INTEGRATED AMP WITH 6CM5.
Informal chat about the
development, use of the 6CM5 or
and its Ra, gm, µ best
triode operating points.
can be used as
direct replacement for 6CM5 but without top cap.
Schematic for 23 watt channel,
for power supply with notes and 2006 picture of the amp.
What the heck is JBS-1?
It stands for Junk Box Special One. An old bloke who was to move
house to apartment sold me a power tranny and two
and a choke which they didn't find the time to use in a
hi-fi project in 1960.
The iron was meant for a a stereo amp using a
pair of 6GW8 in UL PP,
and giving about 12 watts with wind behind it.
The transformers were
all Ferguson branded which were made in a Sydney
suburb of Chatswood back before the 1960s when Australia still
electronic products. Most Ferguson OPTs were definitely
of their "lo-fi" range.
They produced three OPT model ranges, lo-fi, medium-fi and hi-fi
OPTs rarely ever selling because they'd cost twice the lo-fi price
anyone took hi-fi seriously, or were too poor, or both. The iron
cores are low
with high losses which gave high LF distortion in OPTs
and lots of core heating
in their power trannys.
I though a little trio-delic amping would be
appropriate and proceeded to test
the iron. Choke was good, but not many Henrys and I eventually
Power tranny ran scorching hot after 2 hours unloaded, Hmm,
crummy old iron
in this one, and not enough turns per volt. So I
stripped it apart, added lots of other
old iron I had laying about from
the junk and re-wound the tranny with more
turns per volt, to give lower Bac.
Ah, it run not so hot now, and it would cope
with 4 x 6CM5 at the same
idle current as in 6GW8 in UL, but with higher
Ea for triode.
The OPT were low fidelity types. Its always been my bad luck to
from guys who were hi-class young misers back in 1960, and who
almost zero idea
about what quality was, or that wide band OPTs were necessary
for hi-fi. Nevertheless, although the bandwidth of the
OPT wasn't good, and with
saturation occurring at about 35Hz at 12 watts into 8ka-a, and HF
off at about
20kHz with the
6CM5 in triode.
Obviously to me the OPT also didn't have enough turns per volt,
The dc resistances were a bit high, and the amount of iron a bit
but heck the
iron was cheap, and I made room on the chassis to increase the
I ever had
time and acquired better
"free" or "spare" OPT.
I was able to stabilize the circuit with critical RC damping
of global NFB was applied.
Usually with any given OPT, if you have the same dc idle current
powerful tubes than the tubes it was designed for the
OPT will cope OK with the
increase in ac signal voltage and the increased signal current,
stressed even with twice the applied ac voltage, giving 4 times
But with twice
the signal voltage applied into the rated load the frequency of
saturation will double, so that instead of saturation
occurring at say 40Hz it will
be 80 Hz at the higher voltage.
Saturation in OPT is a voltage related phenomena.
So while the amp may
have saturation at 40Hz occur at 300 Vrms anode to anode
which is near
12 Watts into 8ka-a, at 2 Watts Va-a is only 126Vrms, so
at about 16Hz, and below any normal music frequencies, so the
suffer at all
because 2 Watts of power is all most people actually use on most
unless teenagers are present at the volume control.
I was asking to get twice the rated power from the old Ferguson
so output voltage would rise
to about 1.4 times the design figures, and although Fsat would
the amp wasn't ever going to be used at high levels.
The 6CM5 triodes would provide greater control of
bass and better midrange
dynamics as well as dynamic range.
6CM5 in triode gives µ = 4,
600 ohms, gm = 6.6mA/V.
These figures mean that the anode voltage can swing quite
by the Ra for Eg = 0V.
I easily got 23 watts AB1 per channel even with such lossy OPT;
losses are 12% or more; I never bothered to check that, but the
Ea at only +375V was more than a
pair of KT66 in triode.
I run Ia = 50mA,
so anode + screen dissipation = 18.7 watts which is ok because
tube don't go cherry red until about 28 watts is reached.
Their low data rating is
because of the TV use conditions.
There are guys who have extracted 200+ watts with such tubes in RF
in class C! I didn't want power lost in cathode resistors with the
bias across the approx 1.1k Rk.
for each tube so for the class AB fixed bias was used.
