In the Feb. 1 issue, we discussed what you’ll find inside an electron tube and its basic principles of operation.
Now let’s talk about how that tube works in an FM radio transmitter.
The key to amplification in a tube is the control grid, the set of thin wires between the filament and the plate. You feed RF drive into this grid, causing electron emission from cathode to vary at the RF drive frequency and amplitude. It is this small voltage and power that regulate a large electron flow at much higher power. What we have been talking about is a 3CX (triode) tube.
Fig. 1: This is a newly rebuilt transmitter tube. Then comes a fourth element, making the tube a 4CX. It is called a screen grid and is another wire grid between the control grid and the plate. Its job is to pull electrons toward the plate with a positive voltage. Most electrons go past the screen grid and to the plate.
Some electrons are captured by the screen grid and become screen current. Yes, it is an unintended consequence of being in the right place at the right time.
If you use a Harris or similar FM transmitter you know that the power control is a screen voltage control. With no screen voltage, the transmitter typically makes about 20 percent of its normal power. You adjust screen voltage upward to increase power. It is as simple as that.
Each transmitter design is different. Sometimes the control grid in these designs has 100 or more volts of negative DC to help bring all the parameters together for best performance and efficiency.
In Continental FM transmitters, the PA (power amplifier) plate voltage, screen voltage and RF drive are adjusted simultaneously with the raise/lower power control. This is an elegant design but does not easily compensate for tolerance differences between individual tubes, especially as they age. Engineers with Continental Electronics transmitters find themselves changing taps on the screen transformer to accommodate a new or rebuilt tube with slightly different characteristics than the last tube. The idea is to get the volts to current ratio where the transmitter manufacturer designed it to be. Higher-screen voltage means higher PA current. Those transmitters also have a PA bias control that will help with tube-to-tube differences.
Fig. 2: Here’s a 4CX20,000D tube in a Harris FM transmitter.TUNING
You tune or “match” impedances between the RF driver and the control grid of the tube for minimum reflected power to the RF driver. Best not to try for maximum power transfer as that could lead to higher than desired reflected power/VSWR on the driver stage. Tune for lowest reflected power to keep the driver stage happy. Then there is power amplifier tuning where you adjust for best tube efficiency by again matching impedances. Watch all the meters, not just the power output. Tune toward minimum PA current while adjusting for maximum power output. Adjust screen voltage to keep the PA current where the transmitter manufacturer says it should be for your power level.
Once you have adjusted for best efficiency, use a thermometer to verify that the exhaust temperature of the tube is within specifications. Output power meters on transmitters are relative devices. Every transmitter has a control to set the meter to read whatever you want.
Set it to read 100 percent only when the transmitter is running correctly. Never use the output power meter to determine operating power. You might have higher or lower actual power than is shown on the station license.
There is a little-used, FCC-approved procedure for using the transmitter’s power meter, but few stations are equipped to do so. You can verify transmitter power with a calibrated Bird or Coaxial Dynamics brand wattmeter, if you like.
An FCC inspector will look at PA voltage, PA current and rated transmitter efficiency to determine if the station is running legal power. The station will pass or fail an inspection accordingly.
You will find PA efficiency listed on the factory checkout sheet for the transmitter or in a graph in the transmitter instruction manual. An example is 8,000 volts x 3.4 amperes = 27,200 watts x 79.1 percent PA efficiency = 21,515 watts of transmitter power output. The station license allows as much as 5 percent over that or 10 percent under that to remain legal.
Fig. 3: Watch all the meters, not just the power output.TUBE TOO SMALL?
You have a 20 kW FM transmitter with a 4CX15,000A tube. Does that make it a bad design?
No, the 15,000 watt tube dissipation capability is for the inefficiency of the power amplifier. To put out 20,000 watts, the transmitter might achieve 75 percent PA efficiency. That means 26.7 kW of high-voltage DC power is required to enable the transmitter to achieve 20,000 watts of RF at its output. The tube must lose 6.7 kW in heat from the high-voltage supply plus power lost in the control and screen grid. Add to that the filament, which is 6.3 volts at 160 amperes or 1,008 watts.
Eimac specifications show the tube could develop as much as 36.5 kW under the right conditions. That is pushing the equation way too far, in my opinion, and does not take into account any mistuning. A transmitter and tube might have to handle antenna icing in winter conditions. Poor matching because of that can severely mistune a transmitter causing the tube to dissipate more power in an attempt to stay on the air. Most transmitters have overload and protection circuits to prevent that.
Electron tubes wear out. The filament is what wears. You’ve heard it before, and I’ll repeat it again.
Best life from a tube comes from proper filament management. That means running the filament at the rated voltage as indicated in your transmitter manual or tube specification sheet for 200 hours of operation. Then drop it as low as possible before there is a significant reduction in emission, which you see as lower transmitter power.
You can easily double or triple a tube’s life by careful filament voltage management. Some transmitters have filament voltage regulators to make up for power line voltage variations. This is especially helpful when power line voltage sags frequently because of a nearby factory or farm when electric usage increases and decreases.
Tube emission and transmitter power output will decrease as a tube nears the end of its life. You should adjust filament voltage upward to compensate. It is time to replace the tube when you are at 5 percent over the rated filament voltage. It is called “end of life.”
If you have tuning trouble in a transmitter, it is best to reduce PA voltage or drive to keep the tube within specification for dissipation. If you look in a tube spec book, you will see figures for maximum control and screen grid dissipation too. Sometimes, I’ve disconnected one of the three leads on the primary of a three-phase power transformer when tuning and troubleshooting. Do this with all the power turned off! Typically the PA voltage will drop to half. That will put the output at one-quarter power. Do the troubleshooting and initial tuning before reconnecting that third wire. That is a great way of handling a problem without burning up a tube.
Fig. 4: A tube cavity with 4CX20,000CMORE IS NOT BETTER
This is for transmitters with solid-state RF drivers.
Just because an FM transmitter manufacturer recommends the PA tube in your transmitter needs 350 watts of drive, it doesn’t mean that 400 watts is better. You can get into a situation where PA efficiency will suffer with more RF drive.
Do your tuning to match what the manufacturer test data shows. Your numbers may be a bit off because you have a different tube in the transmitter. Adjust RF drive by tweaking FM exciter power. You will find that the tube efficiency is poor when under driven. It will peak and then drop off again as drive is increased.
Find the sweet spot where PA efficiency is best. You will know that when PA current is low and transmitter RF output is highest.
Keep tabs on what condition your tubes are in when removed from service.
You can refer to a serial number on the bottom, which is often difficult to read.
Another way is to mark the tube with a statement something like “makes 90 percent power.” Do not mark on the ceramic. Yes, ceramic is a wonderful place for a pencil to leave legible markings. Pencil lead is a conductor and can easily help start an arc-over when the tube is returned to service. Pencil lead is also difficult for a tube rebuilding company to remove. Best to use a liquid pen and indicate status on the tube anode.
You’ll be glad you did when you return to a broken transmitter in the middle of the night and can’t remember which tube is good and which one isn’t.
DO IT RIGHT
My recommendation: Have one new or rebuilt tube on the shelf for every tube that is in service at the site. Tubes are “wear” items and will wear out or outright fail with time. It is just a matter of when.
You can’t predict when a tube will need replacing. Yes, a tube can get weak so transmitter power falls off. It can just quit in just seconds, putting you in a bind to get a replacement in a hurry.
Knowing the facts will help you get the job done right.
Mark Persons WØMH is Certified Professional Broadcast Engineer by the Society of Broadcast Engineers and has more than 30 years’ experience. His website is www.mwpersons.com.