Making this bias adjustment more usable...

[turbofeedus]

Hey all, working on the bias supply i built for the michailoff amp (thread here).
Here is the supply as it stands now.

michailoff bias supply v1 - voltage.jpg

This is working fine, although the range of bias voltage is huge. So much so that the slightest change on the adjustment pot makes a ~2ma change to the bias current.
I'd like the pot to be less sensitive.
My current ideas are either replace the pot with a multi-turn pot, or change to a 10k pot and add resistors to either side of the pot to effectively reduce the range (something like 10K+10K pot+10k).

Any better ways I'm missing?
Grid leak resistors are 100K btw, caps aren't shown but they're both 47u.

[martin manning]

Put a 6k8 across the outer legs of the 25k pot, and increase the 10k to ground to ~20k.

[Ten Over]

The basic strategy to decrease the range of an adjustable bias supply is to make the adjustment pot smaller relative to the total resistance in series with it. martin manning has you covered by putting a resistor in parallel with the pot so that the combination makes a pot that is smaller relative to the series resistance.

If you are interested in a whole different design, I have a suggestion. R3 is chosen for whatever range you need.

[turbofeedus]

Put a 6k8 across the outer legs of the 25k pot, and increase the 10k to ground to ~20k.

Oh, yes, that seems like the simplest thing to do. Is this going to cause any issues with the 220K protection resistor?

The basic strategy to decrease the range of an adjustable bias supply is to make the adjustment pot smaller relative to the total resistance in series with it. martin manning has you covered by putting a resistor in parallel with the pot so that the combination makes a pot that is smaller relative to the series resistance.

If you are interested in a whole different design, I have a suggestion. R3 is chosen for whatever range you need.

that's interesting, thanks. The original bias was actually similar to this. I based this circuit off what merlin shows: http://www.valvewizard.co.uk/bias.html

[Stevem]

No worries with the 2 220K resistors and the bias trim mod's!
You might want to wire that bias pot jumper resistor and the other one going to ground onto a double pole single pole switch so that if you ever install new outputs that need to bias in way different then you do not need to unsolder anything.

[martin manning]

Is this going to cause any issues with the 220K protection resistor?

Since the pot is now effectively 5k3, you could reduce it to 56k.

FWIW, I think the circuit Ten Over posted is a bit better overall. It has the same number of parts, produces low ripple over the entire adjustment range, and it retains the safety feature of failing "cold" if the pot wiper loses contact. I don't think a particular application of that circuit is as simple as adjusting one resistor, though, and I would reduce the value of the filter caps by at least half to minimize the time to develop the bias voltage.

[turbofeedus]

Is this going to cause any issues with the 220K protection resistor?

Since the pot is now effectively 5k3, you could reduce it to 56k.

FWIW, I think the circuit Ten Over posted is a bit better overall. It has the same number of parts, produces low ripple over the entire adjustment range, and it retains the safety feature of failing "cold" if the pot wiper loses contact. I don't think a particular application of that circuit is as simple as adjusting one resistor, though, and I would reduce the value of the filter caps by at least half to minimize the time to develop the bias voltage.

Understood, like I said I was just using Merlin’s circuit as the basis. I’ll have to double check, I may have actually done 10uf like Merlin recommends.

And just to make sure I understand, I could also just replace the pot with a 5k and achieve a similar result as having the paralleled resistor ( and change the tail resistor)?

[Guy77]

The basic strategy to decrease the range of an adjustable bias supply is to make the adjustment pot smaller relative to the total resistance in series with it. martin manning has you covered by putting a resistor in parallel with the pot so that the combination makes a pot that is smaller relative to the series resistance.

If you are interested in a whole different design, I have a suggestion. R3 is chosen for whatever range you need.

I have used this exact bias design coming off the HT line in a couple builds over the last few years and have been very happy with the results. I use a 50K pot and a 27K R3 value .
Although for less range I should probably use a different value for the pot and R3 .

One of the cool things you can do when you take the Bias off of your HT line is if you want to make your amp switchable between fixed bias and cathode bias it can be done with just a simple spdt switch. There is a schematic on this at el34world.com

Cheers

Guy

[martin manning]

...And just to make sure I understand, I could also just replace the pot with a 5k and achieve a similar result as having the paralleled resistor ( and change the tail resistor)?

Sure no problem there.

[turbofeedus]

...And just to make sure I understand, I could also just replace the pot with a 5k and achieve a similar result as having the paralleled resistor ( and change the tail resistor)?

Sure no problem there.

Great, thanks again Martin. I'll consider the other design, not sure how much I want to disassemble what I've already got here. May try that on another build.

[turbofeedus]

Success! Followed Martin's advice and replaced the pot with a 5K and increased the tail resistor.
Ended up with 18K for the tail resistor, this gave plenty room either side of the adjustment, except now the pot isn't so sensitive.

biaschange-1.JPG

I found this neat old sealed AB pot in my parts bin, it actually has a clever feature of mechanically locking the wiper position. Makes a lot of sense for a bias adjustment.

allen-bradley-rv4lays-j-800.jpg

Curious now though, what is the significance of the actual value of the protection resistor?
Merlin recommends a value 10x the pot value, but what if it's lower or higher?

[martin manning]

Nice pot. Locking trim pots are great for this application. Re the safety resistor, if it's too big it will be less effective at maintaining safe (cold) bias in the event of a pot wiper failure. If it's too small it will reduce the range of the pot since it's in parallel with the element at one extreme. You could compensate for the latter, of course, with a larger pot. 5x the pot value would still be fine, I think.

[turbofeedus]

Nice pot. Locking trim pots are great for this application. Re the safety resistor, if it's too big it will be less effective at maintaining safe (cold) bias in the event of a pot wiper failure. If it's too small it will reduce the range of the pot since it's in parallel with the element at one extreme. You could compensate for the latter, of course, with a larger pot. 5x the pot value would still be fine, I think.

So the being to small makes sense, but how does the resistor being to large make a difference? My understanding is that the current through the bias supply is already really small.
Speaking specifically about this amp, is 100k too large? it's less than half of what it was before, but still 20x the pot value.
Could we make a general rule that the protection resistor should be as small as possible without significantly affecting the range of the pot?

[martin manning]

If the pot wiper fails, that resistor becomes part of the grid leak path, which could lead to red-plating.

[TUBEDUDE]

Nice big resistors! I think Antique Electronic Supply sells similar locking knobs that can be installed on any bias pot.

20191031_073810.jpg