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Author Topic: Attenuators at input of amplifier  (Read 10373 times)
VK4WTN
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« on: September 20, 2012, 09:50:37 PM »

I am just refurbishing a 400W FET amplifier that requires about 6 watts of drive.I am thinking of using two henry radio 6db attenuators in series to get approximatly 6 watts out from my TS590.The reason I want to do this is to protect the FETs from any ALC overshoot and also to eliminate operator error eg putting 100W in after a late night.Its a bit of a waste of 95W of power but should protect my FETs. :)I believe the Henryradio attenuators are flange mounted so will need to be well heatsinked. Has anyone tried this method of protection or have any other ideas
regards Wayne VK4WTN
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KM3F
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« Reply #1 on: September 20, 2012, 11:03:21 PM »

Are you sure the 590 spikes power?
Why not turn the power down with the radios control menu.
You should be able to got down to 5 watts.
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WX7G
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« Reply #2 on: September 21, 2012, 12:28:46 AM »

The attenuators are an excellent idea.

And with many solid state amps needing just 50 watts it would be great if MFJ or DX Engineering sold a 3 dB, 50 watt attenuator to use with 100 watt transceivers. No ALC needed.
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YO9IRF
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« Reply #3 on: September 21, 2012, 12:47:10 AM »

That is the safest way, attenuators on amplifier input protect the sensitive mosfets from overdrive damage and also improve the match seen by the transciever troughout the bands. I have used various types of attenuators for a MRF150's amplifier, very happy with the results.
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VK4WTN
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« Reply #4 on: September 21, 2012, 01:55:52 AM »

Thanks for the replies guys.In relation to does the TS590 have ALC overshoot spikes, the considered opinion of many owners on the reflector site is yes. My biggest concern was accidently transmitting full power into the amplifier which can happen to the best of us.So my thinking is that using a 12bd pad (2 x 6db in series) would eliminate any chance of damage due to overdrive either by spikes or accidently transmitting full dower.The only downside seems to be the waste of 95 watts of power. Regards Wayne VK4WTN
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ZENKI
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« Reply #5 on: September 21, 2012, 03:43:59 AM »

You making life difficult following the attenuator path. This would be a good idea for those Aussie Emtron amplifiers that produce a lot of splatter from ALC and power overshoot.

A simpler method would be to just apply negative  voltage to the ALC connector. You can use a 9v battery and a variable resistor and vary the  ALC voltage. This method produces absolute power control.

This is a good trick for  tube radios like the TS830S  and others. You cant vary the power on these radios since they have no power control. A variable ALC voltage gives you a precise power control.

Here is a link for how about going about it. Make sure you use a decent oscilloscope for a peak reading wattmeter. Most wattmeters that hams use are useless peak reading wattmeters.

http://www.astromag.co.uk/ft857d/



I am just refurbishing a 400W FET amplifier that requires about 6 watts of drive.I am thinking of using two henry radio 6db attenuators in series to get approximatly 6 watts out from my TS590.The reason I want to do this is to protect the FETs from any ALC overshoot and also to eliminate operator error eg putting 100W in after a late night.Its a bit of a waste of 95W of power but should protect my FETs. :)I believe the Henryradio attenuators are flange mounted so will need to be well heatsinked. Has anyone tried this method of protection or have any other ideas
regards Wayne VK4WTN
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W8JI
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« Reply #6 on: September 21, 2012, 04:26:53 AM »

Are you sure the 590 spikes power?
Why not turn the power down with the radios control menu.
You should be able to got down to 5 watts.

Unless the menu reduces IF section or driver section gain, or removes a PA section to reduce gain, turning power down always increases ratio of overshoot to ALC leveled power.

You making life difficult following the attenuator path. This would be a good idea for those Aussie Emtron amplifiers that produce a lot of splatter from ALC and power overshoot.
 

Do they reallly do that? A friend of mine has several of them. I always wondered how well designed any tetrode amp really is.

Overshoot or dynamic regulation problems do not show in a two-tone test. Which do you think it  has?
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AD5X
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« Reply #7 on: September 21, 2012, 04:28:05 AM »

The negative ALC voltage trick doesn't fix the overshoot on some radios.  My IC-706MKII overshoots to 140 watts regardless of output power setting, so I tried the external ALC trick.  This didn't have any effect on the overshoot.  It seems like the IC-706MKII doesn't "look" at the ALC input until after you key the radio.

Phil - AD5X
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ZENKI
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« Reply #8 on: September 21, 2012, 07:15:42 PM »

I suspect its the grid bias and grid current monitoring system in the Emtron amplifiers that is the real cause of the problem. There is no reliable indicator of grid current on these amps. When these amps splatter they must be drawing grid current that is  only detected when there is an excessive amount of grid current.  I have monitored a DX2SP into a dummy load using a real time spectrum analyzer. The resulting  splatter in real time is like square wave on either side of the signal as you get the peak splatter. If this was a normal normal metered homebrew amplifier you would see this fault immediately  on the grid I meter.

