Re: Time to ask for help (John Sherrick, W8OHT)
I encountered the 4CX250B because this tube was the tripler and final
amplifier used in production of the FAA's Glide Slope Projector Transmitter.
This RF unit was used {with a mechanical modulator} in the late 1960's
through the mid 1970's, and only at the nations largest airports. After
making certain FAA desired improvements in the design, my company produced
60 of these dual transmitter/modulator/antenna systems that were always
housed alongside the airport runways.
The driver tube was a frequency tripler. Both this and the final amplifier
used two-inch wide, ten-inch long silver-plated-strip-line output resonators
operating in the 330 Mhz range. The plate voltage was only about 1000 vdc,
and no special neutralization circuitry was required. Of course, the system
was used as part of the FAA's Instrument Landing System, or ILS. All
testing of the system was conducted inside of an RF tight "screen room". I
was chosen as the one to do a lot of the work on the RF system because I had
been a ham since 1953 and had recently graduated from a five-semester
"Technical Institute" program (Valparaiso, Ind.). I also was experienced
with high voltage, having built my own 3000 VDC (pair of 813's) ham radio
transmitter. The FAA's reported tuning problems were fixed by increasing the
amount of tuning control dips (from small to nil) to at least 20 percent for
each of the five resonances tuning controls. Another problem reported had
been that the transmitter's final output stage could sometimes be set to
resonate at 660 MHz, the second harmonic of the desired 330 MHz ILS
frequency (a very real show stopper). This was solved by simply reducing
the size of the rotatable capacitor blade (located at the end of the strip
line) so that the stip only tuned through the ILS band. Thus the final then
only tuned through the ILS band.
-----Original Message-----
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Sent: Saturday, July 25, 2015 7:36 PM
To: amps@contesting.com subject: Amps Digest, Vol 151, Issue 33
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Today's Topics:
1. Time to ask for help. (Roger)
2. Re: Odd query.... (Radio WC6W)
3. Re: Time to ask for help. (Bill Turner)
4. Re: Time to ask for help. (Mike Tubby)
5. Re: Time to ask for help. (TexasRF@aol.com)
6. (no subject) (donroden@hiwaay.net)
----------------------------------------------------------------------
Message: 1
Date: Sat, 25 Jul 2015 13:20:24 -0700
From: Roger <ai7rogerroger@gmail.com>
To: amps@contesting.com
Subject: [Amps] Time to ask for help.
Message-ID:
<CAEahN0+ArMOc3UOWqz02XVgbb=0sBMauL+pqFt0BRVL=uMDn6g@mail.gmail.com>
Content-Type: text/plain; charset=UTF-8
I've been building a 6m amp from scratch based on quad 4x150a's for more
than 3 years. I've never built any type of rf producing device before and
unbeknownst to me at the time, chose a project that was way over my head.
I've read and studied volumes of material written on 6m amps and tetrodes.
The power supply was also built from scratch and is sound. It has been
successfully tested on a 2m rf deck running a pair of 4cx250b's in push
pull. HV: 1680vdc, Screen: 360vdc, Bias: ~ -50vdc, when not jumping off the
bench idle current 400ma. Though not currrently designed for individual
adjustment and not perfect, the tubes have been selected and are very close
to matching. I'm at the point of irradicating runaway, oscillations and/or
parasitics. I have access to quality test equipment and am fairly
knowledgeable with its use. I'm conducting a cold test using a tracking
generator with spectrum analyzer to observe the output while injecting a
signal into the grids. This test is suggested in Care and Feeding Chap
5.5.2 (Feeding the Grid Circuit). This states: "The neutralization can then
be adjusted for minimum feed through." One question of many: 1) Is the
author suggesting that the tester knows the frequency of the parasitic and
that is the frequency being injected and neutralized? Continuing. I have
indications of runaway, in that, when the amp is keyed without excitation,
excessive grid, plate and negative screen current is drawn. Some of the
fixes I've tried include raising the cathodes off ground by as little as 10
and as much as 33 ohms; various configurations of parasitic suppressors,
grid swamping with various values of resistors (all non inductive) and
different methods of neutralization.
I have no intention of dropping this project, but would really appreciate
any advice or suggestions beyond that to rein this thing in.
Thanks so much for your time.
73, Roger
AI7RR
------------------------------
Message: 2
Date: Sat, 25 Jul 2015 13:40:39 -0700
From: Radio WC6W <wc6w_amps@yahoo.com>
To: Larry <larry@w7iuv.com>, amps@contesting.com
Subject: Re: [Amps] Odd query....
