Phil explained to me that :-
""""You need to do an impedance test and stop assuming.
The *soft* iron pole pieces of a speaker magnet have LARGE eddy current
losses and so the impedance rises much less rapidly with F than if
transformer lams were used.
Look at almost ANY published speaker impedance curve :
http://www.electrosmash.com/marshall-mg10
Bottom of page:
7.5ohms at 250 Hz ( = minimum) then
9ohms at 1 kHz
18ohms at 5.5 kHz
36ohms at 16 kHz
Indeed Phil. I forgot the iron effect, but if you extend the graph at the above address to a higher F and then plot a 20dB/decade line, you can work out that
there is possibly a 0.26mH inductance in series with 7.5 ohms resistance.
So the curve tries t eventually become a line with 6dB / octave slope.
But not between 250Hz and say 1kHz. Rate of Z increase is less than the rate at say between 2.5kHz and 10kHz.
The 6V6 amp output tubes have around 100k anode to anode Ra/ OPT has about typical 7k0 : 8 ZR, 875:1, so the 100k appears at Vo as 114 ohms and because this is 15.2 times higher than 7.5r typical minimum speaker Z then amp is a current source, NOT a voltage source, therefore voltage response across speaker
terminals follows the impedance curve - approximately. The 2dB of NFB does not do much to reduce the amp Rout, and Fender knows all about this. They WANT the speaker to give a bit of boost like a defacto loudness control - boosted highs and lows.
So when lots of series voltage NFB is used, this defacto tone control vanishes because the speaker voltage is held constant at all F because amp Rout might become only 2 ohms with enough NFB. Then many ppl say the sound becomes dull and lifeless. Its OK not having much FB, because the amp and its speaker are part of the instrument which the artist uses to wow an audience, so anything goes with regard to response contours and THD and IMD and noise..
Somewhere along the line of activities, artistic talent comes into play, and some musos get far bigger audiences than others, and make many hits and lotsa dough and despite having bloomin very ordinary sounding amps.
Fact is there ain't many ppl like George Benson, Paul McCartney, or even Mozart born every day.
I wonder what Beethoven would have thought about a Fender Deluxe Reverb had been given one, ( with a convenient power supply from the local windmill). Could have changed the course of musical evolution, and just where would we be now? But possibly the powers that were in Vienna might have sent around a bloke to stop all the damn noise. Trubble wooda bin that Beeto wooda given the bloke a beer, and bloke wooda found he liked twangy rock and roll. So Beeto wooda lived on.
Beeto mighta pulled out a 12AX7 to make himself a hearing aid.
Patrick Turner.