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Author Topic: Alpha article about AC line wiring gauge feeding their RF Amplifiers  (Read 4246 times)
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WD5JKO
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WD5JKO


« on: March 26, 2010, 10:02:51 PM »

 Hi All,

 The Alpha folks in their latest newsletter describe an RF linear amplifier at 3KW DC input from the AC mains to get 1.5 KW RF output. The amp runs off 240v and should pull 12.5 amps. Since it uses a capacitor input filter, and I presume a FW voltage doubler, we have stout 150 amp current spikes with an RMS value of 12.5 amps. The 20a fuse is happy with this. Then Alpha goes on to describe how those 150 amp spikes combined with the AC line wiring resistance cause the DC power supply to sag. They make the case for #8 awg wire, and as short as possible back to the breaker box.

  This makes me wonder when we will have active PFC switching supplies in the big commercial amplifiers so that the input current is sinusoidal and in phase with the voltage. Of course, a good old fashioned choke input filter can do the same thing..

Download the Alpha newsletter below..

Jim
WD5JKO  

* NewsletterMarch1.pdf (487.3 KB - downloaded 428 times.)
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k4kyv
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Don
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« Reply #1 on: March 26, 2010, 11:59:31 PM »

Maybe you need one of those $600 power cords made with oxygen-free copper wire.

Interesting that they take 3000 watts DC input to get 1500 watts out, while according to FeeCee Magic anything over 500 watts DC input on AM would exceed the limit.

And Hammy Hambone is supposed to be able to figure that out?
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Don, K4KYV                                       AMI#5
Licensed since 1959 and not happy to be back on AM...    Never got off AM in the first place.

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« Reply #2 on: March 27, 2010, 09:11:32 AM »

Alpha has had too many owners and engineering changes to know whats even going on these days besides charging outrageous prices for a mediocre product.

Since the Ruskie GU74B is no longer made they have switched to 2X Chinese 4CX1000A's in their latest amp. Hopefully those tubes will have the same great IMD of the Eimacs and not require a idle current of 75-80% of Pd to approach acceptable IMD of the Ruskies which are being run well out of their design specs when Svetlana relabled (along with changing specs to 800W Pd from 600W and raising the Ep) them 4CX800A's as a way to con the US market.

Alpha and Emtron are in a neck to neck fight to see who has the least reliable product at the highest prices.

The 1963 NCL-2000 which could do about 1200-1300W PEP out was shipped with a 10-3 power cord. Owners often replaced them with #12 or 14 because they were too bulky. Most current amps are shipped with #14 and I recently recieved a NCL-2000 for repair using #16 AND it still puts out 1250W plugged into my 30A 240V line.

Input line spikes as well as output spikes from the diodes can be controlled. It just takes a little bit of money and parts, something Alpha probably doesnt want to do.

Carl
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W2VW
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« Reply #3 on: March 27, 2010, 10:56:18 AM »

I used an NCL-2000 for a few years on AM. Way ahead of it's time and a lot of power out of a little footprint. No design errors. No wasted space but tank losses seemed OK even with all those parts close together. My only complaint is it was 2 pounds over the limit for USPS shipping when packed after I sold it. 
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KM1H
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« Reply #4 on: March 27, 2010, 11:47:08 AM »

That must have been before UPS raised the limit to 150# Grin

I used mine on AM many times since 64 from the CE100V days up thru maybe 10 years ago with a TS-940. It still works fine but these days it only gets used with the HT-32B and SX-115 vintage SSB station.

Another engineering prototype I converted to 6M in 64 and still use it. That really saw a lot of AM use until I picked up the Clegg Interceptor B and Zeus twins. Now its strictly for CW/SSB.

Both have been trouble free except for routine things like filter caps and tubes. The 67 date code 8122's in the 6M one were installed once RCA fixed the design and were still putting out 750W when I replaced them a few years ago; now back to 1200W. Not many amps can claim that level of reliable service.

