On Sat, 19 Jul 1997 16:05:42 +0100 Ian White <G3SEK@ifwtech.demon.co.uk>
writes:
>Carl, KM1H wrote (about 4CX250Bs and the like):
>
>
>>BUT what is the IMD if one runs say 1700V, proper drive and
>stabilized
>>screens?
>
>Worse than at 2000V for the same "attempted" power output; or
>alternatively less output for the same general IMD levels - see below.
That was not what I asked Ian and I am sure that you know that. I think
it would be safe to assume that a reduction in plate voltage would result
in lower power output.
Please note very carefully that my statement included "proper drive"
which in the case of the AB1 amp means "no grid current".
I will refer you again, as I did many months ago, to the Eimac spec sheet
for the 4CX250R in AB1 service.
At 1500V Ep, 350V Es, -62V Eg, and peak driving voltage of 56V the PEP
output is 262W. Third and Fifth IMD products are -30 and -35dB
respectively. Also included in the spec is "worst case 3rd order IMD as
drive is reduced is -29 dB"
Note that the peak signal voltage is a bit under the bias.
Now, the same tube at 2000V Ep, 400V Es, -80V Eg, peak driving voltage
80V delivers a PEP output of 470W with 3rd/5th IMD of -23/-27. Again,
"worst case 3rd order, etc is -21 dB".
Note that Eimac does not have a spec for absolute "best case IMD" in any
tube spec sheet. One would come to the conclusion that the published
information is close to the ideal for a production tube. It also tends to
blow holes in anyones theory that increasing Ep will reduce IMD if the
Ep/Es ratio is increased.
The IMD chart in "Care and Feeding" shows only the mathematical results
of a PERFECT tube that absolutely follows the 3/2 power law. It is meant
as a guideline only and does not necesarily mean that every tube produced
is perfect.
>
>>And Further...what is the IMD at 2500V using the same criteria? I
>believe
>>I asked this question about 6 months ago and never got a straight
>answer.
>>It would be nice to have a chart showing the proper screen voltage vs
>>plate voltage in say 250V increments from 1000 to 2500V plate voltage
>on
>>the various 4CX250/300/350 types in AB1 service.
>Apologies for not having answered fully. It would be really nice to
>have
>all that information, systematically measured and mapped out in the
>way
>you asked for!
>Unfortunately EIMAC haven't provided it.
After almost 50 years of 4X150, etc production I would think that someone
would have
taken the time to run a few tests.
> Instead, we have to make do
>with "snapshots" taken under specific sets of operating conditions,
>such
>as the ones in the datasheets or the ones that I quoted:
>
>>>Don't expect too much output power. For LINEAR operation the most
>you
>>>can expect is:
>>>
>>>2*4XC250B @ 2000V: 570W
>>>
>>>2*4XC250R @ 2000V: 620W
>>>
>>>2*4XC350A @ 2000V: 580W
>>>2*4XC350A @ 2500V: 740W
>>>
So at 2000V on the 4CX250R what is the screen voltage, bias and drive
voltage to derive your claim of -26 dB 3rd IMD at 310W per tube?
How repeatable is it with a wide range of tubes from NIB to various
pulls?
>So, how can we generalize from these "snapshots" to try and understand
>the whole map of possible operating conditions?
>
>We know from 'Care & Feeding' and other texts that the available power
>output will decrease with Va, and also that IMD will increase if the
>ratio Va/Vg2 is decreased.
Only in C&F, not from real world spec sheets.
> Conversely IMD should decrease if you
>increase Va.
>Unfortunately "IMD increasing/decreasing" is a slippery concept,
>because
>"IMD" is not a single quantity. Changing the operating conditions will
>change different orders of IMD by different amounts and even in
>different directions. There are also specific operating conditions
>under
>which each particular order of IMD can be almost nulled out. Again,
>see
>'C&F'.
Only in the PERFECT tube. I challange you to duplicate that graph with a
sampling of 10 real world tubes....we are discussing ONLY the 4CX250R
here....that include a fair variety of pulls. Try it at say 14, 50, 144,
432 mHZ.
>So when we talk about "IMD increasing" or "IMD decreasing", we can
>only
>mean a general trend in IMD.
> ~~~~~~~~~~~~~
>Similarly, when the manufacturer gives the "proper" anode and screen
>voltages, these can only refer to a certain mix of IMD levels. There
>was
>an interesting posting in r.r.a.homebrew several weeks ago, from one
>of
>the actual engineers who generate the numbers for the datasheets of
>new
>amplifier devices. The "proper" figures reflect his personal judgement
>about the best mix of several different orders of IMD (within other
>device operating constraints of course). He happened to work with
>transistors, but I'm sure that's how tube datasheets are written too.
I'm not so sure. Most tube design engineers were raised in an era where
professional integrity still counted.
>
>>I would also be concerned about the quality of cheap "pulls" from FM
>>service. Many that I have tested show various levels of screen
>emission
>>which would certainly degrade the linearity.
>>
>It doesn't have to, because the linearity depends primarily on the
>screen voltage, not the current. If the screen supply keeps a very
>tightly-regulated grip on the voltage for all values of screen current
>across the drive cycle, the linearity problems shouldn't be too bad.
Hmmm, last year you said quite the opposite and cautioned against any
pull with screen emission "unless it was going to remain in FM service".
>On the other hand, a poorly-regulated screen supply that lets the
>screen
>current yank the voltage all over the place is bound to give IMD
>problems.
We finally agree on a point.
>
>See my web page www.ifwtech.demon.co.uk/g3sek/imd1 (imd-one) for a
>"snapshot" of the effects of different screen supply impedances on a
>spread of IMD levels out to 17th-order. It shows clearly that a very
>"stiff" screen supply can make a real improvement in IMD levels.
>That data applies to 4CX350As under a particular set of operating
>conditions, but at least it's one good set of points on the map.
>
>
>Carl, I'm not at all surprised at the general perception that most
>4CX250/350 amplifiers are bad for IMD - because it's mostly true!
>No argument there. But the reason for that is not the tubes
>themselves,
>but poor power supplies and uneducated users. Whatever the type of
>amplifier, there will always be some users who are greedy for more
>power, and don't know/care how to treat the amplifier correctly.
>
>We have exactly the same problem over here, with amateur and even so-
>called "professional" amplifier builders who think they can get the
>400W
>PEP legal limit out of a single tube. Well, they can, just about, but
>the rest of us suffer for it...
>
>Another big problem with 4CX250Bs is that they tend to be everybody's
>first VHF power amp, and truly there's a lot to be learned.
>>The new tubes from Svetlana are quite inexpensive and there is only a
>$5
>>difference between the 4CX250R and the more robust 4CX400A. I have
>heard
>>it reported that a pair of 4CX400A's will run 1200W out in AB1 even
>at
>>432 MHz.
>
>Certainly the 4CX400A is the "best buy" for a tetrode in this power
>class, to go into an existing amplifier (let's not get into the
>economics of building from new). I hope to be able to report some hard
>data on VHF power outputs in the next few weeks.
I am looking foreward to it Ian. More info on real ham level amps is
required IMO, never mind all the 10KW crap on the AMPS reflector.
Perhaps someone would care to detail a proper test setup for evaluating
IMD with professional equipment available to many hams. By that I mean
test equipment from the 1960-80 era that is available at flea markets but
is still adequate. Never mind the cheap hams that believe life ends at
Radio Shack or the local Walmart or the other extreme being a modern HP
equipped lab.
73...Carl KM1H
>73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
> 'In Practice' columnist for RadCom (RSGB)
> www.ifwtech.demon.co.uk/g3sek
>
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