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Author Topic: Looking for Linear Amplifier  (Read 31140 times)
W8JX
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Posts: 13268




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« Reply #105 on: August 24, 2017, 05:51:56 AM »

K6BRN

You want to great detail to try to sell that you MUST have a 3kw amp to have a clean legal limit amp but that dog still does not hunt. A solid clean 1.5 legal limit amp DOES NOT have to be 3kw capable to be clean.  I think most of your comments are better applied to over rated or over taxed amps that at pushed to their limits.(like AL811 series over driven SB200's AL80's driven to 1000 watts and more and so on) When you do this the tube(s) looses its linearity. You can build a amp with a 8877 or pair of 3cx800's or even a pair of 3-500's that can handle 1.5kw with ease (in cases of 3-500's though you need more power supply, voltage and current than is in a stock SB220, TL922 or L4B). It is down right silly to suggest or try to sell that you MUST have 3kw capability to be clean at 1500 watts.  It is your choice of tube and it operating voltages that determine this. Bottom line is 3kw amps sell better because many want them brag factor and run 3kw and never say a word (some run even more because FCC has had its funding cut years ago for compliance) Nice debate though....
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Ham since 1969....  Old School 20wpm REAL Extra Class..
KM1H
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Posts: 5533




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« Reply #106 on: August 24, 2017, 09:47:10 AM »

As built the SB-220, TL-922, L4B, L7, LK-500 series and several other 2 x 3-500Z amps are very clean at 1200-1300W which is the limit of their PS. With an additional 500V at full load they would still be fine at 1500W.

The Viewstar/B&W PT-2500A and AL-82 with a bit more HV can be pushing almost 2000W with the same fine IMD.

Also remember that the RF efficiency with these, and many other tubes, increases as the HV increases.
You should see sometime how well and how clean a GG 4-1000A plays at 5000-6000V and is still within Eimac specs.

Carl
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K6BRN
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Posts: 1346




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« Reply #107 on: September 09, 2017, 08:39:36 AM »

Hi John:

Quote
You want to great detail to try to sell that you MUST have a 3kw amp to have a clean legal limit amp but that dog still does not hunt. A solid clean
Quote

I said no such thing.  What I did say is that you MUST have adequate headroom above and beyond the average power the amplifier is providing to have a clean signal.   The way to do that is to BACK OFF from the amplifiers maximum output by a reasonable amount to ensure a clean signal.  And the headroom needed will vary based on the peak to average power ratio of the waveform used.  SSB has a very high ratio.  CW, when measured from the "on" keying condition has a ratio of 1 - low.  There is no opinion here - that's simply how it works.

In my career, we always ensured the SSPAs and TWTAs (RF amplifiers) were below their 1db compression points, because if they went into that area, our spurious emissions exceeded ITU limits.  We had to worry a LOT about amplifier backoff needed due to the peak to average power ratios of various modulation types and the interactions of severa user signals in the same amp that could cause peak power excusions exceeding the headroom we built in.  At the same time, too much headroom caused the amps to be used and operated inefficiently, wasting power, weight and space - major concerns in my business.

The exact same principles apply to amateur radio amplifiers.

However, you've always been a minimalist and I'm a belt AND suspenders engineer who had to deliver a robust, working product, so we have different perspectives.

Cheers,

Brian - K6BRN

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K6BRN
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Posts: 1346




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« Reply #108 on: September 09, 2017, 08:40:38 AM »

Oooops!  Quotes can be a little hard to use from a cellphone....
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AC6CV
Member

Posts: 302




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« Reply #109 on: September 19, 2017, 09:19:31 PM »

I drive a pair of 3-500z tubes with 50 watts and get about 1200 watts out.
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KE4LJH
Member

Posts: 75




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« Reply #110 on: September 20, 2017, 06:17:43 AM »

If you are going to use 600 watts to the full legal limit with your amplifier you need to run the amp on a dedicated 240 volt circuit wired right to your electrical box. You need the 20 amp circuit in the room to run the radio, power supplies computer and other accessories and don't forget the lights. Running an amplifier on the same 120V circuit is not going to work. That is way too many amps to draw.

Someone correct me if I wrong but running that much power with 120V circuit is harder on the tubes and will noticeably raise the electric bill not to mention that there might not be enough amperage to draw to run the amplifier at full level.

The reason for using 240 volt service is specifically for this sort of thing. High amperage draw appliances. You are buying amps from the power company. Higher voltage allows fewer amps to be drawn.

As you look at amplifiers ask what the amperage draw for amplifier is at full power at both 120v and 240v.
I think you will see that a 120v 20 amp circuit is just not sufficient.

Wire a dedicated 240 volt circuit. Yes, you need to do this. It is not an option.
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K6AER
Member

Posts: 5743




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« Reply #111 on: September 20, 2017, 03:38:32 PM »

The transformer on high power linears have two 120 volt windings. In Parallel for 120 volts and in series for 240 volts. The 240 volt setup is about 3% more efficient due to less IR drop. Either way the tubes don't care. On a long run from the AC panel you will see less voltage drop due to the current being one half or the 120 volt feed. In lower duty cycle modes such as SSB the AC voltage IR drop is less of a factor.
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K6AER
Member

Posts: 5743




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« Reply #112 on: September 21, 2017, 08:20:27 AM »



Someone correct me if I wrong but running that much power with 120V circuit is harder on the tubes and will noticeably raise the electric bill not to mention that there might not be enough amperage to draw to run the amplifier at full level.

The reason for using 240 volt service is specifically for this sort of thing. High amperage draw appliances. You are buying amps from the power company. Higher voltage allows fewer amps to be drawn.




Wither the primary is 120 volts or 240 volts the secondary voltage is the same. Tubes will not be effected.

From the electric company you pay for KWH. Kilowatt hours. Not amps.
« Last Edit: September 21, 2017, 08:23:09 AM by K6AER » Logged
AE0Q
Member

Posts: 106


WWW

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« Reply #113 on: September 21, 2017, 09:14:44 AM »


Someone correct me if I wrong but running that much power with 120V circuit is harder on the tubes and will noticeably raise the electric bill not to mention that there might not be enough amperage to draw to run the amplifier at full level.

The reason for using 240 volt service is specifically for this sort of thing. High amperage draw appliances. You are buying amps from the power company. Higher voltage allows fewer amps to be drawn.

Wither the primary is 120 volts or 240 volts the secondary voltage is the same. Tubes will not be effected.

From the electric company you pay for KWH. Kilowatt hours. Not amps.

Yes, you pay for watts, it doesn't matter at what voltage you use them.

What DOES change is the resistive loss at lower voltage (120vac) and higher current (called I2R loss).  Your electric kitchen range and air conditioner compressor waste less heat in the wiring when running on 240vac than at 120vac because they use less current that is going through the same wiring.  Less current (I) times the same resistance (R) in the wiring squared is less wasted watts.

Glenn AE0Q
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Glenn and V-NATCH Katie,
HP-O, MXB, MJB, XF, TKI
http://www.hoopsandjumps.com/
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