Quad II Forty re-engineering.
Everyone interested in tube
amps may have seen old 1950s Quad-II monoblocs and listened to
amplified by them. They used
two EF86 and two KT66 and GZ32 rectifier and could make about
15 to 22 Watts with speakers
from 32ohms to 8 ohms. They included the facility for load
optimize the work of the KT66
by means of by altering two links on underside of OPT in the
area, using pliers and a
soldering iron, and a brain that had IQ of at least 110.
There were two possible load
matches - one for 8 ohms and one for 15 ohms.
Many Quad-II were used with
the amp set for 15r0 to power Quad ESL57 which have a benign
impedance variation between
about 33r at low F, 8r at middle F and 1r8 at 18kHz, with an
average of 15r0
for the main audio power band
between 100Hz and 500Hz. In 1950-60, "Normal" dynamic speakers
cones and domes and voice coils
were mostly rated at 16r0 across the AF band, and the Quad amps
them well because the
sensitivity was usually over 93dB/W/M and little power could
make a lot of SPL.
With 16r0 speakers, and OPT
links set for 15r0, the amp produces over about 22Watts in class
because the PLa-a anode load is
about 3k8. If the OPT links are changed to match 8r0, then the
16r0 speakers reflect an RLa-a
load of 7k6 and the 18Watts produced becomes nearly all pure
and you then get the best
music, but with less maximum Po.
But since 1955, most speakers
which people want to use with Quad-II have average Z of maybe
with a number of brands under
that and over that, and average sensitivity of modern speakers
has fallen from
about 93db/W/M in 1955 to
about 87dB/W/M so that today's speakers require 4 times more
power to reach
the same SPL as in 1955. The
benefit is that the modern speaker gives far less speaker
distortion and gives a
flatter F response and far
less box coloration and far better bass. But of course you have
to pay more for this
deal because many speakers on
the market are garbage for poor men whose wives hate big floor
Consider a 3 way well made
full range hi-fi speaker with all 3 drivers having Z at 6r0.
Crossover filters often make
speaker Z low at the crossover band, so 6r0 may reduce the Z to
3r0, and those
speakers should be rated for
When powered by Quad-II, and
with OPT links set for 8r0, the reflected load to tubes becomes
with very high winding losses.
Then the Po has a very low amount of initial class A, high
noise injection from
poorly filtered B+, and damping factor becomes lousy. Po max is
In 1990s, the nostalgia about
old Quad hi-fi gear remained intact, with most people unable to
be rational about
their assessment of the
integrity of Quad designs, including the Quad tube gear. Money
could be made
by selling the nostalgia, and
it could be helped along by including better performance, rather
like re-marketing of
the Mini Minor motor car, which
was a little buzz bomb in 1975, but in later years became a much
small car with Mini Minor
The amp making capability of
Quad was sold to the emerging Chinese entrepreneurs and they
Andy Grove to design a more
powerful version of Quad-II which has two 6SH7 input /
drivers and two
KT88 plus a 5U4 rectifier.
This new monobloc was called
the Quad-II-Forty, because down a steel hill with wind behind
it, it can just
The new amp had a schematic
virtually identical to the old schematic of 1950s, which goes
back to the Quad-1
amps by Peter Baxandall in
The Forty chassis size is
about +12% on each dimension of the old Quad-II. This meant the
cover boxes could contain
better PT and OPT, and indeed they do, but it seems the boxes
were made before
PT and OPT design was
finalized, and when that was the PT and OPT didn't fill
the boxes. Inside the PT
box there are two of the small
PSU electro caps. But anyway, I found the Forty made up to 32.5
when tested with a continuous
sine wave, and very little more than what can be had if KT88 are
of KT66 in old Quad-II. The
extra Forty power was expected with KT88. In fact, with more
clever minds, the chassis could
easily have been slightly smaller to still have plenty more room
to fit much
better PT and OPT without
wasting space inside boxes.
Most shortcomings of old
Quad-II were preserved in the new
Forty, and I could easily see why I had
been lumbered with samples to
repair which had not been used very much since purchase new.
The new Quad-II-40 had horrid
quality printed circuit boards and I concluded it suffered from
prototype upgrading before
production. There should have been far more criticism and
correction of shortcomings
before mass production, but the
Chinese like to make short cuts.
The paint job made the amps
look well, and metal work was nice, but tube socket quality is
atrocious, and when you analyze
what Chinese did, and what they
didn't do, a simple repair could only achieve a little, and to
get the best from these amps
one has to remove the PCB and
all parts and start all over again, like one does with the old
Quad-II from 1950s, which are
full of quality invented by the
Quad company accountant, Bertrand Parsimonious.
