 2p, (70x) dx. This yielded a number of
200 or so, and the below diode/resistor needs to make it up to
470. A reality check would be:
heater power = 120V * 150ma = 18 watts.
Radio B+ power would be 150V (don't forget losses in the rectifier
tube) * 60mA = 9 watts. Half the heater power. We are going to
put in "series" the heater power, and the B+ power plus an extra
9 watts or so of power in the power resistor.
Now we figure the power for a half cycle of the powerline, as the
diode/resistor gives this. The diode makes the other half cycle
zero power. So we do the
integral, from 0 to p, (Y(sine2(x))) dx. And just
keep picking a value for Y that makes things add up with the
above 240 to get a total of 470. Once you nail down a value
for Y, do: 120/Y= value of power resistor for use with the diode.
I got about 600 ohms.
Reality check:
figure (1/2)(diode cuts out half the cycle)*(120v2/600)=12 watts, ahhhh... a little high.
Turns out I need to run a bit more than half the usual heater current
thru the tubes on turn-on to get them hot enough to start functioning
and conducting current thru their cathodes. After that starts, more
heater current starts warming up the tubes more, eventually reaching
a steady state condition.
Your calculus prof in college would probably shudder in horror at the crudeness of the above use of calculus, but it yielded an answer that made a working circuit when built the first time! And the above is roughly what real engineers in the real world actually do when designing stuff. No rigour, no proofs.
An alternative to changing the speaker is to change the output
tube to a 50C6. That's right, fifty-cee-SIX.
But be sure it can physically fit inside the
radio, as it is bigger. Same basing as the 50L6. You'll need to
change the cathode resistor to 220 ohms. The 50C6's load impedance
is rated for 2600 ohms at 200V plate voltage, close enough to
the 2000 ohm load for the 50L6 at 120V plate installed in the radio.
The 50C6 is nearly the same as the 6Y6. BTW the 50Y6 is a
rectifier.
A new B+ capacitor rated for 250VDC will be needed. And a 6K seven or so watts will need to replace the old 1K resistor to run the output tube screen grid and the rest of the set. Take the old section of the old capacitor you replaced just now and add it to the B+ running the screen grid and rest of set.
If you decide to use the 50C6, use a lower value power resistor (around 2.2K) in series with the old 1K resistor. The screen grid of the 50C6 goes to the node between the 2.2K and 1K resistors, along with the section of the filter cap that used to filter directly the rectifier output when the radio ran off of 120VAC. The screen grid tap should operate at about 140VDC, so the 2.2K resistor value may need adjusting. The 50C6 can have up to 135V on the screen, and 200V on the plate (referenced to the cathode). As the cathode in this set is at 12V above ground, this gives some margin. I used a 100 ohm 7W power resistor between the rectifier plate and first filter cap to drop the supply voltage down some, in this case with the 50C6 I got 195VDC. You'll still need this resistor (but size it to 220 ohms) if you keep using the 50L6, by the way.
What about the heater string? If there is no pilot light, we will insert an AC rated 2uF capacitor in the series string, between the 35Z5 and hot side of the line. Remove the tapped heater section connection from the rectifier plate, and connect the rectifier plate directly to the hot 220VAC line thru a 5 to 10 ohm resistor. The heater-cathode voltage rating will still be high enough (360V) to support this mod.
If there is a pilot light, we will have to move it (I've had problems with the pilot bulb popping if power is briefly interrupted to the set) and do the above, or arrange things so the set's plate current flows thru the heater tap section of the 35Z5 on the B minus supply line side. One possibility is to run the pilot light (a #47) from the powerline thru a 1. 5uF 250VAC capacitor. Maybe use a neon bulb run off the line or B+ (with series resistor) to light the dial as an alternative. If you don't mind modern "sand", a pair of 8V zener diodes (wired cathode-to-cathode in series, and this in turn in parallel with the pilot lightbulb, zener anodes connecting to both sides the bulb) may keep the bulb from popping. Don't forget, it's 6.3VAC RMS, 8.3V peak. The first 8V zener plus the 0.7V drop of the other zener should make for 8.7V, high enough to not normally conduct. Haven't tried it though.
We can't place the 35Z5 heater on the high side of the line, or else the heater-cathode voltage rating (360V) will be exceeded. When the AC line hits its negative peak, and the cathode is charged to B+ running voltage. So, we place the rectifier heater on the cold side of the line. {Do this if you do the "zener diodes across the pilot bulb" trick: "Cold" side of line to 6V section of the 35Z5 heater, also to the pilot with zeners in parallel. Other side of the pilot with zeners tied to the heater tap on the 35Z5. This becomes the radio's "ground". The other portion of the 35Z5 heater continues onto the rest of the tubes' heaters.} So the heater-cathode interface sees essentially just the B+ voltage, which is less than 360V. This means that the heater on the 12AV6 (or 12SQ7) will be on average 40VAC above ground instead of about on average 6VAC, but I didn't hear any extra hum because of this. We will insert an AC rated 2uF capacitor in the series string, between the 50L6 and hot side of the line. See the diagram and this should become clear. If you run off of a 50Hz powerline (mains), you may need to add 0.1 or so to get the voltage on the heaters right.
