[AMPS] Re: 2m legal limit amps
km1h @ juno.com
km1h@juno.com
Wed, 23 Jul 1997 11:09:16 EDT
On Tue, 22 Jul 1997 20:15:30 -0500 Barry Arneson <barneson@willinet.net>
writes:
>Hi Carl,
>
>Has anyone tried to water cool a 4CX250R,B running 2200 V,Ep, 400V Es,
>=
>-80V Eg. I have seen this done on 2C39's with dramatic results. some
>=
>60 to 70% more overhead?
>
>Barry, K8SD
Hi Barry,
The 2C39 family has removable anode readiators which makes them easily
adaptable to water cooling. I use a pair of 7289's on 1296 and 2304 with
good results. However those that really push it have poor tube life even
though the anode runs cool; the cathode probably gets stripped bare!
In the 4CX250 family, the conduction cooled 8560AS may be a good choice
to try and water cool. Another tube that comes to mind is the 8072 which
is an 8122 without the radiator. It was used for years as a conduction
cooled VHF/UHF amp in GE Master Pros. Since a pair of 8122's will output
1200W I would think that water cooled 8072's could be attractive; they
are available for the asking these days. How clean at extended
voltages/current? I dont think I would operate it on 144.200 in a
populated area. As with any tube you run up against the peak emission
wall. However I have seen the 8122 used in pulsed applications in test
equipment so there may be some reserve after all.
But it would certainly make a good MS or EME amp.
73....Carl KM1H
>----------
>From: km1h@juno.com[SMTP:km1h@juno.com]
>Sent: Tuesday, July 22, 1997 4:17 PM
>To: G3SEK@ifwtech.demon.co.uk; amps@contesting.com
>Cc: vhf@w6yx.stanford.edu
>Subject: Re: 2m legal limit amps
>
>
>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=20
>>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.=20
>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".=20
>
>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.=20
>
>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=20
>>believe
>>>I asked this question about 6 months ago and never got a straight=20
>>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=20
>>on
>>>the various 4CX250/300/350 types in AB1 service.=20
>
>
>
>>Apologies for not having answered fully. It would be really nice
>to=20
>>have
>>all that information, systematically measured and mapped out in
>the=20
>>way
>>you asked for!=20
>>Unfortunately EIMAC haven't provided it.
>
>After almost 50 years of 4X150, etc production I would think that =
>someone
>would have=20
>taken the time to run a few tests.
>
>
>> Instead, we have to make do
>>with "snapshots" taken under specific sets of operating
>conditions,=20
>>such
>>as the ones in the datasheets or the ones that I quoted:=20
>>
>>>>Don't expect too much output power. For LINEAR operation the
>most=20
>>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?=20
>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,=20
>>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=20
>>under
>>which each particular order of IMD can be almost nulled out.
>Again,=20
>>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.
>
>
>=20
>>So when we talk about "IMD increasing" or "IMD decreasing", we can=20
>>only
>>mean a general trend in IMD.=20
>> ~~~~~~~~~~~~~
>>Similarly, when the manufacturer gives the "proper" anode and screen
>>voltages, these can only refer to a certain mix of IMD levels.
>There=20
>>was
>>an interesting posting in r.r.a.homebrew several weeks ago, from
>one=20
>>of
>>the actual engineers who generate the numbers for the datasheets
>of=20
>>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=20
>>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.=20
>
>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".
>
>=20
>>On the other hand, a poorly-regulated screen supply that lets the=20
>>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!=20
>>No argument there. But the reason for that is not the tubes=20
>>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=20
>>400W
>>PEP legal limit out of a single tube. Well, they can, just about, but
>>the rest of us suffer for it... =20
>>
>>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=20
>>$5
>>>difference between the 4CX250R and the more robust 4CX400A. I
>have=20
>>heard
>>>it reported that a pair of 4CX400A's will run 1200W out in AB1
>even=20
>>at
>>>432 MHz.=20
>
>>
>>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.=20
>
>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.=20
> =20
>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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