When you take off the bottom
cover of Quad-II-40, you see the PCB is crammed in some places
and not other and there
are hot running 390r cathode
resistors swaying in the breeze off long leads. The more you
look, the bigger the frown you get.
Now this amp had smoked and
blown a fuse a few times and made noise. The Quad-II-40 does
R&C networks for each KT88
cathode. The caps were tiny and barely rated well enough and I
with the brown colored electro
caps at the far rear of sub chassis space. The original green
cathode R = 390r,
rated about 7Watts, and in
theory should not get too hot even with Ik = 100mAdc, with heat
But you can see how one 390r
has begun to turn brown because of heat, and you can't hold a
finger on them
while operating normally.
Quality of the pin gripper
forks in ceramic tube sockets from 1990s from China was just
atrocious and not
anywhere near as good as NOS
McMurdo sockets. Some Chinese made tube sockets are now
excellent in 2014,
but before you buy any, check
they are OK by buying ONE, then testing with tube in tube out to
see of pin gripper
function is as good as NOS.
Notice the thermistor used to
slow down heating of the 5U4 directly heated cathode. This has
high resistance when cold,
so the 5U4 cathode heating
takes more time than the usual 3 seconds to begin emission.
There are also 2 paralleled
1r0 green resistors in
series between cathode winding and cathode because the winding
is 6.3Vac, not 5.0Vac.
The PT did not have a
dedicated 5.0V rectifier winding, which suggests that the PT was
chosen from "about right"
cheap generic stock. Probably
the OPT is also from generic stock. But the build quality is
better than some other much
worse Chinese amps. Total
production cost to the Chinese is probably < $200 using what
is virtual slave labour,
yet you pay many thousands for
a pair in the shops in London or Sydney.
The 5U4 works OK for a pair of
KT88, and the high Ra of 5U4 gives enough series resistance to
cannot be excessive after KT88
have begun to conduct some 15 seconds after turn on. The slow
turn on behavior
for 5U4 is not really
necessary; and B+ soars quite before KT88 warm up. Hence the
need for the pairs of el-caps in
series for B+. There is ZERO
NEED for a tube rectifier, other than to satisfy idiot nostalgia
enthusiasts who like to pay
for 1955 technology which does
nothing to improve the music. Sure, the audio amp tubes do work
well for music, but the
tube rectifiers give ZERO
positive contribution. Silicon rectifiers allow for far more
reliable working without the heat
wasted by the tube rectifier,
plus the Si diodes allow lower Vac for HT winding with a doubler
or bridge and far better
natural Vdc regulation than any
tube rectifier can offer. The use of high value electro caps is
then possible which allows
very low ripple in B+ supplies.
On the right hand end of PCB,
there are two small size electro caps in series poking down
under PCB into spare space
inside PT box. If these caps
need replacing, the PCB board must be lifted out to get access
to el-caps and repair is
hugely difficult when it
should not be. The box for the choke on chassis top has L >
10H and Rw = 375r, and this acts
to filter the fixed B+
applied to screens of KT88, and for B+ of input stages, just the
same method as used in old Quad-II.
The choke box is bigger than
it needs to be, and the anode B+ was not well filtered, also
like old Quad-II, so best class
AB operation isn't possible.
Fig 2 shows what I ended up
doing with two Quad-II-40 monoblocs.
PCBs are removed to rubbish
bin. Connector strips installed, using 10mm x 8mm hardwood rods
with 4guage c/s brass plated
cupboard hinge screws as
terminals at 10mm c-c. The soldering heat cauterizes the timber
and releases pressure in 2mm drilled holes.
But screws remain well fixed
and in 500years integrity will be fine. The wood strips are well
varnished. Wima caps are glued
to chassis with Selley's
Silicone 401, acetic cure, good for 200C, and to last
indefinitely. All tiny sized R were replaced with 3/4 or
1W metal film. Wima
polypropylene coupling caps were used to replace existing which
could be polyester, or goodness knows what.
The input V1, &
driver V2, V3 have entirely different schematic to original and
produces at least 1/4 of the THD as the original
amp. I added an extra choke for
PSU to filter B+ applied to OPT CT in anode winding. While 2
original el-caps were retained,
others with larger C values
were added. DC is applied to input tube heaters.
There is an active error
protection board added, left hand side, and its auxiliary PT for
protection is at rear right hand
side. near the added black
painted choke for B+. Notice the 1.6mm dia copper wire 0V rail
above tube sockets.
