Is a Cobra 150 amplifier a Class C or Class AB1?

[doffo]

Just ran into the pictures of when I had my Cobra 150 amp opened up sometime either last year or the year before... But Though the board read A1, Is the circuitry there for it to be a true Class AB1?

 

[9C1Driver]

Just AB. The terms AB1 and AB2 in amplifiers are used for tube amplifiers. Has something to do with grid current draw. Can’t remember now.

 

[doffo]

Just AB. The terms AB1 and AB2 in amplifiers are used for tube amplifiers. Has something to do with grid current draw. Can’t remember now.

Ohhh...... For whatever reason that amp sounded the same on air as the RM Italy 203P.... I was under the impression that Cobra 150 is not a true Class AB amp... Maybe I am wrong... :/

 

[9C1Driver]

A sharper person then I can chime in here and let us know. That white square part to the left of the finals that says PCD might be some kind of a bias part. Pics of class B X-Force and Carl built have a similar component. You have a nice little amp there.

 

[SuperLid]

Ground the emitter of the keying transistor, which will key the amp. Do this with NO RF input. Then measure, the DC voltage from the RF transistors bases, to ground. If there is AB bias, you should see .6-.7 volts on the bases.

 

[nomadradio]

The amplifier is definitely wired to place operating bias on the RF transtors. The question is whether or not it works properly. If the sideband audio sounds worse with the mike gain turned way down than it does turned up, you need more idle current on the transistors. This amplifier has fixed resistors that were chosen at the factory to set proper bias current for the RF transistors that were installed when it was made. Other designs that use a trimmer pot to adjust the idle-current bias can compensate for aging or for a change of RF transistors.

The white 5-Watt power resistor marked PCD serves to drop the 13.8 Volt power supply voltage down to around one Volt. The two smaller resistors next to it divide this voltage down to roughly 6/10 of a Volt to feed the input transformer, where it passes through to the base terminal of each RF transistor.

Would be nice to have a simple way to measure the DC current the RF transistors draw with the relay keyed and no RF drive. They didn't provide a collector-current test jumper like the "mirror" board in a Galaxy SSB radio. See if the advice about checking base voltage on the RF transistors provides a reading. If not, something's broke.

73

 

[doffo]

Thank you all for providing great information. My mind recalled when you mentioned about turning down the mic gain and if the SSB audio gets worse, then the idle voltage isnt right. Maybe some day once I crack the amp back open I will just have to find a spot to get the relay to click over and test the voltage to see if it gets .06v - .07v. I think at the time I paid $50 for the Cobra 150 off Facebook marketplace around 2021.