-
You can now help support WorldwideDX when you shop on Amazon at no additional cost to you! Simply follow this Shop on Amazon link first and a portion of any purchase is sent to WorldwideDX to help with site costs.
low drive amps
- Thread starter ethomas
- Start date
the amount of drive that any given amplifier requires is based on the number and type of transistors used and the amount of padding provided at the various power levels. in the low power (high drive) position the 250 will take much more drive than it will in the (low drive) high power position.
Knowing which transistors your amp has is half the answer so please find out which ones you have.
What freecell was refering too as padding is a circuit which limits the amount of wattage that can be introduced into the amp via resistors usually placed inline with the input to the transistors.
If you look at the resistors at the high/medium/low switch if that's what you have you'll see resistors with colored bands on then and by using this resistor calculator you can determine the resistance and it tolerance which is read left to right and the last colored band is the tolerance and that last band indicates how many watts that resistor will allow through the circuit before it self-destructs and that should give you an indication of maximum drive input, so if it has a %5 tolerance resistor it most likely will stand 5 watts but I would only use half it's rated capacity, say 2.5 watts.
All that being said you still need to know which transistors you have and there input and output capabilities and you can use your inline watt meter to make a final determination as to how much drive you can inject at a given setting on your amp.
The schematics of DX250 is on cbtricks.
This is a two pill 2SC2290.
I only see variable on the DX250HDV version. Otherwise, it looks like a fixed attenuator for AM mode.
One thing to note is that R10/R8 is a voltage divider to provide bias.
Bias is unregulated and the (audio) bypass capacitor is purposefully missing across R8, which means on audio peaks the amp will become class C, meaning you can run more RF drive into it than otherwise possible.
This is a two pill 2SC2290.
I only see variable on the DX250HDV version. Otherwise, it looks like a fixed attenuator for AM mode.
One thing to note is that R10/R8 is a voltage divider to provide bias.
Bias is unregulated and the (audio) bypass capacitor is purposefully missing across R8, which means on audio peaks the amp will become class C, meaning you can run more RF drive into it than otherwise possible.
B
BOOTY MONSTER
Guest
PADDING . interesting.........
another example of padding is selling a amp with two or four 2879's (that can be driven to full clean output by most any tuned cb/export) but advertising/making it so it needs another amp to drive it .
"One thing to note is that R10/R8 is a voltage divider to provide bias."
yeah, forward bias. class c requires the devices to be reverse biased to below cutoff. the 250 doesn't move into class c with increased drive it moves into class ab2 from ab1. it's not a 1X2 and it isn't a 4X, it's 2X.
class c operation is achieved by reverse biasing the emitter-base junction, which sets the dc operating point below cutoff and allows only that portion of the input signal that overcomes the reverse bias to cause collector current flow. that creates distortion and destroys linearity when applied to the amplification of am/ssb signals. there is no reverse bias of the emitter-base junctions in the devices used in the dx250hdv. like all texas star amplifiers with the exception of the smaller models designated as modulators they are all passively (forward) biased class ab1 and as mentioned previously, when the drive requirements of the devices used are exceeded they migrate towards class ab2.
a common-emitter transistor circuit with no forward bias has no emitter-collector current and is therefore in class c. any rc emitter-stabilization circuit produces reverse bias for the base when a signal is applied and emitter current begins to flow through it. this places the stage in class c operation when driven. care must however be taken not to overdrive the base-emitter circuit.
Amplifier classes of operation
http://www.firecommunications.com/trdbase/2sc2290.pdf
"so on low power you can drive more in to it than on high power?"
with the #3 (green) button pressed in (high drive) the variable controls the drive input, when out full drive (no padding) is delivered to the bases of the transistors. (low drive)
linearity begins to suffer when 2X2SC2290's exceed 150WPEP output when referenced against the -24db. mil spec for IMD.
yeah, forward bias. class c requires the devices to be reverse biased to below cutoff. the 250 doesn't move into class c with increased drive it moves into class ab2 from ab1. it's not a 1X2 and it isn't a 4X, it's 2X.
