Hi Roger, I share your pain with 8877 prices these days. In fact, if any of
my amplifiers using this tube looses a tube it will be rebuilt for one of
the Russian tubes.
If you want a low profile 2m amplifier the best choice would be like the
old 4CX250B designs using a pair of tubes in push pull. They were quite
compact. A more capable tube like the 4CX400 or 800 will get the power right
up
there. The 3CX800 is also a good choice but you are back to the big $$
unless you have a source for good pulls. Thestripline design (biting my tongue
here) would also be a compact approach and could easily be implemented with
a pair of small tubes or a single tube like 4CX1600U.
I am very familiar with the coax based 4CX1600U (also known as GS23B) as I
run one on 70cm. That amplifier will deliver over 2400 watts in cw or ssb
for hours with no problem. I can imagine how a pair of these tubes might
play.
The comment about higher plate current/lower plate voltage was meant
primarily for 70cm operation. The principle applies to 2m but the problem is
many times less severe there.
I have a couple of acquaintances that converted the old two tube repeater
amplifiers to high power by using a pair of 8877's to replace the smaller
tubes. If I recall correctly, they would easily output 4 to 5 kw; a lot more
than I need.
The push pull circuits are effective on 2m as are the coaxial resonators
like the W6PO unit and even the striplines are not that bad. Most of the
stripline shortcomings show up on 70cm and to a lesser extent 135cm.
The GS35 is a popular 2m tube with the home builders. They are tough, cheap
and readily available. They are not a low profile tube though.
Please let me know if I can help or if you need a pep talk.
73,
Gerald K5GW
In a message dated 2/12/2010 3:30:32 P.M. Central Standard Time,
sub1@rogerhalstead.com writes:
TexasRF@aol.com wrote:
> Roger, I have built my share of VHF, UHF and Microwave amplifiers
> through the years. In the process, I have learned a lot about how they
> work and will share a few "secrets" with you.
I've build quite a few amps for HF, but that's a different world.
>
> The book you are looking for does not exist. You are left with three
> options: 1>copy a known good design (such as the W6PO 8877 2m amp),
IIRC that is a rather boxy cabinet and changing the cabinet/enclosure
changes the amps performance. That and I'm not real fond of the 8877.
Besides, they are very expensive and I have no source for pulls. OTOH
the last Ei mac 8877's I purchased new cost me $305. I had hopped to
build something that will match my low band amp in size and shape, which
is not exactly small, but it's still a desk top even if it does weigh
over 100#. <:-))
> 2>wing it on your own and be prepared for some cut and try (that is
> how the known good designs were developed) or 3> buy a new or used unit.
I already have a Henry 2002A that I need to get on the bench and test as
I've already committed to sell. That uses a single 3CX-800. What I want
is the legal limit, but I was thinking of either a pair of 3CPX800's
which would probably be better than a single one, or a design to use a
pair of the GS-23B's with relatively high current and low voltage. (It's
really a 4CX1600u but with coaxial connections instead of pins)
>
> You ask: why can't I just design the amplifier? The reason is that the
> internal inductive reactance of every tube model is different and get
> this: the inductive reactance changes with frequency. As the inductive
> reactance rises with frequency, the capacitive reactance between tube
> elements lowers with frequency. There is even a frequency that the two
> reactances are equal and look like a short circuit. Above that
> frequency the reactance between the tube elements turn inductive
> instead of capacitive. This is at the tube connection points, the
> capacitance is still there internally.
>
> The tube manufacturers for some reason have never characterized the
> internals of the tubes to allow us to accurately predict the
> inductance at various amateur frequencies. One of the reasons may be
> because the tube internal connections become part of the resonant
> circuits. If your plate resonator is a stripline the Zo of the line is
> very different compared to a coaxial or waveguide design.
>
> At HF, the variation of this reactance is not as severe and we can use
> the input and output capacitance and be close enough for matching
> network designs. Not so at VHF and UHF. Even a small 15 pF capacitor
> has the same problem. For example, one with .25" leads has enough
> series inductance to make the 15 pF work as if it were 1000 pF or so.
> With longer leads, it is no longer a capacitor for the frequency of
> interest but looks like an inductor.
With this I'm familiar.
