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Author Topic: 4PR400A for 3-500z?  (Read 12000 times)
DG0OLG
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Posts: 7




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« on: October 04, 2004, 05:11:26 PM »

Hi Folks!

Got an L-4B with the 3-500z (by Eimac) installed. Additionally I got a matched pair of 4PR400A also made by Eimac.
My question is now:

Are they dircectly exchangeable?
(That means not one bye one - I mean pair by pair. Just to make it sure.)
Or is there any Modification to be done prior to installing them into the amp?
Got up to now no Data-Sheet of the 4PR400A's.

TNX for helping me.

73 de Sascha (DG0OLG)
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W7DJM
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Posts: 1




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« Reply #1 on: October 04, 2004, 08:13:20 PM »

Well, they "sorta" work.  4-400's are tetrodes, 3-500's are triodes.   Just about any amplifier you put them in, such as yours, or maybe the old Heath SB-220 is going to --want more drive for less output power,  change the input impedance (high swr at input)

You also want to make sure that the sockets in your amplifier actually do have grid and screen connections, and that they are properly connected for these tubes.

4-400's, really, really, don't operate well this way, according to what I've read. And, I did run a pair, for awhile, in an old SB-220.   According to some of the "old masters"  people like W6SAI (William Orr)  the control grid can "suck" more than it's share of the grid current.  Even though you are monitoring "grid current"  what you are really seeing in the case of these 4-400's is the TOTAL grid and screen current, unless you have taken pains to wire the grid meter accordingly.  You really have no idea of whether you are overdriving the control grids, or not.

They probably should have higher plate voltage to be efficient.

They probably should have the screens bypassed and some screen voltage applied.

If you do put screen voltage on them, you are going to increase the gain, and may then have stability problems.

Aren't they taller?  Does your amp have chimmneys, will the tubes fit them, will they even fit inside the case?

They just "ain't" the same tubes.

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I8VYE
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Posts: 16




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« Reply #2 on: October 05, 2004, 03:32:22 AM »

Hello,

this is my experience:

After 7 years of inactivity i switched again on my LK450 (1x 3-500z) amplifier. The tube was gassy and i didn't succeed in burning it again, it arced very hardly. Fortunately I had some tetrodes (4-250a) from pulls. So i checked old handbooks finding some projects of amplifiers with tetrodes with both grids grounded.

So I installed the tetrode, checked that all grid pins were grounded. Than I retuned the input PI. The amplifier is a notune type so I hade to retune also the output PI.

It works. Of course the tube is only 250W in plate dissipation so i can't obtain the same output of the old 3-500z. The 3-500 is 500Wplate dissipation with 2100V of anode voltage under load. But it works giving a reasonable output of 500Wpep without any futher expense. The output is measured by both Bird 43 and Bird digital power analyst. It's true that the total grid current is higher the grid current of the 3-500z.

Distorsion is not much worst than the one i had in the past.

I don't use the amp for contest, but for the usual QSO's it works, plate seems to glow correctly, and I drive the amp with a Ts830 120W out.

No major problems, and only a little work to fine tuning the pi's when i activated the tube.

Surely it's not best way to use tetrodes, and the best substitute for a 3-500z is a 3-500z (may be 3-500zg), but i already had the tetrode in the shack.

'73s de i8vye. Joe

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I8VYE
Member

Posts: 16




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« Reply #3 on: October 05, 2004, 03:45:31 AM »

Just for clearness: I meant that i am obtaining around 500Wpep out from a 4-250 in SSB with a 2100 Volts of plate voltage under load.

again '73s Joe.
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WIRELESS
Member

Posts: 1




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« Reply #4 on: October 08, 2004, 07:49:12 AM »

Electrically they can be interchanged.  4-400a tubes were used interchangeably with 3-400, 4-250, 3-500z tubes.  They are not identical but they will work and a 4-400a will give lower output for the same drive.  I would never go out and buy new 4-400a's to replace 3-500z's but if I already had them I certainly would use them.  Physically, a 4-400a or 4-400c is much more rugged tube than a 3-500z.
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