Tremolo trouble | hopefully solved now

Hi Folks,

as a follow-up I want to let you know about my findings regarding the Tremolo Problems, I came across while working on a ab763 bandmaster top of a friend: My first impression was that changing all HT caps will solve the Problem. Now my friend came along and the tremolo was as weak as when I got the amp. Here’s what I found out:

  • I read in Gerald’s book, that changing all disc caps, bypass-caps and controlling the grounds was his texas-shotgun technique. So I checked everything and found out, that the ground of the first grid is connected to the bias voltage. This cap I hadn’t changed.
  • So I changed the 25uF Cap that filters the ripple from the bias voltage with great success.
  • Then I found one of the – already changed – disc caps to have a far to low voltage rating, so I changed it to a higher rating and the amp’s Tremolo was weak again. The solution was found when I unsoldered the ground connection of the Bypass cap of the second tube section, the one where the 820R resistor from the PI is connected.
  • Unsoldering those ground connections can really be the Clou.

So here’s what I would do the next time, when repairing a Tremolo:

  1. Listen to the Tremolo first
  2. Watch the light bulb, if it’s blinking, if it does
  3. Re-Solder the ground Connections with fresh solder, even if it looked ok before!
  4. If it’s still weak, hook-up a fresh, reliable 50uF/100V Bipolar cap in parallel to the one filtering the bias cap and listen. If it’s better
  5. Change the cap
  6. If it’s still not working you might change the light/opto part to a new one to get a stronger trem.

I know that this might not be a complete list but it worked for me. Any questions, let me know.


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