Ssb audio distorted when using amplifier

[Moleculo]

I did a google search on your zetig amplifier, it has a SSB delay so it will work on SSB.

Common CB myth: "my amp has a SSB delay, so it will work on SSB". No. The SSB delay on most of these amps is simply a capacitor that is switched in with the relay that prevents the relay from chattering because there is no carrier to keep the relay engaged between voice peaks. That alone does not make it suitable for SSB. For SSB, it's important that linearity is maintained which is nearly impossible with a class c amplfier inline.

I compared the schematic with one I found on the web for class c mosfet rf amplifier and the relevant sections look pretty much the same. I also looked at a schematic of an AB biased mosfet amp. Please understand that I'm not very versed with MOSFET PA design so I could be very wrong, but I can't see how that amp you have is much different than the class C design I found. It is possible that you have other issues like drawing too more current from a battery than it is capable of, but I doubt it. FM and AM modes typically draw more current than SSB does and you're not having problems with anything other than SSB. Everything you're saying points to a non-linear amplifier which should not be used on SSB. Sorry to be the bearer of bad news.

 

[Damo666]

Your comments are very much appreciated. I will buy a new power pack and try that route. Failing that, it's Zetagi falsely advertising their products for use on ssb. Have a nice weekend guys, and thanks for clearing that one up for me.

 

[bob85]

some of the italian mosfet amps do sound like class c switching distortion on ssb, they have positive bias but not enough to sound decent,
looking at the b47 schematic a 13.8v supply would put just over 1.7v on the mofet gates through the 33k and 4.7k resistive divider,
from my experiments with cte mosfet amps using the same fets that is not enough bias voltage to get rid of the distortion.

 

[Damo666]

some of the italian mosfet amps do sound like class c switching distortion on ssb, they have positive bias but not enough to sound decent,
looking at the b47 schematic a 13.8v supply would put just over 1.7v on the mofet gates through the 33k and 4.7k resistive divider,
from my experiments with cte mosfet amps using the same fets that is not enough bias voltage to get rid of the distortion.

that makes sense. What's more, a lead acid battery is only slightly over 12v so the bias will be even less. I've done a bit of searching on the net, and the schematic does seem very similar to a class c but with an added resistor. I might try adding a variable resistor in place of the gate resistor R4 and see how we go. Thanks for the great input guys. It's really very much appreciated.

 

[Moleculo]

I knew if Bob85 came along he would know the answer to this problem. Those Italian amps with the FETs are a lot more common on that side of the pond than they are here. I need to learn more about FET design.

Bob, you should do a project build, photo it, post it up and play teacher

 

[hotrod]

Is this a base or mobile? How are both units being powered? Running both on too small a power supply or the amp on small feed wire and a week battery can cause what you are seeing.

yea i agree with this.id buy a 7-10amp for the radio and leave amp on battery .
and also how good is that battery.i tried same thing and ive found that using
a battery charger is a must even with a new battery.i was told i was worbling
sopon as i turned on batt. charger i was clean as a bell.i may have missed it
but what size of wires ya using? i know people that have used these amps with
good results

 

[bob85]

mole i dont know enough about them to show somebody how to build one, i am familiar with how some of them sound on ssb.