Message-ID:
<1437856839.23315.YahooMailBasic@web162101.mail.bf1.yahoo.com>
Content-Type: text/plain; charset=iso-8859-1
Hi Larry,
Back in the 1950's, Hughes created the kilowatt HACON amplifier, for use
on the B-58 Bomber, which had 4x 4CX250 family finals, with large finned
radiators in lieu of the standard air fins, autotuned in a not quite cubic
foot package, filled with one of the DuPont $600/gallon ... fluids.
http://qsl.net/wc6w/wc6wamps/popp.html?fr62c1.jpg
73 & Good afternoon,
Marv WC6W
--------------------------------------------
On Fri, 7/24/15, Larry <larry@w7iuv.com> wrote:
Subject: [Amps] Odd query....
To: "AMPS" <amps@contesting.com>
Date: Friday, July 24, 2015, 12:02 PM
Having run out of "important"
projects, I've been toying with starting a "fun" project just for the heck
of it.
The key words in the project description are:
Unusual
Unique
Weird
Wacky
and last but certainly not least:
CHEAP
Several amp concepts have been running around in my old head but one in
particular keeps coming back around. So with that background in mind,
here's the question:
Has anyone water cooled a 4CX250? Or oil cooled for that matter.
? Discussion??
73,
Larry - W7IUV / WH2XGP
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------------------------------
Message: 3
Date: Sat, 25 Jul 2015 14:00:59 -0700
From: Bill Turner <dezrat@outlook.com>
To: Amps group <amps@contesting.com>
Subject: Re: [Amps] Time to ask for help.
Message-ID: <BLU436-SMTP1071AEA6B464F4E1213867AC0800@phx.gbl>
Content-Type: text/plain; charset="us-ascii"
------------ ORIGINAL MESSAGE ------------(may be snipped)
On Sat, 25 Jul 2015 13:20:24 -0700, you wrote:
>This states: "The neutralization can then be adjusted for minimum feed
>through." One question of many: 1) Is the author suggesting that the
>tester knows the frequency of the parasitic and that is the frequency
>being injected and neutralized?
REPLY:
What that means is this: With the amp completely UN-POWERED, you apply a 50
MHz signal at the input and monitor the output. Adjust the neutralizing
circuit for minimum output.
I assume you are using a grid-driven input, correct? You might consider
changing over to a grounded grid input, which is much more stable and
usually does not require a neutralizing circuit at all. The tradeoff is it
does require more driving power.
73, Bill W6WRT
------------------------------
Message: 4
Date: Sat, 25 Jul 2015 22:28:40 +0100
From: Mike Tubby <mike@tubby.org>
To: amps@contesting.com
Subject: Re: [Amps] Time to ask for help.
Message-ID: <55B3FF88.7040502@tubby.org>
Content-Type: text/plain; charset=windows-1252; format=flowed
Hi Roger,
Is the design single-ended or push-pull?
Have you checked out your RF deck by removing each tube and replacing it
with a carbon resistor of/at/near the resistive load (RL) for the tube
operating conditions (get this from the data sheet) - a 4CX250B has an RL of
around 4350 ohms at 2KV in AB1. If you substitute your tubes with
appropriate resistors connected anode to ground then with no volts applied
you should be able to "look in to" the output connector with a vector
network analyzer (VNA) or antenna analyzer and tune up your output stage for
a good match, ie. 20dB return loss or better.
Again, without knowing your design - you should be able to put the tubes
back in and apply heater volts and a few watts of RF drive and minimize the
coupling between grid and anode using a sensitive power meter connected to
the output, i.e. adjust the neutralization for minimum power feed-thru. I
found this technique remarkable on my W1SL 144MHz pair of 4CX250Bs...
Mike G8TIC
On 25/07/2015 21:20, Roger wrote:
> I've been building a 6m amp from scratch based on quad 4x150a's for
> more than 3 years. I've never built any type of rf producing device
> before and unbeknownst to me at the time, chose a project that was way
over my head.
> I've read and studied volumes of material written on 6m amps and tetrodes.
> The power supply was also built from scratch and is sound. It has been
> successfully tested on a 2m rf deck running a pair of 4cx250b's in
> push pull. HV: 1680vdc, Screen: 360vdc, Bias: ~ -50vdc, when not
> jumping off the bench idle current 400ma. Though not currrently
> designed for individual adjustment and not perfect, the tubes have
> been selected and are very close to matching. I'm at the point of
> irradicating runaway, oscillations and/or parasitics. I have access to
> quality test equipment and am fairly knowledgeable with its use. I'm
> conducting a cold test using a tracking generator with spectrum
> analyzer to observe the output while injecting a signal into the
> grids. This test is suggested in Care and Feeding Chap
> 5.5.2 (Feeding the Grid Circuit). This states: "The neutralization can
> then be adjusted for minimum feed through." One question of many: 1)
> Is the author suggesting that the tester knows the frequency of the
> parasitic and that is the frequency being injected and neutralized?
> Continuing. I have indications of runaway, in that, when the amp is
> keyed without excitation, excessive grid, plate and negative screen
> current is drawn. Some of the fixes I've tried include raising the
> cathodes off ground by as little as 10 and as much as 33 ohms; various
> configurations of parasitic suppressors, grid swamping with various
> values of resistors (all non inductive) and different methods of
neutralization.
