8935 tube - good for anything?

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Bill, KD0HG:
Quote from: KD6VXI on January 25, 2009, 04:43:45 PM


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.




We use 4CX15000As in Class C FM rigs. They'll do 20-25 KW output on VHF with around 250-400 watts of drive. (Or a pair of 4CX250Bs).  VERY rugged grids. Plate voltage typically ~ 9 KV; current around 3 amps. At a more reasonable 4 to 5 KV they'll make about 9 to 10 KW out in Class C. There's piles of used ones out there at almost every high power FM broadcast station. I've got the Eimac spec sheet on them at the office, if anyone wants to see it I'll post it here.

You could probably run one in Class A linear if you wanted clean output...And to run up the electric bill..

Tom, would the IMD specs likely be any better in Class A rather than AB?

That was THE final in a lot of Harris FM transmitters.

If I was going to use one on AM, I'd run it Class C, screen modulated.

K1JJ:
Quote from: Bill, KD0HG on January 25, 2009, 06:19:59 PM

Tom, would the IMD specs likely be any better in Class A rather than AB?
If I was going to use one on AM, I'd run it Class C, screen modulated.


Hi Bill,

You know all this, but here goes... :-)

Yes, I'll bet a 4X15 wud do nicely in Class C screen modulated service since it was designed for that - more or less.  I know the Tron had one for commercial service BC and it worked out just FB at 5KW carrier with big audio.

Yes, idling the tube as high as possible makes a tremendous difference in IMD. (In linear, at cut-off  being the minimum and full power idle for class A.) Most run it somewhere in between.  Pure class A is such a power hog and heat producer, it wud have to be for a very good reason.  If for pure linearity, it might be best to consider the 4CX-15000J or the 3CX-15000(J?) designed for the service.

I like using pure class A for lower level driver stages.

The second important factor is the design of the tube - what is it best suited for?

And third, how hard is it driven? If I remember correctly, simply by reducing a linear to 1/2 power from max pep will improve the IMD by -9db. That's quite a lot.

And lastly, the configuration the tube is in matters a lot for cleanliness.  For example a guy running a grid driven pair of 4X1's with 7kv driving it with an SB-200 is gonna have the lynch mob as his house - or at least people yelling every time he unkeys the mike... :-)  The same tubes running GG, 5kv and running 1/2 pep max power would be as clean as a whistle.


It's all relative to the current "standards" out there. Years ago you could run a spark gap and get away with it. Try that in 1935 when they had DC CW and you were toast. Later, the first ssb rigs drifted and had poor suppression but no one cared. Eventually the Collins rigs set the new standard and you were on the outside looking in with the older ssb rigs.

Today the standard is the modern ricebox and the GG linear using tubes designed for ssb linear:  3-500Z finals, 3CX-800's, 3CX-1200's, 8877's etc.  Get on the air with this rig and you blend in nicely. But get on the air with something of lesser clean design and you may find you start to stick out, even though it was quite acceptable only a few decades ago..

It's all relative.

BTW, we called for you a few times last night. There was a void in the south west - Colorado area... :-)


73,
Tom

k4kyv:
Quote from: K1JJ on January 25, 2009, 01:03:10 PM


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


Slopbucketeers got just as screwed with Johnny Johnston's P.E.P. bullshit as did AM'ers. Since the peak-to-average ratio of the human voice waveform averages something on the order of 7 to 10 dB, if the legal power limit were defined in terms of average power, you could run a clean signal to about 750 real watts output while letting the p.e.p. go where it may, which could be as high as 3750 watts or more on instantaneous peaks.

But still, it is average, or mean power that determines the loudness and interference potential of a signal, not the instantaneous power of occasional voice peaks.

Limited to 1500 watts pep, a clean SSB signal with a typical voice and no processing would average no more than about 300 real watts, as measured by an rf ammeter working into a known resistive load or by an accurately calibrated average-reading wattmeter.  To get the average power up to something on the order of 750 real watts requires heavy processing or else driving the leen-yar well into saturation and thus flat-topping on voice peaks, both of which make for a crummy distorted signal, and most probably, spurious sideband products that make the signal wide and splattery.

So this makes a shitty sounding over-processed SSB signal "legal", while a clean undistorted signal of equal loudness would violate the limit.

KM1H:
Put the tube on Ebay and grab a YC-156 which are readily available as pulls for $300-500; less if you get one from a ham who never finished his amp.

That tube will run 1500W with 100W drive and 3500V for the next 100 years or drive it at around 300W at 6-7 kV for serious smoke. Probably the most linear tube available these days at a reasonable cost. The 8-10 minute warm up annoys some :-\  but the filament power is pretty modest so once on, leave it on for your operating time. No socket is a big plus.

It has huge output C so you need some L before the Tune cap if you want 10/12M. Many have even got them working fine on 6M.

Carl
KM1H

K1JJ:
Quote from: KM1H on January 26, 2009, 08:45:29 AM

Put the tube on Ebay and grab a YC-156 which are readily available as pulls for $300-500; less if you get one from a ham who never finished his amp.

That tube will run 1500W with 100W drive and 3500V for the next 100 years or drive it at around 300W at 6-7 kV for serious smoke. Probably the most linear tube available these days at a reasonable cost. The 8-10 minute warm up annoys some :-\  but the filament power is pretty modest so once on, leave it on for your operating time. No socket is a big plus.

It has huge output C so you need some L before the Tune cap if you want 10/12M. Many have even got them working fine on 6M.
Carl
KM1H



Yep, wonderful advice, Carl!

That wud be a great transaction.

The YC-156 (3CPX-5000) is the perfect ham tube.  You could go thru 3 sets of the common linear amp tubes before a YC-156 would die, if run conservatively.  I figger if they're good enuff for the demands of high-resolution linear magnetic imaging in the medical whirl, they'd good enuff for RF linear amps.

It has the same filament structure as a 3CX-15,000B7, and is indirectly heated so it takes relatively little power to light up -  but with it's smaller plate external anode, there is a greatly reduced limit to its dissapation, of course.

I once built a 6M amp with one, but ended up using an 8877 instead. Yes, the output C wuda been a challenge on 6M.


I wish the used price wud come down so I can buy a spare.. :-)  It will - wait a year or so.

The 8 minute warmup doesn't bother me much. Just have to miss a few of those impulsive DX contacts, I guess...
I run mine at much reduced power and the IMD is great. Have never had a complaint once in several years using it... can't say that about other tubes. I'm now building up a class A solid state driver to give it an even better chance. I'm coming off the low level 10mw ricebox output (FT-1000D...clean -70db 3rd order) and going from there into a class A chain.   This way, the IMD can never be worse than the final itself.

I was surprised to learn that most 100w (class B) riceboxes have trouble doing better than -30 to -32db 3rd themselves. That would contaminate the signal before it even gets to the final.

By idling the YC-156 high, running it at greatly reduced power, and driving it with a super clean class A signal, it should approach -40db 3rd or probably even better ... at least that's my goal.


73,

T

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