>
> At these frequencies, you do not get to choose loaded Q as one might
> wish to deal with various plate load impedance variations caused by
> changing the plate voltage or current. You have take what you get as
> the loaded Q can be 60 to 100 or more at 70cm. Even on 2m typical
> loaded Q can be 20 to 30.
and 2-meters is my target. 440 is out of bounds as our power limit here
is only 50 watts out of the rig.
> There is little you can do to lower these numbers other than using
> good common sense: minimize any shunt capacitance, use the highest
> quality resonator design (strip lines are not the best),
What is better for 2-meters.
> Use the lowest plate load impedance possible with the tube you have
> (maximum plate current, less than maximum voltage).
That sounds like a pair of tubes would be better than one of the same type.
> And finally, pay attention to rf current flow in the plate resonator.
> If a strip line is used, nearly all of the current flows on one side
> of the tube anode. There is that inductance mentioned above and every
> inductor has a given Qu and resulting loss resistance. If most of the
> current is flowing on 1/3 of the tube plate connection, there is 3X
> current compared to an optimum design. Losses being related to current
> squared, they are 9X-2 or 7 times an optimum design. These losses are
> subtracted from your output power and can damage the tube or
> fingerstock. Also, in some tubes, such as GS23B, the material being
> heated by the losses will change dimensions enough that the resonant
> frequency drops. With the loaded Q being so high, it takes a few
> seconds for the power output to drop to half or less. Of coarse you
> can keep one hand on the plate tuning control and bring it back but
> who wants to do that? These problems go away with a circular resonator
> design.
>
Circular resonator design on 2-meters in a desktop cabinet of reasonable
size?
> Did I mention that I am down on stripline designs?
>
> By now, you have found the ND2X website with all of the various
> Russian tube designs.
Yes,
> There is a wealth of information there and most of it is good. A few
> of the published designs by YU1AW are from computer models and do not
> work in practice. Beware of any designs designated "lazy builder".
I was wondering about those as they just didn't look right.
> I recently helped W9IIX with a GS35 222 MHz amplifier project started
> from the YU1AW info and it was a ball of flames at turn on.
>
> I will be happy to critique any design you might wish to try. In fact,
> I will offer unending moral support for your VHF or UHF amplifier
> projects.
What? and here I was hoping for monetary support as well<:-))
What about the design used in the old repeater amps with two tubes in
push pull and parallel lines?
>
73
Roger (K8RI)
> 73,
> Gerald K5GW
> eme capable on 6m, 2m, 135cm, 70cm, 23cm, 13cm, 9cm, 6cm and nearly
> ready on 3cm
>
>
>
>
> In a message dated 2/12/2010 3:36:42 A.M. Central Standard Time,
> sub1@rogerhalstead.com writes:
>
> I've brought this up before on the Amps reflector, so far I've not
> come
> up with any information.
> I'm looking for information (book or software) that doesn't cost a
> fortune, for designing a legal limit 2-meter amp with a fair
> amount of
> overhead so it can just loaf along using either one or two tubes in a
> strip line or parallel lines.
>
> I've spent hours searching, but most just lead me in a circle.
> I know absolutely zip about 1/2 wave and 1/4 wave strip lines and
> how to
> calculate their dimensions.
> The same is true of the parallel lines. How does the construction
> and
> dimensions change with power.
> How about a pair of GS-23B Parallel lines, or strip line, or a single
> GU-84B/4CX2500, or a pair of GU-74Bs/4CX800's. On HF or even six
> meters
> I can come up with a tank circuit, on two about the only thing I
> can do
> so far is to copy the dimensions of the strip line out of my Henry
> 2002A
> which uses a single 3CX-800. If I went to a pair of them, or the
> 4CX-800s how would that affect the dimensions of the strip line,
> or if I
> replaced the one or two 3CX-800's with a pair of GS-23Bs/4CX1600U
> (IOW
> it's a microwave version of the 4CX1600B which was so popular
> until it
> disappeared although I see Alpha has a few of them yet.
>
> At any rate, this is the kind of information I'm looking for. The
> problem is being able to find it, or construction articles with a
> bit of
> theory, books, or design softtware that doesn't cost a fortune.
>
> 73
>
> Roger (K8RI)
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