class c operation is achieved by reverse biasing the emitter-base junction, which sets the dc operating point below cutoff and allows only that portion of the input signal that overcomes the reverse bias to cause collector current flow. that creates distortion and destroys linearity when applied to the amplification of am/ssb signals. there is no reverse bias of the emitter-base junctions in the devices used in the dx250hdv. like all texas star amplifiers with the exception of the smaller models designated as modulators they are all passively (forward) biased class ab1 and as mentioned previously, when the drive requirements of the devices used are exceeded they migrate towards class ab2.
a common-emitter transistor circuit with no forward bias has no emitter-collector current and is therefore in class c. any rc emitter-stabilization circuit produces reverse bias for the base when a signal is applied and emitter current begins to flow through it. this places the stage in class c operation when driven. care must however be taken not to overdrive the base-emitter circuit.
Amplifier classes of operation
http://www.firecommunications.com/trdbase/2sc2290.pdf
"so on low power you can drive more in to it than on high power?"
with the #3 (green) button pressed in (high drive) the variable controls the drive input, when out full drive (no padding) is delivered to the bases of the transistors. (low drive)
linearity begins to suffer when 2X2SC2290's exceed 150WPEP output when referenced against the -24db. mil spec for IMD.
Last edited:
I think you are correct, for lower drive levels.
But W8JI thinks the amp will drop into class C at 10 watts drive, and I have alot of respect for what he says.
http://forums.qrz.com/showthread.php?t=188396&page=7
"Transistors are current operating devices. At ANY drive level they source current back INTO the bias. The polarity is negative with respect to the emitter in an NPN, so with ANY amount of drive they try to pull the bias negative."
If you feel he is wrong, then by all means go ahead and reply.
W8JI wrote:
"At virtually any drive level it will START TO REDUCE the forward bias."
nothing stated by him contradicts anything i posted. furthermore, my statement is correct when the manufacturers drive specifications are adhered to with regard to the type and number of devices in question when used in an amplifier with a properly designed bias circuit. in addition, nowhere in his post does he imply that the devices in question drop into class c operation, (but he does allude to this on page 6 and his statement is an exaggeration at best) he merely states the the forward bias is reduced, (not reversed) which is a given with a poorly designed and regulated bias control circuit. big difference when the idle bias is already running a couple of hundred ma. per device. the bias circuit in the TS amplifiers is cheesy, he's right about that. if you're inputting so much drive into these devices as to actually reverse bias the base-emitter junctions to below cutoff then you need to have your head examined and/or redesign the bias circuit for stiffer current regulation in the event that you can't read and understand the manufacturers maximum device ratings. some of us know what those are and most of us don't.
linearity begins to suffer when 2X2SC2290's exceed 150WPEP output when referenced against the -24db. mil spec for IMD.
"At virtually any drive level it will START TO REDUCE the forward bias."
nothing stated by him contradicts anything i posted. furthermore, my statement is correct when the manufacturers drive specifications are adhered to with regard to the type and number of devices in question when used in an amplifier with a properly designed bias circuit. in addition, nowhere in his post does he imply that the devices in question drop into class c operation, (but he does allude to this on page 6 and his statement is an exaggeration at best) he merely states the the forward bias is reduced, (not reversed) which is a given with a poorly designed and regulated bias control circuit. big difference when the idle bias is already running a couple of hundred ma. per device. the bias circuit in the TS amplifiers is cheesy, he's right about that. if you're inputting so much drive into these devices as to actually reverse bias the base-emitter junctions to below cutoff then you need to have your head examined and/or redesign the bias circuit for stiffer current regulation in the event that you can't read and understand the manufacturers maximum device ratings. some of us know what those are and most of us don't.
linearity begins to suffer when 2X2SC2290's exceed 150WPEP output when referenced against the -24db. mil spec for IMD.
Last edited:
No, the 667 must have a low dead key input of say 1.5-2 watts and maybe 20 ish on the peak end. The TS250 and 350 also don't need much more then 2 watts. A 4 watt dead key would be too much for any of them. Just what I have discovered from first hand use.