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

[Richard Knoppow]

----- Original Message ----- 
From: "Philip (KO6BB)" <ko6bb at sbcglobal.net>
To: "Boatanchor List" <boatanchors at mailman.qth.net>
Sent: Thursday, December 30, 2010 1:52 PM
Subject: [Boatanchors] 'Ruggedized" 12AX7's ??


> 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

   I know the Groove Tube people and their tubes may be 
Chinese but they use mostly Russian tubes. My experience 
with Russian tubes is not good but is pretty limited.
   There was a ruggedized version of the 12AX7 but the tube 
you want is the 7025. This was designed for audio use and is 
low microphonic and low hum. Microphonics are likely to vary 
from tube to tube but the support structure of the tube in 
important. I rather think a lot of technology has been lost 
since tube were manufactured in great quantity many years 
ago. I am not convinced of the quality conciousness of 
either the Chinese or Russians because neither country has 
much of an industrial history.
    AFAIK, the 12AX7A is the same as a 12AX7 but with a 
controlled heater warm up. The "Five Star" version of the 
tube has a lower u, about 70 instead of 100.
    If you have a tube that works I would just stick with 
it.


--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com