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Replacement of RCA 8122 Tubes

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

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  1. SM0AOM

    SM0AOM Ham Member QRZ Page

    8122's had a short history of use in ship's radio transmitters from S.P. Radio in Denmark. I worked with these transmitters briefly during the early 1980's.

    I cannot recall which brand of tubes that were used, but generally speaking the reliability and longevity left something to be desired compared to the 4CX250R's used in the ITT-SRT transmitters.

    Poor reliability was a general problem both for the R/O's,the shipowners and the rental radio equipment provider. From what I can recall, only the
    800 W T-1127 with two 8122's came into production, a 400W version stayed at the prototype stage, as solid-state transmitters rapidly were entering the market.

    SRT had a quite popular transmitter dating from the late 60's, the STR350, that used an 4CX250R in the final. It did however use a quite complicated driver chain and a stabilised screen power supply. In order to make it less expensive to produce, it underwent a design change that resulted in the mid-80's 8874 equipped STR430. But as it was not solid-state, it fell out of favour to the newer transmitters that had much lower maintenance requirements.

    The 8122 surfaced again in the early 90's, when I was project manager for an upgrade project for air-ground services. As one candidate for 1 kW transmitters, the RF Communications/Harris RF-110 or URM-23 was considered, as they could be obtained quite reasonably on the second-hand market.
    The price for a quality-brand 8122 was many times what we had to pay for an 4CX250R.

    After understanding that the Harris amplifier required matched driver and final tubes, and also considering the previous track record of the 8122 in the S.P. Radio equipment, I decided to heed the advice of my Canadian colleagues and buy Collins HF-80 gear instead.

    NL7W, KM1H and AF6LJ like this.
  2. KM1H

    KM1H Ham Member QRZ Page

    Great story Reid and I sure appreciate your taking the time to present it.
    Ive used several Eimac 8122 and 8122W's in NCL-2000's (not 1000's altho there was a complete NCX-1000 transceiver that used a single 8122) Ive serviced here over the decades and found them extremely stable as far as screen current drift from heat. What was different between the two versions?

    I could tell you some interesting stories about the battles between National and RCA over the BS snow job they gave us to keep from using Eimac 4CX250R's as used in the Collins URC-32 in the USN, or the fairly new Eimac 3-400Z.
    The 8122 screen voltage and construction was way over rated for typical ham use of long tune up times.

    Ham since 1955
    AF6LJ likes this.
  3. W6MTF

    W6MTF Ham Member QRZ Page

    HP rack.jpg Here's my test rack (#2) a bit messy but I guess I don't care as long as its functional. I made a rack with wood shelves using two Chatsworth racks, you can sometimes find these for $20-30 a piece and usually come 7 ft high but they also make some 6ft (and even 8ft if you are unlucky!). The two racks are placed back to back and I place steel angles between the two racks to support plywood shelves at whatever height you want. This allows removing whatever you want whenever you need it. And a little air circulating between instruments might be beneficial. I don't use the PTS synthesizers very often so they are sitting one on the other at the bottom.
    KM1H, K9FLY, N2EY and 1 other person like this.
  4. AF6LJ

    AF6LJ Ham Member QRZ Page

    Nice set up and you have given me some ideas. :)
  5. K9AXN

    K9AXN Premium Subscriber QRZ Page


    Thanks for using your time to take us on a vicarious ride through history. Your description and assessment of the Eimac and RCA grid structure is right on. RCA had a tough time aligning the beam channels in the 8122 as you say. And your correct in that it was because the beam slots were not precision cut preventing perfect alignment between the CG and SG. They were later to use the deviations to advantage for class AB1 operations.

    Always wondered why Eimac never published operating range of values in the 8122 data sheet ---- apparently didn't need to.

