Originally posted by TelRay
View Post
Notice in your last video that the controls that will abate the screech when turned still produce a noteworthy pop and audible gain reduction in the range that they also stop the oscillation. So this would be those controls introducing negative feedback, that would be countering the positive feedback that is responsible for the instability. I also suspect that there is enough channel interaction with V1 in it's socket that negative feedback from that channel also serves to stabilize the condition. ***It may well be that the boost switch wiring also served to stabilize the amp because it's leads proximity to others producing some negative feedback. My assessment so far...
The amp as it is is inherently unstable. There are two possibilities. One is that there is a problem with the decoupling function of the power supply. This could be due to a miswire (even a minor one that could be missed the first ten times you look it over) or it could be that this particular model amp bordered on instability as it was made. Many silver face Fender amps were actually. In production they may have found that it worked acceptably when new and fully stock. As in boost circuit in place and V1 installed. Change the formula with new caps, filters, boost removed, V1 removed, any associated lead dress changes, etc. and the anomalies that were keeping the original design stable are gone. My opinion on the matter so far...
Look over the power supply filtering one more time to be sure that it's absolutely correct and decoupling between nodes. If you replaced the old filters with stale ones that could also be an issue. If it's determined that the power supply is correct and the new caps are working properly I say DITCH THE CIRCUIT. If it happens that the amp is only barely stable anyway then you probably never will get good overdrive tones from it in my experience. It will always seem harsh or otherwise odd and off the mark. You have a chassis, pots, tube sockets, tubes, transformers, speakers and a cabinet. Everything you need to build a really great amp.
Should it come to this you could count on a lot of support here regarding lead dress and grounding schemes to (almost surely) make it MUCH more stable and therefor eminently more useful than it ever was, even when it was new. Not to mention losing the old circuit board. Which I consider another problem waiting to happen anyway.
It's a big undertaking if you've never built an amp though. JM2C on this one.
Comment