Vintage Electronics - Antique Radios

Vintage Electronics    Antique Radios  •  Supplies

Vintage Restoration - Vintage Electronics On eBay
SoundCityUSA Vintage Electronics T-Shirts
"vintage"
adjective-
of high quality and lasting value, or showing the best and most typical characteristics of a particular type of thing, especially from the past.
"electronics"
noun-
the branch of physics that deals with the emission and effects of electrons and with the use of electronic devices, a system of interconnected electronic components or circuits.
          "vintage electronics articles and other useful information"

Filament Rejuvenation

from Colorado Radio Collector's "The Flash!!"
Although many things can go wrong with an electron tube, the most typical failure is an open filament or the filament's electron emission has weakened beyond usefulness. In the early days the filament was used as the actual electron source, whereas most modern tubes use the cathode element for the electron source and the filament is used as a "heater" - a common filament synonym. Of course an open filament means you've just became the proud owner of a new high-tech fishing float. However, with certain older tubes there is hope for filament rejuvenation.

Fairly early in the tube design process it was discovered that the oxides of certain rare-earth elements were efficient producers of electrons when heated. Unfortunately these compounds are neither good conductors of electricity nor particulary strong physically. Consequently they were used as coatings on Tungsten filaments. This coating practice is also used on modern cathode equipped tubes. However, it turns out there were a few tube types that used filaments that had the electron producing material, Thorium, actually imbedded in the Tungsten as well as being used as a coating. Impregnated Tungsten was known as Thorated Tungsten. It's this type of tube filament that has a chance to be rejuvenated by a technique known as "Flashing". Fortunately, this group of tubes include the ubiquitous 01A.

The flashing process consists of two steps. First there is a relatively high "flash" voltage application to the filament, then a lower "aging" voltage is applied - each for a specific voltage and time duration depending on the tube type. Fig. 1 is a chart showing the voltage and duration values for each tube capable of using this process. Be VERY careful to note the tube types that CANNOT use the flashing process and, in fact, may be damaged by it's use. You might be interested to know this technique was worked out many years ago in the labs of RCA.

Before you get "flash happy", you're going to want to make sure the tube you're about to flash REALLY needs it. Flashing a good tube will materially reduce It's remaining useful life. So, run your suspect tube through it's paces on a tube tester first. In this case we're only interested in filament emission values. Figure 2 is emission test data to provide minimum guidelines for determining if a tube has impaired filament emissions. Although you could use this information in lieu of access to a tube tester, note that Fig. 2 is unfortunately not as complete as Fig. 1. If you use the Fig. 2 data in a simple hook-up, remember to tie the grid to the plate first as if the tube were a diode.

O.k., now it's time to flash! It's unimportant if the flash voltage is AC or DC. Perhaps the easiest source of an adjustable voltage would be your tube tester. Just make sure that if you do use a tube tester DO NOT press any test buttons, as flashing must be done without any plate current flowing. Other voltage sources might be a Variac (a continuously adjustable transformer), or a variable DC power supply. In any case, your voltage supply should be capable of 2 - 3 times the normal filament current draw for the tube being flashed.

Using FIG. 1, simply apply the flashing voltage, then apply the aging voltage. The object of this process is to drive whatever Thorium is left in the Tungsten to the surface as a replenishing oxide. Be aware that the actual results you get will be dependent on the individual tube and it's condition prior to flashing.
filament rejuvenation
TUBE REACTIVATION CHART - FIG 1
Tube
Type
Flashing
Voltage
(Volts)
Flashing
Time
(Seconds)
Aging
Voltage
(Volts)
Aging
Time
(Minutes)
'99 12 10 4 30
'20 12 10 4 30
'22 12 10 4 30
'01A 16 10 7 30
'00A 16 10 7 30
'40 16 10 7 30
'71 16 10 7 30
'10 16 10 9 30

filament rejuvenation
EMISSION TEST DATA - FIG 2
Tube
Type
Filament
Voltage
(Volts)
Plate
Voltage
(Volts)
Minimum
Current
(Ma)
'99 1.1 50 6
'20 3.3 50 15
'01A 5.0 50 20
'00A 5.0 50 14
'40 5.0 50 14