Oscillation with volume knob down?

[John]

This is a project on my "breadboard" (in quotes because it's nothing like what RicharD and DL have.) Anywho, schematic is as built right now. V1 is a new 6SL7, 6V6 is new also. I swapped in new ones as a test. Both Russian tubes, but I've used them in another build with no problem.

As you can see, very simple circuit. The amp sounds good... except that when the volume is under 50%, there's a squeal which I assume to be oscillation. When I turn it past 50% it goes away.

Very possible it's just to do with wires going all over although I've tried to minimize bad placement. But it puzzles me that it only happens at lower volume, not higher.

Just in case I have goofy values on anything, please point that out!

Voltages: forgot to write them down, but they are in the "normal" range. B+ ~ 350 at node 1, 6V6 plate 335, screen about the same, 17 volts on cathode. 6SL7.. I forget plate voltage, but cathode is 1.1 on 1st stage and 1.25 on second stage. When I get myself rousted around I'll take the voltage on those plates also.

Thanks in advance!

Oops, forgot to put in filter cap values- 47, 47, 22, 22.

[HotBluePlates]

... wires going all over ...

That's most likely the culprit any time you have oscillation. That, or a ground connection that is much more than 0Ω (like couple-hundred ohms).

If you're like me, you see reference to "lead dress" but no one telling you what the right way to do things are. So here's a couple things that may or may not apply to your breadboard:

- Keep grid wires short.
    o  That goes for anything after a coupling cap. The wire length from coupling cap to following stage grid, including anything in between should be as short and direct as practical.

- Keep outputs away from inputs, and away from any high-impedance circuit. Consider any wire that carries large currents as an "output."
    o  Outputs are typically higher-voltage. Running these near an earlier-stage input presents the opportunity for capacitive coupling from output to input, and therefore oscillation.
    o  Current through a wire sets up a magnetic field around the wire, and larger currents create a bigger, stronger field. If that field is near a high impedance point (like a 1MΩ grid reference resistor), there is an opportunity for inductive coupling from the wire to the high impedance. It actually induces a small voltage in that high impedance, which is now a voltage input to an earlier stage and oscillation.

- When possible, cross wires at 90 degrees when they are signal points you want to keep apart.
    o  The magnetic field around the wire expands outward from the center of the wire's cross-section. If you run a wire parallel to the first wire, the field cuts across the second wire and induces a current. Crossing the wires at 90 degrees causes the moving magnetic field to be oriented the wrong way to induce a current in the wire.


All those things aside, you might be able to simplify the volume control/voltage divider network you have to make that portion of the circuit more compact. You have a volume control, then a voltage divider afterwards. At maximum volume, you are reducing the signal by 460k/(280k+460k) = 0.62 or 62% of maximum signal level.

Any setting below maximum volume is irrelevant, because that's just a knob-twist away. So have a single resistor between the 0.01uF coupling cap and the 500k volume control; this will limit the maximum setting of the volume control and achieve the same end result.

The network as you have it does change what the volume control looks like to the previous stage as a load. The parallel arrangement looks like 298kΩ to the previous stage. So if you absolutely want to keep all interactions the same, we'd like the new volume control plus series resistor to look like ~300k.

A 250kΩ pot is a standard part, and If you place a 120kΩ resistor in series between the coupling cap and the pot, the maximum setting of the volume control still yields only ~68% of the output of the previous stage. The total resistance does look like 370kΩ to the previous stage, so the gain of the first stage will increase somewhat, but this seems like a good compromise between total load and volume reduction and good match to what you have now.

If you say, "how do I fit all this between the stage and keep short wire length," make the wire from the plate/plate load long, and keep the volume control and resistor close to the following stage grid. Move the coupling cap itself to achieve this layout.

Also, it seems like your 59k grid stopper for the 6V6 should be between the 6V6 grid and the 220kΩ resistor to be effective as a grid stopper.

[John]

HPB, thanks. I'll redo the pot/resistor thing to what you suggest. I wound up with it that way while fighting the squeal.

And good catch on the grid stopper for the 6V6. With all the reading I've done I should know better, but it seems like I only retain knowledge after I do it wrong. Argh!

I'll change the wires around to more like what they would be in an actual amp too, and report back (as soon as my little heater can take the chill off in the milk house) Thanks again!