I - General Description

The SECO Model 78 Grid Circuit and Tube Merit tester is a very compact and highly accurate tube tester designed primarily for the television technician. Its test coverage includes the majority of heater type radio tubes as well as filament rectifiers. Three basic tests are available, each in turn applies the desired data to a special two stage DC amplifier. The amplified signal voltage which is either D.C. or rectified signal voltage is fed to the meter through the proper amplifier input. These three tests are namely: 1. Grid Circuit and shorts test; 2. Merit test; and 3. Filament continuity. The Grid Circuit and selected leakage and shorts test employs the Seco developed and patented "D.C. testing procedure" for detecting "hard-to-find" faults such as grid emission, small leakage and shorts. Low test voltages (4.5 - 50 VDC) are selected to minimize danger of damage to a good tube. If the tube is free of the above undesired faults, the meter pointer will deflect into the good area on the left of the meter are. The Tube Merit Test is accomplished by feeding a low test voltage of 22 volts through a suitable limiting resistance to the cathode and the plate or grids. This constitutes an emission quality test. The tube under test is always isolated from the actual meter circuit. The circuit employed in the Model 78 makes available an unusually wide choice of test conditions as well as safeguarding against meter damage. A good tube will cause the meter pointer to read in the "good area" on the right of the meter are. The Filament Continuity Test is convenient for locating open heaters in series string operated receivers. If heater continuity is satisfactory, the meter pointer will read in the "good" area to the right. The pin selector and switching circuits: It is possible through proper choice of switches to test as many tubes on the Model 78 with its five sockets as some testers do using in excess of one hundred sockets. The rotary switch "R" is a dual ganged switch. One section is an isolating type while the other is a selecting type. The lever switch "L" is a multi-rotor switch arranged for opening circuits to selected pins. It also transposes the heater voltage to different pins. Study of the chart will reveal that 14 heater pin combinations are available in addition to the 9 pin special socket.

  1. See operating instructions in the cover of your tester.
  2. Test each section of dual and multi-purpose tubes individually. Usually only the switch "R" and the load pot need to be adjusted.
  3. Filament type rectifiers are checked for emission only, a shorted tube will produce no reading.
  4. Lever switch "L" is always left at position marked "normal" unless specified otherwise under the heading switch "L" on the set up data card.
To perform a complete high sensitivity interelement short test or for determining where the short exists:
  1. After tube is thoroughly heated and tested for merit and GCT, hold the FUNCTION switch forward in GCT and Shorts positions.
  2. Rotate Switch "R" through all positions A to H and watch meter. On a single purpose tube, the pointer should remain in the good area to the left in all positions except ONE, the cathode position. Readings in the "red" area in excess of one mean that a short is present and associated with the pin selected by Switch "R" in this position. Consult chart showing electrical detail of Switch "R".
  3. A common dual tube such as the "12AU7" has one cathode for each section. Two set-up entries are shown. The meter will read in the "red" area in two positions, namely the "F" and "G" positions.
  4. If an asterisk (*) appears opposite the tube set-up, this means that an inter-connection exists in the tube which cannot be isolated with lever switch "L". Such interconnections will indicate the same as a short between these pins and cause the meter to read in the red. Consult the tube manual if necessary.
  1. Set up the tester controls as usual.
  2. After the tube is thoroughly heated, turn the LOAD pot to 100. On most types the meter will pin at full scale, depending on the position of switch "R". This is normal and no harm can be done to the meter because its current is limited by tube limiting action in the tester.
  3. "Function switch" should be in "TUBE MERIT TEST" position.
  4. Rotate switch "R "through all of its positions A to H.
  5. All plates and grids will deflect the meter pointer to the right.
  6. Cathodes will deflect the meter pointer to the left.
  7. Pins with no connections, or interconnected pins that cannot be isolated with switch "L", will cause pointer to remain at center. Such interconnected pins will be indicated by an asterisk in the set-up.
  8. The grids or plates nearest to the cathode will produce the greater deflection as the load pot is backed off from 100.