Originally Posted by tylerSC
Originally Posted by ILMRay
Sorry I have not logged in lately.
Yes, we are on our backup transmitter at WECTat reduced power. Our final IOT tube shorted and caused some massive high voltage issues.
We have had to rebuild the enitire HV section in our transmitter and hope to have it back on line in the next few days.
The views expressed here are my own, and are not endorsed by my employer
Apparently replacing a tube is not some simple process. Because supposedly WBTV has been down a tube for months, and has had some sort of problem getting it replaced. Not sure if they ever did; haven't heard an update. But I still get their signal at 100% from 75 miles. But looks like a lot more work than the average nontechnical person would be inclined to believe.
UHF has always been more difficult than VHF. The tubes are completely different and require a different kind of cooling as well as handling. Because of the size and weight of these tubes (technically, most are IOTs, inductive output tubes and are in the 100 lb range) they take a small amount of drive wattage, in the tens to hundreds range depending on the desired output, and produce power in the tens of thousands of watts. With that kind of gain, it takes tens of thousands of volts at several amps to produce that wattage at those frequencies. Example, our old tube channel 8 transmitter used your basic tetrode power amplifier tube (around 15 pounds). 9,000 volts with about 2 amps of current produced 12kW average power (25kW peak power). We drove two in parallel for 25kW and with the gain of the antenna, we got 316kW of power. It was forced air cooled. Now, with our channel 35 transmitter, we run 35kV at 2 amps for 27kW and we run two in parallel for 52.5 kW output, water cooled. As you can see, the UHF is much less efficient than the VHF (because of the frequencies) so it takes more raw power for the same output. We actually step UP the voltage. To give you some reference, most power lines outside your house are 11kV and then are dropped down to 220v with the transformer on the pole. Our tube needs 35kV so that is 3 times what the power line outside the building provides. We have a 1200 amp 480v line coming into the building to build up to those kinds of voltages. With our old VHF, we had a 800 amp 220v service into the building. Much different. The price different is as great as well. When we retired our tube VHF transmitter in 2005, visual amp tubes were running about $10k each. For these IOTs we run now, they start at $35k! They are just completely different beasts. But that is what it takes to produce the power levels we need for UHF digital television.