I can't stan it no more, I got to put in my 2 cents worth.
HIHI
I remember a VHF transceiver that I was responsible for
maintaining while in the Air force. Radio was not my job in the Air
Force but I was a ham, and so filled in on a number of disassociated
assignments. Any way, this rig was a Class C plate modulated thing and
it started having a reduction in output even though the grid drive and
plate current was the same. I had no way to measure the plate
temperature. But replacement of the 4CX150s brought the output back up.
I could not explain it. But others in the work field said they had
experienced the same thing over and over again with that type of rig. I
never had the chance to experiment with the phenomenon of this VHF rig.
I have never experienced any thing of this nature in HF and I
have never built any VHF Class C rigs.
I have however noticed a lot of different things about tubes in
audio work. All phenomenons, I have found the reason for. If one goes
to the trouble to actually graph a full set of plate characteristic
curves on the tubes in question you will see that the tube
characteristics have changed with age. All of which is associated with
emission loss or space charge. Some also associated with the surface
area that is actually emitting into the space charge. The ability for
the space charge to be replenished by emission from the entire area of
the cathode or just a small area that is still working is all part of
the aging process. Most of the deterioration I think, is caused by a
poor vacuum or loss of vacuum over time. Characteristics of tubes and
especially aging are a hard thing to control. In my opinion circuit
design should be adjusted to try to maintain a lower gain than might be
described in many tube manuals and low distortion with changing
characteristics. For instance, 1/2 of a 12AX7 has a circuit description
in most tube manuals having a max gain of about 50. I tend to lean
towards cathode bias for self adjusting bias and I leave the cathode
resistor un-bypassed for a lower gain of about 10 and a lot less
intermodulation and harmonic distortion. I will even test the circuit
by reducing the filament voltage on the tube to 5 or even a little lower
to simulate low emission if I have done my work right the gain will not
measurably fall off nor will the distortion level increase. I have
noticed a measurable drop in noise however and 5.5 volts and below.
When the circuit is designed for max gain and the input resistor is high
the tube takes on the characteristics of a reactance amplifier because
the interelectrode capacitive reactance of the tube is a significant
percentage of the input resistance. If the gain of the circuit is high
and the frequency in question is high the reactance is equivalent to a
capacitor across the input and the output will not maintain the exact
180 deg phase reversal that it does at lower frequencies. If the output
circuit is forced to maintain gain at a higher frequency as with tuned
circuits, the thing will break into oscillation as any un-neutralized HF
tuned amplifier would. Designing a circuit for lower gain and lower
input resistance coupling will help maintain its frequency response with
aging as well as keeping the distortion down. By the way, if a graph is
made of the dynamic transfer characteristics of the circuit with a gain
50 it will be found that the apparent linear portion of the curve is
much less than the linear portion of the curve in the circuit with a
gain of 10 and a un-bypassed cathode resistor. The un-bypassed cathode
resistor is a form of inverse feed back within the current loop of the
output. Even though the audiophiles say don't, I do. I'll compensate
elsewhere for the effective rise in plate resistance. In a circuit of
this kind you can barely detect the difference between the 12AX7 and a
12AY7. In short, yes, aging happens, make the circuit self
compensating.
Good discussion, I love it:
John WA5BXO
