Tubes going "Microphonic" (what the heck causes it???)

I have a '69 Vintage V-4 with 7027's, after recently replacing the 
Power Tubes, thought it was time to replace the pre amp tubes... It 
was only through the re-tubing process that I discovered two out of 
the three Vintage RCA 12AX7's went "MICROPHONIC" (the two used in pre-
amp of Channels 1 & 2).
Now all three of these tubes are of the same vintage, and all test 
pretty decent (middle range of GOOD) on my tube tester with fairly 
close emissions... What the heck makes them turn "MICROPHONIC"??

To reiterate in what I describe as "micro-phonic": with the "standby" 
switched OFF- there is an extremely obvious correlation of noise when 
gently tapping on either one of the RCA 12AX7's (very much like 
tapping on the windscreen of an amplified "microphone") in either one 
of the two independent sockets (both being separated channels) of the 
pre-amp section of my V-4. -I'm going to experiment with one old 
RCA's to try out in my other devices that use a 12AX7 tubes out of 
sheer curiosity to see if any such similar attribute can be 
identified- I am highly doubtful that any of the "hybrid" (most of 
which I believe is total marketing B.S. in feeding 12volts to the 
plate- none the less see if it makes any significant difference) 
devices will make any noticeable difference, (I'll have to look in 
see if my vintage General Dynamics amplifier uses any 12AX7's ??) 
None of my other 12AX7 tubes (including the new JJ's ECC83-S I just 
installed) appear to display this attribute, and since I play with 
the head resting atop of one or two speaker cabinet stacks (I 
know "purists" recommend positioning the head distanced from the 
speakers- where they could be affected by noise coloration or 
potentially shaking the tubes free from their sockets- what is 
exactly what happened with regard to one of my 7027's... although it 
actually FELL in transit), my fear is that undesirable coloration 
from vibration, may permeate into such a "micro-phonic" tube. (also, 
many times positioning the head away is not practical- especially in 
regard to room on stage- plus it just plain doesn't look as COOL!)
So I replaced them with the JJ's... 

I don't know if it's my imagination but it seemed like the Old RCA's 
(even though they don't test as strong) appeared to have sounded just 
a TAD "Warmer"??(I primarily use the V-4 for Bass... sometimes for 
guitar if I want to experience unconscious blackouts from sonic-
inflicted loss of equilibrium) I tried a couple other 12AX7's I had 
kicking around(a newer Sovtek, a vintage Toshiba, and a "China-
Special"), but it seemed like the RCA's had a certain tonal quality- 
microphonic and all...  The JJ's sound pretty decent though, and 
pretty quiet, so that's what I have in there now (matched trio 
actually). I was going to maybe try out some Tung Sols & Electro-
Harmonix (Do the "Gold Pins" models actually make a difference in 
sound/reliability, or is it just marketing B.S. to charge you 
double?), and was thinking of trying to find some NOS RCA's or 
Sylvania's... but some of those NOS tubes can be $$$ and I hate to 
waste the $$$ if they are prone to go "Microphonic" on me?  Military-
Grade JAN tubes are outrageously priced- does anyone know from 
experience if they actually sound any better tonally, have less 
tendency to become "Microphonic", or just plain built to survive a 
Nuclear Holocaust?? (and no difference in regard to tone?) 

OK, open for opinions here... Interested in hearing other 
people's "Microphonic Experiences"...

Thanks in advance,

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AmPIG: "John's Fat & Hairy BEAST..."

I received this helpful reply from Erik of AMPLIKAT (he is an 
authorized dealer for JJ, Tung-Sol, EHX, Mullard, Sovtek, and many 
others- as you can tell from his reply: he most knowledgeable and 
personable- plus he really offers some of the best pricing I've 
seen), I thought I'd share (especially since it specifically pertains 
to V-Series). Also, I've ordered from him some JJ "Gold Pins", New 
Sensor/EHX, and New Production Tung-Sol in 12AX7s and 12DW7's to try 
(also going to try the Spring-Loaded Belton Tube Retainers- Anyone on 
here ever use them ??)
__________________________________________________
What kind of tube retainers does the Ampeg use for the final tubes, 
the 7027As?  Are they the "metal clip" variety?  Or, are they the 
heavy-duty, spring-loaded kind that have a sort of "top hat" that 
sits at the zenith of the glass envelope(s)? They are typically, 
these days, made by Belton.  I absolutely LOVE the Belton, spring-
loaded, final tube retainers. I use them in my own amps and keep a 
stock of them on hand for different sizes: 6550s. 6L6/7027s, 
etcetera. The inside of a guitar combo amplifier (and even a 
bass "head" sitting on top of a cab), as well as the intereior of a 
jukebox, etcetera, is a veritable "torture chamber" for vacuum tubes. 
The cheap metal clips don't do any justice. Spring-loaded Beltons are 
the way to go. A spring-loaded Belton tube retainer serves a multiple 
function: It greatly attenuates or eliminates electro-mechanically 
coupled noises such as "tube rattle" and other annoyances. 
Furthermore, it tremendously reduces 'shot noise', e.g. - stochastic, 
or quantum noise by insuring a sold galvanic contact of the tubes' 
pins to that of its socket (assuming that the socket itself isn't 
looser than Paris-you-know-who, during a Lakers post-game 
celebration). My humor (or attempts at it) are silly; hope you don't 
mind. Sure, the 'shot noise' will be below the threshold of detection 
by the human ear. However, the cumulative effects of said artifacts, 
in cascade, certainly are empirically observable on a scope. The 1/sn 
takes away from the "sweetness of the amp" in a tangible manner when 
completely summed. Worse than 1/sn is the "snap, crackle, and pop" 
that can be heard during quiescence due to - again - poor galvanic 
contact betwwent the pins and socket. Or, the Rice Krispies effect 
can be due to a malformation inside the tube itself, sometimes. Be 
advised that there are many so-called "experts" that adamantly assert 
that the only function of the tube retainer is to keep the tube from 
falling out. I won't name names. Suffice to say, it's a shame that 
they are widely published, dispersing tons of bad (potentially 
lethal) misinformation, and are defiantly proud of their ignorance 
when gently questioned.  Anway, I want to send along some Beltons, if 
you don't have them already. They're not very expensive at all. I'm 
confident that the hole pattern matches. Just use the existing screws 
at the socket base to secure the Beltons. Replace any/all cheap ring 
clips with Beltons. I testify that it does make a big difference. 
Finally, I have access to ALL of the brands of tubes you could 
possibly desire. On a personal note, my favorite preamp tubes (I've 
tried 'em all, practically) are the JJ Electronic Gold Pins Series 
(it isn't just the pins, different alloy mixture, and ruggedized 
internal structure). Even my mom can hear the difference between the 
regular JJ and the Gold. It transformed my Mesa F-50 into something 
truly miraculous and studio-quiet. (Btw, I've owned the Stereo 2:90, 
Triaxis, Walkabout Scout).  I also LOVE the New Sensor/EHX, New 
Production Tung-Sol 12AX7s. I refurbed my friend's Fender 1963 Tweed 
Deluxe and it sounds so awesome! Yes, I used different brands. I put 
a JJ Gold Pins 12AX7 in V1, Tung-Sols in the other slots, JJ 6V6s in 
the finals, and a JJ rectifier. My friend used it in a studio in San 
Jose and the owner begged him to lease it. Pro musicians are now 
making use of that amp for tracking. I never felt so proud. Whoa.  
Gotta run. Take Care, Erik

Erik Scott
http://amplikat.com
559-277-5380 Tel 
559-440-9220 Fax

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AmPIG: "John's Fat & Hairy BEAST..."



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