negative feedback BF Fender style

[sbirkenstock]

Hi again,

I´m still tinkering around with my BF Vibroverb project.
Without Reverb and Vibrato. Same schematic, but (very) different layout, leeds as short as possible handwired, most parts directly at the tube socket lugs.
I was trying (and still am ) to figure out the perfect negative feedback part.
So I put a pot in, instead of the 820 Ohm resistor.
The challenge: I don´t hear any difference.
So it could be that I attach the wrong side of the jack.
I simply tried both and if I go below 500 Ohm, I do get a squel on the one side of the jack (guess positive feeback), but not on the other.
So I guess the circuit is correct.
Even if I go to 0 ohm on the pot, I don´t hear anything.
Actually, I do hear the pot moving if I get close to 0, but the sound does not change.
I play the amp around volume at 5 with the Strat completely open.

Any hint?

Best regards,

Stephan

[martin manning]

Is your OT secondary (speaker jack) grounded?

[sbirkenstock]

I am using a standard switchcraft jack (as in Fender amps, not an isolated one as used in Marshall). They are always "automatically" grounded, right?
Anyway, yes, it is grounded and I only measure very small reistance values to ground.
The com side has .1 ohm, and the other side has about .7 ohm to ground.
Measured with my Fluke 179.

Stephan

[JazzGuitarGimp]

So, you are saying that, between the two scenarios, you are using the one that produces the squeal, yes? And you have swapped the OT primary leads, to change the fb from positive to negative, yes?

[sbirkenstock]

I did not swap the OT primary.
I put the wire going from the jack to the 820 Ohm resistor to the other side of the jack.

[martin manning]

If you moved the NFB wire to the other side of the jack, and the jack is grounded, then you grounded the NFB wire, no? Do you have an impedance selector or are you running the OT secondary directly to the speaker jack? If the OT is directly connected then you can swap the secondary leads to change the primary/secondary phase relationship.

[sbirkenstock]

I did exchange the OT leads on the jack.
Now it works! Huge audible difference now!
I´m not sure if I like it better though.
The bass is supposed to be more stable, but I can´t hear any difference with the amp vol at 6.
NF reduces volume quite a lot though.

I think I know now why.
Guess I did ground the "negative side" of it, that´s why I did not hear any difference....
Correct?

Thank you so very much!
Stephan

[martin manning]

Yes, if you connected the NFB wire to the ground side of the jack you would just be paralleling another resistor (820 ohm) with the phase inverter's 47 ohm tail. There would be essentially no effect.

You could now return to your original plan of trying alternate values for the 820 ohm feedback resistor. You can also bypass the 47 ohm tail resistor with a capacitor to shape the feedback signal. I think something around 5uF would be a good place to start.