Changing the NFB value on the fly?

[rooster]

So, I have an extra ON/OFF switch on my chassis front panel. I can plug it or leave it as it sits, but, it occurred to me that of all the duties I might adapt the switch to perform, I could simply use it to increase (double in series) the 4.7K tail resistor (100K from OT) to 9.4K and thereby change the NFB ratio. Has anybody done this? I assume it will tighten up the amp and this could be a worthy option. Comments welcome....

[Matthews Guitars]

Sure, that'll work. You could even use a potentiometer instead.

I have resistor substitution boxes that I find to be very handy when seeking an optimal resistor value. Every tech should have one or two.

[rooster]

The pot is a good idea, actually! Thanks, Matthew's Guitars. Using a 5K pot (along with the 4.7K tail) the ratio spread is basically 20:1 (current) and 10:1 (altered), and this may be good for my needs. However, using a 10K pot here instead, the ratio spread becomes 20:1 and 7:1, which is fairly close to the Marshall Plexi (20:1) and the '59 Fender Bassman (5.5:1). This particular amp mash-up is a blend of the two amps so this may be the better choice. Of course the pot creates more variables, which is a real plus. Hm. Decision time...

[studiodunn]

I just start with a 100k pot in any build now. It is the easiest addition that has the greatest effect on the feel of the amp for me.

[rooster]

A revisit...

So, I opted for a 3-way switch, and for the reason that I could accurately hit my targets. However, I have come to appreciate the adjustable potentiometer usage - but only for the reason that it doesn't make a 'POP' sound at the speaker when making NFB adjustments. Yep, it is always in the circuit, a win. This said, using my particular 3-way, I have to drop the amp into Standby mode to make the change. I'm OK with this because I know my targets are accurate, but I'm wondering if some clever fellow here might be aware of a way to turn a 3-way 'break contact' switch into a 'make-break contact' switch? Of course, I can simply buy a make-break contact switch but I'm wondering if there's another way?

[pompeiisneaks]

I'd think if you kept a high value like 1M or 10M resistor from the switch input to ground, you'd provide a short term path for the circuit to go to ground. Then when the new resistor is in circuit it has a 'good' path no>?

Edit: I'd wait for confirmation on that... shunting the OT tap to ground with even a high value resistor may not be a super wise idea....

~Phil

[rooster]

Hey Phil. I have considered this idea. But I'm thinking something that works for a cathode resistance change on V1 may not fly so well here. I have to think it would basically create a super high resistance that would simply turn the NFB off, much like a 250K pot on a BF Fender would do.

Funny thing, although it may or may not survive the minimal voltage (and it would look rather stupid on an amp face panel), a Fender 3-way switch would work because it's a make-break switch. Also, there's a company that makes something I could use but it's $35. I was hoping someone here who's more technically inclined might have sorted this for me.