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Author Topic: Svetlana 4CX400 for 4CX250B?  (Read 3513 times)
WY3X
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Posts: 768




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« on: January 29, 2003, 09:52:22 PM »

Was doing searches for AM-6154 and Svetlana 4CX400, because I am aware that some hams are swapping the Svetlana tube for the 8930 in these amps.

I'm purchasing a "Clipperton V" 2M amplifier. To make a long story short, wheels started turning in my head, and I was thinking about the conversion of the AM-6154 series amps to the Svetlana 4CX-400. I have an AM-6154 on 2M.

Now- the interesting part: I have seen numerous websites with info on converting the Clipperton V to 6M, so this is in the plans. The Clipperton V uses a 4CX-250B. How compatible is the Svetlana 4CX-400 to the 4CX-250B? Is it as much a drop in replacement for the 4CX-250B as it is for the 8930?

Thanks, -Web Williams in Myrtle Beach, SC
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WA9SVD
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Posts: 2198




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« Reply #1 on: January 29, 2003, 10:38:33 PM »

Very different specs.  It's not a drop in replacement.   (Wish that it were...)  And you'd probably need nore air flow to provide for the added dissipation, and a heftier power supply.
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DK3QN
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Posts: 124




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« Reply #2 on: January 30, 2003, 06:01:33 AM »

Hi Web,

look at this

http://www.svetlana.com/docs/TechBulletins/appnoteNO.5.html

plus this

http://www.svetlana.com/docs/TechBulletins/appnoteNO.1.html

Hope it will answer most of your questions.

73, Klaus, DK3QN
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WA9SVD
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Posts: 2198




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« Reply #3 on: January 30, 2003, 12:48:54 PM »

Hi Web,
    The Svetlana 4CX400 and the 4CX250 both to utilize the same socket, such as the SK600.  (I don't know if the typical screen bypass ring used with the 4CX250 would be usable.)  That's about where the similarity ends.  (I can't get the dimensions for the Svetlana tube, as their web site is down at the moment.)  Because of different dimensions and internal capacitancess, you would probably have to modify your input and output circuits. I also believe the anode is of a larger diameter than that of the 250, and the filament draws almost an Ampere more current than the filament of the 4CX250.
    In addition, the power input to the 4CX400 (and thus the output) will be limited by the power supply in an existing amplifier.  Even if you use a more powerful tube, it's unlikely you can squeeze much more power out of the plate supply.  You would probably have to increase the screen voltage for proper operation.
    I doubt you would gain (no pun intended) any significant advantage in converting the amp; I don't think it's worth the effort or expense.  New Svetlana 4CX250B's sell for $105; I couldn't find a price for the 4CX400.
    If you were building a new amp, the 4CX400 might be a good choice, as it does have significantly higher ratings than the 4CX250, and it's rated to 500 MHz.
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WB2WIK
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« Reply #4 on: January 30, 2003, 05:26:37 PM »

I'd stick with the 4CX250B, unless I couldn't find any or I got a very good deal on a 4CX400A.

Reasons:

New filament xfmr required for 4CX400A, which is 6.3v/3.15a instead of 6.0v/2.6a.

Anode clamp needs to be replaced, since the 4CX400A anode is 0.36" larger in diameter.

Input circuit for VHF would need redesign, since the 4CX400A is Cin = 24pF, as opposed to the 15.7pF of the 4CX250B.

Bias supply might or might not have sufficient range to adjust for the 4CX400A, which requires -30.5vdc for 100mA ZSAC, as opposed to the 4CX250B which requires -55vdc for the same ZSAC.

Anode and screen supplies would probably work.

Ouput tank might require some modification for the 4CX400A to accommodate its Co = 7pF (the 4CX250B's is only 4.5pF).

4CX400A might need a bigger, or at least a different blower; but maybe not.  The cooling requirements aren't that different.

The 4CX400A will produce no greater output power than the 4CX250B when operated at the same bias conditions, e.g., 2000Vdc Ep, 300Vdc Eg2, etc.  Either one will produce 300W output under these condx, per their graphs.

If I got a killer deal on the 4CX400A, I'd probably use it and make the amp mods required.  But in the interim, I've got lots of 4CX250Bs and they seem to last forever....

WB2WIK/6
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WA9SVD
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« Reply #5 on: January 30, 2003, 07:13:05 PM »

Thanks Steve,
    You were more eloquent in your answer, but I think we're basically in agreement!  (And I couldn't get to the Svetlana site to get the physical and electrical specs on the 400.) I don't have current prices for both tubes, but in the '98 catalog from R.F. Parts, the U.S. 4CX250B was $95, and the Svetlana 4CX400 was $129.  For no significant increase (if any) in output, I wouldn't go through the effort necessary to make the modification or pay extra for the tube.
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WY3X
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Posts: 768




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« Reply #6 on: February 02, 2003, 06:22:52 AM »

Thank you all so much for your answers- I think I'm going to stick with the 4CX250B, at
least for the time being. I was reading up on some of the AM-6154 amp sites, and they
mention that other than having to spread the anode fingers out VERY carefully, that the
tube is a drop-in replacement for the 8930, which to me, looks physically similar to the
4CX250B (I know the 4CX250B has the smallest anode of the three). The reason they were
swapping the tube was because the 4CX400 ran cooler (with .3V less filament voltage than
it wants, I guess it would somewhat) and put out somewhat more power (about 50 watts
greater than the 8930). Perhaps (without taking the time to do comparisons) the 4CX400
specs are more closely related to the 8930's than the 4CX250B's, and that is why it was
chosen for that particular application. I may get one (4CX400) for the AM-6154 while leaving
the other amp I'm getting as-is (but still replace the "soft" tube with a newer 4CX250B). I
don't use my amps that much in any event, so I'm sure it'll last me many years, I just like
things to perform at "peak", (yet as cool as possible with minimum stress on internal
components) when called upon to do their job. Next project- a quieter fan for the AM-6154!
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WA9SVD
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Posts: 2198




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« Reply #7 on: February 02, 2003, 03:32:36 PM »

Remember also that the 4CX250B uses a filament voltage of 6.0 Volts (and needs to be derated above 100 MHz) not 6.3 Volts.  Running the tubes at 6.3 volts would shorten their life considerably; but running a tube at too low a voltage will also damage it.  (So says Eimac.)
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