NF loop question

[dazco]

I lifted a cap on my peavey classic 30 that removes the signal input to the NFL. However, i have a question. The secondary is still ground refernced thru the components in the loop, and i can lift a component there to eliminate that too if need be. But before i pull the chassis agian just to do this, can anyone tell me if it matters and could affect the tone even tho there is no signal being fed thru the NFL anymore? In other words, the secondary is still referenced to ground thru a RC network of .0047/220k, a 100k, 2 paralleled .1 caps, then a 10k. Would that have any affect on the OT's tone?

[Old Tele man]

...any RC-circuit has a "corner" frequency response...however, that frequency may be way way ABOVE or BELOW the human audio hearing range so, it's effect is moot.

...thus, the "real" answer to your question is: (A) yes, there is an affect, but (B) it might be so HIGH/LOW that you probably can't hear it.

...can't really determine which without seeing schematic and doing calculations.

[ChickenLover]

I'm not sure which cap you lifted and how you lift a cap to remove NFB yet still have it ground referenced as you put it but I'm not familiar with that circuit.

But in any case, the total resistance of the secondary is just a fraction of an ohm and one end is grounded. So if you have the other end grounded (ac or dc) through (effectively) a 110k resistor then I doubt it would make much if any difference.

Picking a nit (and hopefully learning something in the process): doesn't 'ground reference' refer to dc? If there is a cap in series with that NFB signal then it's not really 'ground referenced' because the cap blocks dc. It may be 'ac ground referenced' but I'm not even sure if that's a real term. In any case, there's ac ground and dc ground; think of your B+ rail as "ac ground but not dc ground" because signal goes straight to ground (via the filter caps) but dc is blocked by the filter caps.

[ChickenLover]

You didn't lift the blocking cap for the second grid did you? If so, that grid needs that cap as well as the 1M ground reference and those are not really related to NFB...but rather proper operation of the splitter.

[donnyjaguar]

If in doubt, just remove the lead from the output transformer as well. This is an unusual feedback loop in this amplifier and I can only guess as to what they are trying to achieve with it. Generally speaking when you see reactive components in series with the NFB resistor it is to stabilize the amplifier at ultrasonic frequencies. Near as I can guess without doing the math PA circuit will put emphasis on the bass frequencies, although the amount of feedback is very minimal in this amplifier anyway. This is to be expected becuase they've copied the Williamson design deleting one gain stage in hopes the gain of the EL84 tubes can make up the difference. As is typical with Peavey cost-saving measures abound.

I'm sure it sounds wonderful though.

DJ

[dazco]

The fact that the secondary is only a few ohms and it therefore wouldn't affect the tone makes sense. Still i can't help but wonder. Wouldn't be the first head scratcher i've seen. But in any case here's the schematic for those who mentioned not knowing the circuit....

http://www.blueguitar.org/new/schem/peavey/c30schem.gif

I'd just do it but i figured it's easier to ask then pull the chassis and snip leads. I don't like having too many soldered leads if i can help it. and to lift things in this amp properly means a lot of work. If you've seen the inside of a C30 you know what i mean.

[ChickenLover]

And which cap is it that you lifted?

And forget all that nonsense I wrote above about the blocking cap for the second grid:NUTS...that applies to a long-tail PI and this amp uses a cathodyne splitter.

C36 looks like the 'typical' cathode bypass cap but it isn't directly grounded. But other than that...that's some funny business that I'm not familiar with so I don't think I can be much help. But FWIW, it doesn't look like the OT would be affected...but if you lifted C36 it might affect the tone of that stage.

[donnyjaguar]

I just had another look. Removing the NFB lead from the output transformer will not make any difference in tone. I'd like to hear what effect removing the capacitor had though. :)

[dazco]

I lifted C36 which does rtake the FBL out of circuit. It also did affect the tone by making it sound a bit stronger but also a bit too bright. I'm just wondering if the components now left in a string from the secondary to ground would make any difference to the OT.

[Thomas D]

Given that the secondary is operating at very low impedance it's probably negligible.