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AL-1500 conductor heat problems
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by KB9WIS on February 15, 2006
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Hey one quick question.. I got a chance to get my hands on an Ameritron AL-1500, that was converted to have 2 X 8877s instead of just one.. Due to the second tube adding capacitance, the ham that did this conversion retapped the band switch to different points on the coil.. I'm assuming it was done to add inductance, to help cancel out the added capacitance of the second tube.. Again, this is my assumption.. The AL-1500 looks like it also has some doorknob capacitors added to the tank either in series or in parallel to tank, and I would think that the easiest way to correctly readjust the tank for all bands, and keep its current "Q", would to simply reduce the capacitance of these doorknob capacitors, inversely to what was added via the second tube..
The problem I am having, is than on 10 through 20 meters, with only about 1KW of output, the conductors are getting very hot, so hot in fact, that the actual solder is melting off where they connect to the bandswitch, to the extent, that they actually just fall off the band switch due to the solder melting away.. They are the 3/8" flat copper strap "solid" type, so I would think the conductor size is acceptable..
I found this strange, as just a "stock" AL-1500 will easily have a 1,500 watt "output" easily, without this problem..
I am not sure what is causing this "beefed" up 2nd 8877 tube version to cause these conductors and soldering joints to become so hot at even reduced output levels, but someone mentioned that the "Q" of the tank may be too high.., which I'm still unsure how that would affect the extra heat, as I am a novice to amp design/theory. There was some retapping done, and I'm wondering if this is indeed the culprit... Should I retap back to original specs, and perhaps reduce or increase some type of doorknob capacitor instead? I'm not really sure of how these doorknobs are added to the tank, but it looks like their is about two sets of them..
Another thing I noticed, is that the tune and load controls have "very" sharp tuning now, (which I imagine may also be due to a "sharp" or Hi-Q). The load control, doesn't seem to need to be readjusted, once drive is increased, but seems more frequency dependent, not drive dependent.. I'm not sure what cause this, but I feel more comfortable, if the load needed to be increased, as drive is increased, but doesn't seem to be the case with this amplifer.. Not sure if the retapping of coil did this as well..
The ham that did this conversion/these taps, said he used a noise bride to tune for resonance, and he intentional tapped it, so that most bands were tuned, when the actual variable capacitor of "tune" control of the amp itself was in the "40" meter (or about have its capacitance rating) setting.. His theory on this, was that the range.., and tuning would be better, if all bands worked, when the air variable "tune" capacitor, was in its middle (or 40 meter) setting.., due to being about middle of its capacitance range.
Not sure if his theory is correct or not, but I am having a hard time with even running legal limit with this amp on 10-20 meters, due to conductors and soldering connections on band switch getting very hot..! Perhaps the bandswitch isn't adequate, as its a stock Ameritron AL-1500 bandswitch.. However,the stock bandswitch is rated to handle per company specs "6KV @ 20amps".
Any thoughts/input would be appreciated,
Kb9wis
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RE: AL-1500 conductor heat problems
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by WB2WIK on February 15, 2006
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Will he take it back?
Sounds like a poorly done conversion, and there's no benefit to adding a second 8877 to this amp.
What was he trying to accomplish?
WB2WIK/6
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RE: AL-1500 conductor heat problems
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by KB9WIS on February 15, 2006
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Well,
He also added a much larger transformer, so should just loaf at legal limit..
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RE: AL-1500 conductor heat problems
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by WB2WIK on February 15, 2006
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It just loafed at legal limit the way it was.
Nothing is being taxed hard in the AL-1500.
This sounds like a conversion made for the sake of making it, without any achievable goal.
I'd probably convert it "back" to the way it was designed, if at all possible.
WB2WIK/6
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RE: AL-1500 conductor heat problems
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by WB2WIK on February 15, 2006
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There isn't any country that has a higher legal power limit in the amateur bands than America has, far as I know, although some places don't have the 200W restriction on 30m.
Converting an AL-1500 to a "two holer" when the first hole already had an 8877 in it sounds like a science experiment gone bad.
The AL-1500 can easily run 2000W output power and has to be "held back" to keep it legal, to begin with.
