can a amp be underdriven ?

[dudmuck]

It is important to make the distinction between AB biased vs. class C.

In a class C amplifier, the only thing turning on the pills is the RF input. The base of the transistors (pill finals) have a DC path directly to ground. Starting from normal RF drive, reducing the drive power will eventually cause gain to drop, often this can be abrupt, and then gain will be zero where the pills are completely turned off, even though there is RF drive, its not enough to turn the pills on. In this area where the pills are shutting off, the impedance is changing drastically, and the possibility for spurs or oscillation occurs. However, its my opinion if an amplifier goes spurious here, its a fault design and needs negative feedback or something.

Class AB or A is a completely different situation. A bias current, or voltage for mosfets, keeps the pills turned on even with no RF drive. With such an amplifier, if you key it up with no RF drive, you'll have a current draw. This should maintain the gain even at low RF drive levels. This is needed for clean SSB. When you talk far from your microphone on your SSB transmitter, you are underdriving your amplifer and there is nothing harmful about that.

 

[BOOTY MONSTER]

so it wont be any problem with a class a or ab amp but a class c could have issues . did i read that correctly ?

dudmuck , that also makes a lot of sense to me about why class c amps shouldnt be used for ssb .

 

[Switch Kit]

the plot thick'ins good post dudmuck (got'a love that screen name) although it sounds like you might have something there Booty ,? but SSB tends to work OK to a degree on class C as long as theres a delay there . Only thing I was ever concerned about was not giving any given amplifier enough amperage , now I know that will blow a few transistors fairly easy. )-:

 

[cbradiomagazine.com]

For most practical purposes and in most situations there really is no such thing as under driving a pill amp unless there isn't enough input to get it to key or the relays are chattering.

A very very low level input on a high output amplifier may sound muffled but with the way amps are made these days I doubt you'd damage an amp.

In most cases if you can get a large amp - such as a 1600 to key, (even with the minimum input require to get it to key) the output of the amp is enough that it wouldn't be falling into those oscillations.

But the real question is "why would you under drive your amp in the first place"?

 

[W5LZ]

I think that "why would you under drive your amp in the first place" is sort of a 'cover all the bases' thingy. It's also one of the 'draw backs' for any amplifier that's "hot keyed", or turned on by the RF input.
- 'Doc

 

[mackmobile43]

I think that "why would you under drive your amp in the first place" is sort of a 'cover all the bases' thingy. It's also one of the 'draw backs' for any amplifier that's "hot keyed", or turned on by the RF input.
- 'Doc

I resemble that remark

 

[BOOTY MONSTER]

im thinking about getting a AB biased 4x2879 or 4x1446 amp next month but i noticed that sometimes you can find a used 1600dx for $350-$400 . my vehicle couldnt support an 8 pill running full throttle without some upgrades that im not willing to do . i was thinking if i got a 1600 and under ran it that it would last a long long time and be super clean . also if my next vehicle could support the 1600 properly then i could open it up and maybe use my ab filtered 225 to drive it with . not necessarally a plan.....but an idea .

 

[W5LZ]

Not talking about 'driving' it, but 'powering' the thing, introduces a whole new set of "Ifs" and "Buts". Finding that 'under powered' 'sweet spot' can be much harder than you might think. Personally, I think that a 'smaller' amplifier used correctly would satisfy you more, maybe. If that 'big' amplifier was such a good 'bargain', I would tend to wonder why. Maybe the seller didn't wanna go to the trouble of powering it, sort of like you don't want to? One of those 'depends' thingys. The trick there is sorting out the pertinent ones from the 'adult diaper' ones...
- 'Doc

(Hey! Nothing wrong with asking questions. Saves a lot of mistakes in the long run.)