Research: Tube Type "CB" Linear Amplifiers

Relics of a time past, focus on mobile types. Dispelling myths about CB Linear amps.

About this page:

The Bunker of DOOM niether encourages nor endorses the use of any external RF power amplifier on the citizens band. Using an external RF power amplifier with a citizens band radio is illegal in the United States. This equipment is legal to own and use on amateur radio frequencies. The term "CB Linear" is meant to indicate any of the black-market RF power amplifiers designed for CB use, although not necessarily labeled that way. We have not yet seen one of these amplifiers that will not work on the "10-meter" amateur radio band of 28.0 to 29.7 MHz with little or no tuning modifications. This research focuses on tube-type mobile linear amplifiers but does not exclude base units or solid state units including 10M/6M ham radio types if they happen to show up on our doorstep..

Soon enough, we will begin to do schematics by tracing the wiring and also provide some comments on design. It takes time, cut me some slack..

About power claims for "CB linears"

Anywhere you go, whatever CB linear you see, the seller or purveyor will often claim that the unit puts out about twice what is really does. Come on, we all know that a pair of sweep tubes is not going to do 200 watts -for long. And not at all with only 800 volts on them. A measurement of the undistorted output of the amplifier in question, using a decent wattmeter, will show the truth. Several factors contribute to the outlandish claims. The power ratings stated on this page for the amplifiers shown are measured undistorted RF power output under single-tone conditions or at the crest of the modulation cycle in AM operation.

Well-designed, calibrated power meters are extremely critical to accurate measurement. If a military surplus wattmeter, a "Workman" CB wattmeter, a "Dosy" CB wattmeter, and a "Daiwa" ham radio wattmeter are all placed in series and a nonreactive 50 ohm load is connected, which ones agree? Almost every time, the CB wattmeters read 1.5 to 2 times higher than the actual power shown on the military meter, and the ham radio meter almost always agrees closely with the military meter. I do not mean to infer that a military wattmeter is perfect, but that a quality meter designed for accuracy, connected to a proper load, is the only way to honestly measure watts. Another artifact of "CB wattmeters" with regard to AM operation is that most of them "swing" under modulation. Whistle into the mike, and see the power shoot up to the end of the scale! Reality is not so. A real wattmeter used with a properly adjusted AM transmitter will swing the needle very little on modulation. A good ham radio wattmeter costs no more than a CB wattmeter and is more accurate.

Manufacturers have tended to claim power levels vaguely, give amplifiers model numbers that suggest a certain power level, and not specify whether it is input or output power (power used by the amplifier, or power put out to the load by the amplifier, very different things!). Whether it is a tube or solid state amplifier, this practice prevails. That's why a "TX-800" is a 400 watt amplifier, not an 800 watt amplifier as implied. The Twister has four 2SC2879 transistors which are rated for 100 watts PEP output each. Perhaps the Twister's PA stage consumes 800 watts -a far more likely scenario. The HEC 300 shown on this page does make 300 watts -DC input from its power supply to the driver PA. RF Output from the PA is about 165 watts.

Honest CB Salesmen with no proper documentation and CB Technicians with inferior meters have no way to know this. I do not make an accusation that CB radio sales and service folks are ignorant or dishonest; On the contrary, -the information and materials they are given to work with can mislead them and others.

Overdrive is one way to get almost 800 watts out of the "TX-800" 400 watt amplifier, even using a good meter. Just as with a guitar amplifier that has an undistorted sinewave output of 50 watts RMS, but can be driven to total clipping making the output a square wave and increasing the wattage to 100W RMS, an RF amplifier can be overdriven into clipping until the power increases to much more than the undistorted output level. The problem here is that just like the pure tone from the guitar amp becomes rich with harmonics on other frequencies, so does the RF amplifier's output. Unlike the guitar amplifier's harmonics, the harmonics from the RF amplifier are a useless waste, because they are not anywhere near the frequency or "channel" the person on the other end is listening on. This energy is not heard by the receiver and does not move the S-meter at the receiving end. In both cases of the guitar amplifier and the linear amplifier, a high quality power meter will show the extra power. The point is not to be fooled by the demonstration of an overdriven amplifier. Look up the output tubes (or tranistors), and see what the ratings are. Those are the FACTS. Any other claims, except truly scientific laboratory measurements*, are pure smoke.

* -some solid state amplifiers have been hot-rodded/designed to put out 25-33% more undistorted power than normal by using an incorrect turns ratio on the output transformer. The practice is usually to add a turn to the secondary winding. The result is increased current through the amplifer, overheating, and early burnout of the transistors.

About these amplifier types

Most of the tube-type "CB Linears" out there are decently built. They have robust power supplies, and the weakest link is usually the tubes.

The tubes are often run at voltages and currents at or above their "absolute maximum" rated limits in order to squeeze the last available watt from the equipment. This can be an advantage when it is considered that higher plate voltages promote greater efficiency and lower harmonic content in most amplifiers. The key to tube longevity is to avoid pushing the over-voltaged amplifier to its maximum output. Check the ratings for the tubes in your linear amp in one of the tube manuals, and you can see the data for yourself. The rating you want is the plate dissipation rating. This figure, expressed in watts, is generally 1/2 of the tube's maximum safe power output level.

