WKCN-FM 99.3 Harris HT25FM Transmitter 4CX20000A PA Tube Replacement – October 12, 2021

The Eimac 4CX20000A PA tube in the WKCN-FM Harris HT25FM transmitter had gone “soft” after being in service since January 4, 2011. We must admit, a bit over ten years and nine months of service out of a rebuilt tube was really good tube life. Nonetheless, it was time to put a new tube in, since the output of the transmitter was dropping below where we wanted it to be. We had switched the transmitter to manual power control mode, which we had done to keep the transmitter’s built-in logic from trying to pull the screen voltage up to excessive levels. Here’s a detailed rundown on how the tube replacement went, including “before” and “after” photos of the transmitter’s performance, as well as some photos taken during the work.

Photos showing transmitter performance before the tube replacement

Plate voltage approximately 9.2 kV prior to tube change. This is about normal for this model transmitter.
Plate current approximately 1.9V Amps prior to tube replacement. This was lower than needed for the transmitter to being performing the way we like for it to run.
Screen voltage prior to tube replacement was running around 800 Volts, which is more than it should be doing for full power output. This was on of the key indicators that the tube was reaching the end of its useful life.
The IPA VSWR had also been climbing steadily and the IPA input to the PA tube had reached the point that it could not be tuned below roughly 2.5 Watts — ordinarily it should be tunable to virtually 0 reflected power.
This photo of the exciter output power running 13.5 Watts was primarily for reference purposes.

So, here we go pulling out the bad 4CX20000A tube and doing a little “cleanup” before installing the replacement tube

Failing Eimac 4CX20000A tube pulled out of transmitter.
The old tube was an Econco rebuild.
Installation date of the old tube was January 4, 2011. We were very pleased with the fact the tube had survived 10 years, 9 months, and 8 days of service. That’s partly due to filament management over the years. In fact, just a few months ago I had read the filament voltage and the tube was still doing 100% TPO running about 8.6 Volts of filament voltage. I had only recently had to increase the filament voltage to 10 Volts and higher. This tube is rated to run 10 Volts on the filament, so it really held up well right up to the end of its useful life.
Anode-end view of old tube pulled from transmitter.
Filament-end view of the old 4CX20000A tube pulled from the transmitter. Given that the tube had been in service just shy of eleven years, it really didn’t look too bad. I have seen MUCH worse looking tubes come out of transmitters after only a couple of years of service.
PA tube socket in the Harris HT25FM transmitter after raising the chimney and removing the failing 4CX20000A tube. A little cleanup of soot and dust and a very careful, gentle wipedown of the contact fingerstocks was performed after this photos was taken, prior to installing the replacement tube.

Time to put in the replacement Eimac 4CX20000A PA tube

Filament contact markings on the replacement tube. This rebuilt tube was acquired from Kennetron.
Kennetron rebuilt 4CX20000A tube installed in transmitter, ready for chimney to be pulled down and clamp everything back into place. The tube seated nicely and easily into the socket and contacts, which is always a good feeling, indicating that nothing has been damaged. Those sockets are really complex critters and can easily be damaged, especially if prolonged high heat has essentially welded any of the fingerstocks to the contacts on the tube, or if a tube is not removed from the socket very, very carefully.

Firing the Harris HT25FM transmitter back up and getting everything tuned after installing the Kennetron replacement Eimac 4CX20000A tube

After letting the filament and tube warm up for about 30 minutes, I fired it back up. Forward power out of the IPA with the Harris DigitCD exciter running at 13.9 Watts produced just under 275 Watts of forward power.
The input to the new tube was easily tunable for no reflected power going back to the IPA. That’s definitely something you want to see in a good tube.
As part of a proper tube burn-in and preparation for filament management, the filament was set for the tube’s rated filament voltage of 10 Volts. I’ll keep it there for the first 100 hours or so of operation, after which I will begin filament management by reducing the filament voltage and then adjusting it to find the “knee” voltage, which is the point at which increasing the voltage no longer creates a fast increase in output power. Once I find that voltage, I’ll set it approximately 0.2 Volts above that voltage. During each monthly routing P/M visit and inspections, I’ll recheck it and increase it if/as needed. Once the tube has to be run at its rated filament voltage of 10.0 volts, it will be time to start watching its performance closely, as it will mean it’s time to get ready for a new tube. If this one holds up anything like the old one did, that will be several years down the road. In fact, I might be retired by then and never be the person who has to access or eyeball this tube again.
Bias voltage running around 320 Volts.
Bias Current: roughly 54 mA.
Screen voltage running roughly 595 Volts.
Screen Current: roughly 70 mA.
With automatic power control (APC) running, the transmitter’s output was staying between 98-102% of TPO for the transmitter.
Plate voltage after peaking all the PA tuning was right on 9 kV.
Plate current was reading 2.8 Amps.

I will be going back to the site on 10/17/21 to recheck the transmitter’s performance and peak all the tuning again. I will also start the filament management on this new 4CX20000A tube. I’ll try to post that site visit here on the Alabama Broadcast Services, LLC website so visitors can see just how low we’re able to run the filament voltage at that point and produce 100% TPO without excessive screen voltage. The whole point of that is to always run the tube with just enough filament voltage to get 100% power output without so that we don’t burn the tube’s filament up faster than necessary due to excessive heat. When you consider the fact that all of the tube’s power output is essentially produced by heating up a piece of thoriated tungsten to the point that it’s spitting out around 17,500 Watts worth of electrons which the station turns into that beautiful sounding country music which Kissin’ 99 FM (WKCN-FM 99.3) listeners enjoy every day, it’s sort of a miracle of physics and some great engineering that makes all that possible. Some of us will never cease to be amazed at how that works. I’m certainly honored and delighted to be a part of the team which keeps all that happening day-in and day-out. Likewise, I’m delighted that we have the Kissin’ 99 FM transmitter humming along at full power and just doing her thing.