I had some old 6CG7 laying around to make a driver input and long
inverter/driver. From these old pulls I used the best measuring
At first I made the JBs as just a power amp but later it became
useful to lend out to people curious about tube sound while I
stuffed solid state amps.
want to give it back when I rang to say their solid state amps
Anyway, I thought if such a
"garbalogical" amp could change people's minds
about their horrible budget Cambridge and Creek amps, then what
think with something made with parts which were not junk?
( Then I found out as time went buy,
but ppl's wallets rarely
agreed with their brains ).
So the JBS has earned its stripes, and I
learnt one of the best kept secrets
about the 6CM5 :-
Although it is a beam tetrode it will sound very good as a triode.
When I first built it and compared it to a pair of Quad-II amps I
repaired, the 6CM5 seemed to sound better.
The 6CM5 was
meant for line output in tv sets, and capable of as much
current as a KT88. But anode dissipation
is limited to around
18 Watts in a class A or AB situation, so with two running Pda =
about 15 Watts of pure class A was available if a high RL was
6CM5 could be used as a single ended triode with about 4k anode
about 5 watts with Ea = 375V and Ia = 50mA, and methinks it will
Don't try to use the tube in beam tetrode mode; it is too non
most high gm tubes. But In the 1950-60 era Phillips did produce a
of PA amps for supermarkets in the 1950s and 60s which used
either 4 or
6 in tetrode mode to make 80 or 120 watts respectively. These
set up as low bias amps tetrode amps with Ea = 330V
and low idle bias
current for nearly class B, and they had a lot of
NFB. I had a couple of
these amps given to me and they are only suited for "100V lines"
the OPT has no
to have the secondaries rearranged to match
4 to 8 ohms.
The Phillips amps worked OK because women disappeared into early
shopping mall stores to and didn't re-appear till hours later
and dazed and
having spent all their husbands earnings on appalling junk
they didn't really need - such was the seductive effect
of muzak via tubes!
Ultralinear can be used where the screen taps are at 40% or more.
Ea should never be more than +375V
when in UL or triode, or else the
grid bias needed becomes too negative. I found the tube can
out of control if fixed grid bias exceeds -70Vdc.
So don't be tempted to run them with Ea = 425V or 450V.
Anyway, over the years I have had a lot of enquiries about the
schematic which didn't appear on the last edition of the website;
picture of the amp as a straight power amp appeared.
So due to
demand, here are the schematics of the amp and power supply
and a fresh picture of the amp with the
cover grille removed.
HAVE TOP CAP CONNECTIONS
AT +375V DC POTENTIALS.
USE A WELL
COVER OVER TUBES.
This schematic is a straight forward amplifier which takes
nice triode characteristics of the 6CM5 or EL36 to make a PP
AB amp with around 25 Watts output.
Another tube that could be
used is the very little known 6FW5 which is an
octal tube and exactly
like 6CM5 electronically.
The 6FW5 HAS NO TOP CAP
and this is a blessing because top cap
connections are dangerous
because people leave
them exposed and some
victim will reach over and touch the +375V and maybe its
Maybe there are some stocks of 6FW5 lurking somewhere. There
be many on Oz but would be a lot in the US. Meanwhile, the
would have to do. There used to be a 6CM5 in
every second TV set until HV
transistors were finally able to be made with a reliable
over from tubes the 6CM5 was one of the last tubes to be
in favour of solid state.
Because there is not a really high B+ for the driver stage,
from and because the driver has to produce up to about 47Vrms
possible distortion I took the "dead" grid
of the V3/V4 LTP to -25V because
it was available from the zener string which regulates the
which appears in the power supply schematic.
Note the zobel networks used to stabilize the amp even with
of global NFB. Such zobels tame the gain and reduce phase
shift at HF
where otherwise the gain and phase shift
would allow oscillations at HF.
Depending on the OPT chosen and its shunt capacitance and
inductance, the values of R and C shown will not necessarily
Unless you know how to build an amp with
NFB so it is critically damped,
you will find your amplifier may oscillate, especially with a
on the output without any R load.
C6 & R12, C11 & R28, C12 & R29,
C15 & R31,
and C14 & R30 all have to be trimmed
ensure stability at HF!!!
There is also a LF gain / phase shift correction network with
This should always be used regardless of how much inductance
OPT primary has.
