tubes age 198
can't see any obvious advantage. Just as a side point: I recently found out that
the 5814A at one point a half century ago was specced for some folks down the
road from you - ARINC. (ARINC has a nice little museum sponsored by their
radio club near the lobby of one of their Annapolis buildings, too.)
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Date: Thu, 3 May 2007 11:01:38 EDT
Subject: Re: [R-390] triple mica tubes
Some believe the triple mica tubes are better. If your receiver is just setting on
the bench and not mobile or subject to more than normal vibration, the
mechanical benefits of the extra mica are lost in your application. The Navy and
mobile Signal Corp. ask for the extra mica.
Tube noise does make a difference. Try all of your 5814's into the first FR
section of the audio deck. Use a signal generator and power meter (AC volt
meter an 600 ohm resistor on the local output.
Compare Modulation on and modulation off meter readings. At modulation off
the lower meter reading is less tube noise. As tubes are used, the noise goes
up and the power goes down. With the signal generator modulation on the
output will be lower. With the signal generator modulation off the noise level will
be higher. Run the 6C4 in the second mixer and use some Rf above 8Mhz.
Rank all the tubes you have as spare. Put the low noise tubes to the front end,
starting with the best 6C4 in the first mixer.
With your pulls and spares run all of these tubes again in you receiver that now
has better tubes in it than you had in the first pass. (assume some of the
spares were better than what was in the receiver and you now have the best
tubes forward in the circuit.
Now you know which of the tubes are really better from a signal to noise
prespective.
You can do this with all the tube in the receiver.
Do the 6AK6 in the last IF
Do the 5749's in the First IF.
Put the best 5814's in the audio chain you use (local line). Then put good ones
in the AGC. The IF follower is not used by most of us so put a poor tube in that
socket. If you do not use the limiter, then a noisy tube can go in there. If you use
your limiter, then put your best 5814 into that circuit.
(14 pages)
(10 pages)
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