tubes age 180
Once you down to differences you cannot hear you can still measure smaller
differences with test equipment and meters. Just look at all the factors Tim
listed. There is no substitution for placing a tube in a circuit and measuring the
output. Who knows exactly where the resistor values in your receiver circuit will
bias a tube.
Who knows where a tube will fall on its performance curve. No substitution for
lots of tube substitution in the circuit to find the one tube that best fits the circuit
from a signal to noise perspective. Does Tim have a batch of tubes from one
source that behave a lot better than other tubes? I can belive he sure does.
What do we know from all of this. Tim has found some good tubes and some
not so good tubes. What can we take away from this? Our mileage may vary.
Our choice is to keep buying and testing what we find.
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Date: Sun, 23 Apr 2006 15:22:02 -0400
Subject: Re: [R-390] Re: 6C4? 6C4W? 6C4WA?
I seem to remember a thread some years ago about the 6C4 and it being best
if the tube is not too "hot" -- as if a mediocre-reading (middling, not weak) tube
was the better choice -- or one that had been worn in elsewhere. I don't recall if
it got into s/n ratio or whatever. Anybody else remember that one?
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Date: Sun, 23 Apr 2006 15:44:09 -0400
Subject: Re: [R-390] Re: 6C4? 6C4W? 6C4WA?
I believe that argument is most often invoked for the 6BA6W/5749 in the PTO.
There the argument is that constant emission/mu there is most relevant for
long term stability, with some hints that "hot" high emission/high gain tubes
may actually be more likely to have their gain depend on other voltages/circuit
elements/have their gain change more rapidly over time as the tube breaks in.
Contrast this with classical oscillator theory where high gain is used as a Q-
multiplier. That works well for Meecham bridges but for better or worse the
390A oscillators are not so idealized! Me, I use 12BA6's that probably saw
many years of use in AA5 sets :-). One of my summertime projects may be
measurements of PTO and the three crystal oscillators for frequency changes
as a function of AC line voltage/AGC voltage/PTO filament currents etc. After a
burst of Cosmos PTO activities a few months ago I am a little burnt out with
frequency measurements at the moment, though :-).
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Date: Sun, 23 Apr 2006 16:11:53 -0400
Subject: [R-390] 6CW4
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