The author thought the 6BM8 as a popular tube for radio reciever for home-use, forty years before.
After the author utilized this tube as a Super-Triode-Circuit (STC) amp., It becames one of much familier tube.
● First meet on 1958 First time the author met the 6BM8 on 1958, which was installed in a monoral radio-LP
record player set as a push-pull amplifier. At that time, The TV was
mono-chromed, the FM broadcasting was at just biginning stage. Most high
end record player was 3 speeded, most cartrige (so called pick-up) was
not moving magnet type but some kind of electric-crystal one.
● First test usage on 1969 Checked the 6BM8 as single ended amp. and push-pull amp. by pentode connectoion,
ultra-linear, and triode connection to know the basic operation of the 6BM8.
● Applied product?? on 1988 Built a 6BM8 ultra-linear single ended amp. for elder son's audio system.
● Further test on 1992～1994 Tried cahthode NFB on the ultra-linear single ended amp. The result was
not so good by hearing.
２ Challenged the 6BM8 amp. again on fall of 1996
The author already tried the STC amps. by the version 3 type (by the classification
of Mr. Kamijyo, Please refer the "Link Section" of the master home page)
for 807 (1996/3), EL34 (1996/5), 6V6GT/6L6GB(1996/6).
However the author had no experience to build direct coupled type STC, called
version 1 type. Then, tried this type of STC on the 6BM8 amp.
2.1 #1. 6BM8 STC amp.
On the new year vacation of 1997, the author choosed a "cookie can" as the chasis of the 6BM8 amp.
The tube configuration was 6AU6 (first stage amp.) -16A8(triode:driver) -16A8(pentode:final stage power amp.)
On the rid of the can, placed all required parts.
On the upper surface, placed B power-supply transf., fuse holder,
power-switch, tubes, variable resister for the Voltage adjustment, speaker-terminals,
signal input-jug, AC cable and rubber bushing so on.
On the reverse side of the rid, placed heater transf., filter capacitor,
and accomodated the wiring and so on.
The author tried and compared several voltage amp. tube instead of 6AU6. such
as 6CB6, 6CY5, 6DK6, 6EW6, 6AK5, 6AS6.
Then choosed 6AK5, which generated the most clear sound.
Interior of the "cookie can"
following is the circuit schematic of #1 6BM8 STC amp.
2.2 #2 and #3 6BM8 STC amp.
The author tried/compared the driving triode of the 6BM8 which is equivalent
to 6AT6 triode (μ=70) with μ=100 triode like as 6AV6 or 12AX7/2, built
#2 6BM8 STC amp. configured with 6AU6-12AX7-6BM8 (pentode only).
Then built #3 6BM8 STC amp. to try transisterized first stage
amp. configured with 2SC1775A -6BM8(t) -6BM8(p). To reduce the load
impedance and increase the capability of the power stage; tried to
connect two of small output transformer, which primary windings are in
parallel connection, and secondary windings are in series connection. (See
the circuit schematic of #3 6BM8 STC amp. )
