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Author Topic: Looking for Linear Amplifier  (Read 31108 times)
W8JX
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Posts: 13268




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« Reply #90 on: August 20, 2017, 05:25:35 AM »

For any amplifier to be clean at rated output you  need excess capacity. At 1500 watts out with good IMD you need an amplifier with 2-3 dB head room.

I do not really "buy" that you have to have a 3000 watt amp to have a clean signal. If you use a tube that is properly rated for task it will be fine and does not need a power supply with it capable of 3000 watts out. Many use this "headroom: excuse but is just that a excuse.
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Ham since 1969....  Old School 20wpm REAL Extra Class..
K6AER
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Posts: 5743




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« Reply #91 on: August 20, 2017, 05:49:02 PM »

John,

Here we disagree.

Every tube amplifier I have tested for IMD gets its best rating when they have at least 2 dB of head room. Solid state as well.
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G4ZOW
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Posts: 119




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« Reply #92 on: August 20, 2017, 10:23:48 PM »

I can think of several reasons to use a tube amplifier.

* Should a tube go band you can replace it without an $800 solder station.


Michael,

The solder station's no longer required.

I worked at this transmitter manufacturer for three years:

https://youtu.be/Y-m5-K0fLfg

https://youtu.be/AkJhb5Qi8ik

They have sold thousands of transmitters up to and including 2.5kW output and AFAIK none of the very small number of units returned for repair over many years have had LDMOS devices go south.

Many units on-air also on your side of the pond sold through SCMS and other broadcast equipment dealers.

David  G4ZOW/5B4
« Last Edit: August 20, 2017, 10:29:29 PM by G4ZOW » Logged
K6AER
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Posts: 5743




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« Reply #93 on: August 21, 2017, 06:52:41 AM »

David,

Thanks for the link.

Mike
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KM1H
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Posts: 5509




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« Reply #94 on: August 21, 2017, 07:53:22 AM »

Quote
Every tube amplifier I have tested for IMD gets its best rating when they have at least 2 dB of head room. Solid state as well.

But also consider that many TX have their best IMD at some level lower than their advertised output. Then test while varying the amp loading at that level while watching the SA IMD. This should only be done with a real voice playback or white noise.

My TS-940/LK-500ZC combo likes 80 and 1200W respectively for -40-42 dB 3rd and most other 100w rigs Ive evaluated are similar. An exception are KW hybrids which are much cleaner. The several K3's Ive tested are horrible at 100W, havent tried a K3S yet.

Carl
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W8JX
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Posts: 13268




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« Reply #95 on: August 21, 2017, 08:45:36 AM »

John,

Here we disagree.

Every tube amplifier I have tested for IMD gets its best rating when they have at least 2 dB of head room. Solid state as well.

I think you miss the point on what I am saying here...  A 8877 can make about 3k with a proper power supply that can supply needed voltage AND current for that but if it is run with a power supply that can handle current for 1500 out but not 3000 the tube will still be 2 to 3 db below its potential without being 3kw capable. Point is you do not need a true 3kw for a clean signal unless you plan to run 3kw and more (as many do but do not admit it). You also said that most antennas are rated 3kw sooo,,, it makes it easier to crank wick up while tell others its for "head room". (The reality this is for big head/ego headroom)
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K6AER
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Posts: 5743




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« Reply #96 on: August 21, 2017, 02:32:59 PM »

John,

At the point the amplifier reaches 1 dB of compression the IMD will degrade significantly by as much as 10 dB on the third and fifth levels. A 1500 watt amplifier that goes into 1 dB compression will be at a bit under 1900 watts. It is pretty hard to hold drive power to such strict levels. The easiest linearity is to have a larger output capable amplifier run at linear levels below the start of 1 dB compression. Their is no magic to this other than Head Room. 2 dB is a minimum.

The other reason to keep the amplifier highly linear is to reduce the harmonics which the FCC is real fussy about.

When the power supply voltage will not allow linear amplification then the drive level must be reduced.

