~ The QUAD II Valve Audio Power Amplifier ~

QUAD II Specification   (Figures for final factory test from last booklet)
Power Output
15W throughout the range ~ 20Hz to 20kHz
Often Exceeded by a few Watts unmodified ~ Up to 20W with larger C6 or new output transformers or for a while with mains voltage tap incorrectly set
Frequency Response
Within 0.2dB 20Hz to 20kHz ~ Within 0.5dB 10Hz to 50kHz
Often worse than 0.5dB at >35kHz and distorted at <25Hz ~ The spec. does not mention the power output for this test which is likely to be <12W
Distortion (measured at 12W output)
Total of 3rd and higher harmonics <0.1% @ 700Hz ~ High order harmonics alone <0.03% @ 700Hz
Mainly dependant on EF86s V1 and V2 ~ The figure below suggests the 2nd harmonic is highest which is often the case and why QUAD IIs sound the why they do
Valve Mismatch up to 25% (introducing 2nd harmonic) not to cause distortion to exceed 0.18% >0.2% THD at 700Hz/1kHz is common even with good ~ so called ~ matched valves ~ Refers to EF86s and or KT66s ?
THD at 50Hz not to exceed 0.25% This is optimistic and more often >0.7% ~ But still much better than the "QUAD II Classic" and later reproductions made by IAG
Sensitivity: 1.4V rms for 15W output
Load impedance: 1.5MΩ in parallel with 10pF
The input is d.c. coupled to V1 which can cause problems with pre–amps other than QC22 due to a d.c. voltage on their output
Background (noise floor)
–80dB below 15W reference
Just about met when limited to audio bandwidth but can be better than –100dB with higher value and better quality capacitors for C6/C4
Output Impedances (15Ω and and 7Ω)
Effective output Resistance: 1.5Ω for 15Ω output setting
This is actually the d.c. resistance of the output winding and has little bearing on "damping factor"
Power Supplies (Input)
200-250V a.c. single phase (or 95~125V a.c.) 40~80Hz
90W consumption excluding control unit and tuners etc.
Input power very dependant on mains supply ~ When set 250V tap in Europe could be >125W ~ affected by "modern" KT66s and old leaky C2 C3 or C6
HT and LT supplies available for external equipment Via the 6 pin Jones plug ~ 0V on pins 1 and 4
340V d.c. @ 40mA to control unit (often higher on UK mains) HT on pin 3 of Jones plug ~ current is limited to about 40mA by L1
6.3V a.c. @ 4A (centre tapped to 0V) Heater supply on pins 5 & 6 of Jones plug
Weight ~ 18 1⁄4 lbs ~ 8.3kg

Dimensions ~ 13" x 4 3⁄4" x 6 1⁄2"

see picture opposite ~ click for large ~ do not fit in a cabinet like the manual suggests ~ you can not admire them and most likely damage them due to overheating

Valve line up
V1 Driver
or Z729
or 6267
V2 Driver
or Z729
or 6267
V3 output


V4 output


V5 Rectifier
or 54KU
or 5V4G
The following information references component numbers from the QUAD II Valve Power Amplifier schematic drawing 11175 issue 1 which you can download from this list
Like many amps made over 40 years ago the QUAD 11 mains input Transformer has taps for a range of input voltages between 200V and 250V or 95V and 125V with "US" Transformer spec. 1008
To determine the correct input voltage tap to use measure the a.c. Heater Voltage ~ This is best done and safely by measuring the 6.3V a.c. across the transformer terminals and not the 5V GZ32 Heater which is at the HT potential >340V d.c. ~ Each 6.3V terminal to chassis should also measure 3.15V a.c. ~ The correct tap is the one which gets closest to 6.3V a.c. and then then other voltages for HT and 5V should be correct providing there are no faults

