Floating Paraphase Negative Feedback

[navdave]

Is the 220Ω resistor the shunt resistor or is it both the 1k and 220Ω together forming a 1.2K shunt resistor?
Is NF only being applied to pin 3 of the 6SL7?  If there was no 220Ω resistor and feedback was applied at the
shared cathode resistor would this cause phase problem with this particular inverter?

[sluckey]

Is the 220Ω resistor the shunt resistor or is it both the 1k and 220Ω together forming a 1.2K shunt resistor?

Only the 220Ω. The 25µF cap puts the 1K at AC signal ground just as the filter caps put the B+ rail at AC ground.

Is NF only being applied to pin 3 of the 6SL7?

Yes. Same reason as above.

If there was no 220Ω resistor and feedback was applied at the
shared cathode resistor would this cause phase problem with this particular inverter?

I don't see it causing any problems, but it wouldn't do anything. The bypass cap would just shunt the feedback signal to ground.
 
 

[navdave]

Thanks for the explanation Slucky. Been wanting to try this inverter out in a build for a while.
Just wasn't sure were I wanted to inject the NF at the inverter itself or at a preceding stage like the V4.
Any one have any suggestions on how to implement a presence control with this particular inverter?

[38Super]

For the second circuit you posted, you could replace the lower 33 Ohm resistor in the cathode circuit with say a 100 Ohm pot and then place a capacitor from wiper to ground.  This control would selectively bypass all or part of the lower resistor and would also affect voltage gain of input stage to some degree.  This circuit uses a voltage divider (3300 and 33 Ohm) to control the effect of the voltage fed back from the speaker output.

The 4700 FB resistor, the values of the two resistors in cathode divider, a presence cap on a pot wiper (and the cap shunting 4700 Ohm) all play a role in the frequency response of the feedback loop.

Controlling the feedback as a function of frequency is, after all, what most designs achieve with a presence control.

cheers,

rob

[catalin gramada]

[

[catalin gramada]

The bypass cap would just shunt the feedback signal to ground.

Hi.
well, is not quite a shunt. why to not tie the nfb retour in 220ohm / 25uF junction, please ? it is not compulsory to reference the common secondary to the ground but safety reason, and can reduce the oscilations risk of this circuit using screened wires,think.
Cheers
Catalin

[HotBluePlates]

The bypass cap would just shunt the feedback signal to ground.

... why to not tie the nfb retour in 220ohm / 25uF junction, please ? ...

The feedback from the transformer secondary is an a.c. signal. If you pick some mid-band frequency where the cap is not a large impedance (for example, extreme lows where the cap rolls off), then you can mentally replace the cap with a wire.

So when Sluckey says the cap would just shunt the feedback signal to ground, it is because if the feedback injection point is moved to the non-grounded end of the 25uF cap, and that cap is essentially a short circuit for the signal, then all feedback is shorted to ground and there is no feedback.

The 25uF cap working against 1kΩ is -3dB at ~6Hz, well below the lowest guitar note of ~80Hz. Therefore, the 25uF cap would short out the feedback if moved from its present location.

... Is NF only being applied to pin 3 of the 6SL7?  ...

Yes, but...

The 2nd half of the paraphrase inverter is after the 1st half, so the whole inverter circuit is inside the feedback loop and receives the effect of the feedback.

Just wasn't sure were I wanted to inject the NF at the inverter itself or at a preceding stage like the V4.

If it were my amp, I'd try to keep as few stages and coupling caps inside the loop as humanly possible. Each coupling cap (or other impedance) shifts phase; more broadly, any frequency-dependent attenuation shifts phase. If enough individual shifts are inside the loop, and if the feedback signal is large enough, the amp will become unstable and oscillate.

For those reasons, I'd leave the loop mostly as-is if this is an existing amp; OT characteristics can impact feedback loop stability because its performance at the extremes of frequency range are unique to that model OT and contribute phase shift.

Any one have any suggestions on how to implement a presence control with this particular inverter?

For your 1st circuit, maybe an alteration of what 38Super posted: Pick a pot with at least 10x the resistance of the 220Ω shunt feedback resistor, attach one outer lug to pin 3, attach a cap to the wiper with the other end of the cap to the junction of the 220Ω resistor and the 25uF cap, leave the last outer lug of the pot open. Adjusting the pot for zero resistance between pin 3 and the cap shunts highs and boosts presence; turning towards max resistance between pin 3 and the new cap provides stock feedback and no presence boost.

You'll have to select the presence cap by ear or calculating typical values used in amps you like and adjusting for the values of resistance in your new circuit.

[catalin gramada]

The feedback from the transformer secondary is an a.c. signal. If you pick some mid-band frequency where the cap is not a large impedance (for example, extreme lows where the cap rolls off), then you can mentally replace the cap with a wire.

So when Sluckey says the cap would just shunt the feedback signal to ground, it is because if the feedback injection point is moved to the non-grounded end of the 25uF cap, and that cap is essentially a short circuit for the signal, then all feedback is shorted to ground and there is no feedback.

The 25uF cap working against 1kΩ is -3dB at ~6Hz, well below the lowest guitar note of ~80Hz. Therefore, the 25uF cap would short out the feedback if moved from its present location.

Thanks for answer.
Cheers
Catalin

[tubeswell]

FWIW I did this NFB recently on my 'Pro-Bolt MkII' - works well as a NFB for a paraphase inverter.