8935 tube - good for anything?

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Drop Rich Measures a line at . He would be into it. He built a box with a 4CX5000 and then one with a 4CX10000. Here's his URL: http://www.somis.org/ I bet he would have the low down on that bottle.

Yep, Richard Measures does advocate these tetrode tubes in grid driven service.  But again, when you run 10db more power than the average ham amp, you need it to be 10db cleaner than the average ham amp - or they will complain.  A grid driven amp WITHOUT heavy RF negative feedback won't pass the test.  Some grid driven tetrodes in LINEAR service designed for class C operation (many) can't do any better than -25db 3rd order IMD when driven to near full power. Some are even worse. 

Maybe it won't get complaints on AM cuz of the common wider signals and the large variation in bandwidth standards from station to station - but on ssb where everyone demands you be 3kc wide and down 30-40db after that, this is where the problems start.  If the DX signals at sunset (75M DX window) are S7 and your neighbor is S9+60 over, you can imagine that even a "clean"  -35db 3rd order IMD signal is gonna trash you 5-6 kc away and take out the weak DX.   

For linear grounded grid (GG) operation driving the tube in triode config, you will need at good 1500w pep output amplifier for drive. Figure on 10db of gain, more or less for the 4X15 GG stage.  I would think that combo would produce no better than -32db 3rd order too. Still a problem for a QRO amp. 

Adding screen and grid voltage to a GG amplifier (cathode driven) DOES NOT make it easier to drive, contrary to popular opinion.  As you increase screen voltage, you need more grid bias to idle it down, thus making it harder to RF drive the grid back to zero volts.  At least that's W8JI's opinion and based on my own personal experience.

Bottom line, when extreme QRO, you need an overall amplifier chain that yields -40 to -45db 3rd order to blend in.  It is a very difficult thing to do for the average builder.   You can always tell the "-32db IMD" guys running extreme QRO (on ssb) by relativity to other signals - simply tuning off their signals and hearing them up the band 5-6kc.   Once you "normalize" your receiver for the present conditions and the average 1500W signals, these guys stand out like a sore thumb.... especially when angles are high and locals are loud.

When someone pulls off extreme QRO with a super clean signal and no blower noise, now that's really doing something... :-)

I've seen many guys get run off the 75M DX Window due to their splatter. Many come back later with super clean amplifiers once they've paid their dues. I went through it myself before I knew any better.

A single 3CX-3000A7 using a very clean class A solid state Mosfet amplifier (300w) as a driver can run -45db 3rd order right outa the box.  Plenty of power and stability without RF neg feedback required. That tube is possibly the cleanest GG tube out there and available.  Another good choice is the YC-156 (3CX-5000) of  medical magnetic resonancing fame.  No socket required and beautiful specs.  Reasonably priced used pulls.  Loaf these tubes along most of the time at 1/2 peak power and you're golden - clean as a whistle.

BOTTOM LINE?  If you're gonna go through the tremendous effort of building a big amp, give yourself the best chance of success by picking a good, clean tube designed for the particular service,  from the start.


My 4CX3000A is GG and never had IMD problems. Plenty of guys have run tetrodes like 4-400 and 4-1000As in gg. Grids will take plenty of power.
you have to be careful with 4CX1000s as they will fry in gg.
Or you could grid drive it. Either way that is a nice assembly that should not go to a CBer

Quote from: WA1GFZ on January 25, 2009, 03:25:13 PM

My 4CX3000A is GG and never had IMD problems. Plenty of guys have run tetrodes like 4-400 and 4-1000As in gg. Grids will take plenty of power.
you have to be careful with 4CX1000s as they will fry in gg.
Or you could grid drive it. Either way that is a nice assembly that should not go to a CBer


Sure, that's right - plenty of hams run them that way... but at ~ 1500w out.  My point is if someone were to run a LARGE tetrode at extreme QRO in any config without extra precautions to increase its cleanliness, they will have complaints simply cuz of the relative signal increase over the average 1500 watt ham signal on the band. (relative crud increase)

Have you ever measured the IMD for the 4X3 GG at FULL strap?

-32 db 3rd specs is probably being too kind for any non-RF feedback tetrode, even in GG, full power.  

But running it at 1/2 - 3/4 of full pep does wonders.

Bottom line is we can pick up another -10db or more better IMD numbers simply by running a tube designed for the job, like the 3CX-3000A7 running reduced power, but still QRO.

But we all run what's available and cheapest - and that's quite OK... :-)


Quote from: Opcom on January 25, 2009, 11:46:56 AM

It's a nice piece of equipment, but with what Tom said about the IMD on the tube I would hate to go to the expense and trouble of building an amplifier only to operate on several bands at once. The advice to turn it for the $ for a 3CX3000 is good but them CB nuts are cheaper than hams.

I admit no experience GG-ing this tube family but I have been told by one guy who GG-d a 4CX5000 that great care has to be taken during operation that the loading is enough, or the grids will fry before the operator knows it.

I had thought of getting around this by grid-driving it in the same way as the "triode" connection is used with the 807, where the control grid is driven through a resistor and the screen is where most of the business happens. Much less power would be needed for drive of only the grids.

If anyone has experience with GG-ing and driving the 4CX5000-15000 I would certainly welcome some comments.

Maybe a good use would be to sell it or for a tesla coil; I will refer this to my apprentice but he probably can't afford it.

I've run grid driven, as well as strapped to triode, 4CX5000 A and R tubes.  By strapping them both to ground, you have a huge triode.  Problem being, equalizing currents between screen and grid.  It can, and is done, but isn't the "best" way.

You can split the currents, Bill Orr, as others, have designs in his and other radio style handbooks.

The 4CX5K is good for about 4 kilowatts of RF carrier, AM modulated.  Multiply that by the ratio of plate dissipation, and you can figure out what you can get out of each tube.  Figgir, 3Xs the dissipation is what the amplifier will do, PEP.  Of course, carrier is a quarter to a fifth.

Measures never finished the 10,000 tube.  The 5000 tube never worked above 15 meters.

Lots of people using the tetrodes (and triodes) with handles are going to using a small amount of L in front of the Tune cap.  This makes tuneup easier, the Pi a bit more broadbanded, as well as lowers operating Q.

If you can purchase it for a low enough price, the power supply can be ran from single phase, albeit at a lower Pout.  The tube, etc. can be worked on all bands we use it on, as well as others.

I've never measured IMD out of the 15,000, but it can't be all that bad, as it is used as a broadcast tube.

Easiest would be strap it all over as a triode, if the price is right.

CBers see about 35 kilowatts out of that tube, on 11 meters.



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