Replacement of RCA 8122 Tubes

Discussion in 'Amateur Radio Amplifiers' started by N2OTG, Jul 12, 2015.

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  1. AF6LJ

    AF6LJ Ham Member QRZ Page

    I second that...
    Reid stop by more often. :)
  2. W6MTF

    W6MTF Ham Member QRZ Page

    Hi Jim, Eimac didn't often provide range values for bias voltage if thats what you mean. Range values for capacitance, yes, those are generally available and used at QA to track the proper alignment because if physical respects internal to the tube are "off" very much a tube may still perform electrically in terms of delivering plate current and so on but the capacitance will start to wander and that trend may alert you to something that will affect customers using the tube esp at higher freqs with respect to tuning. Also the normal tetrode data sheet (such as provided below) states a specific grid bias in class AB1 to obtain a specified plate current. That bias varies quite a bit with ANY tube and is why circuits with really demanding (ie. Mil) specs for IMD for example will have balancing pots for bias or require using a matched pair. That takes us back to URT-23 and the color dots on Burle tubes.

    During the second 8122 manufacturing period (the first was around 1970 and eventually production was stopped for some reason -(I wasn't there so I don't know the details) I was asked to work on a new data sheet.
    I remember discussing the possibility that some Eimac customers might raise issues over the Burle tubes having color dots and Eimac tubes didn't. The devils advocate said a possibility existed that when military users went to replace tubes and, following the tech manuals as they always do, they looked at Eimac tubes with no color dots they would halt the work and end-up returning the tubes as deficient, ie. lacking color dots. We considered using the same color and putting dots on the tube regardless but Eimac never got the big order for thousands of tubes from Stewart Warner, something that I discussed before (and neglected to mention the prime contractor) so it was a moot point.
    Incidentally some Burle tubes had a color painteed to the small top on the tubulation cover while most tubes we saw (ahem, bought for verification putposes) had 2 small dots, either black, red or blue and I don't know how exactly how many variations of dots they used.
    You think thats an odd story?
    Well, I have another anecdote that is similar and may seem even more ridiculous.
    Eimac was delivering tubes to Continental Electronics for an OTH radar, the one that tracked - who was pres in '84? - Reagan or was it the first Bush? I think it was George Bush Sr. who flew to Iceland, anyhow the radar worked, even though it only a small version of the Russian Woodpecker.
    The radar used a water cooled 25kW tetrode as a driver and a 50 kW tetrode in the PA. The anode of the tube had two water pipes coming out the top and it didn't matter which pipe was used for in or out, due to the internal symmetry. The tube was mounted upside down and the fear was that a air bubble might form and prevent proper cooling, a valid concern but it just wasn't an issue as far as Eimac was concerned.
    But the US Navy or USAF or whoever took responsibility of operating and maintaining the radar came running to Continental asking which pipe was "in" and which was "out". My explanation that it didn't matter fell on rigid miltary ears, so Continental asked me to have the tubes labelled IN and OUT just to satisfy some mil guy in a uniform who wanted to enable his men to do something without getting confused. The Radar system used the East coast and the West coast for isolation, no T/R switch required!
    So I had to reprint data sheets accordingly and we changed work instruction sheets to be consistent with ISO documentation requirements (my opinion of ISO is prety low and not worth going into here!).
    Ironically the FPS-118 radar was mothballed only few years after its implementation, probably because of high operating costs. Parts of it were sold or scrapped. A US Senator proposed to congress keeping it alive or I believe proposed some other use I don't know all the details but that just goes to show another way our Taxpayer Dollars get wasted!
    To this day the tubes are still labelled IN and OUT but I'll wager no one save the folks that read my story here will know why!
    In retrospect, I think the entire radar was a good idea until you consider that Satellites can do that better and now there are drones etc.
    Reid W6MTF
    8122 family data sheet attached

    Attached Files:

    K9AXN and WA4SIX like this.
  3. W0AAT

    W0AAT XML Subscriber QRZ Page

    Love reading the history of radio!

  4. W6MTF

    W6MTF Ham Member QRZ Page

    Glad you enjoyed it. Here's another one!