Fig 3 is the new amp
Notice the revised operating
conditions for KT88. Originally, the KT88 have quite high Ia =
100mA and Ea = +360Vdc,
so each KT88 has idle Pda =
36Watts which is getting up and one may find one might idle at
37W and the other at 33W
if the tubes are not matched.
I have Ea = 375Vdc, and Ia at 68mAdc, so Pda = 25.5Watts. Rk =
630r, not the original 390r.
The KT88 and tube rectifier
are very happy with my easier working conditions.
In Australia, the mains voltage
can often measure 250Vrms, and this makes the HT voltage higher
and hence B+
higher than it should be, so
the Iadc in KT88 is higher, so the product of Iadc x Ea is a
higher than wanted number of Watts
and KT88 can run too hot. The
screen voltage is NOT regulated. I thought of adding shunt
regulation of Eg2 which could be
at +350Vdc, about 50Vdc below
the anode B+ voltage. Its not absolutely necessary for what will
always be mainly be
use in pure class A1.But the
lower the Eg2, the lower the Ek needs to be, so Rk could be less
for the same value for Iadc.
KT88 have high Eg2 ratings, but
that does not mean the Eg2 must be as high as the Ea.
The owner had purchased a box
full of NOS 6SH7. I tested over ten of them and found half
were gassy and noisy,
or highly microphonic or had
all 3 defects. They had been made for the NZ military before
1944 and in unopened riveted cardboard
boxes. Rivets and some tube
pins had turned green with age after storage in a damp shed for
so long. Like so many tubes made at
that time, they cannot be
expected to work well for long or acceptably because of
In pentode mode and used
similarly as in old Quad-II and with very low Ia, 6SH7 do not
offer better performance than the EF86.
The 6SH7 has outstanding gm
when Ia is say 5mA+, hence very high gain, but is low gm with
low Ia. But there is a trade off between
low gm and high RLa and high gm
and low RLa. The high RLa between anode and B+ of 180k limits
the possible Ia to about 1mAdc
where gm is low. The tube data
curves show gain of about 100 is possible with resistance
loading. The pentodes in original are set up for
paraphase to make two phases
for KT88 grid drive.
Much better performance is to
be had with 6SN7 as an LTP to replace one 6SH7, and wiring
the remaining 6SH7 as a triode. The 6SH7
makes a very nice triode, with
low Ra and gain = 25+. But I only used the 6SH7 in triode
because the owner had a few good ones
among those he'd bought, and
among those already in the amps as purchased - which were red
painted NOS from maybe 50 years
ago. The circuit would work
better with a paralleled 6SN7 used instead of trioded 6SH7 for
V1 input. Gain of 6SN7 would be about
4dB less than 6SH7, so input
voltage would be about 1.2Vac with the amount of global NFB kept
So if anyone wishes to improve
Quad-II-40, DON'T use any pentodes at input or driver, use only
a suitable octal triodes and the most
suitable is 6SN7. You could
always remove the 2 input octal tube sockets and install 9pin
mini sockets mounted on a small
round sub-plate under chassis
top. This would look very well, and look like no mod had been
done. Then 6CG7 can be used
and these give excellent
performance which is at least equal or better than good samples
of 6SN7. But ECC99 can be used for
V2, V3, and maybe 12AU7 for
V1, the 9pin tubes are more available.
In old Quad-II amps I have
tried using 6BX6/EF80 instead of EF86. With careful setting up,
the 6BX6 can have lower RLa and
much higher Iadc, and hence
higher gm which puts their operation further into the linear
region of class A signal pentodes.
And I have use the pair of
pentodes in differential mode with common cathode taken to -350V
rail Input is to one 6BX6 grid,
and GNFB is to the other grid,
and this works with 1/2 the THD of the Quad arrangement. While
this is acceptable, and better
than original Quad, the use of
2 triodes as I have them in Fig 3 is the very best which can be
The anode load for V1 triode
could be a CCS using MJE350, but the resulting THD reduction and
gain increase is marginal.
Maybe better sounding though,
so keen DIYers will use the extra CCS.
The V1, V2, V3 as I have the
use much more anode current than original 6SH7. Notice Rg for
KT88 at 120k, so bias is held down
without effects of positive
grid current at idle. The original amps had Rg = 470k = too
Notice my usual critical
damping networks needed for unconditional stability, see R8
& C6, R8 & C7, R28 & C15, R10 & C8.
Old Quad-II OPTs had quite
high Rw and high winding losses, and needed critical damping
networks. Every old
and recently made tube
amplifier which found its way to my bench has needed adjustments
of critical damping networks to make the
amps stable for whatever loads
can be configured with L, C and R or with no load connected.
OPTs in Quad-II-40 are better than
in Quad-II, with less Rw, but
the networks I have are necessary for unconditional stability.
