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  Cathode Interface Remedies
   
Although it is sometimes possible to recalibrate an instrument to "overcome" the effects of cathode interface, the only real cure is to replace the tubes which exhibit the problem. There are basically three choices for replacements:
  • New or unused tubes will not exhibit cathode interface, at least until they age and their cathodes degenerate.
  • Some older tubes fail to exhibit cathode interface, even though they should have aged enough to develop the problem.
  • There are alternate types for some common tubes, which are less likely to develop cathode interface.
To remove problem tubes and replace them with "good" tubes, it is necessary to have a means of testing for the cathode interface problem. Testing can be performed in-situ (in the instrument that you are working on), or by means of a fixture dedicated to this purpose.
 
A scope plug-in such as the type CA makes a good test fixture, because the behavior of a single tube can be easily observed. In contrast, a 545 output amplifier contains so many tubes (12) that it is difficult to determine which tube or tubes are at fault.
 
A dedicated fixture might consist of a high speed pulse generator, a single stage cathode follower, and a working oscilloscope to observe output waveform quality.
 
If you can find them, the following tubes make good replacements for the original types, and (we believe) are less likely to exhibit cathode interface:
 
Original Type Replacement Type
6AU6 8425A
6DK6 8136
12AU6 8426A

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Last modified on  2/29/00 10:08:18 PM