When your signal is S9+20 dB no one will see the difference of 3 dB. What they will see and hear is the excess bandwidth at the start of linearity compression.

Normally this is not a problem with just a vertical or a dipole in the trees. Your excess bandwidth is below most noise levels. This is why many hams can overdrive 1 KW amplifiers and we don't often hear their sidebands because they are using inefficient antennas.

Put a 4 element beam in the sky at 100 feet and your -30 dB  sidebands are S9 to the rest of the spectrum. This is not about ego but having a clean signal at 1500 watts. Even the Flex 1500 uses a pair of 1600 watt transistors but rates the output at 2400 watts max out.
« Last Edit: August 21, 2017, 02:46:28 PM by K6AER » Logged
W8JX
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Posts: 13268




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« Reply #97 on: August 21, 2017, 05:33:42 PM »

John,

At the point the amplifier reaches 1 dB of compression the IMD will degrade significantly by as much as 10 dB on the third and fifth levels. A 1500 watt amplifier that goes into 1 dB compression will be at a bit under 1900 watts. It is pretty hard to hold drive power to such strict levels. The easiest linearity is to have a larger output capable amplifier run at linear levels below the start of 1 dB compression. Their is no magic to this other than Head Room. 2 dB is a minimum.

The other reason to keep the amplifier highly linear is to reduce the harmonics which the FCC is real fussy about.

When the power supply voltage will not allow linear amplification then the drive level must be reduced.

When your signal is S9+20 dB no one will see the difference of 3 dB. What they will see and hear is the excess bandwidth at the start of linearity compression.

Normally this is not a problem with just a vertical or a dipole in the trees. Your excess bandwidth is below most noise levels. This is why many hams can overdrive 1 KW amplifiers and we don't often hear their sidebands because they are using inefficient antennas.

Put a 4 element beam in the sky at 100 feet and your -30 dB  sidebands are S9 to the rest of the spectrum. This is not about ego but having a clean signal at 1500 watts. Even the Flex 1500 uses a pair of 1600 watt transistors but rates the output at 2400 watts max out.

It is kinda funny how you tried to scoot around this. Once again that tube does not know if it has a power supply attached to it that can power it fully to 3kw or not until you try to get there. The voltage can be the same so the tube operates normally at legal limit but voltage will sage and pop a breaker if you push to 3kw. Once again, the ONLY reason you need a full 3kw amp on a 30 amp circuit is because you want to run 3kw because you can easily run legal limit on 20 amp 240 volt circuit but it would have trouble with a 3kw amp and you do not need a 30 amps for head room. Pure BS.

Nothing personal Mike as I know you are a very knowledgeable a good guy. It is just I do not buy into this headroom thing. Its all about profit and gives hams a excuse to buy 3kw plus amps and OEM's to make higher profit 3kw+ amps under the guise of for export only or head room to those with deeper pockets and would buy a 6kw or bigger amp without a bat of the eye if they could. I watched a guy from 4 call district try to buy the 8kw Emtron they had at hamfest in dayton several years ago.
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Ham since 1969....  Old School 20wpm REAL Extra Class..
PA1ZP
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Posts: 688




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« Reply #98 on: August 22, 2017, 09:36:49 AM »

Hi

I hear a lot about headroom bad IMD etc.
The most often dirty sigs are produced because the driver rig has the mike-gain and compressor set way to high.

And I say 99% of those dirty sigs are produced by the exiter not the amplifier.
I have seen the Japanese amp with the 2 x 3-500 in them it is already running for 20 yrs with the same tubes and without any repairs.

I can not find the name of this brand that soon but they went out of buiseness a few years back.

I also have seen Heathkits with a pair of 3-500 running for over 3 decades without troubles in the tube department.

I think an AL82 has plenty of headroom.

I am using a AL572B for a little over a decade now and beside the troubles in getting a quad of decent 572B's the amp didn't give a glitch ever.
So not a bad design exept for the troubles with the bad quality 572B tubes nowadays.