Being on the design team got me "preferred" dibs on the prototypes at $50 each!! Others on the team werent ready for something so small at home or got the others Grin

Carl
KM1H
National Radio 1963-69

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KD6VXI
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« Reply #5 on: March 27, 2010, 02:41:29 PM »

My Harris RF-103 (single 3-1000Z) has a pair of 15A breakers in it, one for each leg, or in parallel if you (gulp) try it on 110.

It states to use 8 gauge wire, specified for 1Kw output, any more.

Rated input (IIRC) is about 3300 watts. 


--Shane
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AB2EZ
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« Reply #6 on: March 27, 2010, 06:07:20 PM »

It is interesting to look at his calculations (considering that he is their VP of Engineering)

1. He states that the 415 volt peak amplitude of the sine wave generator in his SPICE simulator corresponds to 240 volts rms. However, 240 Volts rms x 1.414 = 339 volts... so he needs to recheck his math.

2. In the simple model he is using, he is assuming that the transformer has zero winding resistance (primary resistance + secondary resistance reflected back to the primary side). I have a very high quality 4kVA plate transformer, and the primary resistance is 0.26 ohms. The secondary resistance is 8 ohms. Reflecting the secondary resistance of this transformer, which is 240 volts input and 2400 volts output, back to the primary side, the total equivalent primary side resistance is 0.268 ohms.

3. If I use LTSpiceIV (certainly sufficient for this purpose), and the correct peak source voltage (339 volts) I find the following:

a. With only the transformer's 0.268 ohm resistance included in the circuit, the average voltage across the capacitor (remember, the entire circuit has been reflected back to the primary side of the transformer) is 320 volts; and the peak of the AC is 50 amps

b. If I add in the resistance of 200 feet (two wires, each 100 feet long) of #14 wire (i.e. I add in 0.6 ohms), I get a total of 0.868 ohms of transformer resistance plus AC line resistance. In that case the average voltage across the capacitor is 306 volts; and the peak of the AC is 32 amps.

Therefore the effect of 100 feet of #14 power line is only a reduction in the voltage at the output of the supply of 14 volts out of 320 volts, or 4.3%.

I suspect that the actual effect would be less, because I suspect that the transformer Alpha is using has a higher equivalent winding resistance (as reflected to the primary side). Also, most 220 volt lines would be a larger gauge than #14. With 200 feet of #12 wire (two wires, each 100 feet long), having a total of 0.374 ohms of resistance... and therefore using (0.268 + 0.374) ohms = 0.642 ohms as the total of the transformer and the power line resistance... the average voltage across the capacitor is 311.5 volts. This is only an 8.5 volt drop v. 320 volts (no power line resistance), or 2.7%.

As a separate observation, the 2000uF capacitor shown in the diagram (corresponding to less than 20uF on the secondary side of the transformer... where the rectifier/filter actually resides) is too small to do an adequate filtering job (in my opinion). The residual ripple of the voltage on the capacitor has a peak-to-peak value of about 5.75% of the average value of the voltage on the capacitor.

Stu

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Stewart ("Stu") Personick. Pictured: (from The New Yorker) "Season's Greetings" looks OK to me. Let's run it by the legal department
N3DRB The Derb
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« Reply #7 on: March 27, 2010, 06:39:36 PM »

when I had the garage wired I told the guy to use #12 for the branch circuit even though the closest outlet is only 10 ft from the breaker box. I knew I'd be drawing some soup thru that line someday.  I have the gonset hooked in with 12-3 and a 12 amp fuse, manual says with the 572b's you need a 20, but I've never blown the 12 amp fuse even at a full KW out keydown for a 2 second burst or 2.

I wonder what is the best ac line for Uncle Ray, he draws 1700watts using 220. but he's only going to be 3 ft from the outlet.

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KM1H
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« Reply #8 on: March 27, 2010, 07:10:17 PM »

The SB-220 has a 10A breaker, the NCL-2000 a 12A fuse, and the LK-800A a 15A fuse. All have 2 of each for 240V.

Carl
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