I always use more R&C
stability networks in all old amps which are often designed by
designers who believe shit does not happen.
Fig 4 shows where I have
retained the pair of 82uF (C11, C12) originally used to make
41uF after 5U4 rectifier.
R18 & R19 make 41r between
5U4 cathode and top C11 to limit peak charge current in 5U4.
Vripple at C11 with 161mAdc
total = 8.6Vrms. This is filtered down by L1 4H + C9 & C10
so Vripple at OPT anode CT =
24mVrms, much lower than the original amp.
The rest of PSU needs no
explanations about its integrity.
Fig 5 tells most people very
little because most have no idea how to interpret the graph
The two solid dark line curves
show levels of Po at 1dB below clipping at the two available
Look along the bottom axis for
any speaker load value, say choose 8r0.
Then go vertically up from 8r0,
and you intersect the 4r0 outlet curve at 26Watts and 8r0 outlet
at 32 Watts.
Notice that the maximum Po for
4r7 is 32 Watts for both curves. But the use of 4r0 outlet will
DF, less THD, and better
tolerance of Z less than 5r0.
Nearly everyone with say 4ohm
speakers will plug the speaker cables to the two terminals
labelled "Com" and "4ohm."
With 4r0 and music, drum beats
and short duration signal peaks will give maybe 38Watts maximum
there is any chance of B+ sag
and Ek rise which occurs in class AB amps with cathode biasing,
due to long time
constants in B+ rail and
cathode R&C networks.
There are 10Watts of pure
class A available before the amp works in class AB1. All the ppl
I know use less
than 1 Watt average from each
channel for average SPL of 87dB total with two channels with
or rock or jazz. So each
speaker makes 84dB SPL
If speakers are rated for a
rather low 87dB/M/W, then the each amp need only make 0.5Watts,
and total Po
from both amps = 1.0Watt
average of 87dB SPL.
Both amps can generate 109dB
SPL at 80 Watts total. It is more than enough headroom room for
for most people, most of the
time, ( but maybe never enough for teenage sons trying to
impress their friends ).
If this all seems a poor
outcome, get a pair of 100W amps which will make 200Watts total
and give 110dB SPL.
But you may damage your
hearing, which cannot be fixed. Music is about what sounds
pleasurable at SPL
between say 70dB and about
In 1960, few ppl ever needed
more than 12Watts from a pair of KT66/6L6/807 in triode mode but
modern less sensitive speakers
the 40Watts is needed.
You can conclude that 4 ohm
speakers used on 4ohm outlet can give excessive levels.
What happens if 8 ohm speakers
are used at "com" to "8 ohms" outlet? The same SPLs and damping
and distortion occur as using
4r0 speakers at com-to-4r.
Most people will find using 8
ohm speakers plugged to com-to-4r will give adequate headroom.
The graph certainly indicates
26W max from each amp so total of 52W so you get about 102dB SPL
The amps work mainly in class
A, the damping factor is doubled, THD halved and winding losses
I would conclude that you only
should use the com-to-4r terminals which should power most
speakers between 3
ohms and 12 ohms, which these
days means all modern speakers made after 1970. The 4 ohm outlet
the most pure class A power
possible for most speakers which will please most owners.
The 8ohm outlet probably best
suits old speakers of 16r0, and will power ESL63 and other later
Quad ESL models.
ESL57 have stricter limits on
applied voltage so 4ohm outlet may be best.
Speakers of 4r0 MUST NEVER be
plugged in to com-to-8r0 outlets.
This can lead to your amp
becoming seriously damaged.
The Quad-II-40 has OPT ratios
For com-to-8r0, TR = 22.7 : 1,
ZR = 515 : 1, so for 8r0 load, the load for KT88 RLa-a = 4k1,
for class AB.
For load of 16r0, RLa-a = 8k2,
for mainly pure class A.
For com-to-4r0, TR = 32.0 : 1,
so ZR = 1024 : 1, so for 4r0, the load for KT88 RLa-a = 4k1, for
For load of 8r0, RLa-a = 8k2,
for mainly pure class A.
With 4r0 speakers, you cannot
obtain more than 10.2 Watts of pure class A.
There is no way you can alter
the OPT windings to give an outlet for Com-to-2r0
Please make sure YOU don't get
confused by these figures.
When in doubt about the
impedance of your speakers, ALWAYS ONLY use the com-to-4r0 amp
For driving loads of 2r0, you
need a speaker matching transformer made by Paul Speltz at
I have been able to alter old
Quad-II amps to make almost the same amount of low THD power as
the more recent
Quad-II-Forty, see the page
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