A tube like the 8950 that can dissipate 35 watts can therefore (generally) put out up to 70 watts. Of the 70 watts, some power is lost due to the output circuit, the coils and capacitors. Figure 10% loss from that 70 watts. So your costly 35-watt 8950 can, as a general rule, safely do 63 watts. It is reccommended that this be taken as the PEP level. In AM service, the carrier level in watts should be limited to half the plate dissipation rating. (i.e. the 8950 should be run at no more than 17.5 watts carrier level on AM)


These things said, solid state "CB" linear amps are subject to the same design methods as their tube-type ancestors. Some may be designed to push more power through the devices than the datasheets call for. Perhaps this is why you can find a stack of fried solid state linears in the junkpile at some CB shops! The maker set them up to deliver 1.5 to 2 times the acceptable power output, and the user drove the crap out of it with their tweeked rado. Very sad.. But where did the industry get the idea? Hmm... how many ham operators have put 1000 or even 1200V on an 807 for SSB? Your goal, Mr. CB linear designer, is to wring every last penny's worth of power out of as few amplifying devices as possible! -we must remain competitive!

Improvements which can be made to the amplifers include biasing the tubes to decrease the amount of current consumed at idle. Although most of these amps do not energize the tubes until transmit, there is generally no bias and more than enough plate voltage to cause significant and unnecessary dissipation in the absence of driving signals. Cathode bias provided by a suitably bypassed zener diode is one way to help this. Another way, if the amplifier is a mobile type, is to return the cathode circuit to the 12VDC power source. The zener method is preferrable if the voltage supplied to the amplifier has good regulation (sufficient wire sizes, low voltage drop from the battery to the amplifier, and a power supply with good regulation) because the plate voltage will not change much. If the power supply has poor regulation, using the DC input source (12VDC) for cathode bias can be a good choice because it will change in relation to the change in plate voltage from the inverter. If the 12VDC source is too high, that is, cuts off the tube current too much, a resistive voltage divider can also be used to provide a lower voltage tap-off for bias. The divider should be configured to consume 5-10 times the amount of total cathode current in order to keep good regulation. If the amplifier uses two or four output tubes with 6.3V heaters, the experimenter can try the center point of the heater string to get 6V.

Speaking of heater voltage, it is usually poorly regulated because it is taken from the vehicle electrical system which can vary from 10-15V. This is no good for the tubes! The manuals state that heater voltage should be held to within +/-5% of the rated voltage. So the 6.3V tubes (6JU6, 6JE6, etc) need 6 to 6.6 volts and the 12.6V tubes (12JU6, 12JG6, etc) need 12 to 13.2 volts. What about the 8950? -better go look that "industrial type number" up, and see what voltage it takes. To regulate the voltage, you could maybe put together a self-excited buck converter (switching power supply) to turn the 10-14V into 6.3V (this scheme does not work well for 12.6V tubes unless the vehicle voltage is above 14V). Ok, so regulating the heater voltage is severe overkill for most people but it had to be mentioned.

Adding an attenuator to the RF input stage to avoid overdriving the unit is a good idea. In many cases, these amplifiers were designed to put out 80 to 100% of their rated power when subjected to the 4-watt carrier of an AM transmitter. They are too sensitive, and when subjected to 12 watts PEP, they are well into distorition. A 6dB pad will not only serve this purpose, but will also provide a better match from the exciter to the amplifier and a more linear operation of the exciter. Since the input impedance of these amplifiers is usually not near 50 ohms, a good circuit to try first would be to place a 100 ohm 2-watt resistor across the amplifier input side of the T/R relay. The resistor can be placed from the relay contact to RF input ground. Then break the line from the relay to the amplifier input, and insert a 50 ohm 2-watt resistor. Drive the amplifier and check the SWR between the exciter and amplifier input. It should be around 1:1.5 or better. If not, try adding a 50 ohm 2 watt resistor right across the amplifier stage input. This could make it better or worse, but you get the idea.. some experimentation is necessary. It would not hurt to add a low-Q tuned circuit to the amplifier's input circuit. Using an impedance analyzer like the MFJ probably won't work because the amplifier won't be drawing enough grid current with such a small input signal.

Fans can be added, but be aware that they can generate RF noise which may show up in the receiver. If they are well bypassed or filtered, then this is not generally an issue.

Once the amplifier has been modified for clean operation, AM operation is much harder on these units since the amplifier's efficiency at carrier level is only 33% maximum. (Previous to modification, carrier often meant nearly full power and better efficiency -but there was nowhere to go from there with the modulation except downward.. That guy be back-modulatin') This means that the maximum amount of power under carrier conditions one should try to obtain is roughly equal to half the total dissipation of the output tube(s).

Equipment Pages:

  • Vacuum Tube mobile linear amplifiers
  • Vacuum Tube base station linear amplifiers
  • Transistor linear amplifiers

    Want to help?

    If you have an old tube type mobile Linear laying around, I'd like to have it for my research and collection. Would you consider donating it to the cause? If not, how about some good pictures and the kind of technical info we show in the pages here?

    Does not matter if it works or not: weak, dead, or even missing tubes are fine. These things aren't worth much anymore, why not? At least you will save it from the scrapper! Please contact me below. Note I am not in the business of these, nor interested in using them, only doing research into an interesting part of our American past.