there is no better way to grasp dbm than by working with transformers,
take this Triad output for instance, label says 26 D.B.M.
right off the bat we have weirdness, who uses the notation D.B.M. ?
answer: Traid !
ok so this had us confused, this is a 20K to 600 ohm OPT, so which winding gets the 26 dbm assigned to it?
and is it input or output that gets 26?
calculator to the rescues, punch in 26 dbm at 20000 ohms, we get 90 volts, will this transformer also put out 90 volts on the output?
chart 2 shows that 90 volts on the 600 ohm output would make 41 dbm, so that is a no go,
chart 3 show what happens when we fiddle the input voltage until we get 26 dbm at 600 ohms, 15.5 volts,
conclusion: you can take the dbm on both sides of the transformer, 20K and 600, but you must specify impedance, which will give you two different voltage levels,
90 volts input into 20K = 26 dbm.
15.5 volts coming out the 600 ohm ass end will be 26 dbm
both sides therefore can be rated for 26 dbm but it is up to the user to sort out what the different voltages will be.
if we use convention, we would assume 26 dbm at 600 ohms for the triad, in other words, for output transformers we use the dbm rating for the output, and then multiply by the turns ratio to see how hard the tube can hit the primary, which in this case is
20K/600 = 33.33 Z , take root = 5.77 to 1, so multiply 15.5 by 5.77 = 89.5
lets see if the dbm calculator is telling the truth, 90/15.5= 5.8, yes it works,
to drive this into the ground and hammer it home look at the primary,
90V/20K=4.5 ma
and the secondary
15.5V/600=25.8ma
both sides of the transformer will take 26 dbm, the primary side will have 4.5 ma, the secondary side 25.8ma, the primary will have 90V x 4.5ma= 0.4 watts, the secondary will have 15.5 x 25.8ma= 0.4 watts,
it seems as though this dbm business is tied in with power, not voltage, since 26 dbm can be associated with 90V or 15.5 V in this case,
so dbm is a power level which uses 1 milliwatt as a ref, first derived from a 600 ohm system but not limited by a 600 ohm system,
dbu and dbv are strictly voltage levels, dbu uses about .775 volts as a ref, and dbv using 1 volt as a ref.
when deriving a voltage from a dbm rating, impedance must be considered
dbv and dbu voltages need not have an impedance associated with them.
dbv, dbu-voltage
dbm-power
