Boot Hill Champ with Switchable NFB

[hackworth1]

By boothillamps at 2012-04-18

[celeste]

I am really liking your blue board stock

[hackworth1]

Thanks. The blue is even more vivid than seen here. Orange and Blue are Florida Gator colors. That's why I used the Orange Drop Caps.

The 25uF bypass cap will be turned on or off by the The NFB switch.

[celeste]

A dpdt switch or a pot for feedback and a switch on it for the bypass?

[hackworth1]

dpdt micro switch 6mm (approx 1/4 inch hole) is the plan.

[tubeswell]

I was going to suggest a SPST switch in parallel with the bottom leg of the NFB voltage divider (to short the resistor).

[hackworth1]

tubeswell,

Could that also take the bypass capacitor out of the circuit?

[keithb7]

Are you guys saying, you could possibly install a pot for variable NFB resistance? Just pull the resistor and wire the leads to a pot instead? Is this similar to the presence knob on some amps? Thx.

[tubeswell]

Originally Posted by hackworth1

tubeswell,

Could that also take the bypass capacitor out of the circuit?

Switch position shown in this schematic. It merely shorts the resistor, i.e.: it doesn't break the circuit.



Would work the same way for shorting out the bottom leg of NFB on the driver stage of a cathodyne inverter (like a princeton reverb)

[tubeswell]

Originally Posted by keithb7

Are you guys saying, you could possibly install a pot for variable NFB resistance? Just pull the resistor and wire the leads to a pot instead? Is this similar to the presence knob on some amps? Thx.

Kinda. A presence control is typically just a variable frequency pot in an NFB circuit, that 'boosts' treble frequencies (by cutting the gain of selected mid-treble frequencies fed-back through the NFB loop). (But you can have this kind of control in local feedback loops). If you took away the pot's bypass cap, you would have just a plain old NFB blend/gain control.

[muchxs]

Originally Posted by hackworth1

By boothillamps at 2012-04-18

Like the big ol' hole in the circuit board. I drilled mine for a Weber transformer. It misses the turret for the limiter resistor. Turns out Weber transformers use different centers than Boot Hill...

It's the Hackworth / Muchxs Champ. "Muchworth" sounds arrogant. I like "Much Hack" better.

Y'all are overthinking the NFB thing. That's the "danger" of simple little amps. Make everything adjustable and switchable, it isn't a simple little amp anymore.

There is a short list of rules in a stoic engineering philosophy. First rule is "Parts left out" which means leave out anything you can omit. Find a killer tone and hardwire it.

Somewhere on the list is "Anything adjustable can be mis-adjusted". That means there may be settings that are cool by themselves that sound like crap in conjunction with other settings.

Next in our slide down a slippery slope is a tone control. Then a full tone stack. Then a master volume...

[Telenut62]

.....change the pre-am tubes etc. etc......that's s very true muchxs. I've been playing around with hackworths GA-5, while it's interesting to hear what happens when you do this and that. Ultimately it's just a matter of how much volume and clean/overdrive levels you want at the time and being able to dial it in, that is what I find is cool

[hackworth1]

By boothillamps at 2012-04-22


By boothillamps at 2012-04-22


By boothillamps at 2012-04-22

[printer2]

Originally Posted by muchxs

Y'all are overthinking the NFB thing. That's the "danger" of simple little amps. Make everything adjustable and switchable, it isn't a simple little amp anymore.

As I sit here looking sheepishly.




Aww heck, I can't help myself, got a new idea on switching around a classic preamp that I need to post. Will end up with more switches than resistors yet.