Some Sb-220 Questions.

Discussion in 'Amateur Radio Amplifiers' started by KJ4AQU, Apr 28, 2014.

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  1. KJ4AQU

    KJ4AQU Ham Member QRZ Page

    I have some knowledge of amplifiers specifically AL-811's rebuilt one in fact new circuit board, Parasitic board, Band switch back plate and bottom plate
    due to damage in shipping or it was dropped. That was easy. Did the grounded grid mod on an AL-572 and aligned the input coils as well.

    This SB-220 I am being very cautious around. Manly higher voltages etc. and I don't want to ruin the 3-500z's.

    Plus the original builder did a good job. Whoever owned it next put some mods in that I removed and the soldering was iffy at best. Could literally pull the wires out of the meter board. Really bad soldering job so have been slowly checking it over.

    Also I notice the top of the plate caps are very close to the inner metal screen. Is there a chance of arcing. I know someone put some kind of Corona paint? on the inner metal screen in the area of plate caps. Just wondering. If there is a chance how can I prevent it? I was thinking of ordering the low profile plate caps like Ameritron uses in the AL-80B.

    Better to ask lot's of questions of the experts here like you WB2WIK, Steve, Sue and others than be sorry for it later. :)
     
    Last edited: May 12, 2014
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I've never seen an SB-220 arc from anode cooler caps to the aluminum cover. Make sure the dissipators are inserted all the way onto the anode caps of the tubes, the tubes are fully seated in the sockets, etc. It's only about 3 kV and with flat or smooth surfaces they can be very close without arcing. A sharp point will incite an arc with wider spacing.

    You could add a layer of HV Kapton (or similar) adhesive tape onto the "screen" directly over where the two dissipators are if it makes you feel better.:eek:

    3-500Zs are pretty rugged tubes, it takes quite a lot to make one fail. In my amps, they've only failed over time, usually many years. An arc external to the tubes wouldn't hurt them, but might blow the breakers.:p

    Good idea about re-doing lousy solder joints.

    I think the most "useful" mod I ever made on the SB-220 (and I did it on my own) was to add a 6:1 planetary reduction drive to the PLATE TUNE capacitor shaft. Not a small job, takes some drilling and removing parts then re-installing. But they're available for less than $15 each, and I added them to both capacitors (the LOAD hardly needs it, but it made the amp look nicer having the reduction drives on both) for about $24 or so many years ago -- never regretted that mod. Makes it much nicer to tune up, especially on 10m.
     
  3. KJ4AQU

    KJ4AQU Ham Member QRZ Page

    Those 6:1 reduction drives is what I liked about the AL-811 and the AL-572. Possible addition in the future. I re-did the soldering on the Rear of the Band switch the ceramic one. 3 of the contacts had only one lug soldered. So I added additional solder to fill in both sides of the contacts. Boy that was a pain to get to!
     
  4. W1QJ

    W1QJ Ham Member QRZ Page

    No need to put any goop on the top screen. I usually put a slight bow in the top screen before I screw it down. Although the distance is OK the bow adds an extra measure. I have never seen a stock unit arc unless someone screwed around with the placement of the plate choke or something.
     
  5. KJ4AQU

    KJ4AQU Ham Member QRZ Page

    Good idea about the bow on the screen.

    So I added the standby switch and gave it an additional function. When switched to standby it puts a 100 Ohm 40 watt resistor in line to slow the fan down by about half. Makes it a lot quieter. I used a DPDT switch to do this. The resistor itself is mounted just behind the fan on the perforated screen to allow air to flow over it and keep it cool. It gets a little warm but not hot.

    About grounding grids I was wondering if this is acceptable. See Pic. I am using braiding instead of heavy wire.
     

    Attached Files:

    Last edited: May 13, 2014
  6. W1QJ

    W1QJ Ham Member QRZ Page

    The way you grounded the grids is exactly the way I do it and the way Ameritron did it with the Al-82 before they came out with the "ground plane" tube socket that they now use. The only problem I see is you really need to run the braid from the one tube socket pin over to the ground where the coax is grounded. You want that as a ground point as well because the next ground is too far away.
     
  7. KJ4AQU

    KJ4AQU Ham Member QRZ Page

    So run the braiding over to the coax ground as well? Done!
     
    Last edited: May 13, 2014
  8. AF6LJ

    AF6LJ Ham Member QRZ Page

    The way you have done it just parallels up what is inside the tube.
    I prefer to just ground the the individual grid leads directly to the chassis. This involves drilling a few extra holes in the chassis, because it is bad pratice to use the tube mounting bolts as a grounding point. The reason being has to do with the fiber washers compressing over time and the socket mounting hardware loosening up. A direct short lead to chassis is the best in my opinion.
     
  9. KJ4AQU

    KJ4AQU Ham Member QRZ Page

    Picture does not show it but the one tube socket has all 3 grid pins directly grounded. I just ran the braid along the top to put them all at the same potential. See new Pic. So you are saying get rid of the braiding along the top?
     

    Attached Files:

  10. AF6LJ

    AF6LJ Ham Member QRZ Page

    Others may disagree; I am saying you don't need it, I don't think it is going to cause any instability by having it there.....
     
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