[ARC5] WTD: 3CX3000A7 Tube - UPDATE

Sat Dec 1 22:04:06 EST 2012

Henry used an aftermarket socket that is popular with the CB crowd since it 
costs far less than the Eimac.

I used them in the 80's when I built several 160-10M "Contest Grade" amps 
for others and noticed a variance in assembly tolerances. Never had a 
problem but Id suggest checking it out thoroughly and that all hardware is 
tight.

Sent: Saturday, December 01, 2012 7:43 PM
Subject: [ARC5] WTD: 3CX3000A7 Tube - UPDATE

>A few weeks ago, several members of this list gave me helpful advice
> regarding replacement of the Eimac 3CX3000A7 tube in my ham-radio HF power
> amplifier.  These members' collective advice enabled me to find a good
> replacement at a good price.  I've thanked them individually; now I wish
> to express my gratitude by sharing what I learned from the experience.
>
>
> On a fateful day in early October, I left a low-density polyethylene
> (LDPE) box atop my amplifier, blocking the cooling-air exhaust port.  Of
> course the amp was off at the time.  This amp, a Henry 8K Ultra, uses a
> 3CX3000A7 in grounded-grid.  The amp dissipates 1500 watts just idling,
> and more when it's driven.  Because it generates so much heat and blower
> noise, it is made to be located remotely.  Only its controls and meters
> are on the operator's desk.
>
> On Oct. 19th I switched my amp on and I'd been using it for about an hour
> when I smelled hot plastic.  My nose led me to the next room, where I
> found molten LDPE dripping through the amp's exhaust port onto the tube's
> anode heat-exchanger.  After removing the remains of box, allowing the amp
> to cool, and extracting LDPE stalactites, I was relieved to find that the
> amp still worked normally.  I thought I'd dodged a bullet.
>
> But, the next day, after the amp had run hard for a couple of hours in a
> contest, it failed.  Its symptoms were (1) no RF output; (2) plate current
> constant at 0.48 A DC regardless of RF input (drive); and (3) small
> _negative_ grid current.  These symptoms were exactly those of a
> filament-to-grid short inside the tube, or a filament-to-chassis short at
> the tube socket.  (The grid is connected directly to the chassis.)
>
> The DC plate current was just as expected for a 3CX3000A7 with zero bias
> between grid and filament, and 4500 VDC on the plate.
>
> No RF power can appear in the plate circuit when no RF AC voltage appears
> between the filament and the grid.  For no RF voltage to appear between
> filament and grid despite the application of normal RF drive power, the
> short-circuit must have very low impedance.  The driving source impedance
> is low, of the order of 12 ohms.  So the short must be within or very near
> the tube.  It certainly cannot be on the other side of the bifilar-wound,
> common-mode, RF choke through which filament heating power is applied.
>
> Small negative (DC) grid current is expected if the grid is shorted to the
> filament, because the filament current (in common-mode, the _cathode_
> current) is always negative with nearly the same magnitude as the
> (positive) plate current.  With the grid and filament connected together,
> negative cathode current (of 0.48 A in this case) divides between the grid
> circuit and the cathode circuit in proportion to their DC conductances,
> which are essentially the conductances of the respective ammeter shunts.
> This ratio is equal to the ratio of the meter scales, in this case 1:1000.
>
> After cutting power to my amp and letting it cool, I found that it worked
> again; but then it failed again, and again....  The MTBF became
> progressively shorter; and, after about a dozen cycles, the amp was
> useless.
>
> I opened the amp and removed the tube.  With a Triplett DVM I could detect
> no conductance between the grid and filament of the tube, which did not
> surprise me because now the tube was cold.  Nor could I detect any
> conductance between the chassis and the filament contacts of the socket in
> the amp.  I assumed that short had been occurring within the tube, when
> the filament and/or grid became sufficiently deformed by thermal
> expansion.  I knew that the grid and filament wires were tiny and that the
> separation between them was tiny.
>
> I wondered how to replace the tube, and my friend John posted my request
> for advice to this list.  Based on the helpful suggestions I received from
> list members, I decided not to buy a new Eimac tube, which would have cost
> about $1,400.  Instead, for half this price, I bought a Chinese copy.  I
> planned to spend the other half getting my damaged tube rebuilt by Econco.
> More about that below.
>
> I was afraid of buying a Chinese-made 3CX3000A7 because I'd heard many
> horror stories about Chinese tubes.  I'd heard horror stories about
> Svetlana, too.  To be fair, I should mention, I heard that Eimac tubes
> aren't what they used to be, either.  However, I have had good experience
> with RF Parts, Inc.; RF Parts has been in business a long time and AFAIK
> has a good reputation; RF Parts sells a lot of tubes, including tubes
> bearing its own brand name of "Taylor," which are Chinese-made; and RF
> Parts' warranty on its Taylor-brand tubes looked pretty good; so I ordered
> a Taylor 3CX3000A7.
>
> When it arrived, the Taylor 3CX3000A7 _looked_ fine, but I was cautious.
