Forty-eight (48)
pieces of RCA 6GT5 NOS, New-Old-
Stock, unused Beam-Power Vacuum Tubes.
These were designed
as Horizontal-Deflection tubes,
but make excellent audio output tubes.
I am including two
quad sets of sockets. They are
NOS, New-Old-Stock high quality vintage USA Cinch
sockets. They are nine-pin neonoval
sockets.
Two of the tubes in
the box will be “dead”… One I broke
open to handle to see how rigid the construction was.
The other I painted black so I could use an infrared
temperature sensor to see if I could read how hot the
tubes ran… They didn’t run any hotter than other
variants of this tube that I tested.
I ran them all
through my tube tester when I bought
them years ago. I opened the box years
later and
found that the adhesive had dried up on many of the
little sticky labels and many had dropped off the tubes.
I stuck many back on with tape, but some
are lost.
If you study tube
manuals and data sheets, it is
tempting to believe there were a huge number of tube
types but this is partially an illusion.
Designing,
prototyping, testing,
and manufacturing tubes was a
very expensive
proposition, so once a tube design was
proven successful for
a particular application, the
design was milked for
every penny it was worth.
Deflection tubes are
a prime example of this. The
6DQ6/A/B was an early
original medium-size deflection
tube design that
proved itself in service and was widely
employed by
television designers. As a result, the
original design
spawned a large number of variants that
offered the same
characteristics, but in different
packages. In addition, the various manufacturers would
“re-invent” the same
tube, but with just enough real or
imagined
characteristic differences to apply for a fresh
JEDEC registration
number, whose main purpose was
to avoid paying
licensing fees to the original design’s
manufacturer. Thus the original prototype 6DQ6/A/B
design was
continuously recycled by different
manufacturers as the
6FW5, 6GE5, 6GT5, 6JN6, and
6JT6 and several
others. Functionally, and for all
practical purposes,
these are the “same” tube, differing
only in basing and
shape. RCA re-purposed the same
design
characteristics for the very successful Radio-
Freqency tube, the
6146/A/B, which also serves very
well as an audio
power amplifier output tube.
The 6GT5 is a variant
that was fitted into a short
“Compactron” glass envelope with a 9-pin NeoNovar
base.
I cracked one open
and “handled” it to see how sturdy
the internal construction was, and found that the short
Compactron design
style makes the tube cage more
rigid than the taller
versions fitted into older style Octal
tube bases.
For my first self-designed
and scratch-built pair of
power amps I used 6FW5 tubes connected as triodes,
and came to respect them
highly. I purchased a
number of them, but in the triode amps where I
operated them on a 350-volt regulated power supply,
they were very durable and long lasting. This is a set of
tubes I set aside as spares, but I never needed them.
The 6GT5 is a medium
size beam-power tube,
specifically designed and optimized for service as a
horizontal deflection tube in television receivers. It can
be used to advantage as a push-pull audio output
triode, where it will offer similar performance to audio
classic types 2A3, 6B4G, 6A5G, and 300B.
They worked great as
beam-triode power output tubes,
but I recently tested them in beam-tetrode mode as
audio output tubes, and found they gave much lower
distortion than the 6L6 type output tubes.
I tested them
with screen voltage as the variable, and found that as I
raised the screen voltage the distortion in push-pull
mode dropped to very low levels. Screen voltages
above 150 volts produced very low levels of distortion.
I began studying tube
amplifiers circa 1977, and when it
came to power amplfiers, I soon developed an affinity
for triode-mode output stages. The
ancient 2A3 and
300B tubes are the “gold standard” but I found I
preferred triode-connected beam power tubes.
The
6FW5 became a “favorite” with triode characteristics
very similar to the old standbys, but in a thoroughly
“modern” tube.
This is a bulk pack of deflection tubes typical of the
packaging that was used on
television production lines.