Biggest troubles for me going QRO were the antennas and the antenna tuner department, they need to handle the power too and solving that cost me at least half the price of the amplifier.
And it was that cheap only because I litteraly build all tuners and antennas myself, otherwise that trouble would be even more expensive as the amplifier.

So I would vote for a decent amp with a pair of 3-500.
those amps do not break the bank, and with a bit of luck they will last 20 yrs.

If you realy want to produce a clean signal exite the amplifier with a DSP rig running clean signal processing at the exiters end, and do take care of ALC and compression levels, the mike gain and compressor are the biggest issue in producing bad signals in TX. 

73 Jos
 
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K6AER
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« Reply #99 on: August 22, 2017, 10:03:20 AM »

John,

The guy that bought the Emtron 5 was either NT4I or WV4I, that was back in 2005. It was to be used on a DX expedition. That is their largest amplifier.

Tubes only part of the problem with linearity. The supply voltage must be able to support the RF plate swing. A 3000 watt tube run with insufficient plate voltage might only put out 1500 watts at the 1 dB compression point. Voltage  squared divided by the plate impedance equals wattage.

Alpha uses a pair of 4CX1500B's in the 8410 but they put out no more wattage than the 4CX1000A's did due to power supply limitation of 3300 watts and 3000 volts.  Once an amplifier approaches the 1dB compression point the IMD ratio decreases and the harmonic levels rise quickly. At 2 dB of compression noticeable splatter is apparent on a spectrum analyzer.

Legal  limit amplifiers (1500 watts) that meet the FCC harmonic levels generally will put out above 2300 watts CW. All the Alpha amplifiers from the model 86 and above have 2dB of head room. QRO, Commander, Emtron, FLex, RF Kits (Germany), Elecraft, ACOM, Henry, THP, OM Power all have about 2 dB of head room on their 1500 watt legal limit amplifiers.

In addition many of these amplifiers are not bought for their maximum output level but for their ability to run RTTY at 1500 watts and not do a melt down with a high (100 %) duty cycle.

With the price of legal limit amplifiers coming down hopefully hams will be able to up grade their stations and no longer will be trying to get 1500 watts out of a quad of 572b's.  
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PA1ZP
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« Reply #100 on: August 23, 2017, 08:06:42 AM »

Hi Michael

Tnx for the brands, the name I was thinking for was Tokyo High Power, THP.
their 1k2  2 x 3-500 amps of the 90s are build like a tank, with high quality parts.

And squeezing 1K5 out of a AL572B forget it , at 230V here only 1K3 PEP max on its toes and at modes like RTTY 500 Watts max.
CW max 700 watts to be wise.
SSB about 1K PEP is the absolute max for IMD.

I do not have to tell IMD is awfull at 1K3 SSB because of the only 50Hz in AC here the power supply drops from 2K7 to 2K25 at 1K3
These amps are not build for long duration full pull modes, they are more for SSB.
And the power supply is not well suited for 50 Hz at all.

And IMD for RTTY or digital modes is realy awfull very fast, if you overdrive the modulation or audio input from the PC.
But i am no expert in that I only work SSB and CW.
If i want digital i just use the internet that is way more comfy and faster hihi.

73 Jos
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W8JX
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« Reply #101 on: August 23, 2017, 08:16:53 AM »

And squeezing 1K5 out of a AL572B forget it , at 230V here only 1K3 PEP max on its toes and at modes like RTTY 500 Watts max.
CW max 700 watts to be wise.
SSB about 1K PEP is the absolute max for IMD.

