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Author Topic: Will High SWR harm my tube Amp?  (Read 10466 times)
KI4DSC
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Posts: 65




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« on: October 10, 2013, 06:10:14 PM »

First let me say sorry if a thread exist about this, I looked but didn't find what I was looking for.

I was once told that tube amps are not harmed by high SWR.  The person who told me this explained that 40 years ago no one even looked at swr and swr didn't become an issue until coax cable became popular.

My question is this.  I have an off center Fed dipole that I want to use.  I would like to use it without a tuner and using my antenna analyzer I have found that on the bands of interest (40, 20 and 10) I can get an SWR of 5:1 and less on all three bands if I use 450 ohm window line.

Will running 5:1 or even 10:1 SWR harm my amp?

I use a Swan Mark 1 1965 vintage amp and I don't want to have to replace the rather expensive tubes because of doing something foolish.

Tom
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K2UE
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« Reply #1 on: October 10, 2013, 06:23:45 PM »

First let me say sorry if a thread exist about this, I looked but didn't find what I was looking for.

I was once told that tube amps are not harmed by high SWR.  The person who told me this explained that 40 years ago no one even looked at swr and swr didn't become an issue until coax cable became popular.

My question is this.  I have an off center Fed dipole that I want to use.  I would like to use it without a tuner and using my antenna analyzer I have found that on the bands of interest (40, 20 and 10) I can get an SWR of 5:1 and less on all three bands if I use 450 ohm window line.

Will running 5:1 or even 10:1 SWR harm my amp?

I use a Swan Mark 1 1965 vintage amp and I don't want to have to replace the rather expensive tubes because of doing something foolish.

Tom

You were told correctly.  In a tube amp with tune and load controls if you are able to load and resonate properly then the tube is seeing the correct load impedance and there is no difference between a 1:1 and 10:1 SWR.  BUT the voltage and current at the output jack is very different under high SWR: at 10:1 the circle of possibilities include voltages 3.2X that of 1:1, or currents 3.2x larger; these can cause arcing of the load capacitor, in the case of voltage, or overheating due to high current.  The amplifier manufacturer normally specifies what SWR range it can tune, and by inference what currents and voltages it will tolerate at the output jack. 
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KI4DSC
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« Reply #2 on: October 10, 2013, 06:37:36 PM »

Quote
You were told correctly.  In a tube amp with tune and load controls if you are able to load and resonate properly then the tube is seeing the correct load impedance and there is no difference between a 1:1 and 10:1 SWR.  BUT the voltage and current at the output jack is very different under high SWR: at 10:1 the circle of possibilities include voltages 3.2X that of 1:1, or currents 3.2x larger; these can cause arcing of the load capacitor, in the case of voltage, or overheating due to high current.  The amplifier manufacturer normally specifies what SWR range it can tune, and by inference what currents and voltages it will tolerate at the output jack. 

Thank you for the responce.  I have read the Swan manual cover to cover and no specification as to SWR limits could be found. 
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NO2A
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« Reply #3 on: October 10, 2013, 08:17:04 PM »

It also depends what`s driving that amp. If your exciter sees too high an swr it may fold back power,not delivering enough to drive the amp to full output. Unless it`s a tube transceiver.
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N4JTE
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« Reply #4 on: October 10, 2013, 09:47:40 PM »

Wow interesting what you are seeing from that antenna, to answer your question , nope, amp will be okay but something wrong with store bought antenna I think.; and your efficency on getting out for contacts is suffering, to me not an amp question more like antenna issues.
Bob
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KI4DSC
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« Reply #5 on: October 10, 2013, 10:50:41 PM »

Wow interesting what you are seeing from that antenna, to answer your question , nope, amp will be okay but something wrong with store bought antenna I think.; and your efficency on getting out for contacts is suffering, to me not an amp question more like antenna issues.
Bob

Not sure what you're trying to say. The antenna is home brew off center cut for 40 meter. It loads well on 40 (less than 2:1 across the band) and I see no difference between this antenna and my center fed that loads 50 ohm cable. My question is simply will I hurt anything if I load the antenna on 20 and see a 5:1 swr between the amp and the antenna. The swr between t h e radio and amp is not changed or affected by the changing of the swr on the antenna and  feedline.

I am going to build a fan 80,40,20 soon but until then I hope to use the one I have. I also use a 6BTV and that is a nice DX antenna as I can not put up a beam. I am however going to try a vertical moxon for 20 meter.   The reason I am going vertical is that I can not go high enough to get the proper performance on a horizontal.
« Last Edit: October 10, 2013, 10:54:55 PM by KI4DSC » Logged
WX7G
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« Reply #6 on: October 11, 2013, 03:22:49 AM »

Will running 5:1 or even 10:1 SWR harm my amp?

YES it might. The Swan Mark 1 has no output VSWR specification. Then manual says "Wide range pi output." But 5:1 and certainly 10:1 is beyond its tuning range.

A load that is beyond the tuning range of the amp is indicated by either the LOAD or TUNE control being turned full clockwise or counter-clockwise.
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AA4PB
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« Reply #7 on: October 11, 2013, 04:12:18 AM »

N02A: Why would a high SWR on the output of an amp (one that is within the tuning range of the amp) present a high SWR to the driver? If the load impedance is able to be tuned by the PI network then the tube is seeing the proper impedance and assuming the input circuit is properly designed, the input to the amp will present a proper 50 Ohm load to the driver.

