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Scott LK-48-B Bias Voltage Help

Craig,

Sorry for the confusion - lots of posts between the first one and now.

Originally, the amp wouldn't bias properly. Back in post 13, I changed the bias pot ground ref resistor to 50K from 150K (got that idea from one of your old posts). This allowed me to get the bias range of -12.3 to -18.5. Most of the discussion since then has been about how it got that way and whether this correction is simply masking a deeper problem.

It currently biases at .22mA per tube at -14.5V - so room to play up and down.

I'm now thinking, based on your last post, that it would be a better approach to step the voltage down with the additional resistor and cap before it gets to the bias pots and then readjust that ground ref resistor as required.
 
No need for adding resistor and that would just lower the top and bottom voltage which you do not want. You should not ever need a voltage above -18.5 unless a tube supplier send you a really badly out of spec hot running set of tubes which I would send back if it was me.

Just add the 50UF @ 100V caps to the center wiper lug of the bias voltage pots and call it good. Remember reverse polarity + goes to ground.
 
NOSValves said:
No need for adding resistor and that would just lower the top and bottom voltage which you do not want. You should not ever need a voltage above -18.5 unless a tube supplier send you a really badly out of spec hot running set of tubes which I would send back if it was me.

Just add the 50UF @ 100V caps to the center wiper lug of the bias voltage pots and call it good. Remember reverse polarity + goes to ground.
Click to expand...

Got it - thanks again!
 
Oh common now. Who want's to let go of a good controversy? :) My LK48 has a 150K installed at the bias control return resistor location as well, and also has that value listed on the original schematic that came with it.

I'm with Craig however. I think Scott just blew it on the value -- either that or they were trying to error so far on the safe side that it was beyond safe and clear into the not workable realm! If it was ever changed, I'm sure it was done like so many of the other changes Scott made -- in the middle of the night and undocumented. I wonder if any of those employees became politicians!! :):)

Dave
 
Finished recapping the power supply last night. I elected to restuff the old cans to keep the original appearance of the amp intact.

I cut the bottoms off the cans and use a heat gun to soften up the guts before I pull them out. Epoxy is used to seal everything back up again. Surprisingly, every cap I've pulled so far has tested "good" for capacitance! I don't have anything to test for ESR so that could be another story.

Here's a few pics of the work in progress and a shot of the cans back in place.
 

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hm, maybe I'll have to try whacking them at the bottom like that. I had been cutting them at the "step" on the side, but it always ends up looking sort of ugly. I need to epoxy the caps on the Sherwood back together too. They're held together with aluminum tape at the moment, which looks crappy. I did that in case I needed to open one back up in the event I made a mistake. Good thing too, because I did screw one of them up.
 
NOSValves said:
Just add the 50UF @ 100V caps to the center wiper lug of the bias voltage pots and call it good. Remember reverse polarity + goes to ground.
Click to expand...

Got some 47uF 100V caps today and installed them as recommended. That dropped the bias range quite dramatically and I had to raise the ground ref resistor back up to 100K (I bet Craig knew that would happen :yes:).

All is good now. :music:
 
njcanuck said:
Got some 47uF 100V caps today and installed them as recommended. That dropped the bias range quite dramatically and I had to raise the ground ref resistor back up to 100K (I bet Craig knew that would happen :yes:).

All is good now. :music:
Click to expand...



No I didn't and I have no idea why it did.... some thing is wrong! You better figure out what! Did you connect the positive lead of these caps to ground?

Did you install it on the wrong pots as in balance pots rather the bias voltage pots?
 
I've got to ask you can restuffers. When you look the old once high quality winding of the original capacitor and compare it to what you are replacing it with does it make you wonder about the quality change and how it effects the sound? I'm not going to get in a pissing match but I highly suggest you at least think hard on the point I am trying to make.

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At one time, a can that size might have been good for a single 1uf capacitor. Not entirely sure of the point you are trying to make, other than to point out that capacitor technology has evolved somewhat in the last half century.
 
gadget73 said:
At one time, a can that size might have been good for a single 1uf capacitor. Not entirely sure of the point you are trying to make, other than to point out that capacitor technology has evolved somewhat in the last half century.
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;)

I have used those "pencil" capacitors that I got from Mouser. They are 105ºC and are (from memory) about $3 each. They fit nicely inside of the can.
 
NOSValves said:
No I didn't and I have no idea why it did.... some thing is wrong! You better figure out what! Did you connect the positive lead of these caps to ground?

Did you install it on the wrong pots as in balance pots rather the bias voltage pots?
Click to expand...

Here's a pic of the caps I used. Yes, installed correctly - positive to ground and negative to center wiper lug of bias pots. (Not sure that it really matters which pots on this amp as the center lugs of the bias pots are directly wired to the center lugs of the balance pots.)

The bias voltage range dropped from -12.3 / -18.5 to -7.7 / -11.7. When the ground ref was raised to 100K the range improved to -12.5 / -16.5. Note there is already a 50uf 150V (orig 75V) filter cap in this circuit just ahead of the bias pots. Would this have an effect?

The same voltage drop occurred on my 299 when I tried this mod on the bias pots there (installed by me).
 

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NOSValves said:
I've got to ask you can restuffers. When you look the old once high quality winding of the original capacitor and compare it to what you are replacing it with does it make you wonder about the quality change and how it effects the sound? I'm not going to get in a pissing match but I highly suggest you at least think hard on the point I am trying to make.
Click to expand...

Yeah, makes me wonder but what alternatives do we have. As I understand it, those old caps are destined to fail in time so I would rather replace them now. I try to choose quality caps as replacements - Nichicons with 10,000 hour ratings.

Not trying to steal your trade secrets but are there better alternatives? I'm mostly going on what I've learned on this site.
 
The ONLY reason that your bias voltage would drop from a change in capacitors is because the new caps are drawing some current of their own to create the drop.

It may be that they still need to form, but in any event, I agree with Craig -- if there is a permanent drop in bias voltage because of a change in caps used in the bias circuit, then those caps are suspect. A properly operating cap of any type construction will draw no discernible current at all, other than its charging current.

Dave
 
Okay let me give another hint...again I am trying my best to get a thought process going without getting in a pissing match. I think many folks on this board need to realize the long term experience and absolute track record of the person trying to give away free information to his own monitary detriment before they jump to conclusions. I'm not always right. do not know it all and I know that! but on this subject I'm confident I'm absolutely correct. Time will bear this out.


.f
 
dcgillespie said:
The ONLY reason that your bias voltage would drop from a change in capacitors is because the new caps are drawing some current of their own to create the drop.

Dave
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Meaning they are low quality regardless of the spec (lie) sheet or defective unless installed improperly.
 
Gotta admit, I was a little surprised at how small and dinky looking those Nichicon 100uF's looked. I did use Sprague Atoms for the 50, 25s & 10s. I used Nichicon pencils for the rest.

I did a one-off ebay purchase for these 47uF's thinking Illinios was a reputable brand. I'm now starting to suspect otherwise. When I first fired up the amp this morning, the bias was shooting up past .35mA when I cranked the pots down. Then it quickly dropped back down and I was able to rebias at .22mA. It's been happy for a couple of hours now. I'm guessing something suspicious about the way these caps charge up?

Might have to order some Atoms.
 
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