Using types 30 and 31 as replacement Tubes

The 30 will replace 01A, X99, WX12 and most other amplifier tubes which have "4D" basing. The 31 will replace 20 and 71A in early battery sets.

As long as the sockets in the radio are not dependent on the side bayonet pins and you are willing to change the filament voltage, the 30 and 31 tubes make excellent replacements in early battery radios. These tubes are presently available new at reasonable prices.

Early radios usually have a rheostat to control the volume. This rheostat will not have the same action when using 30's as replacement for the higher filament current 01A's. To correct this, you can either add a load resistor across the filament pins in the radio or use a variable supply for the filament. In any case, the performance of the radio should improve as the 30 and 31 have slightly higher gain than the other tubes.

Advantages:

Cost - The tubes are available new at reasonable prices.

Uniformity - Electrically and mechanically the 30/31 are more uniform. You will not need to move tubes around to find which ones work best in which sockets. It is easier to find a set of tubes that have the same physical appearance.

"A" Supply - It is much easier and less expensive to build an "A" supply for 2 volts @ .5 amps than 5 volts @ 2 amps.

Overall - Performance will improve as the gain per stage multiplies not just adds.

Tubes that have Thoriated-Tungsten filaments with low emission sometimes can be reactivated. These tubes, like the 01A, can usually be identified by their bright filaments.

Test a weak tube then operate the filament at a slightly increased voltage without plate potential for a few minutes and retest at the original setting. If there is some improvement, again apply the increased filament voltage for an hour or so and note the improvement. In some cases, it may take overnight for maximum recovery. If no improvement was noticed, use a slightly higher filament voltage.

The 01A operates with 5 volts on the filament. A voltage of 5.5 to 6.0 is usually enough to increase emission. Do not go above 6 volts as this can have the opposite effect reducing emission and can burn out the tube. I have had the best success with RCA and Cuningham tubes sometimes bringing the emission to the level of new tubes. Some tubes will not recover.

Emission in some other tubes like the 26 and 71A may also be improved by a different method. On these after testing make the tube draw excessive plate current. This can be done by making the grid positive and increasing the filament voltage if necessary. This can be done on a tube tester like the EICO 666 or other setup capable of high current.

Under these conditions the plate current will start to increase. In the case of the 26 and 71A, a green color may appear between the filament and plate. If the green color is noticed, the tube probably has increased emission. Do not keep these high current conditions on the tube for more than a minute or so. Remove power, let the tube cool and retest.

Distortion in a radio is generally caused by leaky audio coupling capacitors. Even the slightest leakage can cause a problem and if not corrected will damage the output tube and can overheat the power transformer.

The plate of one stage is coupled to the grid of the next with a capacitor. The plate has a positive voltage while the grid requires a negative bias. A leaky cap causes the grid to run positive and the tube to draw excessive current.

To determine if this is a problem, measure the grid voltage in relation to the cathode of the tube using a high impedance meter, 20000 ohm or VTVM, as not to load down the voltage. For most AC/DC radios, the voltage on the output tube grid should be -6 to -8 volts. For radios using 6F6, 42 or other tubes with higher voltage on the plates, the grid voltage will be -15 to -20 volts. Some tubes like 45's and 71's may have -50 volt grids but these are usually transformer coupled.

If the grid voltage isn't negative enough or is positive replace the coupling cap. Use a good quality cap which shows no measurable leakage on the highest scale of an ohmmeter. Most radios use caps around .01mfd. A radio will have a better low frequency response if a larger value is used, .05 to .1mfd.

At the same time, the cap is being replaced check the grid resistor. The grid resistors on the output tubes should not exceed .5 meg. Some resistors increase value over time and some radios used values exceeding .5 meg which is not recommended for most output tubes. If the resistor reads high replace it with one of the same or lower value.