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:
- Listen to the Tremolo first
- Watch the light bulb, if it’s blinking, if it does
- Re-Solder the ground Connections with fresh solder, even if it looked ok before!
- 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
- Change the cap
- 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.