Replacement Resistors

Picture: QUAD Acoustical II old tag strip

Tag board from a QUAD II Valve Amplifier as it left the factory

R2 1MΩ ±10% ERIE type 8
R3 1MΩ ±10% ERIE type 8
R4 680Ω ±10% ERIE type 8
R5 180kΩ ±5% ERIE type 8
R6 180kΩ ±5% ERIE type 8
R7 680kΩ ±10% ERIE type 8
R8 2.7kΩ ±10% ERIE type 8
R9 680kΩ ±10% ERIE type 8
R10 100Ω ±5% ERIE type 109
R11 470Ω ±5% ERIE type 109
C1 0.1µF ±20% 150V HUNTS A300

If you need Resistor and Capacitor colour codes look here

Almost any modern resistors can be used to replace the ERIE resistors in the QUAD II as these resistors DO NOT affect the sound of the amplifier unless faulty due to age or the wrong value has been fitted ~ There is however one subtle point often missed when replacing resistors ~ They have a Voltage rating as well as a Power rating and in Valve Amplifiers and other high voltage designs the Voltage rating can often be exceeded before the Power rating

I used to fit UK made Welwyn resistors but they proved very unreliable and Welwyns response to the problem was not good and the problem never addressed ~ I now use other brands of 2W or 3W metal oxide resistors ~ especially for R2 R3 R5 and R6 which are about the same size as the ERIE 0.5W and have Voltage rating 500V d.c.

Power Supply Smoothing Block Capacitor C4 and C6
The PSU smoothing capacitors C4 and C6 are housed together in a "standard" round aluminium can which is insulated from and sealed in the rectangular can so these capacitors tend not to dry out as quickly as some other vintage amplifier types but heat and age do eventually damage one or both sections
C6 connects directly to the GZ32 rectifier and is marked +P indicating that it is made with plain ~ un-etched ~ foils which are silver colour similar in appearance to paper in oil foils

The larger mass of foil probably provides better ripple current handling but testing many failed QUAD II block caps shows the +P C6 section tends to fail more than C4 often due to arcing between the foils along the edges

C4 has a much smaller area of etched foil which looks dark grey like most "standard" electrolytics ~ Providing a "polycap" bypass to standard replacement electrolytics gives as good ~ if not better ~ results if you need to replace the block cap

You may have read that there are limits to the value for C6 set by the Mullard specification for the GZ32 ~ Many web based comments about this do not take into account the current drawn by the QUAD II is no more than 200mA and the GZ32 spec. is based on 300mA @ 300~0~300 Vrms ~ Also the series impedance of the HT secondary and reflected primary impedance allow for a higher value

Fitting a GZ34 or similar efficient valve rectifier should give a higher HT which will not be a problem provided the heater voltage and R12 voltage are in spec. and it may actually make the amplifier run cooler ~ Fitting a solid state rectifier replacement even with current limit resistors is not recommended unless the block cap is replaced with suitable capacitors

Coupling Capacitors C2 and C3

As I remark on the schematic C2 and C3 are "bolted" to the chassis to provide a small capacitance to ground which in conjunction with the driver load resistors R5 and R6 help "shape" the frequency response and possibly aid stability of the amplifier ~ provided the cases are connected to each other and the chassis via the KT66 sub plate and are not insulated by paint which can happen after a strip down and re~spray

If you have separated the internal 0V ground wiring from the chassis to prevent ground hum loops as I have done for some of my modifications then having C2 and C3 cases connected to chassis may be a bad idea as the "C to ground" should be to the amplifier signal ground and not the chassis if this is separated from 0V

The QUAD II has overall negative feedback across only 2 stages of amplification and so is basically stable but substitution of valves other than GEC KT66 can cause problems with parasitic HF oscillation which may be prevented by the capacitance of C2 and C3 to chassis ~ or rather the 0V ground ~ and also by the addition of grid stopper resistors close to the KT66 bases when using high gm valves

The frequency response between 10kHz and 50kHz can be affected by the type and age of C2 and C3 ~ If you have a problem with C2/C3 and wish to keep the amplifier near original then consider a better type of "paper in oil" capacitor but beware of the snake oil types sold for more than you should have paid for the amplifiers

