SB-220 question

Discussion in 'Amateur Radio Amplifiers' started by KC9JYW, Apr 10, 2021.

ad: L-HROutlet
ad: l-rl
ad: Left-3
ad: abrind-2
ad: L-MFJ
ad: Subscribe
ad: Left-2
  1. KC9JYW

    KC9JYW Ham Member QRZ Page

    I have a question that has me stumped. First I have done little work on this amp but recently changed out a noisy fan. Since I have done this I have noticed that when the amp is on OR OFF I see a negative grid reading when switching positions of my switch to the grid position. I have taken out the tubes and this still occurs. Any ideas? could the fan wires be crossed make this happen or does it matter?
     
  2. W1QJ

    W1QJ Ham Member QRZ Page

    When tubes are in an amp and the grid current shows negative going, it’s usually not really grid current. Its usually a grid to filament short on a tube. The grid meter is reading an opposite flow of current. If you remove the tubes and there is still a reading of the grid meter negative, chances are it’s not a tube it is another flow of current like a leaky bypass cap or leakage from the transformer winding into the chassis. Or something of that nature.
     
    K0OKS likes this.
  3. KC9JYW

    KC9JYW Ham Member QRZ Page

    Do you think the fan wiring could of done something if the wires were reversed? This is when it started. I cant say that I think this is what caused it but it was working fine until then.
     
  4. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    It's an AC powered fan with no polarity.
     
  5. KC9JYW

    KC9JYW Ham Member QRZ Page

    Cant understand why the grid would read negative. Not sure where to start looking.
     
  6. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    If this problem occurred coincidentally with the fan change, reflect back on what you did to replace the fan.

    Accidentally bump other wiring or connections? Pinch a wire when putting the enclosure back over the chassis?
     
  7. N0NC

    N0NC Premium Subscriber QRZ Page

    Check Bypass caps on the filament choke. I worked on an AL-811H that had had a tube arc and blew up the MOVs on the input tothe filament choke. Acted like that, negative grid current meter indication with no tubes installed.

    73, Chuck
    N0NC
     
  8. KC9JYW

    KC9JYW Ham Member QRZ Page

    Bypass caps check out within spec on the filament choke. Any other thoughts?
     
  9. K0OKS

    K0OKS Premium Subscriber QRZ Page

    Did you check what W1QJ said above?
     
  10. KC9JYW

    KC9JYW Ham Member QRZ Page

    I did check the bypass caps and all is good but the transformer (filament I assume) I am not familiar on how to check that for leakage.
     

Share This Page