Garey Barrell <[EMAIL PROTECTED]> made an utterance to the drakelist gang
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Darrell -
Yes, the 6EA8 is a direct replacement.
As most know, a tube tester is at best a rough check of a tube's
operation, especially in RF use. Just as an extreme, suppose the all
the internal leads from the elements to the pins each had a four turn
loop in them. The tube tester don't care! :-) Even a _good_ tube
tester only tests the relationship between grid voltage and plate
current, at perhaps 60 Hz or 1 kHz. Lots of ways to go wrong.
Emission is the "easiest" and least important specification about a
vacuum tube, other than to indicate "end of life" for some uses.
Anyway, your only true test is to plug it in to the circuit you want it
to operate in.
As for your tubes, ProComm doesn't make tubes. They buy them wherever
they can find them, put their name on them and out the door. Some good,
some not so good.
I don't know anything about HitRay. Never saw one. Could be great,
could have four loops in each lead wire! :-) Just as an aside, there
was a story circulating when the Chinese started making tubes. Seems
each one was hand assembled, and the assemblers were adding their own
little touches to make "their" tube unique!
Phillips has a good reputation, and so is probably fine.
RCA ... is RCA. All tubes were machine made to absolutely consistent
dimensions, each was tested, and thousands that DIDN'T meet
specifications were crushed each month.
Personally, I'd plug in the RCA and go.
73, Garey - K4OAH
Glen Allen, VA
Drake 2-B, 4-B, C-Line & TR-4/C Service Supplement CDs
<www.k4oah.com>
Darrell Bellerive wrote:
Garey,
Thanks for the tip on the 6EA8 at V-2. From what I read on the web it looks
like a direct replacement. Just plug it in. Am I correct?
I have four good 6EA8's. Testing them on my Mercury Model 990 tube tester show
the following emission levels:
ProComm (Gt Britain) Triode 81 Pentode 81
HitRay (Japan) Triode 80 Pentode 78
Phillips Triode 79 Pentode 77
RCA Triode 76 Pentode 70
The tube tester good range is from 50 to 100.
From your message it looks like the oscillator triode is the problem with the
6U8 and not the pentode (1st mixer) section.
Which of the above tubes would you use and why?
73,
Darrell
VA7TO
On Wednesday 23 July 2008 20:07, Garey Barrell wrote:
The 2-B came with Sylvania or RCA tubes when new. By the way, Zenith
didn't make tubes, and all Zenith branded tubes were made by Sylvania.
V2 can be problematic. The 6U8(A) is marginal on gain, and as it ages
the 18.0 MHz crystal will sometimes fail to oscillate. The oscillator
circuit is a little different in that it operates with fundamental
crystals below 18 MHz, and 3rd overtone crystals from 18 MHz and up,
with no switching. Anyway, the 18.0 MHz crystal is a 3rd overtone, and
seems to be "on the edge" in more ways than one. Changing tube to a
6EA8, with its higher gain, will make this circuit more reliable. Quite
a few 18.0 MHz crystals have been replaced over the years, when a simple
tube change would usually "fix" the old one.
The 8BN8 is correct also. The very first 2-A's had a 6BN8 here, but at
s/n 1000, it was changed to an 8BN8 for hum reduction. There is an
article at www.wb4hfn.com explaining the mechanism involved.
You're off to a good start!
73, Garey - K4OAH
Glen Allen, VA
Drake 2-B, 4-B, C-Line & TR-4/C Service Supplement CDs
<www.k4oah.com>
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