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KE7VOB
02-09-2009, 07:00 PM
Hello group,

I recently acquired a Swan 1200-x to use with my Yaesu FT-757.

The amp has the fan mod and 6lq6 tubes.

The amp requires 100-120 watts to operate.

In CW mode, after tuning the amp, my radio will only output approximately 50 watts. With the amp bypassed it puts out 120 watts.

SWR after the amplifier and before the antenna shows 1.2:1 on two different meters. I tried momentarily unplugging the alc line to see if that would make a difference which it did not.

Does anyone have any idea why my radio would be folding back the power like this? It basically defeats the purpose of the amp if I cant get 100 watts to it.

Thanks

Dave

KD7MSC
02-09-2009, 07:09 PM
Hello group,

I recently acquired a Swan 1200-x to use with my Yaesu FT-757.

The amp has the fan mod and 6lq6 tubes.

The amp requires 100-120 watts to operate.

In CW mode, after tuning the amp, my radio will only output approximately 50 watts. With the amp bypassed it puts out 120 watts.

SWR after the amplifier and before the antenna shows 1.2:1 on two different meters. I tried momentarily unplugging the alc line to see if that would make a difference which it did not.

Does anyone have any idea why my radio would be folding back the power like this? It basically defeats the purpose of the amp if I cant get 100 watts to it.

Thanks

Dave

Hi Dave,
I am not familiar with that amp. Does it have a tuned input?

KL7AJ
02-09-2009, 07:18 PM
Hello group,

I recently acquired a Swan 1200-x to use with my Yaesu FT-757.

The amp has the fan mod and 6lq6 tubes.

The amp requires 100-120 watts to operate.

In CW mode, after tuning the amp, my radio will only output approximately 50 watts. With the amp bypassed it puts out 120 watts.

SWR after the amplifier and before the antenna shows 1.2:1 on two different meters. I tried momentarily unplugging the alc line to see if that would make a difference which it did not.

Does anyone have any idea why my radio would be folding back the power like this? It basically defeats the purpose of the amp if I cant get 100 watts to it.

Thanks

Dave


Do you have a small antenna tuner you can insert BEFORE the amplifier? It can make a world of difference.

eric

WB2WIK
02-09-2009, 07:21 PM
The 1200X does not have tuned input and was designed for use with its vacuum tube transceiver counterpart, the Swan Cygnet 260 (or 260B or 270), which has a pi-network output tank and can load power into a mismatch pretty effectively.

Getting a solid state rig to work with the 1200X can be a challenge.

I doubt the ALC output would be compatible with the FT-757, either, but then you already disconnected that, so that's not the problem at this point.

WB2WIK/6

WG7X
02-09-2009, 08:19 PM
Logic dictates that if your rig, which is one of the new ones that is (overly) sensitive to load variations is cutting back power, then the load that it is seeing (your amp) is not 50 ohms resistive.

The load on the other side of the amp (usually an antenna) can sometimes add problems. This is why a 50 ohm dummy load is an important tool.

But, as already stated, that amp will be a difficult beast to tame as far as the input goes. A good bet is to acquire one of the small (100 watt) auto-tuners such as the LDG or MFJ and put that between the amp and the rig. Unfortunately, load impedance will vary a bit between the loaded and unloaded states of the amp, so there might be a bit of hunting going on with the tuner.

If your rig has a built-in tuner, this is a good use for it.

73 Gary

WG7X
02-09-2009, 08:21 PM
Dave,

Don't forget that your rig probably will have a problem with the relay in that old amp.

Unless the aforementioned mods covered that, I would also use a keying interface of some sort.

73 Gary

KD7QPW
02-09-2009, 08:34 PM
would this be the Swan 1200x I saw on Craigslist last week up north in Mt. Vernon area?? I talked with the owner also mainly in ref to the antenna.. J.

KD7MSC
02-09-2009, 09:49 PM
The 1200X does not have tuned input and was designed for use with its vacuum tube transceiver counterpart, the Swan Cygnet 260 (or 260B or 270), which has a pi-network output tank and can load power into a mismatch pretty effectively.

Getting a solid state rig to work with the 1200X can be a challenge.