If the radio uses 6 tubes (RF, converter, IF, detector and first
audio, 35L6, 35Z5) you could substitute a 50L6. The heater string
will add up to higher than 120V, but we have 240V available.
One radio I modified, my second GE 422 (a 6 tube radio), I used a
12SG7 for the RF,
and the 50L6 in place of the original 35L6. Used a 19HR6 in place
of the 12BA6 in the IF.
Added up to 140V heater
string. Used 2.15uF capacitance to drop the 240V to 140V to run
this heater string. The pilot light was replaced (and rewired) with a neon
bulb that has a bayonet base, and the 35Z5 stayed at the hot top
end of the heater string, just "under" the capacitor. That is,
the capacitor went between the 35Z5 and the "hot" 240V line.
This keeps the heater-cathode voltage below the rated max of 360V.
Another separate mod is to use the higher B+ we now have available on the plates of the tubes in the RF, IF and converter stages. The screens need to be run on the lower voltage though. Note the 22K resistor on the screen grid of the IF tube. As the AVC reduces the current thru this tube, the screen voltage will go up. This actually decreases the AVC action of the IF tube itself. The AVC being a feedback system, this causes the RF and converter tubes to work harder to reduce gain on strong stations. The signal level at the IF stage's input is thus lower. This keeps the strong stations from getting too distorted by the remote cutoff curve of the IF tube, as the signal strength is quite high there. And higher and more prone to distortion if we didn't do this. And the IF tube acts partly as an AVC amplifier to boost the amount of detected AVC signal. This extends the radio's ability to handle strong signals. Another way to do this is to use a high impedance voltage divider on the AVC line to bias the IF tube grid circuit. High impedance to avoid loading the AVC detector circuit.
The radio with the 50C6 option consumes about 50 watts.
The radio with the elevated voltage on the 50L6 consumes about 40 watts.
The 12BA7 pentagrid converter is a separate modification from the 240V modifications (i.e., not needed for 240V operation). Click here for details. The 12BA7 yields about twice the signal gain of the 12SA7 or 12BE6.
Click here for a radio using a compactron
38HK7 power pentode/diode with 270VDC rectified directly from the
250V powerline (mains). This tube replaces the 50C5 and 35W4 in
an otherwise ordinary AA5.
In an AA5, to get higher B+, use a power resistor of about 300
ohms between the rectifier tube heater and the rest of the string.
And a 3. 7uF cap.
I got 125V B+ with 350 ohms and 4uF cap. Using a scope to measure heater
to cathode voltage in the 35W4, I'm slightly exceeding the -360V
rating. Tubes are a little more forgiving on this sort of thing
than solid state is.
Looking at the voltage waveform across the
heaters and resistor with a scope, I see a 157V RMS somewhat distorted sine wave with
a DC bias of -75V DC. Doing some crude approx. math to calculate the
power in the heaters and resistor (total 1150 ohms), let's say the positive portion of the
waveform looks like a sine wave of 105VAC RMS, and is active for 40% of the
time. I get an answer of 3.8W from P=0.4(Vsquared/R). And for the other
60% of the time (negative portion), the waveform looks like a somewhat distorted sine wave of
210V RMS. That gives me 23W. Adding them together gets me 26.8W, call it
27W. As the 350 ohm resistor is just slightly less than half the resistance
of the heater string (which should consume 18W), it looks to be about the
right amount of power is heating the heaters. The color of the cathodes and
heaters looks about right. Even with the error range dictated by the crudeness of
these measurements and calculations, we seem to be pretty close.
A note on the heater-cathode voltage as seen by the 26A6.
It appears to be exceeding the 90V max rating on the spec
sheet, but I haven't had any problems with the tubes I'm
using. These tubes were mil surplus made by RCA and packed in Dec 1962.
The spec sheet is dated July 1946. My tubes may have improved heaters,
as manufacturers would use heater stock used for many different
types of tubes (some with higher H-K ratings) in production. Or they
never characterized the 26A6 beyond 90V figuring nobody would ever
need it higher. Or else the tube just hasn't blown up yet (though I
have put about 50 hours on it so far)... If this design were ever
to go into large scale production, another tube would be used (maybe 26CG6)
unless the 26A6 were characterized to take the higher H-K voltage.
