[Boatanchors] 'Ruggedized" 12AX7's ??

[Alex Whitaker]

I have heard from friends of mine who were in the broadcast industry back when it was all tubes that 12AX7 tubes are a perpetual source of trouble.  I guess that they kept a stash of them in the control room for the turntable amps....the tubes would get noisy and microphonic with use and frequently needed replacement.  In my limited experience with them, I have had to go through several used tubes to get one that actually sounded good in the application I used it in.  So, you probably need to get a stash of them if you're going to use that amp a lot.  I've never heard of a ruggedized version of the tube, although I'm sure that it exists.....don't have my tube manual handy, or I'd look it up.  

73,

Alex 
AA9XY

--- On Thu, 12/30/10, Philip (KO6BB) <ko6bb at sbcglobal.net> wrote:

From: Philip (KO6BB) <ko6bb at sbcglobal.net>
Subject: [Boatanchors] 'Ruggedized" 12AX7's ??
To: "Boatanchor List" <boatanchors at mailman.qth.net>
Date: Thursday, December 30, 2010, 4:52 PM

Hi,
Sorry if this is considered OT, but at least it's not CFL's, AND we are 
talking about real vacuum tubes ;-)

I have a one month old Fender Super Champ XD guitar amplifier I 
purchased to use at the church.  MUCH smaller than I'm used to playing 
through, but I have a mic on it and it only needs to provide enough 
volume to sound good on stage.  With only 15 Watts I can push the output 
tubes pretty hard for good sound without being excessively loud (unlike 
the larger amps I traditionally use).

This amplifier uses 1/2 of a 12AX7 as a voltage amplifier, the other 
half is a Phase Inverter for the PP 6V6 tubes running Class A, 15 Watts 
to a single 10" speaker. All the pre-amp and DSP circuitry is SS.

Problem is, the 12AX7 has already started to get a little microphonic.  
Not really bad, but it puts a sort of 'haze' over the sound while 
playing (the tubes are only a few inches behind the 10" speaker cone), 
along with the occasional "crackle".  I dug around in my junkbox and 
found a "no-name" 12AX7 in a generic box, plugged it in and it sounds 
very good now, actually better than when the amp was first brought 
home.  I have no idea if the tube is a NOS American one or what. . .

Anyway, I could send the amplifier in for service under warranty (5 
years, except for tubes, 90 days), but chances are, they'd put in 
another Fender 12AX7 that might not hold up.  I understand Fender goes 
to the cheapest source for their tubes. There are premium tubes sold for 
guitarists under the "Groove Tube" name, among others.  But I've read 
where these are basically Chinese tubes that have been tested and 
matched where need be (outputs etc).  Needless to say, these tubes take 
a beating in the typical combo guitar amplifier, even though I don't 
play rock or drive my amps into heavy distortion etc.  I play a lot of 
"clean" single note lead, rhythm only when accompanying my own singing.

QUESTION:  At one time there were a lot of "Military nomenclatured" 
tubes that were supposed to be ruggedized. They typically had a 4 digit 
number instead of number/letter/number.   Are there any such for the 
12AX7, and if so, are they likely to be still available at the usual 
tube suppliers (Antique Electronics, RF Parts etc)?   And in this case, 
would 'ruggedized" mean freedom from microphonics, noise, etc?

-- 
73 de Phil,  KO6BB
http://www.qsl.net/ko6bb/   (Web Page)
http://ko6bb1.multiply.com/ (My OTR Blog)
http://cbsmysterytheater.multiply.com/ (My CBS
Radio Mystery Theater Blog)

RADIOS:
Yaesu FT-2000 Xceiver for "hamming" and LF Beacons.
National NC-183 16 Tube Receiver (circa 1949)
Zenith "American" Royal-7000 Transoceanic. (circa 1970)
Zenith "Taiwan made" 7000-2 Transoceanic. (Circa 1971)
Realistic DX-380 Digital Portable.

ANTENNAS:
88' Long balanced dipole.
"Mini-Whip" (active antennas) for LF.

Merced, Central California, 37.3N 120.48W  CM97sh

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