If you do a real dynamic IMD test  using a voice signal you will hear this leading edge splatter burst thats indicative of  grid current draw.    What other conclusion can you come to when  its a given that drawing grid current on SSB will cause splatter. While  I have done 2 tone measurements on one of these amplifiers and the resulting 2 tone numbers are respectable, the dynamic on air  performance is terrible.  Who do you blame the amplifier manufacturer or the radio, manufacturer?

All my homebrew  Tetrode amplifiers have always a sensitive op-amp driven grid current I meter.  Any hint of grid current flowing could be detected. How do you do this on a tetrode control  that uses a  LED display with coarse poor current monitoring hysteresis and one one that has no active grid current display?

In the hands of the average ham these poorly designed tetrode amplifiers are  nuisance on the bands. Try tuning the websdr.org SDR receivers in Europe and listen on 40 and 20 meters you can easily spot these tetrode amplifiers they that obvious on the waterfall display.  Its very rare to hear  splattering triode amplifiers, its always a joker with tetrode amp that causes the damage. When something as simple  a single LED grid current indicator could help these operators tune these amps, one has to wonder why they leave such critical monitoring  off the front panel and only give an alarm when there is really excessive current drawn. Its then no surprise that just about every commercial tetrode amplifiers causes excessive splatter.


You making life difficult following the attenuator path. This would be a good idea for those Aussie Emtron amplifiers that produce a lot of splatter from ALC and power overshoot.
 

Do they reallly do that? A friend of mine has several of them. I always wondered how well designed any tetrode amp really is.

Overshoot or dynamic regulation problems do not show in a two-tone test. Which do you think it  has?
[/quote]
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K8AXW
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« Reply #9 on: September 21, 2012, 08:24:11 PM »

I use a 2db attenuator in line with my SB-200, to prevent overdriving and overshoot.  Works great but unless the attenuator is built into the amplifer, after the input relay, the attenuator is also in the receive line.  2db isn't objectionable but if your going to use a larger attenuator, the cut in received signal might be intolerable.
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KM3F
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« Reply #10 on: September 29, 2012, 12:59:39 PM »

I had replied mistakenly to this in another thread so won't get lengthy here.
My TS480 has no overhoot under any conditions with or without the use of ALC when turned down to it's lowest power setting.
I have just tested it with a peak reading power meter set on 5 watts and see no needle movement whatever as long as there is no audio inputs on SSB and the balanced modulator is set correctly from the factory or othereise.
I could still check it with a scope to further verify.  I might see some signal but it won't be a spike.
I use the radio to drive two different Transverters who's' inputs are 1/2 watt max by using switched attenuators built into the 10 meter in/out ports.
I use the amplifier delay option and  home brewed external keying interface with many ports due to many pieces of equipment that has to be keyed in various combinations on the bands.
I understand other radios may spike but this one does not do it.
Good luck.
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AD5X
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« Reply #11 on: September 29, 2012, 02:38:26 PM »

...My TS480 has no overhoot under any conditions with or without the use of ALC when turned down to it's lowest power setting.
I have just tested it with a peak reading power meter set on 5 watts and see no needle movement whatever as long as there is no audio inputs on SSB and the balanced modulator is set correctly from the factory or othereise....

You need to test the spike WITH an audio input, or a CW keying input.  It happens with the first syllable or first dit, after which the ALC takes control and sets the power correctly.  I'm not saying your radio does this, but you must test it with a signal to see a spike (or not see a spike).

Phil - AD5X
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W8JI
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« Reply #12 on: September 30, 2012, 04:48:19 AM »

I had replied mistakenly to this in another thread so won't get lengthy here.
My TS480 has no overhoot under any conditions with or without the use of ALC when turned down to it's lowest power setting.
I have just tested it with a peak reading power meter set on 5 watts and see no needle movement whatever as long as there is no audio inputs on SSB and the balanced modulator is set correctly from the factory or othereise.
I could still check it with a scope to further verify.  I might see some signal but it won't be a spike.

The overshoot is caused by ALC rise time lagging behind signal envelope rise time. The radio has too much gain for the desired output power level, and depends on ALC to reduce gain.

This means we always have to have a signal present to test for overshoot, and it must always be from a zero or low ALC state, like on a leading edge following a pause.
 


When a test is incorrect, the results don't mean much.
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K9FV
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« Reply #13 on: October 01, 2012, 05:40:28 AM »

Would the proper test for a spike be to connect radio to dummy load, set power to lowest power setting (1 to 5 watts) in CW mode. Connect a DSO to the dummy load, key radio to capture the CW signal on DSO with time set to capture an event that only lasts a few mSeconds.

Shouldn't that saw a problem?  If the radio does have this issue, should it show up every time the rig is keyed from idle to a long CW tone?  Or only some of time?

Thanks for any guidance,

73 de Ken H>
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KM3F
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« Reply #14 on: October 01, 2012, 09:02:38 PM »

I can test with a signal but as of yet have never lost a transverter or SS amplifier to spiking.
In actual service, what would it sound like to the receiving end.
No one has ever made mention of anything at first sound.
With audio in from the mike, there are no spikes showing on a peak reading power meter set on 5 watts full scale.
Seems if this would be a chronic situation  a spike after every pause in audio would take place.
My head set boom mike does not pick up anything between words to keep the internal ALC under control.
Sorry to be a pain in the a** but this in not an issue with my 480.
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