6SB7 replacing 12SA7 in octal AA5 radios for more sensitivity

[wa2ise]

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, AFAIK. One can modify the radio to accept the 6SB7Y by rewiring the heater circuits. Several ways of doing that are shown in the schematics.


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.

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.

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.

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.

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.

[David Roper]

It's a brilliantly conceived mod, but will the average AA5 even benefit from the difference? It makes a lot of sense to do this with, f'rinstance, a set with short wave band that conks out above 15mhz, but very few AA5s have that issue.

[dieseljeep]

After reading about subbing a 6SB7 in Bob's website I changed the 6SA7's in my Halli comm sets. I'll bet that change will improve the S38 series greatly.

[bob91343]

Improving the S-38 sensitivity might be a good idea but the stability and selectivity are the main deficiencies in that unit. Of course one must appreciate the context, that it wasn't intended to be a serious communications receiver.