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Author Topic: ACOM Amps and the advertised 3:1 SWR parameter  (Read 3120 times)
K3VAT
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« on: March 08, 2014, 03:20:52 PM »

ACOM Amps (at least the current 1000 and 1500 models) advertise "The amplifier will perform the functions of an antenna tuner for VSWR up to 3:1, thus enabling you change antennas faster and use them over a wider frequency range (saving tuning time)" < http://www.acom-bg.com/amplifiers1000.html

Several veteran "Amplifiers" forum members also have noted this as a positive attribute.  See:
http://www.eham.net/ehamforum/smf/index.php/topic,90087.0.html among others.

QUESTION: is this due to superior output matching circuit components or something else (or a little bit of both).  Other commercial amps seems to quote "...up to 2:1 VSWR ..."

Thanks, 73, Rich, K3VAT
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W8JI
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« Reply #1 on: March 08, 2014, 04:04:06 PM »

Rating a 160-10 meter amplifier for 3:1 VSWR without any restrictions or complications makes a design very difficult.

3:1 VSWR means anywhere around the 3:1 VSWR circle on a Smith chart, and that is pretty difficult to do on 160 or 80 meters. It isn't that the 3:1 is anything spectacular, most pi nets will do far more than that on ten meters or higher bands, the problem is doing that on 160 meters or 80 meters at both ends of the band and at various power levels.

3:1 is nothing if it is 150 j0.

3:1 is tough if it is 17 ohms or partly inductive on 160 meters.

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K3VAT
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« Reply #2 on: March 08, 2014, 04:29:59 PM »

Rating a 160-10 meter amplifier for 3:1 VSWR without any restrictions or complications makes a design very difficult.

3:1 VSWR means anywhere around the 3:1 VSWR circle on a Smith chart, and that is pretty difficult to do on 160 or 80 meters. It isn't that the 3:1 is anything spectacular, most pi nets will do far more than that on ten meters or higher bands, the problem is doing that on 160 meters or 80 meters at both ends of the band and at various power levels.

3:1 is nothing if it is 150 j0.

3:1 is tough if it is 17 ohms or partly inductive on 160 meters.



Thanks Tom,  Any ACOM Amp ops out there care to comment on operating their amp on the two lower bands viz-a-viz with a 3:1 SWR?
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W8JI
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« Reply #3 on: March 08, 2014, 06:40:54 PM »

Run the numbers on an amplifier with the tank values of the Acom. If you know the values, plug them into a spread sheet. Then you have the answer without asking anyone.
Smiley

The limiting factor is virtually always the amount of capacitance range on 160 and 80 meters. (won't tune)

On 20-10 meters, the limit is usually component current and voltage ratings. (burns up)
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G3RZP
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« Reply #4 on: March 08, 2014, 11:06:43 PM »

I haven't tried anything above 2:1: my sloping dipoles are about 1.5:1, the Steppir gives as close to 1:1 as I need, and the tuners handle the rest. However, as the feed for the folded unipole blows in the wind the SWR varies between 1:1 (indicated, which may be actual!) and about 1.5:1. The Acom 1000  handles that OK, and its protection circuitry is sensitive enough that if you f**t the wrong way, it trips!

It didn't like the arc in the antenna insulator for the folded unipole at 300 watts on 160, and tripped every time.
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F6DEX
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« Reply #5 on: March 09, 2014, 11:45:52 AM »

Hello

The amp appears to work fine up to 3:1 (no more, but depending on the band). However, running an antenna @3:1 high power is not recommended unless it is specially designed for such use. You may have high voltage and lastly arc faults... Reducing power is a good precaution if your antenna is not designed for this use (ie. a trap antenna near the band edges).  A friend had tube failure after such experience !... 2:1 is my limit.

73, Laurent
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W8JI
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« Reply #6 on: March 09, 2014, 02:29:35 PM »

Hello

The amp appears to work fine up to 3:1 (no more, but depending on the band). However, running an antenna @3:1 high power is not recommended unless it is specially designed for such use. You may have high voltage and lastly arc faults... Reducing power is a good precaution if your antenna is not designed for this use (ie. a trap antenna near the band edges).  A friend had tube failure after such experience !... 2:1 is my limit.

73, Laurent

Respectfully, any amplifier with a tuned tank system does not pass SWR along to the tube.

The tank will either match the load, or it will not match the load. If the tank does not adjust to the load then the amplifier should not be operated.

The tank circuit output capacitor can arc from high load impedance, but that is because of load impedance and not SWR. For example with a pi network the output connector voltage with 1500 watts and a 150 ohm j0 load (3:1) might be sqrt of 1500*150 = 475 volts times 1.4141 = 670 peak volts. The tank output capacitor must handle that voltage. A pi-L would depend on the impedance set at the load capacitor by the L section.

All of this can be done on paper very easily.

Any load mismatch (SWR) is only seen by the amplifier as a new termination impedance, so as long as the tank adjusts to the new value without running out of range, the tube has no idea anything is wrong. Nothing changes at the tube.

