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Featured VT: The 6BQ6

Posted 18th January 2013 at 02:45 AM by Miles Prower

The 6BQ6 as an audio final came to my attention as a chance result of finding
an old article from a Portuguese (Brazilian? -- it was written in Portuguese)
ham magazine. This described a simple AM plate modulator that claimed an output
of 30W (fixed bias) or 25W (cathode bias) that used a push-pull pair of these
VTs. You would expect to see 6L6s or 807s used in this particular application.
Why 6BQ6s, and what were they?

The 6BQ6 is a large signal beam former. It has no audio pedigree
whatsoever, and the spec sheet makes no mention of its use as an audio final.
During the 1950s, screen sizes and deflection angles increased, giving a larger
viewing area, and a shorter CRT for more compact TV sets. This development meant
that the usual audio finals and RF types became increasingly unsatisfactory for
horizontal deflection duty. New types more suited to the task were developed,
and one such type was the 6BQ6, in several different iterations for large
screen, B & W TV sets.

The 6BQ6, as with all HD types, is capable of pulling big currents through
the horizontal deflection coils. This was accomplished by cathodes much
larger than those found in comparable audio and RF finals, such as the 6L6-oids.
Compare heater voltages and currents:

6V6: 6.3V / 0.45A (2.835W)
6L6: 6.3V / 0.9A (5.67W)
6BQ6: 6.3V / 1.2A (7.56W)

This makes possible a maximum cathode current spec of 400mA. In terms of
solid state, that isn't very spectacular, but for a nominally high voltage, low
current device, it's pretty good. Other design features include a low voltage
capability, made possible by the close proximity of screen grids to the cathode,
and that's what you want for drawing big currents through deflection coils, and
for operation from transformerless power supplies that derive the working DC
directly from the AC mains. The latter consideration has spawned versions of
this tube with odd heater voltages for operation in series heater strings.

As an audio final, the high current and low voltage capabilities make for
lower load impedances, and therefore, for easier to design OPTs. The lack of any audio final usage
requires one to draw up loadlines to find suitable operating points. Here is one
such audio loadline:

Click the image to open in full size.

As you can see, the load is just 1K1 per phase. This is a Class AB loadline,
so you would need an OPT with a primary that can match 4K4 (P-2-P) to your
speaker load. It's is a good deal easier to manufacture an OPT with excellent
high and low frequency performance than it would be for the 10K (P-2-P) load a
PP pair of 6V6s need. It's also a convenient value since there are off-the-shelf
OPTs that were designed for Class A, PP, 6L6s that match this value. The only
drawback to using one of these OPTs is that they are a bit under powered for the

As for why no audio uses are specced, the plate characteristic tells all. It
is not possible to use the 6BQ6 in Class A. The most linear part of that
characteristic lies well within red plate territory. You are committed to Class
AB, push-pull only. That isn't necessarily a bad thing, though.

If you stick with the usual convention for determining the static plate bias
current, you are deep into Class AB. The closer you come to Class B conditions,
the more cross-over distortion you have. The estimated H3= 5.0% is
barely acceptable. There is something you can do about this. The 6BQ6 was
specced very conservatively, since its main purpose is to output max RMS
power for hours a day, all the while maintaining a reasonable service life.
Audio amplification is a good deal less demanding. If you increase the bias, and
bust the plate dissipation spec, the performance improves greatly.

Click the image to open in full size.

Doubling the bias current to 50mA per plate, makes for 17.5W of static
dissipation: exceeding the spec by 6.5W (the 6BQ6 is rated close to the 6V6).
This is of no consequence, as you don't get any color on the plates. The RCA
gray plates can handle up to 70mA, with just a trace of color. The Sylvania
black plates aren't showing any color even at this extreme 24.5W of static plate
dissipation. 50mA of bias won't harm this type, even if it's outside of the
spec. The increased bias current, and the much lower H3 estimate,
does make a noticeable -- and improved -- sonic difference in practice.

As to the sonic performance of the type that had no audio pedigree, in a
word: excellent. The 6BQ6 easily matches the performance of the legendary 6V6,
and it pumps out more than twice the power. Running open loop, the 6BQ6
doesn't have any of that expected "pentode nastiness" until it's nearly at the
point of clipping. There's just an "edginess" or "aggressiveness" to the sound.
That would account for why the Portuguese (Brazilian?) hams elected to use it in
their plate modulator project (it was an open loop design). 6L6s tend to sound
much worse open loop.

The Twin-T test also bears this out. After nulling the fundamental, the
residual is almost a perfect sine wave at three times the frequency. There is
little higher order harmonic components. Though high fidelity isn't legal on the
ham bands, the 6BQ6 plate modulator would indeed sound better. For excellent
sonic performance, all that's required is enough gNFB to take off the edge. The
6BQ6, like the 6V6, doesn't require the additional help of local NFB. That is a
good thing in that the low screen voltage spec pretty much eliminates
Ultralinear. Operate them in full pentode mode with a regulated screen supply.
As with any pent that can't run the plate and screen at the same potential, the
best sonic performance results when the screen voltage is tightly regulated, and
supplied from a source with a very low impedance, as is the case with an active

The 6BQ6 also seems to be a good deal more stable in operation. It doesn't
produce the snivets that are seen with other types, even if you don't include
plate and screen stoppers. Even if not strictly necessary, it does no harm to
include these anyway. Screen stoppers can be 680R/0.5W C-comp resistors.
Plate stoppers can be made from ten turns of #18 wire, space wound 7/16ths inch
ID, with 100R/2W (or four 470R/0.5W C-comps in parallel) de-Qing
resistors mounted inside the coil. That should take care of any possible
instabilities, or tendencies to make RF, due to lay-out issues.


