Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: chocopower on January 23, 2012, 08:26:55 pm
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Hi,
i have an old chasis with two 6V6, one 5Y3 and one 12AX7.
No marked speaker impedance, so..
I aplied 36,2 vAC to primary/plate taps.
I get 0,65 vAC in secondary.
so:
36,2/0,65 = 55,6923
55,6923 square = 3101 ohms
If a pair of 6V6 at 285vdc need to "see" 8k, i have a dilema:
With a 2 ohm load i get 6k2
With a 4 ohm load i get 12k4
the closer value i get is with the 2 ohm load (maths say 2,56ohms).
Close enough?
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Two-6V6 were often loaded in 10K.
This gives "3.2".
3.2 is another name for a 4 ohm speaker. In the old days, speaker impedance specification was not standardized. But DC resistance is clear and easy. A 3.2 DC ohm coil will be about 4 ohms around 400Hz, which is where speaker impedance specifications are now usually given.
Call it 4 ohms and play.
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Cool.
Thanks!