I'm thinking you have some kind of goofballism in your power supply, for want of a better term. The PS on this amp is...semi-bizarre. It is two voltage doublers stacked on top of each other, and judging by your reading of the 6L6 plate voltage, you're missing about 100 volts somewhere.
Consider the stack of caps, from low to high, C29, C28, C27, C26. The neg of C29 should be ground or zero volts. Moving up that stack, the "+" of every cap should have about 120-125 more volts than the prior one, eg; the "+ of C29 and the "-" of C28 should be about 120 volts, the "+" of C28 and the "-" of C27 should be about 240-250, the "+" of C27 and the "-" of C26 should be about 360 volts, and the king of the hill, C26 "+" should be your 480 volts. Seems to me like one of those four caps could be bad. But see if you have that progression of voltages up the stack. Realize that if, for example, one of the lower members of the totem pole is bad, it will take about 100-120 volts away from everyone above. Right?
The bad news is that if one of those is bad, the wise move would be to replace all of them.
The second highest point in the stack, the junction of C26 and C27 (should: about 360 v) feeds the choke which then feeds the preamp stages (the drawing is a tad confusing, the voltages are high on the left and low on the right for those two series 68K resistors which is sort of backwards. Check those volts. The electrolytic cap C25 the 3-way, could have a bad section.
Although the symptom you describe "motorboating" is not especially a pointer to "bad caps", there is no avoiding checking those caps and the voltages which should be present on them. We call those points in the power supply "nodes" because they are the source points for a series of voltages that are fed to the various stages in the amp. You have to be very certain the voltages on those nodes are within reason....they can vary somewhat....but start with the C29-28-27-26 stack and make sure that is in order.
By the way, the death cap in this amp, C31, as switched by SW4, is something you probably want to remove in favor of a three-wire AC input cord.
And by the way #2, you noted a mis-wired repair. Of course, you'll have to measure volts and all that. But also, inspect solder joints things carefully ("all over the place") and look for a difference in patina or finish, eg; look for any "new" (meaning, non-factory) ones. That could lead you to look at other things.
In the 80's I worked at a factory where I was in charge of fixing stuff that came right off the assembly line and failed initial test = first power-up. Normally, when you open something up on your bench you're entitled to make an assumption---that it was indeed working at some point. I couldn't make that assumption. Sometimes, Consuela put all her diodes in backwards that day and the things flat out would not work. Of course, I was used to looking for such things, especially if a flood of that particular model suddenly started failing at the "initial power-up station". Sometimes there would be 50 units (these were little CRT monitors) that could be repaired in about 1.5 minutes each by flipping those diodes around. Sometimes she got the electrolytics in backwards...and above the gal who ran the power-up station, the foil insulation in the ceiling looked like it was blasted by a shotgun with the number of holes made by blowing-up e-caps. Incidentally, this is where I encountered backwards-marked diodes and if you think that would fry your brain, that would be correct.
With a botched repair or an incomplete repair (which it sounds like you have) you really have to look at just about everything that might have been messed with. Nuff said.
But to reiterate, you have to have that power-supply cap stack set up right as a prerequisite for any form of good operation.
Topic: Silvertone 1484 motorboating (Read 6475 times)