one could be added for tube matching. Reduced cathode
electron emission is a common cause of failure in tubes —
especially power tubes. If there isn't enough plate current,
the output power drops.
The ideal way to keep your tubes matched would be to
have both types of testers, then test the tubes (or new tubes)
and record the numbers. Then, later the tubes could be
re-tested to determine if they are getting weak. Mr. Asselin's
suggestion to check for bad or unbalanced components is
important for vintage amplifiers — especially for changed
values of carbon resistors and leaky capacitors. I like his
suggestion to use a dual-trace scope to compare the input
waveform to the output waveform. I had never thought of this
but I am going to try it.
Another method of tube matching is to use an adapter
which goes to one of the output tubes and measures the
current, such as the Bias King and Bias King Pro, the
Alessandro Bias Meter, and the Bias Probe from JHD Audio.
If the unit has two adapters (like the Bias King Pro), use only
one of them.
You need a good amplifier which uses the same
tube type you are matching. Place the adapter in one
of the output tube sockets of the amplifier, plug the
tubes to be matched into that adapter one at a time,
and record the current reading for each tube. This is
measuring them in an amplifier circuit, and each tube
is measured under the same conditions.
Several years ago, I decided that these adapters
were rather high priced, so I built one for much less. It
measures both the cathode current and the grid bias
voltage of the tubes (grid to cathode or grid to
chassis), using a digital panel meter for both bias and
current measurements. I am planning to add a
measurement of plate to cathode voltage, and a
switching circuit with four or six tube adapters, so all
of the tubes in a set can be measured without
changing tubes in the adapters.
One thing I learned back in the tube days is avoid
cheap tubes. Buy only name-brand tubes from a
reputable supplier (counterfeits are
always possible). Back then, there were
a couple of suppliers of cheap tubes,
with their own name on the tubes. I
never knew the source of their tubes
(they were not manufacturers), but I
suspected they sold either used tubes
or factory rejects. The suppliers I use
most are Parts Express and Antique
Electronic Supply (www.tubesand
more.com). If you need tubes quickly,
most music stores that sell guitar amps
will have tubes in stock — often at fairly
Bill Stiles; Hillsboro, MO
Thanks Bill. Lots of pearls to consider.
In the October Q&A, Figure 6
under the Tesla Coil Project is missing.
Under the LTspice Tutorial sidebar,
Figure 6 seems to be mislabeled.
Figure 7 and Figure 6 as presented
do not appear to have supporting text.
Thanks for the tutorial.
Peter A. Goodwin; Rockport, MA
I see the problem. I had two Figure 6s, so the numbering
is screwed up. The one for page 21 is in Figure A. I don't
know if I sent it originally because I failed to save the sent file.
In the text on page 23, Figure 6 in the LTspice tutorial should
be Figure 7 to go with the figure on page 24. Figure 6 on
page 23 should have been Figure 8 and the text where it says
"see Figure 7" should have said "see Figure 8." The INA128
diagram is shown here in Figure B. I apologize for the errors.
The files for these graphics are available at the October
Q&A article link
FEEDBACK Continued from page 7
December 2013 73