Today I am presenting a TV Damper tube with series string heater : The 25DK3.
The 25DK3 was designed for damping diode in the horizontal deflection unit of color TVs. Like other TV damper tubes it can be used as rectifier in power supplies.
The 25DK3 has a 9-Pin Novar base which is not to be confused with the much more common 9-Pin Miniature base which has the pins arranged in a smaller diameter circle. The Novar base is very similar to the Magnoval base as used on the EY500 but the latter has slightly thicker pins. The cathode connection is wired to a small cap on the top of the tube. As mentioned above the 25DK3 was intended to be heated in series with other tubes with the same heater current of 450mA. If that current is applied the heater drops 25V. There is also a 6.3V parallel heater version, the 6DK3. Like the EY500 the 25DK3 and 6DK3 have very large current handling capability. It can deliver up to 400mA per tube and can handle current peaks of 1.2A. That is coupled with an insane peak inverse voltage rating of 6.5kV and a very small voltage drop between plate and cathode. Detailed technical specs are available in the General Electric data sheet. With these specs a pair of the tubes can deliver up to 800mA.
An ideal candidate for high current power supplies. Like other TV dampers the 25DK3 has a very slow warm up which causes the output voltage to rise gently at start up. Especially series heater tubes can be found at low prices. The heater can be operated from a suitable transformer. Readily available 24V transformers will do the job.
I only have 25DK3s made by Philips ECG. So let's have a close look at these.
The plate is made of copper:
Some more details:
The base pins in comparison to a EY500:
Quite an impressive little device!
Now lets open one up:
Some close up shots:
Removing the heater:
The spiral around the heater wire ensures the high voltage insulation against the cathode.
And lastly some photos with the heater lit up:
I hope you enjoyed the presentation of the 25DK3!