Mojo 5F11 (variable resistor) bias pot *simulation*

King Fan

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Trying to learn simple circuit simulation, I've been using the online sim at falstad.com. Y'all recently helped me learn from my 6G2 / Doug Hoffman bias sim. A slightly different bias pot solution comes up when folks build the Mojotone 5F11 (a kissing cousin of the 6G2), cuz Mojo wires the pot as a variable resistor instead of a voltage divider, and because their 5F11 PT has a bias tap instead of knocking down the HV tap off the rectifier, as in Doug's classic Princeton Reverb mod.

Mojo's layout is a bit hard to follow; the essence is this.

5F11_bias_Mojo.png


Pretty simple. Except I can't get a potentiometer to play nice as a variable resistor in the falstad sim, so I just stuck in a 10k pot and change it to 10Ω (remove the 'k') to visit the other extreme of resistance. It looks like this with the bias pot set to maximum resistance (so with bias voltage at its most negative).

1672933668846.png


Here's a link to the actual sim, if you want to play with it. *To see the minimum end of the negative voltage range, just double click the 10k resistor and delete the 'k' from its value.*

https://tinyurl.com/2ezkgpdc

Even if I did this right, it's a sim, with several assumptions and approximations, so it won't generate exact actual voltages. My goal is mostly to study the direction and magnitude of swapping in different 'range' (their 10K) and 'tail' or shunt resistors (their 33K). I set up the voltage source to produce 50Vrms; the diode is their generic version. And again, I'm just learning -- don't take my word for it.:)
 
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King Fan

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Though smart folks won't be surprised in any way, here are the simulated max negative voltages with different resistor values.

1672934281986.png


Example 4 shows the result of a slight drop in 'range' resistance. The other examples just vary the tail resistor from 33 downwards or, in 5, upwards to 46K, the value that would reproduce (at max negative) the voltages produced by Fender's original (56K) fixed resistor that the pot replaces.

FWIW, Merlin on bias points out that for the 6V6, even with screen voltage as high as 350V, a max neg bias voltage of –35V should "take the valve almost to cutoff." Oh, and a shout out to @Lowerleftcoast -- he suggested the 8.5K range resistor in a 5F11 thread.

Again, before you heat up your soldering iron, this is just a simulation. Real results will vary. I'm mostly doing this for fun, and to learn from my betters if I've made mistakes or my sim could be improved.
 
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2L man

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Try swapping 10k and diode and test does it change voltages?

I am used to rectify 50VAC and have capacitor which filter it to about 70VDC and then add voltage dropper, then another capacitor and then adjust circuit.

Yes Screen voltage has significant effect how control grid effect to anode current and max output power.
 

King Fan

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Try swapping 10k and diode and test does it change voltages?

I am used to rectify 50VAC and have capacitor which filter it to about 70VDC and then add voltage dropper, then another capacitor and then adjust circuit.

Yes Screen voltage has significant effect how control grid effect to anode current and max output power.

Thanks. I should emphasize these aren’t full-featured bias circuits like we'd want in bigger amps. But they work well in these smaller, simpler amps. Also, lots of folks add 'em to lots of these smaller amps we see built here. Best of, they’re simple, which perfectly matches my sim-building skills. :)

I like the idea of reversing the resistor values. It gets to the bigger idea that we might want to design a circuit that spans a range that's wide enough but not way too wide. That’s a bit trickier than just reaching a certain max negative. Most folks leave the resistor balance alone and just use a 25 or 50k pot to widen the range. But my experience tells me Merlin is right that you can 'do it all' with a 10KL pot if you get the resistors in the right balance… and he points out (reference above) why smaller may be better.
 

King Fan

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Heh, of course reversing the range and tail resistors dramatically lessens the negative bias voltage. The 'range resistor', true to its name, moves the range. With 33K range and 10K shunt, sim bias voltage ranges from about -6 to -10V. "Tubes melting down in 3, 2, 1...."

But to boost the *span* of the range, of course, it helps to cut the tail resistor (so that -- ding ding -- the 10k pot swing has a bigger *relative* impact). But that requires reducing the head resistor to maintain the absolute value of the negative voltage, as seen here (range pink, max neg blue).

1673029207076.png


Now, per Merlin (above) bias voltage of -39 or more negative should chill out a 6V6 even at screen voltage 390. So just boosting the tail (to 57K or more) will get you there -- but it narrows the adjustment span. OTOH, knocking down the range resistor lets you get there too, while widening the adjustment span. Again, probably obvious on first principles, but a lot more obvious to me after playing with the simulator. :)
 

2L man

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Thanks. I should emphasize these aren’t full-featured bias circuits like we'd want in bigger amps. But they work well in these smaller, simpler amps. Also, lots of folks add 'em to lots of these smaller amps we see built here. Best of, they’re simple, which perfectly matches my sim-building skills. :)

I like the idea of reversing the resistor values. It gets to the bigger idea that we might want to design a circuit that spans a range that's wide enough but not way too wide. That’s a bit trickier than just reaching a certain max negative. Most folks leave the resistor balance alone and just use a 25 or 50k pot to widen the range. But my experience tells me Merlin is right that you can 'do it all' with a 10KL pot if you get the resistors in the right balance… and he points out (reference above) why smaller may be better.
I meant that does the simulation make DC RMS voltage after the diode -70DCV when it is first and filter capacitor is on its anode and the rest of the citcuit follow?

Bias circuit is where I usually have to change a resistor, sometimes two to get adjust range about the same both directions. Often I use 22k potentiometer and I believe installing any same tube type the range comes wide enough even tube gains are different.
 

King Fan

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I meant that does the simulation make DC RMS voltage after the diode -70DCV when it is first and filter capacitor is on its anode and the rest of the citcuit follow?

Bias circuit is where I usually have to change a resistor, sometimes two to get adjust range about the same both directions. Often I use 22k potentiometer and I believe installing any same tube type the range comes wide enough even tube gains are different.

Thanks, I see now. You're right, Merlin notes it is often necessary to experiment somewhat on resistor values. He also notes there seem to be an infinite number of bias circuits out there. A bigger pot will certainly expand the range with few resistor changes.

I've been interested in his idea that to maintain optimal low resistances in the bias circuit, "A 10k bias pot is ideal." I like the 10K value because CTS happens to make the Fender-style 10KL with an externally-adjustable (but safe) slotted actuator, and simulation helps me estimate what resistor balance might let it work. In no way is 10K the only choice -- I like your 22K, or 25K, and in fact I used 50K on my 5G9; with its easy-access tweed-style chassis, internal adjustment is no problem.

1673032881486.png
 
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