[AMRadio] Testing tubes

[Jim candela]

	Bill,

	I liked your response very much. I few questions do come to mind however.
Many an audiophile will make statements about "sounds warm", or a "little
tinny", etc. I was wondering about your statement about "losing high
frequency tone with age." Is there a way to quantify this with test
equipment, or is this one of those things left to the listener of a home
audio system to judge for themselves? I can see for example on a scope when
a triode isn't biased correctly, the AC sine wave as it starts to fill out
the available dynamic range will compress more on one side than the other.
You might say that the peak to average ratio of a pure sinusoidal waveform
is being altered each 1/2 cycle, and in terms of distortion products, a rise
in even order distortion products. The listener may then start using all
kinds of catchy phrases praising the quality of the sound, etc. while
advocating the use of Teflon insulated wire, and $100.00 / pair RCA to RCA
cables, etc.
	I guess I am trying to get at a way to sift through the "bull", and
quantify the merits of one circuit versus another, and maybe even a given
circuit with one tube brand or another (all the same type, i.e. 12AX7). I
was warned by one audiophile that a tube like the 12AT7 which is in current
production today for audio use may not work in a RF circuit like a TV tuner
where a old NOS 12AT7 works fine. If true, that means that the standards for
manufacturing a given tube type have been violated in recent years. Things
like interelectrode capacitance, and tube interconnection paths / lengths
may not be constrained like they used to be.
	I have a "Realistic Lifetime" 12AX7 that I just put into my Sherwood S8000
amplifier. It has gold colored pins (really gold?), and says it is made in
Poland. It too warms up with a brilliant glow that is brief in duration. I
wonder if the inrush current may be too high, and if ramping the filament up
in a soft start circuit may be of any benefit?
	You also say "All tubes tend to loose high frequency tone with age." Going
to say 20,000 cycles / second for most any tube should be no big deal. So
what do you think happens with aging tubes? Is it the cathode oxide changing
every 1000 hours or so, or is it a rise in Miller capacitance due to some
mechanism of tube aging? Also consider that many a tube amplifier with
12AX7's use DC on the low level stages, and sometimes less than 12.6/6.3
volts to heat these tubes. My Fisher X-101-B amplifier for example runs 4
seriesed 12AX7 filaments as the cathode resistor for the 7591A output tubes,
and the cathode voltage is set to 40 volts. That means 10 volts per 12AX7
filament (if they share evenly). Maybe a situation like this where the tube
is "emission starved" will show a degradation in emission with rising
frequency?
	Sorry for all the commentary, but with me a nerve is struck whenever I hear
the catchy phrases that often come from a audiophile peddling some form of
"snake oil" to line their wallets with...I prefer a technical explanation
for things which means taking the subjectivity out of the equation.

PS Still looking for a "live one" to buy my pair of Genelax "Gold Lion" pair
of KT-77's. First $250.00 takes them. Very rare, best sounding EL34 out
there with such magnificence in the sound that is indescribable in the
purest of tunes. listening to an amplifier with these tubes will bring tears
to your eyes, and thereafter no other amplifier will even begin to sound the
same.... I bought a Gonset G50 from Ebay for $43.00, and the KT77's were in
there as modulators in place of 6L6's! Big surprise!

Regards,
Jim Candela
WD5JKO


-----Original Message-----
From: amradio-admin at mailman.qth.net
[mailto:amradio-admin at mailman.qth.net]On Behalf Of DOXEMF at aol.com
Sent: Monday, September 29, 2003 6:46 PM
To: amradio at mailman.qth.net
Subject: Re: [AMRadio] Testing tubes


Gentlemen,
If you look at the mfg. of the tubes that light up like that
you will probably find that they are Telefunken or Amperex
(Holland). While not all tubes labeled such will have that
characteristic that does indicate they were made in the same
facility regardles of label. I don't remember the exact reason for the
momentary incandescence but believe it has to do with
the construction of the filament from that Mfg.
It's actually a good sign since that type are regarded highly
by the audio world.
As for the offscale readings... check the tester calibration
as "new" tubes readings will depend on the mfg. and year made.
I have quite a few new Phillips/ECG late production 12AX7's
that do not test close to what a NOS RCA or GE, or even
some of the used Mullards do.
My Cardamatic, the WE ,KS series version of the Hikock 123A
tests 12AX7's with a different card than ECC83's in that the bias is
slightly different. Actually the parameters of the 5751 and ECC83
are the same.
All tubes tend to loose high frequency tone with age. Kind of a softening of
the high frq. response. But will still test good in a
standard tester that uses line frq.as a signal source.
The exception is the Weston testers which use an internal
osc. at 5 khz which gives a more realistic test of the sound of
the tube.
Beware of most of the Sov. 12AX7's as they tend to be very
thin sounding with their small plate structure.
IMHO the long plate Euro mfg. or NOS US long plate styles
have the best overall tone.
Excuse the long reply. Hope it helps.
Bill KB3DKS



--- StripMime Report -- processed MIME parts ---
multipart/alternative
  text/plain (text body -- kept)
  text/html
The reason this message is shown is because the post was in HTML
or had an attachment.  Attachments are not allowed.  To learn how
to post in Plain-Text go to: http://www.expita.com/nomime.html  ---
_______________________________________________
AMRadio mailing list
AMRadio at mailman.qth.net
http://mailman.qth.net/mailman/listinfo/amradio