This type appeared early on in TV development. The first iterations were in
ST glass. As TV screens got bigger, new versions appeared. The 6BQ6GTA has the
small profile, tubular glass envelope. This type has smaller cathodes, and is
not quite so robust as later iterations. As such, it will red plate badly if
used at the Q-Points mentioned previously.

The 6BQ6GTB has the same small diameter envelope, but does work as described
above. The 6BQ6GA has a larger bottle, but otherwise biases identically to the
6BQ6GTB. As for sonic performance, the 6BQ6GA may have a slight sonic
edge, but I'm not 100% certain of that. Either version sounds great, and will
stand up to the spec busting for long service life in audio amplification
applications, which is what we're interested in here. Of course, audio
amplification is not the same as horizontal deflection duty, or "brick on the
key" RF modes (FM, Packet) either.

The 6BQ6 really is a tougher, higher powered, 6V6. The main difference is the
greatly reduced screen voltage spec that precludes Ultralinear, and makes
pseudotrioding problematic. As a pseudotriode, since the screen voltage never
exceeds the plate voltage, you may be able to get away with running at
higher than specced screen voltages without either poofing the screens or plate
current run-away. That remains to be seen, if you're so inclined to experiment
with it.

The advent of TV sets running transformerless power supplies has also led to
the development of *BQ6s that have some odd heater voltages:

12BQ6: 12.6V / 0.6A
25BQ6: 25V / 0.3A

These types were intended for use in series strings, as there was no PTX to provide heater power. That such types
exist is beneficial in case the 6.3V version should become harder to acquire,
and/or the price becomes excessive. Five years ago, you could get 6BQ6s for
under a dollar a pop. Due to inflation, and probably the publication of designs
that use the type, the prices have been going up. Still, it's not like
audiophool expensive, even for quality that's dubious, of finals that do have
that audio pedigree. Availability should not be a problem, since the 6BQ6 was
widely used in a great many brands of wide screen, B & W TV sets.

6AV5 and Other HD VTs:

This type is very close to the 6BQ6 in terms of specs and sonic performance.
The main difference is that the 6AV5 lacks a top cap connection to the plate. This
would be a plus for designers who're leery of plate top caps in audio amps that
potentially expose end users to dangerous voltages.

If you can wind your own OPTs, or don't mind paying extra to have custom OPTs
wound, other possibilities present themselves. As B & W screens increased in
size, new types that could handle the increased current demand were developed.
This would include types such as the 6CB5 and 6DQ5 -- both capable of even
higher output power.

With the advent of color TV, the demands put upon horizontal deflection
subsystems grew even greater: color TV CRTs operate at much higher voltages,
require more cathode current, and also had the large screens and deflection
angles. To meet these demands, some truly awesome power tubes were developed for
increased current sourcing, and power handling.

Click the image to open in full size.

This beast is the 36LW6: Pd= 40W, Imax= 1.75A

It was one of the last generation of Octal HD finals before the advent of the
Duodecar (12 pins) and Novar (9 pins) all glass tubes that began to appear
during the early 1960s. Part of this development was to allow for putting more
tubes in the same envelope. You had a single bottle that might include two RF
pentodes for TV IF amps, or combination power triodes and small signal triodes
for vertical deflection duty, or triode/pentodes for sync sep/horizontal
deflection time bases. FM demodulator/audio voltage amps and power pentodes were
likewise included in the same bottle. It was common to put the HD final and the
damper diode in the same bottle.

The 6.3V version has become extremely difficult to find these days since a
good many were diverted to ham use as RF finals. Even worse was the diversion to
illegal CB linears during the mid-1970s CB craze. These illegal rigs were all
too frequently badly designed, and poofed the finals too quickly. However, there
are quite a few still available if you go with the odd heater voltages, and the
prices aren't exorbitant either. You'll have to either wind your own heater
PTXs, or have them custom wound. The characteristics remain the same. This type
was also designed for operation from a DC rail derived from a transformerless
voltage doubler. The spec sheet mentions that it needs just 280VDC to
work, and the published plate characteristic doesn't extend beyond that
particular voltage. That would account for the variety of heater voltages. This
is a definite benefit for audio design: low load resistance, and the avoidance
of high plate voltages.

Click the image to open in full size.

This looks promising indeed. That's nearly 95W of audio power from a single
pair of PP finals, running conservatively, and with a Vpp=
300VDC. It is helpful that the grid voltage swing isn't out of line
with the more common audio power pents, as this allows for lower distortion from
the front end. A good many power finals for RF use in particular would need to
run at twice -- or more -- the DC plate voltage and likely Class AB2
operation. Getting that much audio power would require PP-parallel operation of
the more usual audio power finals running Class AB1.

The estimated H3 for this loadline comes in at about 0.2%. That
bodes well for sonic performance. Given the low screen voltages, there is
another possible way to use this: screen drive. Use a MOSFET source follower to
drive the screen grid, with the control grid bypassed to AC ground, and used
only to set a Q-Point current. This becomes a type of Class AB2
operation that can boost the output power while maintaining a reasonable plate
dissipation. It is also helpful in that the screen current remains low until the
Vpk is taken to very low levels indeed.

I'm definitely going to have to give this a go some day.
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