>
> I have no intention of dropping this project, but would really
> appreciate any advice or suggestions beyond that to rein this thing in.
>
> Thanks so much for your time.
>
> 73, Roger
> AI7RR
> _______________________________________________
> Amps mailing list
> Amps@contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
------------------------------
Message: 5
Date: Sat, 25 Jul 2015 17:54:31 -0400
From: TexasRF@aol.com
To: ai7rogerroger@gmail.com, amps@contesting.com
Subject: Re: [Amps] Time to ask for help.
Message-ID: <9f0cf.739586cc.42e55f97@aol.com>
Content-Type: text/plain; charset="US-ASCII"
Hi Roger, a couple of thoughts come to mind. The 400 mA idle current is
problematic. This would not cause an oscillation but it allows the tubes to
operate pretty much at maximum gain and that can make the taming process
more difficult. In addition, the plate dissipation rating is being exceeded
before there is even any rf drive applied. There may be thermal run away if
the screen voltage is not stiff enough. The negative screen current will
try to charge the screen voltage upward and depending on the screen voltage
source impedance can rise as high as the plate voltage.
Under these conditions the tube gain is way greater than that expected.
The purpose of neutralization is to purposely create a feedback path from
plate to grid that is of opposite phase in respect to the normal internal
grid to plate feedback path. In addition, the amplitude is supposed to be
such that the internal feedback path is effectively canceled.
You have the perfect test equipment to make the test and adjustment. This is
not related to any parasitic suppression as that is a separate issue.
While I don't have direct experience with this aspect of using a 4CX250, I
have tested other larger tetrode tube amplifiers. They showed about 40 dB
input to output isolation when an rf signal was connected to the input and
measured at the output. Also, as a check. the signal can also be applied to
the output and measured at the input. Of course this is done with the
amplifier completely cold and at the expected operating frequency.
I would expect the neutralization null to be fairly narrow band but that
will depend on the degree of neutralization needed to make the null happen.
Once you get to this point, you can then start looking at possible parasitic
oscillation issues. The 4CX250 family of tubes are quite small physically so
it would seem that they would be a lot less prone to parasitics than a
larger glass tube that inherently has more inductance in the internals.
Actually, I am thinking your root problem is more likely due to a run away
issue as discussed above.
The idea of swamping the grid is a valid method of reducing the gain but if
that does not help, that again points to the run away scenario.
Please let us know how you make out.
73,
Gerald K5GW
In a message dated 7/25/2015 3:20:45 P.M. Central Daylight Time,
ai7rogerroger@gmail.com writes:
I've been building a 6m amp from scratch based on quad 4x150a's for more
than 3 years. I've never built any type of rf producing device before and
unbeknownst to me at the time, chose a project that was way over my head.
I've read and studied volumes of material written on 6m amps and tetrodes.
The power supply was also built from scratch and is sound. It has been
successfully tested on a 2m rf deck running a pair of 4cx250b's in push
pull. HV: 1680vdc, Screen: 360vdc, Bias: ~ -50vdc, when not jumping off the
bench idle current 400ma. Though not currrently designed for individual
adjustment and not perfect, the tubes have been selected and are very close
to matching. I'm at the point of irradicating runaway, oscillations and/or
parasitics. I have access to quality test equipment and am fairly
knowledgeable with its use. I'm conducting a cold test using a tracking
generator with spectrum analyzer to observe the output while injecting a
signal into the grids. This test is suggested in Care and Feeding Chap
5.5.2 (Feeding the Grid Circuit). This states: "The neutralization can then
be adjusted for minimum feed through." One question of many: 1) Is the
author suggesting that the tester knows the frequency of the parasitic and
that is the frequency being injected and neutralized? Continuing. I have
indications of runaway, in that, when the amp is keyed without excitation,
excessive grid, plate and negative screen current is drawn. Some of the
fixes I've tried include raising the cathodes off ground by as little as 10
and as much as 33 ohms; various configurations of parasitic suppressors,
grid swamping with various values of resistors (all non inductive) and
different methods of neutralization.
I have no intention of dropping this project, but would really appreciate
any advice or suggestions beyond that to rein this thing in.
Thanks so much for your time.
73, Roger
AI7RR
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Amps mailing list
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http://lists.contesting.com/mailman/listinfo/amps
------------------------------
Message: 6
Date: Sat, 25 Jul 2015 18:35:57 -0500
From: donroden@hiwaay.net
To: amps@contesting.com
Subject: [Amps] (no subject)
Message-ID: <20150725183557.94552gh2o7kp8z8d@webmail.hiwaay.net>
Content-Type: text/plain; charset=ISO-8859-1; DelSp="Yes";
format="flowed"
If you have under 200 ohms hung off the grids , a quad won't turn into
an oscillator. Remove the connection between the tubes and the plate
cap and see if the tubes are stable by themselves
Don W4DNR
------------------------------
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