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Alan K1313

redlght

Active Member
Feb 1, 2013
131
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38
In this amp there are 2 resistors R2 and R3 (maybe a voltage divider for the Hi Lo Sw3 whick looks like Hi Lo input adjust?) the schematic shows the values of 68 Ohms each but in the amp it has R2 15 Ohms and R3 47 Ohms. Was this done to change the input range? What would two 50 ohms do there seeing i have those in stock My calculation would be the same as 2 68 Ohm ones, yes?
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Those resistors appear to do as you say, to limit drive to the tube. The values could be changed depending on the available drive from the exciter if it is fixed and cannot be adjusted. If you have an exciter with an adjustable RF drive output you can eliminate those resistors all together and simply adjust the drive level accordingly. Whoever changed the values did so to accommodate whatever fixed value of drive his exciter had to satisfy the tubes needs.
 
I changed it to 2 50 ohms works fine, I did find a schematic of another it seems to have an extra circuit in it using IC2 a pic and 4Mhz xtal some kind of clock timing function?
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IC2 is a fig leaf covering the private parts, so to speak.

It's supposed to be a frequency-counter sort of function that disables the keying circuit outside the legal ham frequencies. The 4 Mhz resonator is the time base for the PIC microcontroller's clock. In reality, trying to count the carrier frequency of an AM radio with a lot of modulation is a waste of time.

Unhooking TR2 will prevent IC2 from disabling the keying circuit.

This circuit was a lame effort to make this a legal "ham" amplifier by disabling it from amplifying CB frequencies. It still fails to meet other FCC rules about what is or is not a legal ham linear.

A single tube like this can only take about half the drive power of a normal legal 40-channel radio. The two attenuator resistors should absorb about half the radio's drive power if you use a single-final radio.

And a dual-final "10-meter" (smirk) radio will overdrive a single tube even with those resistors.

Then again, if you have a 1965 tube-type CB that busts a gut to deliver a 2.5-Watt carrier and 9 Watt peaks, you could probably skip the resistors. The transmit power of most legal 40-channel radios will be about twice that large.

It never hurts to place a SWR meter and coax jumper BETWEEN the radio and the amplifier's input socket. If your resistor values are too far out of whack, the SWR the radio feeds into when the amplifier is keyed could be too high. Anything under two to one should not be a hazard to the radio.

73
 

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