[R-390] Found something interesting about tubes
Barry Williams
ba.williams at charter.net
Tue Dec 4 21:39:07 EST 2007
Anybody remember Nolan Lee's R-390As that ran 24/7?
TUBE ROTATION AND REPLACEMENT
Date: Fri., 15 Jan 1999 22:34:22 -0600
From: Nolan Lee <nlee at gs.verio.net>
Subject: Re: [R-390] MTBF (was tubes and 3TF7 ...)
Every six months, I pull the tubes out of the R-1051B's and install new
ones. They'll last longer than that but I've got plenty of them. I think
that I paid .25 a piece for the 6AN5WA's when I bought them and only
about .70 each or so for the 6BZ6's. At the current rate of consumption
I think that I have enough for 30 years or so. :-) It costs me about 4
dollars a year to retube both receivers. Cheap, huh? And, yes I can
notice a difference in sensitivity before and after retubing. The
performance difference and the low cost of the tubes is the reason that
I don't even bother to test them.
The antenna couplers, have a switchable meter on the front panel that
monitors the ten pairs of 6922's in each one. When I see the numbers
drop to a level a little below the recommended reading, for one pair, I
replace that pair. I used to just replace all of them but my supply of
good quality Amperex 6922's is getting low and they now cost more than
the coupler is worth thanks to the golden eared audio crowd. When
they're finally gone, I'll have to start using the Jan 6922's but in the
mean time, I'll cook Bugle Boy's in the couplers.
I've make little cards up that I keep on each of the receivers. On the
card is the testing date, the location (V#) in the receiver, the SN of
the tube tester, and the tested value of the tube expressed as xx/yy,
where xx is the tester reading and yy is the minimum value of the tube.
I try to test the tubes about every six months. After a couple of
decades, you see trends. It's been my experience that for the R390A's
the tubes that seem to have to be replaced the most often are the
26Z5W's, and the 0A2WA's. I typically let them run 24/7. I don't trust
the regulator tubes as far as I can throw them. I hate the damn things.
Anytime I install new rectifier tubes in an R390A, I select and install
a new regulator tube. Amazing how many are a couple of volts or so
"off". I've also had "noise" problems in the receiver that I've traced
to them. It's amazing the difference in sensitivity the tubes in the RF
section make as they start to deteriorate. Since it's a gradual thing
and you don't notice.
About the least trouble that I've had is the 6 or 7 dozen or so tubes in
the old Tektronix scope and the CA plugin that's usually in it. Over the
last twelve years or so, I've changed a set of 5 of the 5642 HV
rectifiers, the 0G3 regulator and a pair of 6CW5's that come to mind.
Very well engineered piece of equipment. As long as the tubes aren't
noisy or shorted, they'll usually work fine in it even when their test
values are well below the normal "worn out" numbers that the tester
manuals list. I've had MORE transistors fail in the 547 than I have
tubes fail. Finding matched pairs of those old early 1960's transistors
can be a real pain, too. No BA collection is complete without a hundred
plus pound tube type Tektronix scope setting on the bench sucking up a
thousand watts or so. They're magnificent pieces of gear that are first
class examples what American industry was capable of producing at one
time. Quality engineering, craftsmanship, and best quality materials
available, but mainly, when people took pride in their work.
Tubes, as a whole, are a hell of a lot more reliable than a lot of
people believe. There were some really suck designed tubes/applications
in the 1970's, such as those damn compactrons used in televisions and
used in applications like RF amplifiers that they really weren't really
up to. Ditto to the tubes in the power supply for the IP-173 series
panoramic adapters. I've not real impressed with the life of the 6AK6's
used in the R390A's either. They don't seem to last as long as they
should. For what it's worth, the RCA's seem to last longer then the
other brands. The 6DC6's on the RF deck are another one that should be
replaced every 6 months of 24/7 in the R390A's if you're interested
in peak performance.
On a side note, try to never install a new 5749 in the PTO module of an
R390A. Always use a good strong used one. The alignment will hold longer.
Just for kicks, measure the sensitivity of your receiver, remove and
label the tubes on the RF deck and replace them with new tubes. Then
retest the sensitivity. I'd almost bet money that you'll leave the new
ones in there. :-)
Oh, about the only piece of gear that I have that gives a time frame for
tube replacement is my AN/PRM-10 Grid Dip Osc. The 955 acorn tube in the
head is supposed to be replaced every 300 hours. I was curious about
this a while back, so I tested the tube then let it run for a couple of
weeks and sure enough, the tube was flatter than hell. ;-) Which reminds
me, I need to pickup a few more spares, I only have a couple of them
left. ;-( Thinking of the GDO, 6X4 rectifier tubes don't impress me much
either. I've went thru a lot of them in the GDO, the RC and RCL
bridgers, the old CV-116 when I used it
(it has five of them in it), and the pair of them in the TV-2 tube
tester. The 6X4 is one of my least favorite tubes.
One factor on "tube life" that's over looked is the application that the
tube is used in. As an example, lets look at the 6AK6 used in the R390A
as an example. There's three of them in there if I remember right. Two
on the audio deck and one on the IF deck. The one used as the 4th IF
amplifier on the IF deck seems to last the longest, followed by the line
output amplifier and then the local audio amplifier on the AF deck. Same
tube, way different "life". When I have to replace a 6AK6 used for local
audio, I always install the new tube on the IF deck, and take that tube
and install it in the line audio position, and then place the line audio
tube in the local audio position. Hmmm, maybe we need a tube rotation
schedule. <grin> Nolan
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