If the load control is within range:

1.) The things that can change large amounts are voltages and currents from the antenna port to the load control.

2.) A minor change is the voltage from contact to contact on the bandswitch and current there, but that is a minor change.

The rule is if the load control can tune the new load and the load control system does not arc and conductors overheat, it does not matter what the SWR is. If the loading control cannot match the load impedance, then you should not use it no matter what the load SWR is.

This is why load SWR is a meaningless rating for any amplifier that has an adjustable network, unless someone finds the very worse case on the worse frequency. Then that one band will be at the edge, and the rest will be more conservative, some very significantly conservative.

73 Tom
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F6DEX
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« Reply #7 on: March 09, 2014, 03:50:12 PM »

Tom,

Thank you for clarification. No problem with the tank circuit, while used within the limits. But my concerns are the consequence at the antenna side if the antenna itself is not designed for such use (like a  vertical with traps used with a borderline swr and high power). In the case of my friend, a succession of events (antenna side) finally caused the tube failure, first at the antenna and then... the amp after several warnings by the acom 1000 which was not in fault. The user was in fault in that case....

Btw, I replaced the tube but he now has to repair the antenna...

73, Laurent
« Last Edit: March 09, 2014, 03:58:19 PM by F6DEX » Logged
W8JI
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« Reply #8 on: March 09, 2014, 05:03:39 PM »

Tom,

Thank you for clarification. No problem with the tank circuit, while used within the limits. But my concerns are the consequence at the antenna side if the antenna itself is not designed for such use (like a  vertical with traps used with a borderline swr and high power). In the case of my friend, a succession of events (antenna side) finally caused the tube failure, first at the antenna and then... the amp after several warnings by the acom 1000 which was not in fault. The user was in fault in that case....

Btw, I replaced the tube but he now has to repair the antenna...

73, Laurent

Don't they have protection for the tube?

What do you think happened, screen current too high?
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K3VAT
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Posts: 704




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« Reply #9 on: March 09, 2014, 06:12:49 PM »


... The tank will either match the load, or it will not match the load. If the tank does not adjust to the load then the amplifier should not be operated.

The tank circuit output capacitor can arc from high load impedance, but that is because of load impedance and not SWR. For example with a pi network the output connector voltage with 1500 watts and a 150 ohm j0 load (3:1) might be sqrt of 1500*150 = 475 volts times 1.4141 = 670 peak volts. The tank output capacitor must handle that voltage. A pi-L would depend on the impedance set at the load capacitor by the L section.

...

Any load mismatch (SWR) is only seen by the amplifier as a new termination impedance, so as long as the tank adjusts to the new value without running out of range, the tube has no idea anything is wrong. Nothing changes at the tube.

If the load control is within range:

1.) The things that can change large amounts are voltages and currents from the antenna port to the load control.

2.) A minor change is the voltage from contact to contact on the bandswitch and current there, but that is a minor change.

The rule is if the load control can tune the new load and the load control system does not arc and conductors overheat, it does not matter what the SWR is. If the loading control cannot match the load impedance, then you should not use it no matter what the load SWR is.

This is why load SWR is a meaningless rating for any amplifier that has an adjustable network, unless someone finds the very worse case on the worse frequency. Then that one band will be at the edge, and the rest will be more conservative, some very significantly conservative.

73 Tom

This explanation is exactly what I needed.  Now I can restudy my amplifier texts and even do some basic calculations.  Thanks again Tom.  73, Rich, K3VAT
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F6DEX
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« Reply #10 on: March 10, 2014, 12:00:07 AM »

Tom,

He had repetitive arc faults messages. HI think he  had a problem with saturation of the tore of the transformers (base of the antenna) which caused higher mismatch during transmission. The protections were effective with explicit messages but not understood for their values. After 2 / 3 months... the tube died (as the antenna !). This may be an extreme case however.

The amp is well conceived, no problem but there is limits for all.

73, Laurent
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W8JI
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« Reply #11 on: March 10, 2014, 07:06:16 AM »

Tom,

He had repetitive arc faults messages. HI think he  had a problem with saturation of the tore of the transformers (base of the antenna) which caused higher mismatch during transmission. The protections were effective with explicit messages but not understood for their values. After 2 / 3 months... the tube died (as the antenna !). This may be an extreme case however.

The amp is well conceived, no problem but there is limits for all.

73, Laurent

Tubes with gold plated grids are subject to accumulation or addition of small damages at every fault. If the current is momentarily severe, the electrons can dislodge gold even without time to heat the entire grid. Also cathodes must never be saturated or they will poison.

Even with protection tubes with oxide cathodes and gold covered grids may slowly be damaged if overloaded again and again.

This is why I do not like those grids and cathodes for things that are abused.
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F6DEX
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« Reply #12 on: March 10, 2014, 11:21:14 AM »

Tom

Thank you, interesting point (I was not informed).

73, Laurent
« Last Edit: March 10, 2014, 11:24:08 AM by F6DEX » Logged
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