Positive Feedback?

[shaun]

So, I built 2 amps the same - 6L6 SE's. But one sounds smoother - and tonal nicer - than the other. Took me ages to realize that, on one, I'd accidentally wired the 680k M.Vol resister to the hot side of the spkr jack instead of the ground side. Weird thing though - this is the amp that sounded considerably nicer (subjective, of course). The design also has a switchable NFB btwn the hot and the cathode of the driver, which means I not only have NFB, I have positive FB; the positive signal must be going back to the input signal of the 6L6, which is also positive, if that makes sense.

I'm guessing it will effect the output impedance the OT sees, so instead of 8ohms, it must be seeing 8ohms sort of in parallel with whatever the impedance is at the input of the 6L6. Nothing has overheated or smoked, and the 6L6 is drawing a respectable 71mA, but I'm guessing this might not be so good long-term, so I'm just wondering whether I can keep it 

Any thoughts?  The schem indicates how it the 680k should be wired, not how it was wired.

Thanks, as always.

[PRR]

> not how it was wired.

Don't make us guess. I can't see your build from here. Is this it??

[acheld]

Since the output of the 6L6 is inverted, wouldn't that mean that you're simply providing additional NFB?

A 680k resistor forming a divider with your formal NFB circuit doesn't seem like it should affect the reflected impedance much (I could be wrong).

[shaun]

> not how it was wired.

Don't make us guess. I can't see your build from here. Is this it??

Yes, that's it. My apologies.

[shaun]

Since the output of the 6L6 is inverted, wouldn't that mean that you're simply providing additional NFB?

A 680k resistor forming a divider with your formal NFB circuit doesn't seem like it should affect the reflected impedance much (I could be wrong).

Yes, I also wondered about that. It seems to behave just like an NFB circuit, but I thought the idea of attaching NFB to the cathode was because the cathode is 180 degrees opposite the signal, more or less.

[PRR]

The 6L6 with OT has voltage gain less than unity, so about no effect.

[tubeswell]

Your 68k is connected to ground through the MV pot, so the way you had it wired means that at worst, it’s the same as adding a 68k resistor across the OT secondary in parallel with the speaker (as MV pot rotation gets closer to CCW), which won’t affect reflected load on the OT. As to effect on NFB, depending on how much interleaving the OT windings have, there might be a phase shift at high frequencies between OT primary and secondary windings due to interwinding capacitance, which could be acting in your favour. I’m only speculating on that because you’re saying the two amps are identical in all other respects yet you can hear a difference. So either your ears are playing tricks on you, or there is something else causing the difference.

[shaun]

So either your ears are playing tricks on you, or there is something else causing the difference.

Yes, my ears are always playing tricks:). But you you are right about the end result - it seems to have behaved just like NFB, with the most prominent effect being the smoothing of top frequencies. In fact, when I compare the result of my mistaken wiring to the NFB, they seem almost identical. Thanks.

[shaun]

The 6L6 with OT has voltage gain less than unity, so about no effect.

Very interesting, PRR, and a study I will get my teeth into. I have a lot to learn about the voltages either side of the OT. Thanks.

[PRR]

> learn about the voltages either side of the OT.

Primary must be hundreds of volts: we feed it 300V-400V and don't want to waste any of that precious DC power.

Secondary is obviously 8V in 8 ohms for 8 Watts; or numbers very near to that (6L6, 6V6, 4r, 8r...).

6L6GC datasheet shows 18V peak on grid goes-with about 350V supply, pencil 300V peak on plate. So tube has voltage gain of 300/18 or 1:16.

5k:8r is 625:1 impedance or 25:1 of voltage.

16*(1/25) is 0.66 of voltage gain, a bit less than unity.

[shaun]

Thank you, PRR. That's a great study.