need help

A couple of points in regards to Record-changers post--
As I understand it (I could well be incorrect !) any tube type amp must never be operated without a (speaker) load.
Transistor types, at first were said to follow that same rule, however, time proved that to be wrong, and they survive with no problem if powered up without a load---
Impedance: Any value will measure slightly lower this way--and can be "guessed at" when using an ohm meter. A lot depends on the meter used, but a typical "16 ohm" speaker will read 12-14 ohms. An 8 ohm, will read in the 6 ohm range.
Ron Rich

Ron, when I asked about this in RCA broadcast school the reason given was that since a tube amp is transformer coupled the absence of a load on the secondary would cause a large kickback to the tube plates in the case of a transient signal.

This is what you'll often see a resistor/capacitor that is rated @1kv across the secondary or much higher if located across the primary.

I've found the speaker measurements you cited to be accurate and my results over the decades.

The simplest way to accurately measure and track speaker impedance:

Start with an amplifier with a low source impedance, S.S. amps are best for this purpose. Connect a signal generator to the input and connect a wirewound pot of at least 5 watts and at least 20 ohms in series with the speaker under test. Set the resistor to have zero resistance.

Connect an -accurate- AC meter across the speaker's terminals and feeding a low frequency signal (200Hz or lower) use just enough power to get a usable reading, then adjust the pot so that the speaker reading is now half of the full (zero resistance) reading.

Remove and measure the resistance of the pot -that is the AC resistance of the speaker at that frequency.

This test does have some built-in limitations due to the likelihood that the variable resistor will have a value of inductance that would affect the reliability of of readings at higher frequencies and the fact that most people will be using the AC function of a DVM which are generally inaccurate at anything much above 1KHZ.


As variation of this test can help find the speaker's free-air resonance:

Use a power resistor of approx 50-100 ohms @5 watt or more. Then, with an AC meter across the speaker terminals sweep the generator across the low frequencies from approx 20hz to say 1khz.

The reason for the high impedance of the series resistor to eliminate the damping effect of the amp's internal resistance and thus make the peak(s) stand out.

With jukeboxes it can be interesting to see the varying impedance. Most Seeburg machines had free-air resonances in the 75-92 hz range (LPC's closer to 100hz) While Wurlitzer in it's 1950's machines that used three woofers, had one with a 34hz point.

Rob

Rob,
Of course there is always my "Wheatstone Bridge", that has been sitting up on my shelf for 29 or so years, without being touched--
Wanna buy it ? Ron Rich