Heathkit SB-230 -- Mine uses a new $25 tube and is still cooled by conduction!

Discussion in 'Amateur Radio Amplifiers' started by KI4ZX, Nov 1, 2017.

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  1. KI4ZX

    KI4ZX Ham Member QRZ Page

    The SB-230 has a bright future, thanks to the brainchild of VA1DER, whose modification I worked up in parallel with his. Our SB-230 amps now run on GI-7BT tubes with modified anodes. We cut off the threaded rod intended for a finned cooling assembly and, instead, we press the flat anode surface against the BeO cooling block, just as the 8873 was configured. The circuitry supporting the tube is still essentially the same. At a minimum, all that needs to be done is eliminate the tube socket and bring up wires for the filament, cathode and grid (or just leave the tube socket there to serve as a terminal strip and bring up the lines). The cathode bias must be increased to 24 volts, but that is just a matter of installing a different zener diode. Also, an anode clamp has to be fabricated -- that's very doable for hams that are mechanically adept in the workshop.

    Oh yes, the GI-7BT tube has a 12 volt filament instead of 6 volts. I simply installed a small transformer, but there are alternate ways of dealing with this.

    Kurt, VA1DER is working up a detailed description of the tube mod and will post it soon (although he's been a bit busy lately). But I wanted to get you owners of SB-230 amps aware that your "quiet amps" have a bright and quiet future after the 8873 tubes bite the dust!
    NQ1B, AF5CN and KK4RSV like this.
  2. W9GB

    W9GB Ham Member QRZ Page

    Tony King, W4ZT (sk, 2009) and Ron Wilson, K4POZ attempted
    retrofitting the GI-7b in the SB-230 amplifier, with poor results.

    The Eimac 8560AS ceramic tetrode is the only other RF tube soometimes discussed
    as a replacement for the Eimac 8873 ceramic triode, while maintaining the cooling method.
    The 8560AS was used in Motorola MICORĀ® equipment.

    Look forward in reading about your conductive cooling approach.
    Last edited: Nov 1, 2017
  3. W1QJ

    W1QJ Ham Member QRZ Page

    I actually have an SB-230 conversion to a horizontally mounted GI-7b done by Tony and Ron. These guys are old friends of mine. Tony was s wonderful guy, very innovative. He had a heart transplant many years ago and when he passed away in 2009 I thought that maybe it was his heart but unfortunately he had died from a melinoma. Tony, Ron and Lawson were experts at changing the obsolete tubes in the Dentron line for more obtainable tubes. Great conversions, but rather involved and not conducive for customer based jobs. Great as a one off done by the owner.
  4. KI4ZX

    KI4ZX Ham Member QRZ Page

    Thanks Tony. We've looked over a number of these mods. Ours is simple by comparison and it works. Well, I say "works," but we can't offer up any long-term usage beyond a few months of operation. The main trouble I encountered has nothing to do with the mod, but with my addition of a fuse and glitch resistor to the HV line. Strangely, a 1 amp, fast-blow fuse likes to blow at random times in the HV line. There are mysterious current spikes that have forced me to a 1.5A, slow-blow fuse.

    The other SB-230 mods I have seen have crowded up the tube cage -- not this one -- one tube leaves, another comes in. Some mods have modified the relay circuit, increased the number of poles, and/or changed the metering scheme. Not this one.

    The GI-7BT is the more robust, high-temperature version of the GI-7B. Kurt chose this after reviewing the specifications and performance curves.

    I hope to post a few photos in the way of a preview of this mod, but I'm not ready to post a photo in the way qrz requires it.
    NQ1B likes this.
  5. W1QJ

    W1QJ Ham Member QRZ Page

    Glass fuses, unless specially identified for HV use are NG and should not be used. Drake had the right idea by using a small 2 watt .82 BWH type resistor as a glitch in the B+ line. These resistors (BWH) type are made for the purpose of acting as a fuse. Although they look like a regular carbon comp resistor, they aren't exactly that. They can handle a very quick overload of current but for too long it will blow open without the arcing of a typical glass fuse. It is little known that Heathkit used the same resistor in the SB-220 and it acts very well as a fuse in a fault situation. Unfortunately these resistors are made to be sacrificial and when they do blow, most hams replace them with other than the "like" resistor and the protection of the "fuse" goes away.
    NQ1B and N2EY like this.
  6. KI4ZX

    KI4ZX Ham Member QRZ Page

    Thanks for the tip on fuses! The fuse you describe is still readily available on EBAY (new old stock). I don't know how the HV line in the SB-230 compares to the Drake amp, but perhaps current surges in these amps tend to be similar within certain bounds. At any rate, I may buy a package of 5 later today.

    I need to find a place to upload my photos that allows 3rd party sharing without costing a lot of money. I may or may not succeed on that venture. Might wait for Kurt to post his article.

  7. W1QJ

    W1QJ Ham Member QRZ Page

    How much do they want for them on eBay?
    F5LUX likes this.
  8. KI4ZX

    KI4ZX Ham Member QRZ Page

    You get 5 of them for $4.50 (incl shipping).
  9. W1GCI

    W1GCI Ham Member QRZ Page

    There is a concern with handling Berilium Oxide BeO in that it is considered a carcinogen. This is not a problem is not disturbed in its solid state but it can be if it's machined. If the mods to fit the BeO to the GI-7B doesn't involve touching the Berilium block, then it sounds like a really good fit.
    N2EY likes this.
  10. KI4ZX

    KI4ZX Ham Member QRZ Page

    I handled the BeO block with my bare fingers as I cleaned it and applied a new thin coat of thermal compound -- and then I washed my hands because there was nasty thermal compound on them. :) I spent over 33 years working as an engineer at the Oak Ridge National Laboratory here in Tennessee. At the nearby weapons plant, they machined Beryllium for many years. Although the machinists used some personal protection, it proved to be not enough for some of those workers. They now get free health care and other Federal compensation. The mod I'm talking about certainly does not involve machining, filing, or even scraping the block. It's a non-issue for this mod! I really like the BeO block -- it's a great electrical insulator and better thermal conductor than most metals.
    N2THD and N2EY like this.

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