Lets say you replace a 27K resistor with a potentiometer, and find that spot that makes your amp sound just right. You measure the pot and it reads 23.5K. Besides searching all your 22K and 27K for aberrations, parallel two or more resistors, or order from Mouser for 3-5X standard value price (if available), what else can you do? Thanks!
If your resistor value is THAT critical, then tack in a small trimmer pot and set it to exactly what you want.
If you need 23.5k, then find a dead on 22k and a dead on 1.5k and put them in series. Parallels work too, but don't forget series when you are fussing about.
1% small resistors are not very expensive these days. Even if the price is 5 times lower precision one, we are still talking cheap. It isn't like you need a hundred of them.
As to "what else" you can do, well, what was wrong with the things you listed? Why do we need "else"?
I rebuilt an old Silvertone amp head for a guy, then he wanted it modified, so I did. Then he wanted to try this and that value resistor and maybe some other value, and won't it sound better if we made it this value. So in the preamp stage we were bothering over, I took out the resistors, mounted tiny trim pots in place of each, and told him he could tweak away until his heart was content. Everyone now happy.
You will find some commercial amps that use parallel resistors as trimmers. A transistor in the bias circuit of a solid state power amp often has a trim pot to set idle current. Some amps were made with the basic resistor where a trimmer might be, then two or even three parallel higher value resistors. Once the amp was running, one or more of the resistors could be snipped out to effectively raise the resistance of the original one. I can't bring a specific model to mind at the moment, but the schematics usually have some sort of notation. Sometimes the resistors are marked as "optional" on the print.
Considering that guitar amps are not precision circuits, I am always suspicious of "it's gotta be this exact value" situations. COnsider that your B+ moves typically 3 or 4 volts for every single volt your mains AC varies. So if my mains are at 118v today and tomorrow they are up to 123v, then that represents a 15-20v change in my B+, and the resultant effect on the sound and performance. One wonders if that ultra precise resistor will still be the exact value then.
If you need 23.5k, then find a dead on 22k and a dead on 1.5k and put them in series. Parallels work too, but don't forget series when you are fussing about.
1% small resistors are not very expensive these days. Even if the price is 5 times lower precision one, we are still talking cheap. It isn't like you need a hundred of them.
As to "what else" you can do, well, what was wrong with the things you listed? Why do we need "else"?
I rebuilt an old Silvertone amp head for a guy, then he wanted it modified, so I did. Then he wanted to try this and that value resistor and maybe some other value, and won't it sound better if we made it this value. So in the preamp stage we were bothering over, I took out the resistors, mounted tiny trim pots in place of each, and told him he could tweak away until his heart was content. Everyone now happy.
You will find some commercial amps that use parallel resistors as trimmers. A transistor in the bias circuit of a solid state power amp often has a trim pot to set idle current. Some amps were made with the basic resistor where a trimmer might be, then two or even three parallel higher value resistors. Once the amp was running, one or more of the resistors could be snipped out to effectively raise the resistance of the original one. I can't bring a specific model to mind at the moment, but the schematics usually have some sort of notation. Sometimes the resistors are marked as "optional" on the print.
Considering that guitar amps are not precision circuits, I am always suspicious of "it's gotta be this exact value" situations. COnsider that your B+ moves typically 3 or 4 volts for every single volt your mains AC varies. So if my mains are at 118v today and tomorrow they are up to 123v, then that represents a 15-20v change in my B+, and the resultant effect on the sound and performance. One wonders if that ultra precise resistor will still be the exact value then.
Well, I *have* shaved carbon film resistors to get an exact value without trimmers.
Both for bulk and cost reasons.
Mind you: value goes UP only so start with a lower one.
And it's irreversible, you can't replace what you took out.
But it does work.
And I can get the value I want in a minute or 2, compare that to mail order or even going downtown.
Both for bulk and cost reasons.
Mind you: value goes UP only so start with a lower one.
And it's irreversible, you can't replace what you took out.
But it does work.
And I can get the value I want in a minute or 2, compare that to mail order or even going downtown.
I have increased the value on modern film resistors with a dremel tool. The resistive element is a spiral wound coating on a creamic form. The element is just below the protective coating and it is about 0.030 inches wide, thinner on high valued resistors. Only a very small amount of material should be removed. Doing this will create a local weak spot because the resistive element now has a thin spot, so use a resistor with excess wattage capacity.
The old carbon composition resistors are "trimmed" by cutting a V shaped groove in the side with a triangular file or dremel tool.
In either case cover the "wound" with some fingernail polish or other protective coating.....you wouldn't want any of the electrons to escape!
The old carbon composition resistors are "trimmed" by cutting a V shaped groove in the side with a triangular file or dremel tool.
In either case cover the "wound" with some fingernail polish or other protective coating.....you wouldn't want any of the electrons to escape!
Not necessarily, the usual one is cut "like if an axeman were chopping a tree".
Mind you, desired adjystment is tiny, 1 or 2% usually.
Any more, you just get the next higher standard value.
By the way, essentially the same is made by important manufacturers.
Ever wondered what "stage gain/balance laser trimmed to 0.5% (or whatever)" means?
Yes, they laser "shave" a couple resistors straight at the die, before encapsulating.
Mind you, desired adjystment is tiny, 1 or 2% usually.
Any more, you just get the next higher standard value.
By the way, essentially the same is made by important manufacturers.
Ever wondered what "stage gain/balance laser trimmed to 0.5% (or whatever)" means?
Yes, they laser "shave" a couple resistors straight at the die, before encapsulating.
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