I doubt the ALC output would be compatible with the FT-757, either, but then you already disconnected that, so that's not the problem at this point.

WB2WIK/6

I was hoping Steve would see this. He knows his stuff! I did'nt think that amp had tuned input. You can do as Eric said and insert a match box between the rig and amp. I have a couple amps that dont have tuned inputs but I use tube rigs to drive them so it is not an issue.

GL and 73, Sean

KE7VOB
02-09-2009, 10:27 PM
Thank you for the replies.

I put my fc 757at tuner inline and that helped. It was only able to match at just under 3.1:1 but that keeps the radio's alc circuit from kicking on.

I don't have the radio setup to key the amp. It has a hand switch on that line to that. I am thinking of using an old sewing machine foot pedal instead.

So now I have a hundred watts out of the radio no problem. Only getting 450 out of the amp after careful tuning though. It should do 600 but thats okay for now. Maybe it is time for some new tubes in which case I will only run it at around 400 anyway.

Appreciate all the help guys.!


Dave

WB2WIK
02-09-2009, 10:46 PM
Thank you for the replies.

I put my fc 757at tuner inline and that helped. It was only able to match at just under 3.1:1 but that keeps the radio's alc circuit from kicking on.

I don't have the radio setup to key the amp. It has a hand switch on that line to that. I am thinking of using an old sewing machine foot pedal instead.

So now I have a hundred watts out of the radio no problem. Only getting 450 out of the amp after careful tuning though. It should do 600 but thats okay for now. Maybe it is time for some new tubes in which case I will only run it at around 400 anyway.

Appreciate all the help guys.!


Dave

::450W carrier power is about correct, it really should not do more than that key-down. If you tune for that and then switch to SSB, using a true PEP meter you'll probably see about 600W PEP, which is what the amp was rated for when it was brand new. Don't prolong tune-ups with sweep tubes, that kills 'em faster than anything else.

WB2WIK/6

KE7VOB
02-09-2009, 10:49 PM
I am using a PEP meter,

And thanks for the advice on the tune up. I read something somewhere about using a cricket on CW for tuning up..

Dave

WB2WIK
02-09-2009, 11:09 PM
I am using a PEP meter,

And thanks for the advice on the tune up. I read something somewhere about using a cricket on CW for tuning up..

Dave

::These do work and help find correct tuning point without overstressing the tubes and power supplies:

http://ameritron.com/Product.php?productid=ATP-102

WB2WIK/6

N6DCN
02-09-2009, 11:52 PM
Logic dictates that if your rig, which is one of the new ones that is (overly) sensitive to load variations is cutting back power, then the load that it is seeing (your amp) is not 50 ohms resistive.

If your rig has a built-in tuner, this is a good use for it.

73 Gary

:confused: I thought you couldn't or shouldn't use your built in tuner when running a amp ? :confused:Can someone explane and clear this up for me

N7WR
02-09-2009, 11:59 PM
ki6dcn
to get around the problem discussed here (lack of tuned input on the amp causing the exciter to fold back drive power) using the built in tuner helps as several have discussed. If you have an amp with a tuned input you don't need to do it

W8JI
02-10-2009, 12:47 AM
:confused: I thought you couldn't or shouldn't use your built in tuner when running a amp ? :confused:Can someone explane and clear this up for me

Wet blanket time. :-)

Sweep tubes are history. If you buy an amplifier with sweep tubes you better hope they never go bad, because if they do you will be in for a rough time replacing them.

The Swan, as Steve said, is only good for about 400 watts carrier when tuning, and a tad more than that on SSB using PEP if the tubes are good and fresh. The tubes won't last long however at that power level on CW or carrier.

As for using a tuner, tuners are generally a bad idea compared to a tuned input. In a grounded grid amplifier the input is in series with the output. The plate conduction angle is short and this causes pretty high level harmonics, and since the input is in series with the output those harmoncs appear at the cathode. For the tubes in the amplifier to have good efficiency and good linearity, the cathode has to see a low impedance for harmonics.

If the cathode happens to see a high impedance at the second and higher harmonics then the efficiency drops, the tube runs hotter, and IMD increases. This is why experienced designers generally put low pass CLC pi networks or some other network with a shunt C at the tube cathode as close to the tube as possible.