    Have a great evening Regards Jim

    Last edited: Jun 23, 2017
  6. W6MTF

    W6MTF Ham Member QRZ Page

    OOPS I intended to refer to the NCL2000. I'll fix that error, thanks!
    The problem with the 8122 as you go up in freq the type of socket used begins to be of inportance with respect to circuit stability. Some Erie sockets made for these tubes had built in screen bypass caps (and were gold plated - I still have some and they should make nice paper weights!). The cheap plastic 11 pin sockets did not have integral bypassing so disc ceramics were used and their inherent parasitic (>!) inductance caused the URT-23 transmitterrs (exciters) to show instability with Eimac tubes but they were stable with Burle tubes, as discussed in my little history on the 8122. If I hadn't been there and witnessed the issue with Bill Orr I would probably have doubted such a story.
    >! BTW I hesitated to use the term "parasitic" because old wounds never heal - you never know when some obsessive Troll might get his juices worked up on such a word, ha!

    A "W" suffix in the type number indicates a ruggedized structure but the electrical performance should not differ.
    On that note, RCA (later Burle) made a 4657 which was a ruggedized 8072 because it had to operate in mobile radio transmitters and the 8072 grids would move if the tube received shock or vibration as expected in a mobile rig, so they added something mechanical to keep the grids stable.
    When Eimac made a few 4657 on order the tubes were simply 8072's labelled 4657 since nothing was necessary to add stability to an already stable structure.
    I always wondered why the EIA designations in this family of tetrodes are so far apart? Like the 8121, 8122, etc. all have the same basic internal structure then you have the 4657 8072 same basic tubes but without air cooling fins. Note the wild range in EIA type numbers. BTW the EIA used to supply historical data sheets from their Washington DC location on Eye Street (its funny how I remember little things like that!) - I wonder if they are still operating?
    I'm sure some savvy ex-Burle or RCA employee would know.
    Reid W6MTF
    N2EY likes this.
  7. AF6LJ

    AF6LJ Ham Member QRZ Page

    I have one of those 4657s with a Motorola part number on it, still in the box with the silver paper wrapped around it. If I ever had a Signal One CX-7A I would have a spare final for it.
  8. WA4SIX

    WA4SIX Ham Member QRZ Page

    Got to love Carl. When he knows, he surely knows. Ticks some people off.

    KM1H likes this.
  9. WA4SIX

    WA4SIX Ham Member QRZ Page

    My best suggestion is to call Merrit at Tell him what you have. You might make measurements of your bias to get better matching results. Get a matched pair, or even 4 matched tubes for future use, if at all forseen.
    I think this has been the most informative thread on this subject that I have seen. Great to have you folks still around to teach us youngsters a thing or 8122.
    When I ordered my matched quad of 811As for an 811H, I specified what amp it was going into. They specifically sold me a matched quad for my amp, with the cavet of performing the updates to my older unit (Direct grounding, etc...). I have always used RFParts, since the 80s. They are the Go To company for this stuff. Partly for their supply chain & partly for the knowledge they have about all of this.

    AF6LJ likes this.
  10. KM1H

    KM1H Ham Member QRZ Page

    National and most others Im aware of used the Johnson socket and it was impossible to tame the tubes in a NCL-2000 converted to 6M. The cure to that was to use the regular 4CX250B separate screen bypass rings, which fit perfectly, worked like a charm and my NCL-2006 I converted in 64 is still in regular use. The early RCA tubes, 63 to late 66 dates, had unsupported screens which would drift into the plate with heat if they didnt fail first from skyrocketing screen current. The 67 date codes I wound up with lasted until 2013 when the power was down to 700W with increasing IMD and now have 83 codes from when I picked up a distributed amp with 13 in them all reasonably matched and visibly looked brand new, even perfectly clean fins. I suspect it was retubed and soon scrapped.

    :eek::rolleyes::rolleyes:o_O Ah yes, I still remember the time I called Eimac and talked to you about his QST "article". He hasnt been heard from in awhile and Ive heard his health isnt very good. Anyway we still have our own Troll on QRZ who is always good for laughs.

    My question there was what did they do to ruggedize it?

    Keep in touch on QRZ Reid, its always a pleasure to read about Eimac.


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