This is very close to the original power supply. If your
transformer does not have a bias winding then you may use a
transformer using one taken from a
canobolised solid state amp. I have a
few old small transformers with a 240 primary and a few
used to make a suitable bias voltage by a voltage doubler or
quadrupler, and then apply RC filtering. The filtering of the
above is a bit
excessive, but I like quiet bias supplies which retain their
charge when the amp is turned off then on again while cathodes
The bias voltage is also shunt regulated but really need not
when the mains voltage rises say 10%, The B+ will
tend to rise since the B+ is
not regulated. However the combined
parallel Ra of all four output tubes is 150
ohms only, so the B+ rise
with a change in mains voltage will not be much,
since the source
resistance of the B+ is probably much more than the Ra
or the 4 tubes
To counter the rise in B+, if the grid bias is also
allowed to increase with
a mains rise then the tubes tend to be biased to conduct less
grid bias. I used to be fanatical about regulating and
bias circuits but it isn't always necessary in
a triode amp. But I also wanted
shunt regulation of the voltage applied to control
the constant current source
Q1 MJE340 for the cathode current to V3, V4. Again, this
if a voltage divider is used to replace the zener string and
value caps of 1,000 uF are used as bypass caps. Such
with humungous cap sizes is legitimate, does not
spoil the music and the
caps for the voltage wanted are small and cheap.
The B+ rail is not regulated. The transistors MJE340 and
BU208A make it
look like it is regulated but what is there is an "electronic
otherwise called a capacitance amplifier but really
what the bjts actually do
is act as a giant emitter follower with the two bjts connected
pair, with very low output resistance and very high input
frequencies. When I first built the amp it
was regulated with a string of zeners
to hold the MJE340 base at about +377 V.
Some years later a friend asked me to lend the amp to him to
try with a
of horn speakers he'd built which were about 105dB/W/M
Unfortunately there was hum to be heard and I altered the
you see above and the hum disappeared. In fact
there is only 10mV of hum
at the bjt output emitter, although ripple voltage at the
input of the
about 11Vrms. So the above active hum reducer acts with an
about 0.001. To get the same amount of attenuation with a CLC
C1 = 50uF
as shown, ( C2 & C3 in series, ) and C10 = 100uF, the
choke has to have XL = 1,000 x XC, and if C10 = 100uF, XC at
= 16 ohms, so XL must be 16,000 ohms so L is 25H which would
10 times more costly than a couple of R&C and two cheap
mounted on some scrap
aluminium for a heat sink.
The mechanism of the hum persisting in the above emitter
because there is some ac collector hum current current since
works into the collector resistance
which is perhaps about 10k. HFE of the
pair of transistors is about 200, so about 0.005 mA flows at
since the base resistance is 1k, about 5mV of hum must appear
The passive filtering of R2 &
C6 and R5 & C8 attenuate the 11Vrms of ripple
from the top of C2 by 0.00011 and in any case the resulting
at top of C8 is phase shifted by nearly 180 degrees and
counters the hum from the collector to base resistance path.
So about 1.0 mA of ac hum current flows in the collector and
current must flow and since there is a 1k series base
there is a
tiny hum voltage at the base. The 1k is needed to allow the
base voltage to be
quickly pulled down by the current in the four
1N4007 diodes in series between
base and output if the output is ever
shunted to ground in a fault.
The action of R8, 2.7 ohms acts to allow a voltage of 1.4V to
across itself when I out = 0.5A approx. If this occurs the
2.5V across the 4 diodes is
exceeded and the base is pulled towards ground
and the excessive current should cause the mains fuse to blow.
The resistors R 2 and R5 which total 300k have a dc base input
so that when 230mA flows in the collector-emitter path, base
0.16mA, thus base voltage
is about 50V below the voltage at the top of C2.
The transistors don't seem to be able to be killed in the
if a short
Complicated? sure is, and much has to be
included in a high voltage regulator or ripple reducer
or else the use of
solid state as slaves to the tubes lets the smoke
out of the devices, and once
it has come out, it won't go back in!
