Sb-200 weird thing happening

Discussion in 'Amateur Radio Amplifiers' started by KN4CTD, May 15, 2019.

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  1. HAMHOCK75

    HAMHOCK75 QRZ Member

    Not sure if you have ever restored other Heathkits but they are famous for having carbon composition resistors that increase in value and go out of spec. I replaced quite a few in my SB200, DX60's, etc. This is a problem with carbon composition resistors and not a problem specific to Heathkit.

    The cheapest spectrum analyzer like tool is an RTL-SDR. They are available covering 100 kHz to 1.7 GHz for about $20. The important feature is that they provide a visual display of amplitude vs frequency like a spectrum analyzer so you can search a wide frequency range quickly for signals.

    As I recall the reason the SB200 does not have its grids grounded is so that C14 and C15 can be used to tune out the grid inductance of the 572's. One drawback is that 200 pF cannot provide a very good ground on the 160M band which is why most true grounded grid amplifiers cover 160M but the SB200 cannot.
     
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  2. KN4CTD

    KN4CTD Ham Member QRZ Page

    I still need to check a couple of resistors at the tube sockets, but the two 33 ohm grid resistors measure good. I removed the 200pf caps and one checks about 202-203pf and the other 207pf according to my LCR meter. That’s within tolerance. The only thing I’ve found wrong so far has been the out of tolerance parasitic suppressor resistor and the missing plate choke bypass cap. Bypass cap is installed now and I rewound the suppressors on 5 watt metal oxide 47 ohm resistors. I’ll try to locate some original carbon types to redo them with. My plan is to take the bandswitch and air variables out and clean all that good by hand and reinstall.....it needs to be done regardless so I will do that. I’ve checked every bypass cap in the amp and none show to be bad. The plate choke looks original and undamaged or modified. I’ve found no cold solder joints or loose connections on any part of the input or output band switch. I do want to get some sort of scope or analyzer (and learn to use them) so I can tell more about these things when I’m using/working on one. I also plan to put a quick 10 meter dipole together one day this week and see if the amp exhibits the same symptoms into a resonant antenna.
     
    Last edited: May 19, 2019
  3. WA7PRC

    WA7PRC Ham Member QRZ Page

    See my post #25 regarding a source of NOS 2W carbon composition resistors. They don't HAVE to be precisely 47Ω... anything around that value will work as well.
     
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  4. KN4CTD

    KN4CTD Ham Member QRZ Page

    Thanks will get some on on order. I’m waiting until I finish checking everything else out so I can make a list of stuff I need all at once.
     
  5. WA7PRC

    WA7PRC Ham Member QRZ Page

    Sure...
    That's what I'd buy. If you have your heart set on 47Ω, you'll pay $2.50 each (http://www.ebay.com/itm/401551785876).
     
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  6. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    The Ohmite type OY resistors are good replacement for carbon composition; they're ceramic resistors that are better at high temperatures and drift less, and are non-inductive to a few hundred MHz.

    This is what's used for parasitic suppressors in most of the "new production" tube amps.

    https://www.ohmite.com/ox-oy-series/
     
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  7. KN4CTD

    KN4CTD Ham Member QRZ Page

    I’m not gonna post anything else about this amp until I take the bandswitch and air variables out to clean.......I whipped up a quick dipole this evening. I need to cut it down a bit, but I’m gonna try it out just to see if this craziness continues. If this amp happens to be stable into a dipole cut resonant for 28.5 MHz, then what? What would a person chock that up to? Like I said in earlier posts.....I care not about 10 meters.....BUT, the amp was designed to work there. I’ll update probably tomorrow after I cut this thing down and verify it’s resonant in the middle of the ssb portion of the band. Using my antenna analyzer of course! I just cannot find anything else wrong based upon the original build schematic. I’ll remove, clean and reinstall the air variables and bandswitch, but after that, I’m just at a complete loss with this thing. I’m not very mathematical and electronically inclined by any stretch, but I do have a basic understanding of what’s going on with a grounded grid amplifier. Going by the schematic, I can’t seem to find any reason for this except maybe I need carbon resistors in the parasitic suppressors. This amp isn’t some basket case full of unauthorized mods. I will, BTW, swap the Cetrons in when I test into the dipole cut for 10.
     
  8. WA7PRC

    WA7PRC Ham Member QRZ Page

    Q: How much reactance does Ohmite specify?
    A: They say nothing about reactance or inductance.

    OTOH, the Allen-Bradley datasheet for the CC resistors sold by eBay seller 'bigsmythe74' specifies Z/R ratio very close to 1. They're cheaper, too. My OE CC resistors lasted four decades of nearly daily use. :)
     
  9. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    An interesting thing to consider with SB-200s is that no two are alike. Differences in layout can make the performance between two units diverge as the frequencies go up.

    I've recently had 2 SB-200's in my shack. Mine is about 7 years in my hands and a bucket of parts when I got it. The tube sockets, associated parts, suppressors, and power supplies were complete toast. The input section was intact and built fairly well. I rebuilt it as a directly grounded grid amp for the fun of it and wound up using a 15 ohm resistor to feed the cathodes to turn the gain down a bit. I believe that the topology change plus the fact that my radio at the time was a Kenwood hybrid, added up to, "I never noticed that 15 or 10m was tough to drive." My SS radio can drive it pretty well on 15 and some of 10m.

    A friend purchased a well built completely stock SB-200 that I put a keying kit into. My ICOM doesn't like it on 10m unless I tweek the input coils for a particular part of the band, and that tuning is fairly narrow. From what I've seen, heard, and read, this seems to be pretty standard for the SB-200, i.e. not an easy drive across the whole band of 10m. If I had to use that amp and a SS radio that had trouble driving it, I'd consider modding it like mine is. A couple of Ol' Timers here did razz me pretty hard about messing with the original design, but I must say that my SB-200 does seem to work very well.

    My point is that your amp may be working reasonably well. I would go with the tried and true suppressors (carbon comp resistor with 3-1/2 turns of 18ga tinned copper) to reduce the variables. I would also consider going over the chassis and extracting parts that don't belong, putting parts in that do belong, and putting them in the correct locations and dress. This would give you the opportunity to examine all of the connections and correct any bloopers.

    You could tune the input section to work reasonably well in a portion of the 10m band but working the whole band may be problematic with a SS radio driving it no matter what you do.
     
    Last edited: May 20, 2019
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  10. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    There may be an engineering ap note on this, I haven't searched for one; but I see the type OYs are what Ameritron uses in all their "built today" tube amps. Tom W8JI likely specified those, and he likely characterized them; I recall in the AMPS reflector years ago he commented they're as good as or better than A-B carbon comps for use in HF parasitic suppressors.
     

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