Building of 6BM8 amp.

Hiroshi Uda

#‚Pamp. called "Cookie-can"


@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.

œ@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.

œ@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.

œ@Applied product?? on 1988
@Built a 6BM8 ultra-linear single ended amp. for elder son's audio system.

œ@Further test on 1992`1994
@Tried cahthode NFB on the ultra-linear single ended amp. The result was not so good by hearing.

‚Q@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 (ƒÊ=70) with ƒÊ=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. )

Outlook of #2 6BM8 STC amp.


Interior of #2 6BM8 STC amp.

3rd 6BM8 STC amp.

Outlook of #3 6BM8 STC amp.

3rd 6BM8 STC amp. schematic

Circuit schematic of #3 6BM8 STC amp.

End of this article.