If the input to the collectors is shunted to ground then
could flow backwards through the bjts. This means instant
death to a
bjt if the
reverse voltage from emitter
to collector exceeds about 6V!! So hence the
diode between base and
collector allows any back flow to harmlessly bypass
the bjts. There is a 100 ohm series R4 rated at 10 watts on
is normally about 23v across this R and about 5 Watts is
About 25V is across the
bjt, so 5Watts is dissipated in the bjt.
With an increase in current, the voltage across the 100 ohms
and voltage across the bjt reduces, so short circuits kill the
resistor, not the bjts.
The maximum current
flow possible from the 425V at top of C2 is limited
to 4.25 amps. This is less than the
BU208A maximum collector emitter
current rating. Series pass element regulators should always
Maybe you are not impressed with the idea of enslaving power
to the whimsical current desires from the tubes. OK, use a
B+ instead. See my pages on
power supplies and follow your nose to where
I discuss the benefits of using large capacitances and
to get a CLC input circuit with very low ripple with
470uF, 2.5H choke
and 470uF with diode rectifiers. It means though that
the B+ winding with
a full wave bridge needs to be about 284Vrms. About
142Vrms is needed
for a doubler circuit with the two input caps at
470uF each which need to
be rated for 250V each, but the same 2.5H and 470uF still
can be 450V rated where there is approx 375V across the caps.
Selecting a power tranny with slightly too high
HT winding is better than
selecting one which gives a disappointing low B+ voltage.
Having say +400V B+ at 230mA for the
2323 can easily be reduced by placing
series R between the diodes and caps to be charged to reduce
charging currents in high value electros with silicon diodes.
Using a B+ = +350V will give much less power. When I wind
for such amplifiers I usually have about 4
taps at about 15Vrms steps in from
the ends of the HT winding to
accommodate different output tubes and be
able to adjust the B+ voltage
without using too many series trimming
There is shunt regulation of the anode supply to the preamp
a string of zener diodes at the first stage of the power amp.
This prevents any possibility of LF instability.
There is no active protection on the amp because it is a Junk
and I have a lot of spare 6CM5, and if I fuse an OPT then I
I'll just have to wreck another few Sundays
to make replacement OPTs
which would withstand a short circuited or saturated tube for
trannies would. The wire used in 1950/60s OPT was usually
very thin and prone to easy
fusing. The use of thin transformer
was an unscrupulous capitalist plot to maximize meager company
and have you come back to buy again later.
( But when
transistors displaced tubes in 1960, no wonder people happily
their parked and defunct tube amps. And the strange thing was
although the cost of production of early SS amps was much
they initially sold for very little
less than the tube amps they replaced,
and of course ppl needed two amps, because stereo had arrived
If I was keen I would place 200mA fuses into each cathode
each output tube. These should blow if a tube saturates and
OPT but maybe
not. I am wary of fuses because they don't always blow
when you want them to. See my other pages on active
The amp in 2006. It gave no troubles after 11 years, and in
goes fine. Even
the black felt
pen markings on transformer tape is easily readable.
JBS-1 amp, 2006.
The power transformer at the rear has a 75mm stack of 38 tongue
using questionable core material. Even with the B reduced from
Tesla used in 1960 to 0.9 Tesla
the core still gets quite warm in hot weather,
but not excessively hot.
I have piles of this iron from old trannies I have stripped down
now only use it in chokes.
The damned ugly aluminium "thinge" in the middle of the chassis at
front is an early attempt of mine to farnarkle solid state devices
willing slaves to their masters which
are the tubes surrounding the
( Farnarkling is a Sunday R&D process which means you keep
get something to work and not let smoke out before dinner time.
such things take to dinner time on Monday
to get right. )
The "Thing" is a bit of channel section with a heat sink insulated
mounted inside the channel. The high voltage transistors with B+
collectors are also well insulated in their mounting so they are
insulated from the channel which is at 0V. Gradual breakdown of
between bjts at 400V and metal heat
sinking at 0V is almost inevitable
at some time and I have fixed a few commercially
made amps with
such problems such
as a high end Centrepoint amp.Using TWO
insulation washers and plenty of silicon sealant around the
bjt and where the base and emitter pins project through the 5mm
holes in the sink metal is essential to prevent
little tiny leaks of current
which can destroy a solid state device so
easily. I found the size of the
ripple reducer to be a bit excessive, but at
least it runs cooler than if I'd
been a miser and made it too
Keep the home triodes burning!