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John
Broskie's Guide to Tube Circuit Analysis & Design

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07 March 2009

TCJ 10 Year Anniversary
In
March of 1999 the Tube CAD Journal was born. I had been writing
the Tube Circuit of the Month articles for my GlassWare
website and the response was just tremendous. This surprised me, as
schematics were not rare on the Web, but enthusiasm-dripping e-mails I
received told me that something extra was going on. I asked and the answer
I got was that I just didn’t show how a circuit was laid out; I
explained why it was designed that way. I realized that something
more than one short article a month was needed. Thus the Tube CAD
Journal was created. It, however, did not become what I had intended,
as I wrote back in 2002 in an answer to a TCJer’s
letter:

The original intent of this journal was to mimic
the Math CAD Journal, a magazine put out by the MathSoft people
to assist users of their Math CAD program by providing
math-related articles that illustrated how their software could be used.
For example, users of Math CAD who explain how they found the
program useful write many of the articles. Well that was the intent, but
the Tube CAD Journal went off in another direction altogether.
Part of the reason was that many of the tube fanciers needed, as you
pointed out, to be technically brought up to speed. Another part was my
hope of expanding the tube-audio horizon, which I felt was collapsing
into a few simple-minded topologies and practices.

Trying to put out a one-man-operation webzine, with an
occasional helping hand from John Atwood, was thrilling, but exhausting.
From webzine to just single articles to blog entries was the path I had to
follow to save my sanity. Yes, I know that blog is not a proper blog,
being to infrequent and being too long. But this format is a comfortable
one and it is here to stay.

So here I am, as Thomas Stearns E. might have put it, in the middle
way, having had ten years—ten years largely wasted, the years of
l'entre deux enfants terribles. Trying to use words to describe
circuits, and every attempt is a wholly new start, and a different kind of
failure. Because I have only learned to get the better of words for the
circuits I no longer wish to explicate, or the way in which I am no longer
disposed to say it. And so each blog entry is a new beginning, a raid on
the enigmatic, with shabby equipment always deteriorating in the general
mess of imprecision of high-end audio—undisciplined squads of fabricated
rumor, advertising myths, and science most pseudo. And what there is to
conquer, by acumen and acquiescence, has already been discovered once or
twice, or several times, by men whom one cannot hope to emulate—but there
is no competition—there is only the fight to recover what has been lost
and found and lost again and again; and now, under conditions that seem
unpropitious, but perhaps neither gain nor loss. For me, there is only the
trying. The rest is not my business.

Tube Tester: Ig & Vg

Back to testing tubes. Last time we saw how an auto-bias circuit could be
implemented that forced the tube under test to conduct a fixed current by
adjusting its grid voltage, while its cathode-to-plate voltage remained
fixed. (Had we tried a constant-current source in series with the cathode,
the cathode-to-plate voltage would decrease by the cathode voltage, which
would throw our results off; maybe not a lot with a 12AX7, but by a huge
amount with a 6AS7.) Now, we look at how we can derive some useful
information with the auto-bias circuit, such as the grid voltage and the
grid current.

In the schematic above, we see three extra OpAmps. With
these OpAmps we can extract the voltage drop across the two 2M resistors
that attach to the grid. Now, 2M in parallel with 2M equals 1M, so any
voltage that develops across these resistors will reveal a current flow
equal to V/1M. Thus, for example if we measure 2V, then 2/1000000
milliamperes of current flows through the 1M resistance. In addition, this
circuit allows us to measure the exact voltage at the grid, without
dragging down that voltage in the process of measuring.

The four-position switch allows us to set the desired
idle current. At the first position, there is no resistor and the NE5532
OpAmp sees only the 2500-ohm feedback resistor. At the second position,
the 625-ohm resistor is in parallel with the 2500-ohm resistor, which
means that the OpAmp effectively sees 500-ohm feedback resistor. Because
the auto-bias circuit will strive to maintain an output voltage of -25Vdc
for the bottommost OpAmp, the current flowing through the feedback
resistor (and by extension, the tube under test) must equal 2.5/Rfb. Thus,
for example, 2.5/500 equals 5mA.

The voltage reference is made up from a buffered
voltage-reference IC, whose set voltage is 2.5V. To be honest, the RC
filter that following the voltage reference is just so much extra frosting
and it can be left out.

One last feature worth mentioning is that while most of
the OpAmps see a +/-12V power supply, the two OpAmps closest to the grid
work off of +5V and -20V rails. Why? The tester needs to be able to
control a wide range of tubes, so extra negative grid bias voltage is
needed and at no time should we expect a tube to need positive grid bias.
At the time I designed this tester, I preformed a survey of output tubes
and I concluded that with only 150Vdc for a cathode-to-plate voltage all
the tubes would be able to idle at 100mA with a grid voltage not less
negative than -19V; perhaps I was wrong. More testing is required.

Next Time
We will look
into how to measure transconductance, noise, and linearity. And I hope to
announce the arrival of more PCBs.

Since I began with T.S. Eliot, maybe I should end with what my fellow
Capricorn, Henry Miller might have written about the Tube CAD
Journal: What is this then? This is not a tube-Wiki website. This is
useful, original, sonically sound. This is not an electronics website, in
the ordinary sense of the word. No, this is a prolonged cheat sheet, a gob
of hot solder in the face of high-end audio, a kick in the pants to
proprietary circuits, Audio Research, Conrad Johnson, VTL, MJ,
AudioXpress… what you will. I am going to spill the beans for you, a
little sloppy perhaps but I will spill.

//JRB

Kit User Guide PDFs
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image to download

E-mail from GlassWare Customers

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