The Horizontal
Deflection or “Sweep” tube service was
the most brutal application that any vacuum tube faced
in common “civilian” service. It was
essentially a large-
signal pulse amplifier, delivering full-power 15kHz
sawtooth waves to the magnetic deflection coils of a
television picture tube. There was no
“volume control”
on these tubes… they operated constantly at full power
anytime the television was on.
If you study tube
manuals and data sheets, it is
tempting to believe there were a huge number of tube
types but this is partially an illusion.
Designing,
prototyping, testing, and manufacturing tubes was a
very expensive proposition, so once a tube design was
proven successful for a particular application, the
design was milked for every penny it was worth.
Deflection tubes are a prime example of this.
The
6DQ6/A/B was an early original medium-size deflection
tube design that proved itself in service and was widely
employed by television designers. As a
result, the
original design spawned a large number of variants that
offered the same characteristics, but in different
packages. In addition, the various
manufacturers would
“re-invent” the same tube, but with just enough real or
imagined characteristic differences to apply for a fresh
JEDEC registration number, whose main purpose was
to avoid paying licensing fees to the original design’s
manufacturer. Thus the original
prototype 6DQ6/A/B
design was continuously recycled by different
manufacturers as the 6FW5, 6GE5, 6GT5, 6JN6, and
6JT6 and several others. Functionally,
and for all
practical purposes, these are the “same” tube, differing
only in basing and shape. RCA
re-purposed the same
design characteristics for the very successful Radio-
Freqency tube, the 6146/A/B, which also serves very
well as an audio power amplifier output tube.
The 6FW5 is the only
variant that was fitted into an
octal-based single-ended envelope (no top cap for the
plate connection).
Horizontal Deflection
specifications were quickly
standardized after the invention of the television, so that
after FCC standards were adopted, all televisions
produced in the USA operated under identical
specifications for 525-lines and 30-frames-per-second.
The larger the television picture tube, the more powerful
the deflection pulses had to be in order to deflect
[sweep] the electron beam[s] across the full width of the
picture tube.
The requirements for
a sweep tube were understood
early on. All sweep tubes are low-mu,
that is, they have
a low amplification factor… the lower the amplification
factor, the more zero-bias plate-current the tube will
deliver for any given plate supply voltage.
Most sweep
tubes have a mu between 3.8 and 4.5.
This is exactly what
we need for audio output triodes,
where we need the tube to deliver as much current as
possible, balanced against the required input voltage at
the control grid. The ancient 2A3 audio
output power
triode had a nominal mu of 4.3, and the larger and
much coveted Western Electric 300B had a designed
amplification factor of “3.85.”
I studied and worked
on tube amps for many years
starting in the late seventies, and decided that, from a
theoretical standpoint, triode output tubes were
preferable to pentodes and beam power tubes.
But for
me it was “silly” to spend hundreds of dollars for old
2A3 / 6B4 or 300B tubes when there were a lot of
sweep tubes available at much lower cost.
A beam-
power sweep tube is converted to triode operation
simply by connecting the screen grid (g2) to the plate,
usually by means of a low-value (100 ohm) resistor.
Easy as could be.
For my push-pull
triode amplifier, I settled on triode-
connected 6FW5 tubes. These are a
re-packaged
6DQ6B type, attractive because of the octal base and
the single-ended design [no top-cap for the plate
connection]. I chose this class of tube
because of it’s
similarity to the classic 2A3 and 300B tubes, with mu =
4.1 and transconductance = 6600µmhos.
The slightly
higher transconductance of the more modern and more
advanced 6FW5 gave it a slightly lower plate
resistance, and so it is able to put out a bit more power
than the earlier tubes [the 2A3 and 300B had
transconductance of about 5250 and 5500umhos
respectively]. You must be aware that
these are
“target” values. In the real world of
tube manufacturing,
you will find some fairly wide spreads on
transconductance from sample to sample.