    As we approach Independence Day 2017 let us remember when America came dangerously close to being a subject of the USSR, during the Cuban Missle in October 1962. President John F Kennedy carefully, in non-negociable terms (except for what was supposed to be a secret agreement for the US to remove its missles from Turkey) created a better world for both nations.

    The Cuba missle incident led to the U.S. building the FPS-85 radar to maintain vigil on the area South of Florida in case such a thing might be attempted a second time. The radar has operated continuously, except for minor maintenance and a fire I describe below, and today it is an important asset for watching space debris.

    If Eimac had been unable to supply a couple of special tubes in the past few years, the radar would have either been shut down or it would have to be converted to solid state, either of those paths would have been a lot of down time and that just wasn't in the cards.

    Eimac comes to the aid of an aging military radar – twice!

    There is an interesting story behind tubes used in the FPS-85 radar at Eglin AFB in Florida. Not one, but three tubes that Eimac makes are necessary to keep the radar there operating.

    This is a well-known radar system; it was built during the 60’s and was intended to be a significant tool to be used as a threat deterrent after the US discovered Russian missiles on the island of Cuba. Today it serves the important function of tracking space debris, stuff left in orbit after the launch of a satellite or other rocket and it is one of several sites dedicated to that purpose.

    The FPS-85 is a phased-array radar and it was built around a large wood structure that holds the array of antennas with an rf amplifier at each antenna.

    Unfortunately, the structure caught fire shortly after it was completed and it burned to the ground because there were no means of putting out the fire. Perhaps the fabric cover that encompasses the entire array caught on fire, after all high voltage is routed across the entire antenna and either that or a lightning strike from the frequent electrical storms that occur at the South end of the Floridian peninsula may have been responsible.

    There is a beautiful fire house located on the site now, located near the entrance to the radar, the side away from the antenna array. I imagine a story about life as a fireman there would not be very interesting as hardly any tourists even venture this far South besides the armed guards at the entrance make it less than a friendly vacation spot.

    The electronic system at Eglin consists of a pulser downstairs feeding pulses of current at high voltage to the array of 5184 individual UHF amplifier modules placed across the face of the array, each antenna is fed by an Eimac 4CPX250K and the rf radiated energy is combined in space.

    The specs for 4CPX250K are quite impressive for a tube of its small size (2.8 inches long and weighing only 4 ounces). Tubes are tested at Eimac and must deliver to the load a minimum output of 10 kW at 442MHz for 250 microsecond pulses at 0.005 duty. At an NAB show one year I met an individual who was involved with a special series of tests on samples provided for consideration when the radar was in a development phase. Basically, he said both RCA and Eimac tubes had been submitted (to Bendix?) for evaluation to determine which would ultimately be used in the rf modules for the FPS-85. I believe he indicated the first radar at the site used a GE triode that was designed into the radar but couldn’t deliver the power necessary to meet performance objectives.

    It was said that RCA tubes sparked during the peak power test. I learned that Eimac had considered this so they had processed the samples using a special high voltage processing called de-barnacling in the industry, this allowed 4CPX250K’s to reliably hold-off over 5000Vdc. Ultimately Eimac was awarded a contract for thousands of 4CPX250K and many more have been delivered over the decades the radar has been in operation.

    Now, enter the Westinghouse WX-4778. This is a unique tube having a small oxide cathode triode with a Mu (amplification factor) of 20. It is also designated 5960-00-892-9475 and this tube is truly “extinct”. Westinghouse made glass special purpose and receiving tubes at their Red Bank, NJ factory and the WX-4778 may have been made at that facility. I believe the WX-4778 is the highest power tube having an external anode with an oxide cathode that Westinghouse ever made. It uses a small glass bulb below the anode and the base appears unique with two pins extended for supplying heater power. In the samples I have seen the base is made from fused glass, a material Eimac used in early 4X250 tetrodes and discontinued after it was found to be susceptible to vacuum leaks.

    The grid flange on the WX-4778 is part of a cup that terminates in the glass base insulator and there is a heliarc weld around the relatively thick rim which constitutes the grid terminal.

