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24.22.235.48
In Reply to: RE: 71A driver at 10mA - too low to sound good? posted by dave slagle on January 22, 2015 at 10:15:53
Because you can swing a maximum of half the current, relative to the 20mA point, the transformer inductance must be twice as large for a given bass limit.
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Since the job of the 71A is already defined (driving the grid of a 50), I do not see an issue with running 10ma of current. The image below represents a 20hz loadline on an 801A (didn't have a 10 model) with an IT that has 50 (green) 100 (blue) and 1,000,000hy's of inductance.
It is interesting to note that in this case, the finite inductance takes its toll in voltage gain before current cutoff is reached.
Interesting - thanks for the pictures!
I am startled at the reduction in peak output, imposed by the zero-grid line.
Not sure how comparable the 801A and the 71A are, but in the graph you posted, the output voltage capability is +/-50v with the low inductance, and +/-75v with huge inductance.
A Type 50 can want +/-84v at its highest operating point - which suggests the 71A may not have enough plate voltage for this application, irrespective of the interstage transformer's inductance.
OOPS!... I must have had 10Y on the brain. The 71A is below and things look a little different. Green 40hy, Blue 80hy, red a bazillion hy.
Keeping in mind that this is 75V peak of swing @ 20hz I still think it is worth a try since simply moving up to 40hz as a full power low frequency limit has the same net effect of doubling the inductance and dropping to 10ma still seems workable.
as an aside... when running the distortions on the 801 version, I was actually getting less distortion at the lower current operating point which makes me believe that I was entering the distortion summation zone.
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