Calculating the value of Rparallel

[J M Fahey]

Yes, in pratical everyday life that's the way to go.
After all, we care mainly about what can be heard, which is not such an exact measuring instrument.

[Enzo]

And if it is some odd value, two clip leads and a pot to the rescue. 20 seconds later I can measure on the pot what resistor I need.

[Howlin' Mad Mac]

All this algebra is making my poor head hurt... Personally I like the idea 205k is pretty close to 220k... Just pretend like you work for the DNC & fudge it a few ohms LOL

[lowell]

I like Algebra. It's fun.

[Chuck H]

I like Algebra. It's fun.

They make drugs for that...

2.2M resistors are common. 220k resistors are REALLY common. The parallel sum is 200k. Pick a couple of resistors that read a little high (as they most often do) and you'll be ignorably close to the somehow determined ideal figure of 205k. Two resistors. Very little fuss. Or just use a pot?!? Or better yet, just use a 220k and call it close eF#@*en nuff. You could even adjust other parameters to help make 220k the ideal value I suppose.

This is starting to go really long for a basic "ohms law" discussion.

[guitarmike2107]

20 posts is hardly long considering the lengthy discussions we have had in the past about more trivial topics

Why don’t you use excel, (or similar spreadsheet program) to create a table of standard value resistors in parallel, easy to do and quick to reference…that way you never have to think too hard again

[kg]

ahh, get it within 10%, call it done.

[imaradiostar]

ahh, get it within 10%, call it done.

Jeez, no kidding. It's a tube amp, not a nuclear power plant. 15/220=.068. That's well inside the acceptable tolerance for most tube things I've worked on.

JT

[Chuck H]

+/- 20% tolerance for the resistor values used in all those coveted vintage amps...

[Enzo]

Wall charts, spreadsheets, etc etc. All those are fine ideas if you have to make these calculations all the time. I kinda thought this was a one-off for Lowell. Seems to me if one finds himself trying to parallel a bunch of resistors all the time to come up with various values, ought not one just order the values he needs?

[J M Fahey]

Since it's a one-off, why not rethink the problem a bit and focus it from another end? (a.k.a. lateral thinking).
Adding resistors in series is much easier on brain health
180K+22K= 202K ; 180K+27K=207K.
Take your pick.

[jmaf]

Since it's a one-off, why not rethink the problem a bit and focus it from another end? (a.k.a. lateral thinking).
Adding resistors in series is much easier on brain health
180K+22K= 202K ; 180K+27K=207K.
Take your pick.

Have you tried collateral thinking? Blow it up. Then try to figure out why it blew up. Happens all the time.

[Chuck H]

I like lateral thinking (as opposed to collateral). Amp failed=bad output transformer. When new output tranformer fails to fix the amp, take it to a tech. Tech fixes problem that caused the failure. Recieve the now fixed, properly biased and tuned up amp, which sounds much better than it did, now tell everyone how good the amp sounds with the new output transformer. It's important to forget that the output transformer wasn't actually the problem in the first place.

[jmaf]

I like lateral thinking (as opposed to collateral). Amp failed=bad output transformer.

Hihihi. And bam! Instant lucrative amp business.