resistor 101

[EWBrown]

For those who may not be familiar with how to calculate series and parallel resistor networks, the method is pretty easy: Most "networks"can be broken down into simple series and parallel combinations.

Resistors in series, the values are simply added, and for parallel, the formula is R1 X R2 divided by R1 + R2. Just think of those old math "word problems" where Johnny takes 3 hours to finish off a case of beer, and Jimmy takes 6 hours, how long does it take for them "working" together to slam down the case :smileyinthejar: ? :drinking:

Answer is 6 X 3 (18 ) divided by 6+3 (9) or 2 hours. Same applies to two resistors in parallel.

Now, on the ST35, the biasing network for each EL84 consists of a 470 ohm resistor (call it R17), a 1K resistor (R18), a 10K pot (R 19) and a 10 ohm resistor to ground (R43).

R 18 and R19 in series can be considered a single resistor with which the minimum value is 1K, and maximum is 11K.
In teh following examples, I'm rounding numbers off to the nearest 10 ohms for simplicity, so please don't "spank" me for sloppy math :lol:

This 1K / 11K resistor in parallel with the 470 ohm resistor (R17) can be calculated as R max = (470 X 11,000) / 11,470 which results in approx 450 ohms. The R min will be (470 X 1,000) / 1470 which results in approx 320 ohms. Add in the 10 ohm resistor R43 in series and this yields a max resistance of 460 ohms, and a minimum of 330 ohms. At a setting that results in 400 ohms total, the pot will be set to approximately 1400 ohms, the 1K resistor makes this 2400 ohms across 470 ohms or darn close to 390 ohms, add in 10 ohms and the grand total cathode to ground will be 400 ohms. Bear in mind that the 470 uF cap to ground throws in a little bit of time delay, so allow a few seconds for the meter reading to settle down.

The alternate formula for 2 resistors in parallel is: 1/((1/R1)+(1/R2))
This can be extended outwards for 3 or more resistors in parallel.

Capacitors work in just the opposite manner: Caps in parallel simply add together, in series the resistor series formulae apply.

Inductors get more complex, especially if there is any mutual coupling between individual coils, I won't go there for now... :crazyeyes:

/ed B in NH

[erichayes]

Hi All,

There's a very handy variant to the R1 x R2/R1 + R2 formula for when you know the value you need, but need to calculate the shunt resistor value: Rshunt = Rknown x Rneeded/Rknown - Rneeded.

If, say, you have a 1000Ω cathode resistor that you need to get down to 330Ω, you'd multiply 1000 by 330 and divide that total by the difference of 1000 - 330. The result is 670Ω, which is close enough to 680Ω that nobody except real anal types will care.