while i am tuning my amp i run out of load adjustment. what does this mean? i feel the amp can do more about the load knob will not allow it.
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running out of load adjustment
- Thread starter NEONNIGHT34609
- Start date
To start, I'll guess that your Load control has an end stop. That is, you can turn the knob only a half turn end-to-end.
First thing to check is which direction has it "bottomed out".
With the plates fully meshed together all the way?
Or with the plates fully apart as far as they'll go?
All amplifiers are not created equal. A small amplifier with two color-TV "sweep" tubes tends to behave a bit differently from one with six of those tubes tied together.
You may hear the advice to adjust the coil. There is a wire coil mounted between the Tune and Load controls. It works together with the two variable capacitors to step down the high-voltage low-current RF energy from the tubes to a low-voltage high-current impedance that matches the coax and antenna.
But it also must tune to the frequency where you're operating.
Some small linears were built with a chintzy Load control that should have had more plates on it. The builder saved money by cutting that corner and using one that's almost big enough.
But that's not always the problem.
Most important is to make sure the "Tune " control is not running to one end of its range.
If the Tune control has no end stop it will spin 360 degrees. It should show a peak on the wattmeter somewhere between the two extremes of adjustment. Because there is no hard end stop, you may get to one extreme of its range and see what looks like a "peak" on the meter, but really is NOT.
Any control that has no end stop must show * TWO * peaks in one whole 360-degree turn of the knob.
Those two peaks might be close together, might be 180 apart. That's not what matters. If you see only ONE peak, this means that this control has ALSO bottomed out.
If I just play the percentages here, I would guess that the Load control has gone to full mesh, with the plates fully together when it reaches the end stop.
If so, placing a fixed capacitor in parallel with it will have the effect of adding plates to the control, giving it more adjustment range.
Once you have enough added capacitance wired in parallel with the Load control, you'll see that the Tune control now shows a peak with the plates as far apart as they can go.
That tells you the coil needs to be adjusted. In * THIS * case (not all cases) you would spread the turns of that coil farther apart and try again until the Tune control now shows a proper * TWO * peaks in one full turn of the knob.
BUT THIS IS JUST A GUESS ! !
Sight unseen, your box may have been built by someone in his garage who never got it adjusted right from day one.
Or, it could be a factory-built unit that has been tweaked into the twilight zone by three tweaknicians in a row.
But hey, pay attention to which direction the Load control has "bottomed".
And see if the Tune control really has two separate peaks.
73
First thing to check is which direction has it "bottomed out".
With the plates fully meshed together all the way?
Or with the plates fully apart as far as they'll go?
All amplifiers are not created equal. A small amplifier with two color-TV "sweep" tubes tends to behave a bit differently from one with six of those tubes tied together.
You may hear the advice to adjust the coil. There is a wire coil mounted between the Tune and Load controls. It works together with the two variable capacitors to step down the high-voltage low-current RF energy from the tubes to a low-voltage high-current impedance that matches the coax and antenna.
But it also must tune to the frequency where you're operating.
Some small linears were built with a chintzy Load control that should have had more plates on it. The builder saved money by cutting that corner and using one that's almost big enough.
But that's not always the problem.
Most important is to make sure the "Tune " control is not running to one end of its range.
If the Tune control has no end stop it will spin 360 degrees. It should show a peak on the wattmeter somewhere between the two extremes of adjustment. Because there is no hard end stop, you may get to one extreme of its range and see what looks like a "peak" on the meter, but really is NOT.
Any control that has no end stop must show * TWO * peaks in one whole 360-degree turn of the knob.
Those two peaks might be close together, might be 180 apart. That's not what matters. If you see only ONE peak, this means that this control has ALSO bottomed out.
If I just play the percentages here, I would guess that the Load control has gone to full mesh, with the plates fully together when it reaches the end stop.
If so, placing a fixed capacitor in parallel with it will have the effect of adding plates to the control, giving it more adjustment range.
Once you have enough added capacitance wired in parallel with the Load control, you'll see that the Tune control now shows a peak with the plates as far apart as they can go.
That tells you the coil needs to be adjusted. In * THIS * case (not all cases) you would spread the turns of that coil farther apart and try again until the Tune control now shows a proper * TWO * peaks in one full turn of the knob.
BUT THIS IS JUST A GUESS ! !
Sight unseen, your box may have been built by someone in his garage who never got it adjusted right from day one.
Or, it could be a factory-built unit that has been tweaked into the twilight zone by three tweaknicians in a row.
But hey, pay attention to which direction the Load control has "bottomed".
And see if the Tune control really has two separate peaks.
73
nomadradio thank you very much for making this so clear and understandable. i will check a few things and reply back. you are correct my tune spins 360 but my load has an end stop. the amp is a Super Scout that has had some work done to it it looks clean but i do understand that doesn't mean it was done correctly
What amplifier? What band? A little info would certainly help!!
An Ameritron Al-811 on 80? Not uncommon. Just need more padding capacitor.
An Ameritron Al-811 on 80? Not uncommon. Just need more padding capacitor.
Uhh, pretty sure the Super Scout was four sweep tubes driving eight. Meant for 11 meters.
Placing eight tubes in parallel presents a challenge to the design of the output circuit. There is a functional or "parasitic" capacitor to ground inside each tube from the plate cap to ground. The tube's internal structure is the root of this. With the tubes' plate caps wired in parallel, the capacitance of the eight tubes adds up. To make the output coil resonate at 27 MHz, it has to be cut down so small that it can be hard to get a proper impedance match and couple the output power to the antenna efficiently.
If the Load control goes all the way to full mesh without showing a peak, I would put a 100 pf capacitor in parallel with it. It will have to be a stout specimen with plenty of metal in it. A ceramic capacitor won't have enough metal for the job unless it has a really high voltage rating.
One other way to improve the current rating of a capacitor is to use two, three or more in parallel. Silver-mica capacitors tend to handle high RF current better than ceramic disc types, but only if they are big enough.
Old military-surplus "transmitting mica" capacitors with a threaded brass insert at each end are meant for high current if you can find one.
100 pf might or might not be enough to do the job, but it will at least help. And if it moves the peak setting of the Load control back between the end stops, that's what matters.
I would be worrying more about the plate tune. If it doesn't have the "Twin Peaks" this can be hard on the tubes.
73
Placing eight tubes in parallel presents a challenge to the design of the output circuit. There is a functional or "parasitic" capacitor to ground inside each tube from the plate cap to ground. The tube's internal structure is the root of this. With the tubes' plate caps wired in parallel, the capacitance of the eight tubes adds up. To make the output coil resonate at 27 MHz, it has to be cut down so small that it can be hard to get a proper impedance match and couple the output power to the antenna efficiently.
If the Load control goes all the way to full mesh without showing a peak, I would put a 100 pf capacitor in parallel with it. It will have to be a stout specimen with plenty of metal in it. A ceramic capacitor won't have enough metal for the job unless it has a really high voltage rating.
One other way to improve the current rating of a capacitor is to use two, three or more in parallel. Silver-mica capacitors tend to handle high RF current better than ceramic disc types, but only if they are big enough.
Old military-surplus "transmitting mica" capacitors with a threaded brass insert at each end are meant for high current if you can find one.
100 pf might or might not be enough to do the job, but it will at least help. And if it moves the peak setting of the Load control back between the end stops, that's what matters.
I would be worrying more about the plate tune. If it doesn't have the "Twin Peaks" this can be hard on the tubes.
73