Sometimes you can get away without doing that, but the result is a system mostly operating by luck. Change the input cable length, the exciter, or other things and suddenly plate efficiency can drop or IMD increase.

This is the reason why the reliable accurate recommendation is to install a tuned input and to NOT use an external tuner. Now if the amp already has a low pass tuned input an external tuner won't be much of an issue because the harmonic filtering is at the cathode, even if the impedance is mismatched and needs touched up.

The tuned input does far more than just match the exciter, as a matter of fact it may be impossible to match the amplifier with a high pass tuner because the harmonics will blow right back through the high-pass network tuner!!!!

I designed an amplifier for Heathkit that had about 20 inches of coax between the 3-500Z cathodes and the low pass pi, and that amp mysteriously had about 30% efficiency on ten meters. It turned out it was the distance the low pass input network was from the tubes, so this is a very real effect!!

If someone already has a sweep tube amp, especially one without a tuned input, and if it is running then by all means use it. Common sense says it is a very poor investment today for a used equipment purchase. It isn't worth working on to upgrade with a tuned input or tracking down tubes for one just to get 400 watts or so output. They were OK when we were paying $3 for a new 6MJ6 tube.

Better to find something used using 811 tubes today.

73 Tom

KE7VOB
02-10-2009, 01:03 AM
I'm a pretty low budget operator here . I still rotate my antenna manually with a rope! hi hi. The amplifier was a freebie. Of course it isn't free if it ends up causing harm to my radio or tuner. http://www.download.n7tgb.net/yaesu/FC757/FC-757AT.pdf Now I wonder if it will?

It seems to be putting out power and I only intend to use it on 20 meters so I wont be retuning it from band to band.

Hope it will be okay.

N6DCN
02-10-2009, 04:05 AM
ki6dcn
to get around the problem discussed here (lack of tuned input on the amp causing the exciter to fold back drive power) using the built in tuner helps as several have discussed. If you have an amp with a tuned input you don't need to do it






Wet blanket time. :-)

Sweep tubes are history. If you buy an amplifier with sweep tubes you better hope they never go bad, because if they do you will be in for a rough time replacing them.

The Swan, as Steve said, is only good for about 400 watts carrier when tuning, and a tad more than that on SSB using PEP if the tubes are good and fresh. The tubes won't last long however at that power level on CW or carrier.

As for using a tuner, tuners are generally a bad idea compared to a tuned input. In a grounded grid amplifier the input is in series with the output. The plate conduction angle is short and this causes pretty high level harmonics, and since the input is in series with the output those harmoncs appear at the cathode. For the tubes in the amplifier to have good efficiency and good linearity, the cathode has to see a low impedance for harmonics.

If the cathode happens to see a high impedance at the second and higher harmonics then the efficiency drops, the tube runs hotter, and IMD increases. This is why experienced designers generally put low pass CLC pi networks or some other network with a shunt C at the tube cathode as close to the tube as possible.

Sometimes you can get away without doing that, but the result is a system mostly operating by luck. Change the input cable length, the exciter, or other things and suddenly plate efficiency can drop or IMD increase.

This is the reason why the reliable accurate recommendation is to install a tuned input and to NOT use an external tuner. Now if the amp already has a low pass tuned input an external tuner won't be much of an issue because the harmonic filtering is at the cathode, even if the impedance is mismatched and needs touched up.

The tuned input does far more than just match the exciter, as a matter of fact it may be impossible to match the amplifier with a high pass tuner because the harmonics will blow right back through the high-pass network tuner!!!!

I designed an amplifier for Heathkit that had about 20 inches of coax between the 3-500Z cathodes and the low pass pi, and that amp mysteriously had about 30% efficiency on ten meters. It turned out it was the distance the low pass input network was from the tubes, so this is a very real effect!!

If someone already has a sweep tube amp, especially one without a tuned input, and if it is running then by all means use it. Common sense says it is a very poor investment today for a used equipment purchase. It isn't worth working on to upgrade with a tuned input or tracking down tubes for one just to get 400 watts or so output. They were OK when we were paying $3 for a new 6MJ6 tube.

Better to find something used using 811 tubes today.

73 Tom

Copy....... Thanks guys. Ron

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