Matching the load impedance to the tube with a PI network is no different than matching the load with an external antenna tuner except that the tuning range will normally be more limited.
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NY0P
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« Reply #8 on: October 11, 2013, 04:50:14 AM »

With a 5:1 standing wave you would have 450w reflected for a kw forward power.  That could also create other problems like coax heating for one.  You will pick up a 1.9 db of transmitted power by dealing by tuning that antenna. 
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NU1O
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« Reply #9 on: October 11, 2013, 07:54:10 AM »

Your tubes can only handle so much heat.  My Alpha 8410 manual says a 3 to 1 SWR is the limit.  At legal limit out that would be about 375 watts of reflected power yet the reflected power LED bar graph only goes to 250W and then the amp starts soft faulting.  Your tank circuit is a matching device but it does not have the range of a dedicated transmatch that maybe to handle a 10 to 1 mismatch so tubes are a lot more tolerant of reflected power than transistors but there is a limit as to how much reflected power they can handle.
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NO9E
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« Reply #10 on: October 11, 2013, 12:33:01 PM »

K2UE had a good answer.

In the 1980s I used Swan 500 with off-center fed dipole set for 80m. Matched all bands from 10 to 80. However, a vertical tuned for 20m was much better for DX on 10-20m.

Ignacy, NO9E 

 
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KH2G
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« Reply #11 on: October 11, 2013, 05:58:47 PM »

Too high an SWR is never good for any amp. It is true that the older rigs tolerated a fairly high SWR but Your not going to have a very high SWR with a conjugate match. As has been stated, HI SWR tends to cause unwanted high voltages in the final tuning stages and with older equipment you may have some dielectric breakdown problems. I would not run any higher than 3:1 and prefer lower. (With a balanced line and balanced tuner, you should have a very acceptable SWR.) Remember too, that SWR is not king because a 1:1 could be bad if it not the right match and your dumping your power into ground = hi It's all in the match.
Regards,
Dick KH2G
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KI4DSC
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Posts: 65




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« Reply #12 on: October 12, 2013, 04:15:45 AM »

Will running 5:1 or even 10:1 SWR harm my amp?

YES it might. The Swan Mark 1 has no output VSWR specification. Then manual says "Wide range pi output." But 5:1 and certainly 10:1 is beyond its tuning range.

A load that is beyond the tuning range of the amp is indicated by either the LOAD or TUNE control being turned full clockwise or counter-clockwise.
WX7G: Thank you.  That is good information!  I have found that one one band I have to tune the amp full clockwise on the Load and way farther clockwise on the tune than I do with the vertical.  I will take that as being outside of the limits of the amp... 
With all the other answere I have gotten I will not push my luck.  I will put the tuner in line and play it safe.  The trouble is my tuner is not a balanced line tuner so I will have to run a short run of coax before going to the twin lead.  In a few weeks I will have my fan dipole up and it will be a non-isue.
Thankyou all for your help.
73,
Tom
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NO2A
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« Reply #13 on: October 12, 2013, 05:42:03 AM »

N02A: Why would a high SWR on the output of an amp (one that is within the tuning range of the amp) present a high SWR to the driver? If the load impedance is able to be tuned by the PI network then the tube is seeing the proper impedance and assuming the input circuit is properly designed, the input to the amp will present a proper 50 Ohm load to the driver.

Matching the load impedance to the tube with a PI network is no different than matching the load with an external antenna tuner except that the tuning range will normally be more limited.

What if he doesn`t use the amp? The transmitter,assuming it`s solid state,would still see a high swr,too high for the transmitter. He would still need a tuner or resonant antenna.
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K2UE
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« Reply #14 on: October 12, 2013, 01:44:13 PM »

Your tubes can only handle so much heat.  My Alpha 8410 manual says a 3 to 1 SWR is the limit.  At legal limit out that would be about 375 watts of reflected power yet the reflected power LED bar graph only goes to 250W and then the amp starts soft faulting.  Your tank circuit is a matching device but it does not have the range of a dedicated transmatch that maybe to handle a 10 to 1 mismatch so tubes are a lot more tolerant of reflected power than transistors but there is a limit as to how much reflected power they can handle.

There seems to be a misunderstanding to the effect that if there is reflected power it shows up as heat in the tube(s).  If you are still able to tune and load the amp correctly within the range of its controls there is no change in the plate dissipation of the tube(s)  -- the power is reflected again by the amplifier, and bounces back and forth between amp and antenna until consumed by line losses and the portion that is radiated on each bounce.  The tube end always reflects, it is never "conjugately matched" -- a conjugate match has a efficiency limit of 50% max.  Useful (efficient) power amplifiers are typically current sources matched to a load impedance that extracts maximum power.  As a result when they are tuned to deliver maximum power the impedance looking in from the antenna is not 50 ohms even when the load resistance is 50 ohms. It is instead somewhere on an impedance circle that is defined by the phase shift in the plate matching network and the ratio of the plate current source impedance to the load impedance.  In broadband amps that have no tuning, poor SWR will increase tube dissipation, not from the reflected power, but from the load line being elliptical due to being off resonance, and/or improper in resistance.
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