Changing the NFB value on the fly?
Moderators: pompeiisneaks, Colossal
Changing the NFB value on the fly?
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....
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Re: Changing the NFB value on the fly?
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.
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.
Re: Changing the NFB value on the fly?
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...
Most people stall out when fixing a mistake that they've made. Why?
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Re: Changing the NFB value on the fly?
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.
Re: Changing the NFB value on the fly?
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?
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?
Most people stall out when fixing a mistake that they've made. Why?
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Re: Changing the NFB value on the fly?
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
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
tUber Nerd!
Re: Changing the NFB value on the fly?
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.
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.
Most people stall out when fixing a mistake that they've made. Why?