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donkeystomper amps
- Thread starter roger460xvr
- Start date
Those 10 ohm resistors from base to ground are there on a biased amp also. Texas Star has them too.
The chokes from the input transformer to ground are what ground the bases and make it class C. It's most likely C unless someone took the time to put the bias circuit under the board.
The one thing that's not visible in the pics is a method of activating bias current only when the amplifier is keyed. You can't leave bias current running in standby mode, so there has to be a way to apply bias current only while it's keyed. More than one way to do this, but none are visible.
Looks like a product sold by someone who has no idea what "Class AB" means.
Except that "they want to hear this in the description".
73
Looks like a product sold by someone who has no idea what "Class AB" means.
Except that "they want to hear this in the description".
73
The resistor IS the biasing resistor. It connects to the base of the transistor and pulls the base of that transistor downwards and sets the operating DC bias point below the cut-off point in the DC load line so the collector flows less than a 180 degrees (half cycle) of the AC signal. CLASS C. The inductor and capacitor form a tuned circuit (tank circuit) and is used for either generating signals at a particular frequency or picking out a signal at a particular frequency. The transistor produces a series of current pulses according to the input which flows thru that resonant circuit.
Look up Class C biasing and you will see that a resistor is used on the base of the transistor to ground. In a class AB amp the base is lifted off of ground and supplied with enough voltage of 0.7V so the transistor is turned on. So ask yourself how can you have a class AB biased amp that has the base of the transistors grounded out??
The resistor is a current sink, leading back to the emitter potential.
Forward bias for class AB must come from a current source. None visible.
73
Forward bias for class AB must come from a current source. None visible.
73
I do have amps with .7 volts on the base with these 10 ohm resistors in place.
Those resistors are10 ohms. Is that going to prevent you from putting .6 to .7 volts on the base? No. Try ohms law and you will find that 10 ohms across .7 volts is going to have about the same effect as 2.8k ohms across a 12 volt power supply. The resistor would dissipate 50 mW or less in either case. Will that short out the p/s? No. It wouldn't even burn an 1/8 watt resistor.
What makes it class C is the red wires wrapped 3 times thru the ferrite on each input transformer that are soldered to ground.
If what you say is true then please explain why these 10 ohm resistors are left in place when a bias circuit is added and why other biased amplifiers use them. Why do the bias circuits I have added to these comp amps work with these resistors in place?
I could not see the solder connecting those inductors, chokes or whatever you want to call them to ground. What I have done is drill the board and put the bias circuit under it. The amp still looks class C but you have to look closely. The inductor slips through the board where you would expect it to be soldered. Not saying that was done here. I would bet it is class C
You don't have to believe me but please take a close look at a Texas Star. You will find these same 10 ohm resistors.
Those dual 1/2 w resistors you are talking about are shunt resistors between the base emitter junction.
Yes. Which ones were you talking about that make it class C?
These are the 10 ohms I was referring to.
So whats the best way to bias the transistors in a amp, the diode and resistor method. Or the voltage regulator IC with a pass transistor?
Jay in the Great Mojave Desert
Jay in the Great Mojave Desert
Regulated voltage with a diode in contact with the transistor to track heat is better than the Diode/ Resistor set up like is used in Texas Star.
I agree with 453.
That is a class C amp.
Another Comp box.
73
Jeff
I agree with 453.
That is a class C amp.
Another Comp box.
73
Jeff
There's no returns. But if I'm correct and an eBay item is grossly misrepresented in its description doesn't that mean the buyer has a legitimate reason to request a return/refund?
If the seller doesn't agree then eBay should and most likely will side with the buyer. And inevitably refund the buyer for the full purchase price. If I bought an amp based on the description that the amp was AB and finding out it was class C after receiving it i definitely would not want to keep it, and would want to return it. There is a large difference between C and AB. I'd consider that a gross misrepresentation of the amplifiers description. If I'm buying the amplifier based on it's advertised AB bias and it turns out to be C I would fell like I got robed! In my opinion advertising class AB but selling C is not a small discrepancy. But a large chasm. Does anyone agree?
All the amp guys selling on eBay say "no returns due to part swappers but check my feedback it will not be DOA". If they sell you a dud don't be surprised because you have been warned. Caveat Emptor. Of course you could bring a claim for a refund and raise hell and maybe it would work; maybe not. It's a risk no matter how you slice it. Buying direct from the builder has worked best for me as they almost always accept returns.
Are all transistorized amps class C? I thought that transistorized amp can be configured AB or AB1 or can be modified from one class to another?