Weird electronic magic

[lowell]

This is really odd. I just about finished a custom amp that I designed. 2 6l6 and 2 6v6. I'm ironing out some things here and there. There is a really loud motoboating sort of sound when I have all power tubes "on." If just the 6L6 or 6V6 are on then it is gone. To try to narrow the issue down to one area I disconnected the coupling caps 1 by 1. It turns out when 1 of the caps that goes to 1 6L6 is disconnected the problem is gone. So to confirm, I pushed the cap's lead to touch the turret and bang, motorboating. I then tried alligator clipping the lead to a different turret, which is connected under the board to the same point and no motorboating. I then alligator clipped to the turret where the cap was originally soldered in and... no motorboating. To confirm this I left the cap's lead disconnected and soldered a wire from it to the original turret and the problem is gone. Now, if I push the lead to touch the turret, motorboating, even w/ the wire soldered in. I cannot understand how this is happening. I checked with my scope and meter to confirm that it is indeed electronically connected and according to the circuit it's exactly right.

So to sum this up, there is motorboating with the cap's lead touching the turret, but NOT motorboating when a wire is connecting the cap's lead to the turret. Need help.

[Jazz P Bass]

Wire Capacitance

There is a certain amount of capacitance associated with that wire.
That may be a hint.

[Steve Conner]

It has nothing to do with capacitance. Motorboating is a low frequency phenomenon, and the tiny capacitances between wires are only significant at high frequencies. (Well, except in those rare cases when the motorboating is actually intermittent HF oscillations, pulsing themselves on and off by the grid blocking that they cause - radio hams called this "squegging" )

It's all about what other ground connections share the point you're tapping onto. Maybe when you tap the wire to the offending turret, some of the power tube's cathode current gets back down the preamp ground bus, gets into the signal at an early preamp stage, and gets amplified along with the wanted signal.

We assume that the lines on a schematic all have zero resistance, but real wires have a little resistance, so any ground wire shared by two circuits can couple a small amount of signal from one circuit to the other. Some EEs call this the "hidden circuit".

Try giving the power tube cathodes, and their cathode bypass caps if any, a separate ground return to the (-) of the same filter cap can whose (+) the OT center tap is connected to.

[lowell]

Tried rerouting grounds to no avail. Then realized I have my power tube grid wires connected to pin 5 instead of pin 1 through the 1.5k grid stopper. Whoops. Didn't think that'd be the issue but rewired it and the problem is totally gone! I didn't think those grid stoppers were THAT important. It seems extreme to me that the whole power amp would oscillate because of lack of grid stoppers.

[Jazz P Bass]

6V6 pinout

I think that you proved what a grid stopper resistor does.
Cool!
Pin #1 on a 6V6 is not connected to anything, internally.
The assembler used that #1 lug as a place to solder in the 1.5K R.

[lowell]

Just a note, Steve I think that that wire must've had just enough resistance to fix the issue. I guess it may have acted like a very minimal grid stopper. Good call.

[Steve Conner]

Well, in this case I eat my words, because it MUST have been one of these cases where the motorboating is really HF oscillations that "squeg" on and off.

Here's how grid stoppers work, in finest geek-speak :

Inductance in the cathode lead of a tube (or the source of a MOSFET or the emitter of a transistor, they all behave the same) interacts with the gain of the tube and the Miller effect in such a way that it gets transformed into a negative resistance in the grid circuit.

The stray inductance and capacitance of the grid wiring already forms a resonant circuit, and enough negative resistance in this circuit will overcome the ordinary resistance, giving it a Q of infinity, which causes it to oscillate. The grid stopper resistor prevents this by adding more "positive" resistance.

So in slightly less geek-speak terms: The longer and more convoluted your cathode wiring is, the higher its stray inductance, and so the bigger a grid stopper resistor you need to stop oscillations. In poking around the wiring, you were probably changing the stray inductance of the cathode circuit, flipping it over the boundary between stability and instability.