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previous: JCS Hi All, I have a SFV that is motorb... -- 7/21/2000 3:03 AM

Re: Silverface Vibrolux motorboating ?

7/22/2000 1:39 PM
R.G.	Re: Silverface Vibrolux motorboating ? If you just changed all the electrolytic caps, that's not the problem. The Debugging Page is really oriented to existing amps that develop a problem, not as much to new mods that have a problem. The "B+ decoupling caps" are some of the electrolytic caps you just changed. Motorboating is a low frequency parasitic oscillation, unlike the common idea of high frequency oscillation. Motorboating is why there are all those RC decoupling stages in tube amps. The series R introduces some series resistance, and the capacitor shunts any signals to ground. The idea is that you can't have the gain stages interacting with each other through the power supply's impedance if the shunt caps give each stage an isolated (kinda, by that resistor) and low impedance (by the shunt cap) power supply of its own. Less than three stages can't motorboat, so it's common practice to use a decoupler for every two stages. Old, dry electrolytics that can't shunt signal to ground anymore are the most common cause. However, if you just replaced them, it's not old, dry caps - unless you put in old, dry caps. You could try using a 47uF/450V cap in parallel with a 100K resistor (for bleed down) with a cliplead to ground at the power supply tack-soldered to the + lead of each capacitor in turn to see if one of them happens to be defective. Be %&*%$%^#$#&() careful with that thing, as it will store an ugly amount of voltage - one hand in pocket, tack solder in place, get out of the amp before turning it back on, let the bleed resistor bring it down before moving it to a new position, etc. The life you save could be your own. It's also possible that you missed a ground connection on one of the new caps, maybe, in cleaning up the lead dress??? There are other causes, such as having the input low frequency rolloffs all be the same and putting feedback around three or more stages, but those are rare. Maybe a wiring mistake in cleanup? That's the problem with new mods. An introduced problem is usually part of the new mod wiring, you just have to track it down, sometimes by backing out pieces of the mod, one at a time.

7/22/2000 1:39 PM

R.G.

Re: Silverface Vibrolux motorboating ?

If you just changed all the electrolytic caps, that's not the problem. The Debugging Page is really oriented to existing amps that develop a problem, not as much to new mods that have a problem. The "B+ decoupling caps" are some of the electrolytic caps you just changed.

Motorboating is a *low* frequency parasitic oscillation, unlike the common idea of high frequency oscillation. Motorboating is why there are all those RC decoupling stages in tube amps. The series R introduces some series resistance, and the capacitor shunts any signals to ground. The idea is that you can't have the gain stages interacting with each other through the power supply's impedance if the shunt caps give each stage an isolated (kinda, by that resistor) and low impedance (by the shunt cap) power supply of its own.

Less than three stages can't motorboat, so it's common practice to use a decoupler for every two stages.

Old, dry electrolytics that can't shunt signal to ground anymore are the most common cause.

However, if you just replaced them, it's not old, dry caps - unless you put in old, dry caps. You could try using a 47uF/450V cap in parallel with a 100K resistor (for bleed down) with a cliplead to ground at the power supply tack-soldered to the + lead of each capacitor in turn to see if one of them happens to be defective. Be %&*%$%^#$#&() careful with that thing, as it will store an ugly amount of voltage - one hand in pocket, tack solder in place, get out of the amp before turning it back on, let the bleed resistor bring it down before moving it to a new position, etc. The life you save could be your own.

It's also possible that you missed a ground connection on one of the new caps, maybe, in cleaning up the lead dress???

There are other causes, such as having the input low frequency rolloffs all be the same and putting feedback around three or more stages, but those are rare. Maybe a wiring mistake in cleanup?

That's the problem with new mods. An introduced problem is usually part of the new mod wiring, you just have to track it down, sometimes by backing out pieces of the mod, one at a time.

Replies:

JCS Thanks R.G. for the responce.I'll g... -- 7/23/2000 3:24 AM