Hi,
Usually I'm bothered when I see a transistor amplifier without an emitter resistor. The gain should be set by the ratio of the collector resistor to the emitter resistor and be set significantly below the gain of the transistor. This is so things will be stable and you can do large production runs of products and not have to stop production to determine how to fix the product (or add adjustable pots that will cost labor time to adjust) every other time you get parts from a different supplier or batch. The other important thing the emitter resistor provides in stability of the bias point of the transistor. As the emitter provides a negative feedback based on how much current flowing through the collector. This works to keep the voltage around 1/2 the Vcc with wide variations in the gain of the transistors.
In the case of Q70 the next stop for the audio is a volume control, so if the gain is higher or lower in various batches, so be it. Likely nobody will notice. In the case of the "wide open" Q70 it is important to note that the bias resistor is connected from the collector to the base. That is an alternative form of negative feedback for keeping the bias level from going out of bounds.
If a batch of transistors with higher gain were used, the gain of the transistor would tend to want to conduct more and drive the collector voltage down. Maybe so close to ground, the amplifier would not work. But, the collector voltage goes down, also reducing the bias current. That limits how far the collector voltage can go off from the optimal middle point. Low gain transistors would be the same idea but the reverse would happen, keeping the collector voltage from going too high from center. The impedance of the audio feeding Q70 from the mixer is low enough that the 100K bias resistor should not provide much negative feedback at audio frequencies and lower the audio gain.
It is surprising that higher gain transistors did not make a noticeable difference. Although they would need to be more than twice the gain to be noticeable to the ear, per common wisdom. Maybe changing Q70 to a darlington and adding an emitter resistor to tame it down some might work.?
Another thing that has been discovered by folks that have had low transmit power, or trying to boost it further, is the supply voltage (especially below 12/13 volts) has a major effect on the obtainable transmitter power. A large contributor to that seems to be that the gain of the IF chain drops significantly below these voltages. As the same IF chain is used for transmit and receive, running those circuits at 9 volts may be a problem. Your measurements on your radio will let you know if this is an issue.
Tom, wb6b