I have a old Dentron Clipperton QRO with 4 ea 572's and it runs a 1000 watts SSB and CW with out breaking a sweat and has for 25 years now. I will handle 800 watts PEP on AM and 800 RTTY. It will make more on SSB/CW but I see little point in it. It still has original Cetrons. I modified stock cooling shortly after a got it and have it sucking air out and not blowing it in and reversed top cover to place inlet holes over band switches and coils. As a result front panel stays cool to touch even after hours of usage. I have no experience with AL572 but I am sure that like other Ameritrons in that class that focus is more on quiet fan that proper airflow for good cooling at high power levels.
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Ham since 1969....  Old School 20wpm REAL Extra Class..
PA1ZP
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Posts: 688




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« Reply #102 on: August 23, 2017, 05:36:27 PM »

Hi W8JX

Mishandled my AL572B for 3 yrs running high power levels.
Cooling isn't a problem, bad chinese tubes are the problem.
They have bad cathode material so electron emmision is going down rapidly.

Also lots of arc problems with others due to bad grids and vacuum.
The only problem I had are with bad tubes, the rest of the AL572 worked rock solid for 10 yrs now.
First set of tubes are in reuse after 7 yrs of the second set of other bad Chinese tubes (bad vacuum and VHF oscilating problems high in the anode) still get 1K2 SSB out of the missused first set of tubes.
The stupid part is they were realy bad when I removed them only 700 watts CW with 100 watts drive, 7 yrs in the attic in a dark big cartbard box and in original tube boxes with plastic bubble wrap reactivated them to 90% again for as long as they will relast again, no idea.

My friend Nico has an exact same AL572B bought 3 month later with the Svetlana tubes from Ameritron, great tubes still like new after 10 yrs those Svetlanas.
He was lucky getting a great set of Svetlanas, I had bad luck getting a bad set of Min Hows.
But my Russian friend Natasha has an even more impressive set of round boulbes tubes with lovely anode caps on the front hihi.

But I still go with my recomandation a tube amp with a pair of 3-500s is still hard to beat.
Or go with the one with a triple set of 3-500s in it.

73 Jos 
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KM1H
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« Reply #103 on: August 23, 2017, 06:35:28 PM »

Quote
But my Russian friend Natasha has an even more impressive set of round boulbes tubes with lovely anode caps on the front hihi.

My Tanya (Tatiana) was born near the Svetlana factory in Leningrad before it reverted to St Petersburg and the #2 pencil erasers on the front are very lively Roll Eyes
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K6BRN
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« Reply #104 on: August 23, 2017, 06:42:12 PM »

John (W8JX):

Michael (K6AER) is correct regarding amplifier headroom.  Unless you are transmitting a signal whose envelope is fixed (not AM, not SSB, not two FM signals or carriers mixing a few HZ apart, etc.)  amplifier back-off from maximum available average power output is needed to avoid distortion on signal envelope peaks.  This applies to ALL linear amplifiers (exciter, power amps., stereo amp, etc.).  I've worked with SSPA and tube amps professionally for years in commercial communications long-haul apps and minimizing back-off has always been important to maximizing power efficiency to limit installation costs - we'd love to get rid of the need for back-off, but to maintain required spectral purity, could never escape it.

The reason is simple and obvious.   When average power is set just below maximum output, the envelope of signals like AM and SSB (or two mixed, offset carriers) will have peaks that rise well above the average power level and can push the amplifier into saturation on those peaks.  If the amplifier does not have sufficient headroom to cover the resulting transient power demand, it will saturate (become non-linear), distorting the waveform peaks and causing a variety of spurious emissions.

Some amateur amps, like your venerable SB-200, have a certain amount of "extra" short-term dynamic headroom above and beyond their maximum CW ratings (as long as the power supply voltage does not have time to sag as it does under CW conditions), and this helps to reduce distortion on SSB when the amp is driven hard.  ALC is another attempt to at least shape the resulting saturation characteristic and somewhat control distortion products  (better ALC implementations only) as the amplifier approaches saturation (which is ugly in the extreme) and also protect the amplifier.

Back-off is not a marketing ploy by any amp vendors - they really never discuss it.  Its just a fact that running your rig and amp well below its maximum AVERAGE power rating when running SSB or AM modes is absolutely necessary to have a clean signal.  Which is where a good peak reading watt-meter can help by telling you where (most) of the peaks are, if its response is fast enough.

Brian - K6BRN

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