The 26C6 triode has a much lower mu (16) than a
12AV6
or 12SQ7 has (100), so the grid
needed to be biased to about -3V. I tapped the pentagrid converter
tube (26D6) oscillator section to get some negative bias. This
bias is usually -5 to -8V. The orginal grid leak resistor of 10
megs is used along with another 10 meg resistor tapping the
pentagrid tube to create a voltage divider to get the -3V
for the triode grid. The plate resistor was reduced to 150K as
well to have the triode operate in a reasonable region.
To make up for the lost gain, I added a cathode bypass cap to
the audio output pentode (117L7). The 26C6 triode is nearly the same as
that of the 6SR7.
There's also
another tube, the 26BK6 (heater of 26. 5V and
70ma), that is nearly the same as a 12AV6. With
a mu of 100, plate resistance of 80k, gm of 1250, and grid bias of -1v
(same as the 12AV6). Soon as I get some, it should fit nicely in this radio
without the 26C6 mods... Just got them in... The 26BK6 works well,
just like a 12AV6, once the 26C6 mods are removed. Also removed the
bypass cap on the 117L7 pentode cathode, as there is plenty of audio
gain now. But the 26BK6 is
as tall as a 35C5. The detector
diodes don't arch around the plate, but are housed inside a shield
which is tied to the cathode (thus to ground). This would cut down on
IF harmonics from jamming reception of stations at 910KHz. The diodes are near
the tube base, and the triode up top.
This radio had a pilot light, but I couldn't figure out a way
to incorporate a pilot light in the new heater string. Without
putting undue voltage stress on it upon power up. So I decided
to use the pilot light as a cathode resistor on the audio output
pentode tube (117L7). The pilot light is a 1819 which is rated for
28V at 40ma. I'm using it at 15V and it passes about 30ma at
that voltage. The 117L7 usually develops -5V of cathode bias
by itself. Here I rigged a voltage divider across the cathode
resistor (the pilot light) with a tap at 2/3 from ground. This
tap is what biases the control grid. Resistors 470K and 1.2meg. This
makes the tube pull enough current thru the bulb to get the
difference between the tap and the cathode to be 5V. Thus
the 15V.
The output impedance of the 50L6 is rated at 2Kz. The 117L7
is rated at twice that, 4Kz, for similar B+. An easy way to accomodate
that is to replace the 4 ohm speaker with an 8 ohm speaker so
the output transformer will now look like 4Kz instead of 2Kz.
However there will be less bass response in the audio. With audio transformers, when
operating at impedances below those specified,
the frequency spectrum is shifted downward, conversely, at
higher impedances frequency spectrum will be shifted upward.
To a first order approximation, in this radio, the bass
response thru the transformer will be reduced
to about twice the old bass frequency. However, the 117L7 will only produce about 3dB less audio power
than the 50L6. These factors do not cancel out, and you will still lose that
bass. The inductance of the output transformer's primary Lp doesn't
change even though the
load impedance (as seen at the primary via the impedance transformation
of the transformer) Rload did. Also the winding resistance of the primary
stayed the same, but here it's about a hundred ohms and can be ignored.
So this looks like an RL high pass filter which is not affected
by voltage or power level. But this radio had a larger than average transformer,
so this loss of bass isn't quite as serious as with a set with a tiny
transformer.
A sub for the 117L7 looks like the CV2556.
This set lends itself for switchable or wire-able 117V or 240V service.
There's two heater strings designed for 120V, and the rectifier circuit
that wants 120V input. Change the connections from the 240V configuration
and configure it for 117V:
Or use a slide switch of the sort found on PC power supplies, the ones
marked "115V/230V" and require a small screwdriver or such to change:
The 60HL5 needed a "grid stopper" resistor of around 4.7K to avoid oscillations
at supersonic and RF frequencies. This tube has a high gm.
The resistor and the stray capacitences in the
tube form a low pass filter that kills gain at frequencies above audio, thus
stopping the oscillations. These oscillations can cause audio distortion and
can also trash some AM band frequencies. I used a surface mount resistor
to bridge across a cut I made in the circuit board trace feeding the 60HL5's
control grid. Makes for a neat install.
This particular set is a clock radio, with the usual Telechron 120VAC 60Hz drive.
I added a 5K 10W resistor in series to run it off my 250VAC 60Hz line. The
resistor will see around 160V, as the field coil has a fair amount of
inductance. Below is the modified circuit board:
A third method using a 2. 5uF 250VAC rated capacitor in series with
the entire AA5 set circuitry is possible. At powerup, the heater string
is the only thing that will conduct. After the heaters have the
tubes warmed up, the cathodes of all the tubes start to emit
electrons. At first thought, one would think the AC line dropping
capacitor would interfere with the half wave rectifier (35W4)
supplying B+ in the AA5 radio. Because capacitors won't pass DC.
