I wouldn't buy them from that internet place unless they were from someone
that guaranteed them and has tested them. Most of them are sold as-is with
comments like "they worked when they got pulled" or "I was told they
work"...
www.elexs.com lists them for about $200+ for NOS and $85-$120+ for used. He
guarantees them and tests them all and can give you the performance curves
as well.
Still, for $160 each, 3-500ZG's from RF parts really can't be beat...
Check on the classifieds at www.qth.com. Good tubes show up there all the
time. A single 4-1000A (a tube which I use in two amps) will run you
$200-$300 and can take all kinds of abuse. I had one broadcast pull go over
20 years of heavy use including RTTY contesting before it gave up (in the
middle of a contest). Running the tube at 4000V with 100W out will give you
legal limit and won't get too bad on component ratings. Contrary to many of
the comments I have heard here, I get legal limit with 4000V (full load
voltage) and 100W drive. I wouldn't recommend higher voltages because the
increase in component ratings and risk of generating X-rays outweighs the
benefits. There are a ton of articles in the various (older) Ham
publications using this tube and the glow of the plate going almost white
hot during key down RTTY is a sight to see!
Mike, W1NR
----- Original Message -----
From: "Chris Howard" <chris@yipyap.com>
To: "Joe Subich, W4TV" <w4tv@subich.com>
Cc: <amps@contesting.com>
Sent: Tuesday, October 24, 2006 12:11 AM
Subject: Re: [Amps] More information for "What NOT to buy"..
>
> Is the 4-400A a good choice for something cheap to start with?
> There seems to be a steady stream of cheap used ones on that internet
> place.
>
>
>
> Chris
> w0ep
>
>
> On Mon, 2006-10-23 at 22:01 -0400, Joe Subich, W4TV wrote:
>> Clint,
>>
>> You have gotten good advice ... but a couple more thoughts
>>
>> Almost any amplifier is a good starting amp but be prepared to make
>> some changes and learn from the experience.
>>
>> 1) The MLA-2500 is to be avoided - as others have said - because of
>> the fragile and out of production 8875. Yes, it can be converted
>> to other tubes but the small chassis is a significant handicap to
>> a clean conversion.
>>
>> 2) home built amplifiers using ex-USSR tubes are probably not a good
>> vehicle for learning. Replacing the ex-USSR tubes can be a problem
>> as many are not being made and surplus sources are not reliable.
>>
>> 3) External anode tubes (8874/3CX400, 3CX800, 8877/3CX1500) are
>> expensive and have fragile grids. The 3CX800 and 3CX1500 are
>> great tubes in the hands of an experienced designer but are
>> probably not a good place for the beginner to start.
>>
>> Recommendations:
>>
>> A solidly built single 3-500Z or pair of 3-500Z amplifier ...
>> SB-1000, AL-80, SB-220, L4B, L7, AL-82, etc. You can learn
>> much but be prepared to eliminate the "floating grids", add
>> the proper glitch resistor, improve the cooling of any external
>> power supply, fix the bias and metering circuits, and convert
>> the T/R switching to low voltage and (possibly vacuum relays).
>> Converting any of the 3-500Z amplifiers to one of GM3SEK's
>> "triode board" controllers would be worthwhile from both a
>> reliability and learning prospect.
>>
>> A clean amplifier with FOUR 572Bs is not to be avoided. With
>> the proper voltage 4 x 572B will produce nearly the legal
>> limit and the tube cost will not break the bank. Again, be
>> prepared to fix many of the "floating grid," bias and T/R
>> switching issues. I would avoid the two tube 572B amps if
>> only because they are generally too small, undercooled have
>> poor layout are difficult to service and cost as much or more
>> than a single 3-500Z amplifier.
>>
>> 73,
>>
>> ... Joe, W4TV
>>
>>
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>
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