The tuned filter input will not be a significant benefit to you for the cost and space involved. Most modern transceivers provide a signal clean of harmonics, so there will be nothing to filter out on your amplifier's input. Since the amplifier is "broadband," it IS prone to generating/amplifying harmonics and does need an output filter.
Originally Posted by KB3HLK
A low pass filter on the amplifier output will minimize any signal beyond the filter cutoff frequency from reaching your antenna. Lets say you select a single low pass filter with a 30 MHz cutoff. That means that if you are transmitting on 40M (7MHz) that you may also be passing a signal of twice that (14MHz), three times that (21MHz) and four times that (28MHz). So what good did your low pass filter do for you if it allowed you to transmit on four amateur bands at the same time?
You will need low pass or band pass filters designed for each band of interest.
Bias is not a specific voltage, rather one that is adjusted for the best linearity for your particular amplifier. It will be somewhere in the range that you mentioned, about +0.6 to 1.0V DC. The difference lies in the variables of the components themselves. Each has a tolerance within a specification range, thus you will need to adjust the bias voltage for your particular amplifier's collection of components. I suggest an IC voltage regulator/ driver transistor combination mounted on the same heat sink to aid in preventing thermal runaway. Each 2SC2879 pair (modules) should have their own bias supply and adjustment, so you will need four supplies built into the amplifier housing. Start off with the bias set for 200MA of collector current for each pair of 2SC2879 transistors. Each module's linearity should be tested and bias set separate from the other modules. When all modules have their bias properly set, their RF inputs and RF outputs may be combined in the final configuration.
Eight 2SC2879's will provide about 800 watts output PEP with an intermodulation distortion of about -24dB. I would not run them any higher than that. Too much drive power will short the emitter base junction of the finals. Build a 3.5 to 4dB input attenuator in the amplifier box to prevent your 100 watt transceiver from over-driving the amplifier. 40 -45 watts of drive should do it. The input attenuator will also present a nice 50 ohm load for your transceiver.
If the amplifier dos not have a fan cooling the internal components and external heatsink, add one......or two.
This project will be a good use of parts from a former CB amplifier.
Last edited by K7FE; 05-08-2010 at 11:43 PM.
Terry Graves, K7FE
Chief Editor, QRZ.COM
"Some people call CW a MODE but in
reality it is an autonomous LANGUAGE."