Doofus-Proofing Bias Supply

[Justin Thomas]

Hey All,

Finally have made excellent progress on my 4th build after a years. Now wanting to improve the Fender BF-style bias supply in it now. I have the negative voltage output coming from the wiper of the pot. I've come to understand that the "moving part" of the pot is the most likely to fail.

Merlin & Wombat recommend adding a large-value resistor to keep bias voltage in the case of a bad connection between wiper & track. Others here recommend tying the wiper to one end of the pot.

Pictures or schematics of either of these help me to understand. I can find Merlin's on his website, the adding of the "safety resistor." Is there a schematic available of the other way? I believe it's wiring the pot as a rheostat or variable resistor instead?

Thanks all, just wanna doofus-proof this Most Essential Circuit In The Amp!

Justin

[glebert]

I think it should be like this, but not necessarily these values. If the wiper open up then the bias voltage goes to the full negative supply or coldest possible bias.

[Dave H]

Is there a schematic available of the other way? I believe it's wiring the pot as a rheostat or variable resistor instead?

Edit: I don't like the one above. It's possible to set the pot to zero bias voltage.

Try this one.

[Justin Thomas]

Okay, I'm the doofus I'm trying to proof here... The arrow across the resistor symbol... How is that connected?

Jusrin

[eschertron]

As far "wiper open" condition, as long as the wiper is tied to the end of the track both circuits are safe with max neg bias.
Dave's JTM45 schem is more 'doofus-proof' with respect to actual doofus operation, though

edit: tie the wiper to whichever end convention decrees to be max neg bias

[glebert]

Edit: I don't like the one above. It's possible to set the pot to zero bias voltage.

Good point.

[glebert]

Okay, I'm the doofus I'm trying to proof here... The arrow across the resistor symbol... How is that connected?

Jusrin

One end is wiper, one end is track. Orientation shouldn't matter. I would use a linear pot and not a log taper one.

[Justin Thomas]

Schematics!

Okay, here is what I'm working with, and 2 possible scenarios. I am thinking #1 as the solution in case of an open wiper, as it will set me up for maximum negative voltage - in this case, about -55V as wired, which is respectable for a couple 6L6s @ ~450-475V.
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EDIT: One, thanks to Bloomfield for pointing out my improper polarity on my caps! I promise they are not wired like that. Also, in scenario 1 below, that won't work at all. It will just send max negative voltage to the grids & bypass the pot completely. Correct?
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I had to grab an extra pot to figure this out; I usually struggle with working with pots just because of having to orient them in my head, so it's easier to handle a concrete example...

Justin

[Enzo]

There is one more part. In the example above, the 25k pot can turn to zero. SO we replace the pot with a pot and series resistor. Instead of 25k pot, we use a 10k pot with a 15k resistor in series. That way all the way down is still 15k. Not only can we no longer turn to zero, but the whole range of the pot becomes useful, rather than just one end of the adjustment.

[Bloomfield]

I'm sure you're aware Justin but the caps are drawn wrong polarity in your sketches.

[Justin Thomas]

I'm sure you're aware Justin but the caps are drawn wrong polarity in your sketches.

D'oh! Yes, you are correct! They're not wired like that, though. Hence, Doofus! Jusrin is having a field day today!

Justin

[Justin Thomas]

Enzo,

What I'm using is in my sketch. I have range roughly between -45 to -60V over the full travel of my pot. I just realized that my #1 scenario will completely bypass my pot & send max -V straight to the grids all the time. Scenario 2, in case of wiper disconnevt, would send in my case -4tV to the grids full time, correct?

Merlin & Wombat have suggested a large resistor on the pot in case of wiper failure; any thouhts on that?

This is all new to me... Most of my builds were cathode-biased or used the existing power amp so I didn't mess with bias supplies...

Justin

[Dave H]

Scenario 2, in case of wiper disconnevt, would send in my case -4tV to the grids full time, correct?

Merlin & Wombat have suggested a large resistor on the pot in case of wiper failure; any thouhts on that?

I prefer the JTM45ri circuit with the extra resistor over your #2 circuit. The #2 circuit reverts to max bias current (-45V) when the wiper loses contact. The JTM45ri circuit reverts to min bias current.

The large resistor works best when the bias control is wired as a potentiometer (connect it between the wiper and more negative terminal). If the control is wired as a rheostat the lower control resistance will dominate.

[Justin Thomas]

Thanks Dave,

So re: large resistor: this would actually be the easiest for me to do to what I already have, correct? Since I'd basically just tack the resistor in.

Do you or anyone have any opinions on whether you like BFR or the JTM45 way better? I say "simple is good too if it works...

Justin

[Chuck H]

I tend toward the design Dave posted. As Enzo pointed out, it allows useful range over the pot without allowing for a dangerous bias condition to be dialed in. I also like the additional filtering over the BF type circuit. Not sure it makes a huge difference, but in the event of mismatched tubes.?. However...

The circuit values should be adjusted to your particular circumstances. Looking at resistance values in your sketch I'll guess that (like me) you carry many standard values of resistor, but not many values not typical in classic guitar amps. And even then the inventory is like beat up Mr. Potato Head with missing pieces and the ear in the wrong hole So...

I assume you have 10k resistors on hand. Otherwise you could parallel two 22k resistors. If you have a 47k or a 56k resistor you could use that instead of the parallel 100k's shown in the drawing. I haven't tapped bias direct from one side of a full wave rectifier before, but I assume we get a starting point of half HT. I'm sure I'll be corrected if I'm wrong. Anyway, I think this circuit with these values should work out. Wiring the pot like a rheostat as shown should, I think, be better in the event of a wiper failure. Might even allow you to finish a gig. So why not.