WB2WIK/6
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RE: AL-1500 conductor heat problems
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by N9NRW on February 15, 2006
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Hi Scott,
I half to say I agree with Steve on not being sure why, other then the obvious reason, one would convert to two tubes but…
Since you already have the amp I suppose you could at least make it work.
I know I would have a tough time not working out the problems.
I’m no expert, heck I really just a beginner, but it sounds like QL is high and causing excessive current in the tank coil. You may want to work on the values changing the tank C and L values for something around 8 to 10 for the loaded q.
Like I said though, I could be wrong, listen to what others have to say and I hope someone corrects me if what I’m saying sounds off base.
Bob
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RE: AL-1500 conductor heat problems
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by WB6BYU on February 16, 2006
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The reason why the output network had to be modified in
the first place is not the tube capacitance but the operating
impedance level. Just to pick convenient numbers, let's
say the original runs 2000V at 1 amp, for a plate impedance
of 2000 ohms. If you add a second tube that also draws
1 amp, you now have 2000V at 2 amps, or 1000 ohms plate
impedance. The output network has to match this impedance
to 50 ohms, and will require different values for different
impedance levels.
A pi-network has three components to choose based on the
plate impedance, output impedance, operating frequency
and operating Q. There is exactly one solution that will
work, so normally the Q is allowed to vary somewhat to
accomodate the available C and L values. But if you try
to constrain the plate C to a particular value as well,
then the Q goes all to heck.
So the problem is that the tank circuit was poorly
designed with too much plate capacitance on the higher
bands if they tried to use the use the same capacitor
setting (unless they are using a small tuning capacitor
with fixed shunt capacitors across it for each band.)
Meanwhile, proper way to tune up your amp would be at
FULL power output, then reduce the drive to keep it
legal when you switch to the antenna. If you are
trying tune up the amp at a lower power level, the
IMD products may be poor and it may not work as well.
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RE: AL-1500 conductor heat problems
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by KB9WIS on February 16, 2006
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WB6BYU,
You are probably right.. I honestly am a novice at amplifier-related design/theory.. I just want to make this thing work, as I have a lot invested in it.. I perhaps felt too assured, that the ham had everything set up properly..
So, since we are indeed, adding a second tub, I the simple multiplier would be perhaps (x 2) and the impedance has indeed doubled.. What would be the easiest way/fashion, to bring Q back to near needed spec.. Can I simply tap the coil, or change a doorknob capacitor, or add a capacitor, to bring Q for bands back in, or will the Pi Network need to be completely redone with both difference capacitors, inductor taps... for each band..
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RE: AL-1500 conductor heat problems
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by W8JI on February 16, 2006
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The only reason the second socket cut out was added was to allow a monoband two tube amp to be made for ISM applications.
Nothing inside the amp is correct for bandswitched operation.
The amplifier already has too much C and more Q than needed for higher bands. Adding a second tube just aggrivates the problem, and there is no way to correct it.
Finally, the operating impedance of a tube is oproportional to time-varying plate current and time-varying anode voltage at the operating point being used. This means the ideal impedance into the tank does NOT change when you add a second tube, unless that second tube changes the RF power level.
When you run a kW of RF out with one 8877 or run it with two, the loadline is nearly the same.
Now if you doubled power at the same voltage, the load impedance would be half....but that isn't what you are doing. You said you were running only 1kW. In that case the optimum load impedance at the anode is higher than it would be with one tube running at 1500 watts.
What has been done by adding the second tube is extra capacitance has been added, and that extra fixed capacitance increases the minimum available Q on higher bands. Since the Q is higher, circulating currents that are already very high are increased.
I really can't think of a way to fix that problem, short of installing a different tank with a vacuum capacitor.
Anything else you do will only make things worse, or not change things at all. If you reduce coupling cap size, nothing much will change.
By the way, the worse possible thing to do to the tank circuit is to tune it for full power at reduced drive. So you have multiple things going the wrong way.
I have an AL1500 here that is 20 years old. We use it in contests, sometimes with two radios alternating through the amp on CW. That's like having a continuous duty CW transmission. All I did to it was install a high speed blower and a few airflow mods. A single 8877 will run 3 kW if you hammer it. I can't imagine why anyone would want to run two 8877's.
In don't think there is any way to make that amp right, short of installing a new tank with vacuum caps or pulling a tube back out.
73 Tom
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