    The anode cooling fins on the WX-4778 have a series of dimples to increase air turbulence, a simple technique but not as effective as the louvers on Eimac air-cooled tubes which provide increased cooling efficiency. The rated dissipation for the WX-4778 as indicated on the Westinghouse data sheet is 600 Watts which is pretty impressive considering the fact its anode isn’t much larger than that on a 4CX250B! Westinghouse states this dissipation requires air flow at 30 CFM with a modest pressure drop of 0.5 in water.

    Looking at the overall system one sees there are 16 banks of pulse modulators for the over 5000 4CPX250K’s. The first tube in each pulse modulator is a type 5687, a low power glass dual triode (think audio!) which drives the WX-4778 and that drives four 4PR60C tetrodes in parallel, these tubes remove bias momentarily from the final tube in the chain, a big F8147A triode which drives a pulse forming network and that then supplies pulsed plate current to groups of 4CPX250K’s. The total combined rf output from the array is 32MW peak. The F-8147A was designed by ITT in the 1960’s and is a whopper, weighing in at 22lbs, it has a 7.5kV max anode rating and can deliver up to 150 amperes peak current but to do that requires the grid be pulsed up to approx. 2000v.

    Now, the first “Bail-Out”: Suddenly, at some point around 1999, Eglin’s stock of spare tubes was nearly exhausted and ITT had been sold to Triton Services which had deteriorated to the point it could no longer supply good working tubes (in 2008 Triton would declared bankruptcy and close its doors) so a government contractor responsible for maintain the radar asked Eimac if it could make an equivalent to the F8147A. Having nothing similar, Eimac developed the EI8147A (5960-00-720-0615) and CPI continues to supply that tube for the FPS-85, the only system that uses it.

    Then, an opportunity for a second “Bail Out” occurred: In 2007 the USAF asked Eimac if it could supply an equivalent to the WX-4778. The situation was getting urgent because they had almost depleted their stock of spares of this unique tube. As is normal practice for government systems, they had purchased a large number of WX-4778 and warehoused the tubes. Subsequently Westinghouse discontinued that tube when it went out of business in the late 1960’s or early 70’s. Fortunately the operating life of the WX-4778 was good so the use-rate was fairly low, nonetheless they suddenly found themselves with almost no spares and another crisis was at hand!

    I was asked to recommend something so I compared the constant current curves of all the triodes Eimac made in 2007 to those of the WX-4778 and I was able to find one tube that had very similar curves: the 3CX1000A3. Although this was a tube with thoriated tungsten filaments and was quite a bit larger than the WX-4778, theoretically it appeared it should work with little or no other changes. The filament transformer was different and there were a few mechanical details associated with the socket. Chris at Universal Transformers in Farmersville, TX supplied a custom filament transformer and delivered it to the contractor at Eglin and, when everything was installed in place of a WX-4778 the radar worked perfectly and it has now been retrofitted to use Eimac tubes in both critical sockets thus CPI successfully rescued the FPS-85 radar twice!
    KE0ZU, KI6NQT, KM1H and 5 others like this.
  5. W0AAT

    W0AAT XML Subscriber QRZ Page

    Surprised they haven't moved the drivers to solid state.
  6. W6MTF

    W6MTF Ham Member QRZ Page

    That was studied and the task must have been enormous. If anone could do it it would be Diversified Technologies:
  7. W6MTF

    W6MTF Ham Member QRZ Page

  8. W6MTF

    W6MTF Ham Member QRZ Page

    DJ7WW, KM1H and AF6LJ like this.
  9. K7BIT

    K7BIT Ham Member QRZ Page

    Cheers Carl, I just picked up NCL 2000 without tubes for $25, some one removed the band switch but I found it, I did find schematics, still looking for manual, I am debating if I use 8122 or mod using 3-500ZG (PentaLabs), adding 10VAC 15A filament transformer, updating to W7RY QSK, reed/vacuum relay. Transformers are good, plan to replace all caps and resistors, pull air caps and clean them, update to 30AMP relay with 2 min delay to replace flaky time delay relay, replace all the diodes, and relays. Hard however to find much mod info on this amp. Any tips would be greatly appreciated. I also understand you sell parts do you have a web site? Thanks K7BIT
  10. AF7XT

    AF7XT Ham Member QRZ Page

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