NFB pot releasing light & smoke

[Meteorman]

I replaced R27 (100K) with a 100K pedal-sized potentiometer, in an attempt to have an adjustable NFB circuit.
At the end of the sweep, I got light and smoke out of the pot. Not optimal, i'd say.
The rest of the sweep seemed to work fine.
I replaced that small pot with a full sized CTS 250K pot. Same result.
What's going on ?
Are pots not capable of handling this circuit - it needs to be switched with various resistors in place ?
thanks.

 

[WalthamMoosical]

I don't know nuthin' about amps but "the end of the sweep" sounds like it's where you turned the resistance R27 to zero, allowing I suppose lots of current to flow. You might instead try replacing R27 with a series combination of, say, 68k and a 50k pot, and make sure the pot is beefier than a "pedal pot." I can't guarantee that even turning *that* one down to zero won't let some more smoke out; that will depend on the wattage rating of the pot and of the 68k resistor and maybe also of R16 or even the other "nearby" components. Maybe 47k in series with the pot would be ok. Maybe you need to go no lower than 82k.

 

[tubejockey]

Your circuit needs to go from 100k ohms to infinite, not to zero.

 

[Meteorman]

Your circuit needs to go from 100k ohms to infinite, not to zero.

I don't know nuthin' about amps but "the end of the sweep" sounds like it's where you turned the resistance R27 to zero, allowing I suppose lots of current to flow. You might instead try replacing R27 with a series combination of, say, 68k and a 50k pot, Maybe 47k in series with the pot would be ok. Maybe you need to go no lower than 82k.

[..light bulb goes off in deep recesses of cranium]
there's a reason I work with wood and not electrons.
I will try a 47K resistor in series with a 250K or 500k pot

 

[tubejockey]

[..light bulb goes off in deep recesses of cranium]
there's a reason I work with wood and not electrons.
I will try a 47K resistor in series with a 250K or 500k pot

You might find that a switch works better in that position. You already have very little feedback in that amp. I would solder tack in a 1M pot, with the 100k still in place, and see if you like the results. You might find that the difference between a little fb and no fb is quite subtle.

 

[Peegoo]

Or perhaps a multi-position switch that selects between two or three beefy resistors.

 

[D'tar]

.

 

[2L man]

Effect pedal size potentiometer might be only 0.125W and when turned to low resistance it overlaods and burn. Potentiometer rating is between ends and when it is used to adjust resistance feed thru "slider" the power capacity drops proportionally...

There is 12V RMS on OT output when it deliver 18W to 8 ohm speaker and phase inverter end can be considered zero to high freguencies because of 0,1nF.

If potentiometer is used there should be series current limiter resistor.

 

[chas.wahl]

I think that Uncle Doug's implementation of this (where I first learned of it) has a fixed resistor establishing the basic low threshold of resistance to NFB (meaning the high end), and then the pot allows one to reduce NFB further by increasing resistance. On a 5F2-A-ish schematic I'm planning, I have a 10k resistor following a 50k pot, which would mean starting with twice as much NFB as stock circuit (where a 22k resistor is used); so a max. of 60k resistance to NFB, reducing NFB to about a third of the stock circuit.

A neat trick might be to use nail polish on the "high-resistance" end of the pot's wafer so that the circuit is effectively broken there, resulting in no NFB at all -- that's your "switch".

I don't know how much wattage a pot in this location might be expected to dissipate. All my "projects" use only 6V6s; more powerful iron & bottles presumably would require greater dissipation.

 

[jrblue]

"smoking" "pot," eh?

 

[hamerfan]

I built a 5f1 clone and tried a pot at first, just to see that there are just two settings i like. The nfb off (with a high resistor like a 1meg to smooth some overbearing grunt) to get the raw tweed sound and 11k for a more filtered sound in the higher gain settings. I don’t recommend a complete switch off.
The sweet spot for a filtered sound depends on the output tube(s), the gain setting and also the pickups (humbuckers need more filtering aka lower resistor). A push-pull pot or a on-on switch is enough for just two presets.

 

[Skyhook]

I got light and smoke out of the pot

 

[2L man]

6V6 and EL84 PP amp produce max 18W and if 8 ohm output is where Feedback is taken the RMS voltage is 12V.

24mm potentiometer power rating is at least 1/4W. Then there should be at least 2.5k additional series resistor but for safety margin it is better to use 5k or more and 24mm FB potentiometer should not burn. And FB what the 5k produce should be enough for all amps. Potentiometer value can be what ever and 5k series resistor will keep current thru its "slider" low enough for these low power amps.

 

[King Fan]

Good discussion. To, um, complete the loop on pot v. switch(es), the suggestion I’ve seen (maybe from the ever-wise @Wally ) is to start with a pot inline with a minimal or baseline series resistor. You want the pot to span all useful values. Then find the one or two pot settings you actually like and use, measure resistances, and put it/them on a switch. To belabor the obvious, most of us don’t want to have to 'dial in' our preferred settings or even try to remember where they are on the dial…