Heathkit SB-230 (8873 tube) Amplifier Questions

Discussion in 'Amateur Radio Amplifiers' started by AD5GG, Mar 21, 2018.

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  1. AD5GG

    AD5GG Ham Member QRZ Page


    I recently came into possession of a Heathkit SB-230 amplifier. Everything in it seems to be stock with the exception of the high voltage supply electrolytic capacitors. They've been replaced at some point, but I have no idea when. Looks like at least a decade ago.

    My questions are as follows:

    1. There is no grid protection circuitry. While this would be nice to take care of the tube if there are any overdriving/tuning mistakes, is it really that critical so long as tuning up is done at low power, and the amplifier isn't overdriven or doesn't see excessive grid currents?
    2. I have read that people are installing a low value, high power resistor in line with the B+ supply to the tube anode. What type of resistor is being used here? Carbon film or similar?
    3. Regarding the B+ resistor, how much protection are you actually getting from it? I ask since there is a decent sized choke in the B+ supply before the anode. Would a good power conditioner (from 120V or 220V) eliminate the need for this B+ resistor?
    4. Are there any other components which are notorious for wearing out and/or failing in the SB-230? I'd like to do a good job preserving this amplifier and especially the tube!
    Also, any hints or tips would be welcome.
    Thanks in advance, and 73 from AD5GG.
    Last edited: Mar 21, 2018
  2. WB1E

    WB1E Ham Member QRZ Page

    A couple of years ago, I thought I remember hearing that RF Parts was considering rebuilding the 8873 tube.
    AD5GG likes this.
  3. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    Tuning at lower power doesn't solve anything. Tuning while watching grid current does -- if the grid current starts to rise, increase LOADing to bring it back down.

    That's a "glitch" resistor and probably not needed with a ceramic metal tube; they rarely ever develop internal arcs. They're much more likely to suffer grid damage which a B+ resistor won't help.

    The B+ resistor is there to limit the energy available for an ion arc inside the tube. 8873s are not known for this at all; I probably wouldn't bother. A power conditioner won't do anything, and you'd need a really big one for an amplifier. The arc comes from inside the tube if the tube starts to lose its vacuum or of B+ surges ridiculously high; both very unlikely to happen.
    WA6MHZ, WA7PRC and AD5GG like this.
  4. AD5GG

    AD5GG Ham Member QRZ Page

    Excellent, thank you very much for your advice.
    Care and feeding are my primary concerns with this amplifier.
  5. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    There are two "critical" things about the 8873-4-5 tubes: Cathode warmup time (60 secs min, 90-120 secs recommended) and grid current.

    I think the SB-230 already has a "warm up timer," as most commercial amps using such tubes do.

    I don't think the SB-230 has a grid current trip circuit which monitors Ig and automatically puts the amp into "bypass" mode if grid current peaks rise above a preset value, and if it doesn't, that's one circuit I'd definitely add (set to maybe 30mA max) and also would not be difficult to add, especially at such low current. A quick glance at the schematic just now seems to show R17, a 33 Ohm resistor in series with the grid. If that develops 1Vdc drop across it, that would be about 30mA Ig and could be used with a simple differential amplifier with a 1Vdc reference to pull in a relay that opens the coil of RY1; bingo, quick grid trip circuit. There's many ways to do this, but that could be one.

    In any case, when tuning such an amp I'd only look at output power and grid current. Plate current doesn't matter much, and looking at HV is kind of a waste of time since you can't control it.:)
    WA7PRC, AD5GG and WD4IGX like this.
  6. AD5GG

    AD5GG Ham Member QRZ Page

    Fully agree on the HV. Good for checking every now and then, and possibly showing up problems with the HV supply, but that's about it.
    Thanks for the hints on the grid trip circuit. It's almost like they put a 33 ohm resistor there just for that purpose - very handy, and an elegant idea.

    And yes, there is a 60 second (ish) delay timer which prevents the amplifier from being keyed during warm-up.

    Thanks very much!
  7. KM1H

    KM1H Ham Member QRZ Page

    I doubt that since it is an indirectly heated tube which are considered unrebuildable at that power level; the 8874 and 8875 are identical internally. The 8877 is also in the same category unfortunately.

    The 3CX1000A7 in my HB 2M amp is considered the smallest rebuildable tube but after 30 years on a pull it still puts out an easy 1500W...plus I have a full power spare.

    One of my SB-230's came without a tube so I modified an 8874 to do the job as they were cheap as pulls back in the 80's. I also completely rebuilt that awful bias circuit to a traditional GG circuit. It is now unconditionally stable as a 6M monobander after getting working properly on HF. It is a portable/rover amp that loafs at 500W altho it easily does 650 without complaints with the same tube. Ive converted about 20 others for customers and regularly get a stock one in for various repairs.

    Another is bone stock and I will likely sell it after making the same changes.

    Tuning at low power is an important first step for anyone not familiar with the very fragile grid in that tube. Using an external wattmeter tune at about 20w drive and use the Tune control to get some indication (NEVER use the Relative Power meter position as you always want to monitor grid current).

    Bring drive power up in small increments while using the Tune and Load controls to PEAK the grid current at around 10 ma. With about 50W drive and peaked the power should be around 500W with a good tube. Be satisfied with that considering the 8873 replacement cost. When QSYing always keep an eye on grid current and use the Tune control as needed to peak it.

    If using it heavy with processed SSB, CW, digital, etc, mount a muffin fan externally on the heat sink. Reduce power for FM and AM.

    Make up a tuning chart as you go along to simplify things.

    Here is an obviously used tube being sold as NOS so be careful when shopping

    AD5GG likes this.
  8. K0OKS

    K0OKS Premium Subscriber QRZ Page

    Most electrolytic capacitors of any size havea date code on them. Typically this is the year and week of manufacture... e.g. 9817 would be the 17th week of 1998. Sometimes the numbers are reversed like 1798.

    Perhaps that will help you identify their age.
    AD5GG likes this.
  9. AD5GG

    AD5GG Ham Member QRZ Page

    Thanks everyone for your invaluable inputs - I'm a semiconductor guy, and this is my first foray into particle accelerators.

    How robust are these tubes to poor VSWR? 2:1 and greater, for example?
    With proper cooling, of course.
  10. KM1H

    KM1H Ham Member QRZ Page

    As long as grid current isnt exceeded during the tuning/peaking process VSWR is not a tube issue unless it causes arcing/oscillations which are tank circuit related.

    I have modified my amps to work into what loads my antennas present. A pi-network is an impedance matching device and is often easy to modify to something well beyond the 50 Ohm 2:1 limits in the manuals. Why buy an external tuner and add loss when the amp can do the same job? I dont own a tuner.

    First of all I start with 75 Ohms since I use CATV RG-6, 11, and hardline coax since the late 70's. I can cover 160 and 80/75 end to end with simple fan dipoles cut for 1825 and 1925 as well as 3600 and 3800 where the VSWR can be up to 5:1 elsewhere. At those frequencies the coax VSWR added loss is too low to bother with.
    On other bands where yagis have a high VSWR at the high ends where most AM activity is I move coil taps a very small amount so grid current can be brought down to safe levels.

    This is a nice feature of tube amps vs SS.

    N8VIL and AD5GG like this.

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