#### CJ

##### Well-known member

a tube is just a variable resistor, or a pot to the beatnik, (not weed, they call it tea)

so put a battery on a pot and turn the knob real fast, maybe 20 times a second if you are wired up like a cross country beatnik,

so now we have current moving at 20 times a second, hey, we could connect a speaker to that and dig the rebop,

if the pot has a high resistance, we will need a high voltage to get the same current to drive the speaker,

if it has a low resistance, we only need a few volts to get the same current, like a transistor,

now speakers do not like DC, so we need a way to get the battery out of the circuit,

so we use an isolation device called a transformer.

our 20 Hz AC signal will go right through the transformer, but not the DC.

that means we can drive the speaker.

if the tube has a high resistance, then we can wind the transformer with less turns on the speaker side,

this is called a step down transformer.

voltage will be stepped down, but power going in and out of the transformer will be about the same, so current will go up.

this means we can wind transformers to match the the plate resistance to the speaker load, a high plate resistance means low current so we wind less turns on the sec to get more current,

a low plate resistance like a 6L6 means our turns ratio will be lower since the tube is already pumping out 40 ma, our speaker might only need 1 amp of current,

1 amp / 40 ma = 25, so we need a 25:1 turns ratio to match the tube to the speaker.

what if we use a a 12AX7a tube to drive the speaker?

1 amp sec speaker current/ 1 ma tube plate current = 1000, so we would need a 1000:1 transformer,

but if our plate voltage was 200 volts, now we have 1 amp of speaker current but only 200/1000 = 0.2 volts to drive the speaker, this is not enough power, .2 x 1 = 0.2 watts,

so we use a tube that can handle 200 volts on the plate and 40 ma, maybe even 400 volts on the plate and 40 ma, that = 400 x .04 = 16 watts, now were talkin,

ok so what does this have to do with 600:600 vs 10K:10K?

they are both 1:1 transformers.

well, 600 ohms at 200 volts = 200/600 = .333 amps

our tube only puts out 1 ma, not 333, so if we saddle up that 600:600 ohm transformer to our poor lil 12AX7a, we have a mis-match.

we have 200 plate volts 12AX7a /.001 ma plate current 12AX7a = 200,000 ohms,

so we need a 200,000 ohms on the pri of our transformer otherwise the tube will not have enough poop to transmit power,

so 600:600 or 10K:10K is all about matching the transformer with whatever is driving it.

propeller heads use fancy words like Source Impedance to describe these details, but it is really just volts and amps.

now when they say 600 ohms when talkin transformers, what they really mean is 600 ohms of Reactance. this just means that the ohms change with frequency.

XL=2 pi f L, XL being our "ohms" for the transformer.

now we can see fro the formula that as frequency goes up, Reactance or "ohms" goes up, so if we are running at 10,000 hz, we will have plenty of ohms for our vacuum tube, but down that at 20 and below, the stuff hits the fan, so we need to get our inductance, "L" up there to jack up the ohms,

so a 10K transformer will have more inductance than a 600 in order to get our ohms from 600 to 10K.

how much more?

easy, since our formula is linear, inductance and ohms are the same, so we have 10,000/600 = 16.67 times as many Henries in the 10K over the 600.

Turns wise? take the root and we get a 4:1 ratio, turns of the 10K to turns of the 600.

but with less turns we get higher flux, 4 times as much, so we watch the core when building these things,