> I applied rated filament voltage with no grid or plate bias and let the
> tube bake for 24 hours, hoping that filament heat (around 400 watts) alone
> would be sufficient to activate the getter and at least partially clean up
> the vacuum, in case the tube had sat on a shelf for a while.  Then I
> connected a Variac autotransformer ahead of the primary of the
> high-voltage transformer of my HVDC supply; and, still with rated filament
> voltage, I increased the plate voltage gradually, starting with just a few
> hundred volts, while keeping the grid bias zero or small so the plate
> would draw relatively high current and would dissipate relatively high
> power at low voltage.  My reason for keeping plate voltage low was to
> avoid avalanche breakdown in case the tube was gassy.  My reason for
> keeping plate dissipation high was to get the getter to work.  Little by
> little, I increased plate voltage and dissipation over the course of two
> days, until the tube had spent a few hours with plate voltage near 3 kV
> and dissipation near 1 kW (with air cooling, of course.  I monitored
> external anode temperature with a thermocouple.)
>
> Then, I began applying RF drive power, watching the grid current and
> calculating the DC-to-RF efficiency.  Everything looked just fine.  So I
> declared the tube operational and used it at normal voltage and power for
> 26 hours in an HF DX contest, as usual transmitting 1500 W on all bands
> from 160 meters (1.8 MHz) through 10 meters (28+ MHz).  This tube's
> performance was indistinguishable from that of my original Eimac tube
> before that tube failed.  For the same RF output power at the same
> frequency, it required the same RF input power; it drew the same (small)
> grid current; and it drew at least approximately the same plate current.
> (I do not observe plate current as closely as I observe input and output
> RF power and grid current.  I don't care about plate current unless it's
> WAAaay out of line.  I always monitor grid current, for safety; and I
> always monitor RF output power, for legality.)
>
> For a given radio frequency, the plate-circuit (pi-L network) capacitor
> and inductor settings for maximum output from the amp were also the same,
> from which I infer that the Taylor tube's inter-electrode capacitances and
> other "stray" reactances and electron ballistics are also
> indistinguishable from that of my original Eimac tube.
>
> Bottom line: I am very happy with the new tube, which cost half as much as
> a new Eimac tube would have.  Only
> time will tell how long it lasts.
>
> I sent my original Eimac tube to Eimac's sister company Econco for
> rebuilding.  A few days ago I got a telephone call from an Econco engineer
> telling me the results of his "acceptance" or "qualification" testing of
> my tube.  I had expected to receive such a telephone call telling me
> whether the tube could be rebuilt, and if so, for what price.  However,
> this guy told me that he could find nothing wrong with the tube!  He said
> that the tube shows no sign of having been overheated or having been
> damaged in any way.  In particular, there's no darkening or discoloration
> of any exterior surface, metallic or otherwise; and the vacuum is
> effectively perfect.  He had tested the tube with 15 kV plate-to-grid and
> (more impressive IMO) 5 kV filament-to-grid.  Filament emission was just
> fine, typical of a tube that's been used for relatively few hours (as mine
> has been).
>
> He agreed with my interpretation of the symptoms I'd observed, namely that
> they implied a grid-to-filament short; but he thought the short must have
> been external to the tube.  He said that, if the filament and/or grid of
> my tube had been deformed enough that a short could occur when the tube
> got hot, then the tube could not have passed his 5 kV grid-to-filament
> test.  He said that, when a tube of this type is overheated, the first
> thing to go is the vacuum, because a metal-to-ceramic seal fails -- NOT
> the grid or the filament structure.
>
> Bottom line:  Econco is returning the tube to me, at no charge except for
> UPS Ground shipping.  Wow!  How many times have you paid someone more than
> $100 just to tell you that he couldn't find anything wrong with your [fill
> in the blank] ? !  Ya gotta love Econco.
>
> I'm left wondering how a filament-to-grid short could have occurred
> external to the tube.  I can only guess that a loose screw or other piece
> of metal found its way into the space between the tube socket and the
> chassis, began shorting one of the filament terminals to the chassis, and
> then fell out when I pulled the old tube out or pushed the new tube in.
> Eventually I'll take the amp apart far enough to look for a culprit.
> Meanwhile, my amp is working like a champ.  Last night I spent another 12
> hours or so using the amp in a contest, again with no sign of trouble.
>
>
>
>
>
>
>
> ======
>
>
> A friend has a Henry Linear Amp and the tube has died. I'm posting for
> him. He needs a replacement.
>
> It's an Eimac 3CX3000A7. Preferably new, but would consider known good 
> used.
>
> Also what do folks know about the Svetlana 3CX3000A7 or the Chinese tubes?
> Svetlana claims that it's an exact replacement for the Eimac tube.
>
> Thanks,
>
> -John
>
> =========
>
>
>
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