As it turns out, the DC current circulates thru the heater string
from ground to the point where the rectifier tube plate is connected.
This current heads into the capacitor for half of the powerline cycle,
and back out during the other half.
Then it goes thru to the rectifier tube cathode, and after filtering,
supplies the B+ to the set. Then the DC returns thru the circuits
back to ground. (Remember current is in the direction opposite to
electron flow).
The AC capacitor supplies the drive to make this circulation
happen. The B+ will be a bit low, somewhere around 90V
instead of 110V, but the set will operate. If you (carefully!)
use an oscilloscope to view the voltage waveform across the
heater string, you'll find a crude sine wave with a negative
DC bias on it. The positive peaks will be about +100V, the
negative peaks around -180V. A tradeoff to consider,
if the capacitor is increased to boost B+, the heater string will
get too hot with excessive voltage. I did this to an FM only
set that had a selenium rectifier. A 3uF cap along with a 35W4 added to the top of the
heater string (for a total of about 150VAC) replaced the selenium
rectifier, and its heater allowed higher B+ to be developed.
However, one needs to be mindful of the max cathode-heater
voltage (which for the 35W4 is 360V). A 2. 5uF capacitor on 250V
60Hz lines is about right, about 3uF value capacitor should be
right for 50Hz mains for an ordinary AA5; the FM set needed
a little more. The AA5 radio consumes about 23W, the FM set
uses about 30W.
Ian Robertson in Australia writes to tell me of a Japanese made AA5 modified for
250VAC operation. It's somewhat similar to the above, except that
the heater cap is in parallel with the rectifier circuit. This yields
better results, as there is a higher reactive voltage across the cap to
feed the rectifier tube 35W4. Thus yielding higher B+ voltage without
needing the power resistor in the heater string. Below is a detail of the
set's diagram he drew up and I added a few notes to. The heater
string also provides peak charging current limiting to avoid exceeding the
peak current rating of the rectifier tube. The 35W4 heater tap
with or without pilot light does this in regular AA5s.
Yet another radio:
Well this time I used 4 tubes:
117L7 audio output/rectifier,
26A6
IF pentode,
26C6
triode/diode, and
26D6
pentagrid converter.
That's 195V worth of heater string. An 800 ohm 3W resistor pads it
to the 240V line. But the 117L7 draws 90
ma of heater current, and the 26x6 tubes draw 70ma. Well, what
I did was pass the plate current thru the 117L7 heater so as to
divert it from the 26x6 tubes. The radio's ground is a point
between the 117L7 and the 26x6 string. The 117L7 rectifier
plate is connected to one side of the 240V line, and filter
cap goes to the radio ground. So all the B+ current to operate
the circuits of the radio (RF, IF, audio) comes off the line, rectified
and filtered, and then finds its way to the radio ground. And then
it goes thru the 117L7 heater to reach the other side of the 240V
line. Only thing the 26x6 current plus the plate current from the
radio circuits is a little more than what the 117L7 wants, so there
is a 4K3 3W resistor across this heater to take the excess.
When the set is first warming up, the 26x6 tubes see a bit
more than the usual heater current. This isn't a real problem
as tube heaters are designed to withstand higher than rated
current, and this condition goes away as soon as the 117L7
is warm enough to start conducting. Visual inspection shows that
the tubes don't get too bright during warmup anyway.
Some
oscilloscope measurements and some calculus (below) show that the
heaters are getting about the right amount of power.
The radio using the 26BK6.
With these oddball heater voltage waveforms going on, and
with the radio ground not tied to either side of the 240VAC
line, one should check to see if the rectifier portion of the
117L7 is not being overvoltaged for peak inverse voltage (PIV) rating.
And yet another:
Here we are using 100ma heater tubes.
The front end, IF, and detector/AVC/audio
driver tubes are the 18V 100ma versions of the miniature AA5 tube line up. Their B+
is around 110V. But, as the 100ma AA5 tubes (32ET5 or 34GD5, and 36AM3) are
not designed to take the higher voltages from a 250V AC line, we need better
tubes here. There's the 60HL5, which can take 330V plate voltage, and the
38A3 (AKA UY85)
which can accept the 250V AC line. This set develops 260VDC
for the output tube, and a resistor dropper provides the screen voltage and
supply for the rest of the set, 110VDC. We could also use a UY82 rectifier as
well. The heater string doesn't come up to 250V, but more like 152V (with the 38A3)
or 170V (with the UY82). So we use a plain old fashioned power resistor of 850
ohms to drop the line down. Or a cap of 1.4uF 250VAC line rated. The cap
avoids 8.5 watts of heat inside the radio.
In the USA and other parts of the world using a pair of
120V powerlines (240V grounded center-tap), you don't have
to wonder if the chassis is "hot", either way
the powerline is connected, the chassis is always "hot".