I operated my
push-pull 6FW5 triodes on a +350 volt
B+ power supply and they required about –72 volts of
fixed bias, meaning the peak-to-peak audio signal
required at the grids was over 140 volts to drive the
amps to their full power output of about 17 watts per
channel. Not easy, but do-able. The tubes proved to
be very reliable in this service, offering trouble free
operation for many years.
If you study the tube manuals, you will see that most
sweep tubes carry maximum Grid no.2 ratings of
175-225 volts. This would seem to disqualify them
for higher power work as audio triodes, but no…
this rating applies only to sweep tube service. In
sweep-tube service, overheating of the screen grid
is the “Achilles’ heel” of any deflection tube.
Higher g2 voltages in sweep tubes will result in
excessive “steady-state” pulse currents in the
screen grid, causing them to overheat. Audio
signals are very different than the steady full-
power pulse trains seen in sweep-tube service,
and audio work is a “walk-in-the-park” compared
to deflection service. As evidence, see the
ratings for the RCA 6146 tube. This was a
6DQ6 re-purposed and optimized for radio-
frequency modulation service. Same mu,
same transconductance, same heater power,
same power rating… but RCA specified that
the 6146 could be operated as a triode with
up to 400 volts B+ when g2 was connected to the plate.
Though their main
attraction to me is as a triode, the
6DQ6B family including the 6FW5 can be used as a
push-pull audio output tube in “normal” beam-power
tetrode mode… RCA Australia published guidelines in
the RADIOTRON magazine in March, 1962.
Shown
above – “Operating Conditions for the 6DQ6A as a
Class AB1 Audio Frequency Power Amplifier.”
Even
though it is a relatively small tube, it compares with the
6550, delivering “71 watts” of power with a B+ of 460
volts and a 4500-ohm center tapped output transformer.
Like all beam-power and power pentode tubes, its
efficiency is much higher than in triode mode, requiring
“only” 72 volts peak-to-peak at the control grids to push
it to full power output of 71 watts.
This is half the signal
input level it took me to get only 17 watts out of the
tubes as triodes. The 71-watt figure
quoted is
theoretical, and may be a “peak” power output rating.
Due to losses in the output transformer and in the real-
world power supplies, you would probably get more like
50 or 60 watts RMS out of a pair.
SAFETY CAUTION – if
using sweep tubes with top-cap
plate connections, please be safe. Using
a top-cap
tube brings the full B+ power outside the protective
walls of amplifier chassis, and presents a potential
threat of dangerous electrical shocks. A
perforated
metal cage should be installed over the output tubes to
prevent physical contact with the tubes when they are
energized. Never place an un-enclosed
amplifier of this
type where children or unsuspecting adults might be
endangered.
I have added several
information pictures….
One shows a set of
triode curves for the 6FW5 type of
tube, and the next shows the zero-bias output curves
for this type of tube compared to a 2A3 and a triode-
connected 6L6GC. The 6L6 has higher mu,
and can’t
deliver the higher currents that the low-mu tubes can.
One of my favorite
old hi-fi books is “High Fidelity
Techniques” by James Langham. He favored
triode
output tubes, and the 6A5 was an indirectly heated
version of the 2A3. The 6FW5 behaves
similarly, but
will put out a bit more power.
Next is a circuit
diagram of the 6FW5 triode amps I
designed and built, followed by a picture of one of the
stereo pair of monoblocks with regulated power
supplies.
Next, to show that
this type of tube can be used as
triodes with higher B+ supplies are data sheets for the
6146, which is the same type of tube, that is a low-mu
high-perveance beam power tube with practically
identical characteristics as the similar sweep tubes.
RCA Radiotron
magazine page showing recommended
usage data for this type of tube as a push-pull beam
power tetrode in audio amplifier applications.
Finally, a couple
pictures of my audio work bench.
Data sheet for the
6FW5 can be seen at:
https://frank.pocnet.net/sheets/093/6/6FW5.pdf
See my other listings... I have a similar large lot of GE
6FW5 tubes listed as well.