What are you going to dive it with? hopefully not a cb on 27 mhz!

The amp is actually built as well as most ham sets of the day including a bias supply and negative feed back making for a fairly clean imd as log as it's not over driven!

It does need a harmonic filter though.

Drive on that 2 stage amp is only a few watts pep for max clean output.

I honestly don't know where Hams come up with the stuff they do.

Here is a schematic:

http://www.cbtricks.com/Amp/txstar/dx667_dx667v/graphics/ts_dx667v_sch.pdfYou'll see that is the most half-arsed attempt at a bias circuit anyone would have.

First, the driver stage and PA stage share common bias on half of the output side, this puts (2) 10 ohm, one 2.7 ohm, and a 6.8 in parallel sources from a 25 ohm series to Vcc. That means we have a 1.4 ohm driven from a 25 ohms, so bias at zero mA base current (no drive) is 5.3% of supply, or .68 volts with 13 volts supply on one output side. This is UNregulated.

It has .5 amps quiescent current in the bias, so every 100 mA of base current from the driver and PA base currents summed will push the bias about .15 volts lower. The transistors are already just at or in cutoff with no drive, so with any base current they move to class C.

The other side of the PA has (2) ten ohm in parallel with 1.8 ohms, for a lower leg resistance of 1.32 ohms. That side has 5% of applied voltage for bias, with zero base current. With no drive it has .65 volts which will put the transistors in cutoff.

That side does not sum driver base current (from RF drive) into the load on the bias, but will still roll off bias at about .15 volts per 100 mA of base current. So one side starts barely into class C, and by the time 150-200 watts is reached will be pretty deep into class C.

Apparently some of us thing transistors do not have base current, and that an unregulated bias source that is already just at or below cutoff is a good bias system.

That design clearly is to give the amp "RF AGC", which is typical for CB amplifiers. The more drive is applied, the deeper into class C the amp moves. This gives audio RF clipping, makes the amp less sensitive to drive power changes (because it is non-linear with built in automatic gain compression), and can make meters dance around on AM which puts CB'ers over the moon with joy.

A real bias system would have up around .75 volts bias, and be very stiff with varying base current. This system is clearly is just CB junk.

73 Tom

while the other half runs with more bias voltage!!!