The QUAD II driver circuit is deliberately unbalanced and the output transformer has inherent imbalance so fitting matched pairs or expensive 1% or 2% capacitors for C2 and C3 is a waste of money ~ If you could fully balance the push drive so that the even distortion was minimum you would most likely not like the sound

Picture: Hunts L45 B406 paper in oil capacitor
HUNTS L45 B406 0.1µF ±20% ~ Foil and paper in oil as originally fitted in the QUAD II ~ 350V d.c. @ 70°C 200V a.c. @ 70°C ~ C to aluminium case 15pF~25pF (many tested over the years) nominally 22pF
ITT L45 / 406 / T53K paper in oil capacitor
ITT L45/406/T53K 0.1µF ±20% ~ A good replacement type ~ 500V d.c. @ 70°C ~ 250V a.c. @ 70°C ~ C to aluminium case 18pF~27pF ~ Often with blue or clear plastic insulation which needs to be removed for QUAD II
TCC CP175N paper in oil capacitor
TCC CP175N 0.1µF ±20% ~ Brass case with ceramic end caps = "Super Metalpack" ~ 350V d.c. @ 70°C ~ 200V a.c. @ 70°C ~ C to case nominally 20pF ~ Good for humid conditions and always low leakage ~ The best British vintage option
Erie Duolectric WF49 paper in oil capacitor
ERIE "Duolectric" WF49/AF411K 0.1µF ±?% ~ 800V d.c. @ 70°C– C to aluminium case 12pF~15pF ~ Good low leakage choice of paper capacitor ~ Remove clear plastic insulation where fitted
Dubilier 470C paper in oil capacitor
Dubilier Type 470C 0.1µF ±20% ~ Very similar to the Hunts L45 ~ 350V d.c. @ 70°C ~ Often lower capacitance to case 10pF~15pF and just as leaky after a few years in service and when heated ~ Remove clear plastic insulation
Plessey Metalmite paper in oil capacitor
Plessey "Metalmite" 0.1µF ±20% ~ 350V d.c. @ 70°C ~ 175V d.c. @ 100°C ~ This one only tested 15pF to case ~These later 1970s types tend to have lower leakage ~ Remove clear plastic insulation
Picture: Russian made K40Y-9 paper in oil capacitor
Russian K40Y~9 0.1µF ~ 400V d.c. ±10% ~ Foil and paper in oil ~ 8 tried Spread in C to steel case from 22pF to 32pF ~ Slightly wider than the Hunts L45 at 14.3mm ~ A good obtainable PIO replacement for the QUAD II with better leakage than any of the above but shorter leads
Russian K42Y~2 0.1µF ~ 630V d.c. ±10% ~ Metallised paper in oil ~ 12 tried spread in C to steel case from 20pF to 34pF ~ width and length less than the Hunts L45 at 11mm and 36mm ~ leads are short but can be fitted with thin PTFE sleeving to fit QUAD II ~ Green or pink coating needs to be removed
The capacitance to case is not a feature specified by any manufacturer and neither is the insulation resistance to case although those shown above all had a much lower leakage from the outer foil to case than across the capacitor itself

Some of the capacitors have a band around one end ~ The connection to this end is normally the outside foil of the winding which is nearest to the case and will have a lower insulation to case than the other end ~ This is why QUAD placed the band to the grids of the KT66s which are (or should be) near to ground potential

If you intend to substitute a metal cased capacitor for C2 and C3 in the original position consider the capacitance and insulation resistance to ground and ~ if marked ~ place the outer foil end to the KT66 grids ~ If you replace C2 and C3 with insulated body types it may be best to keep them away from the chassis ~ or even mount them across the tag strip away from the heat

With plastic body capacitors you can provide additional capacitors of about 18pF to 22pF from the Grids of the KT66s to ground if required ~ I have lowered the range of these additional Cs as higher than 22pF is often not needed to flatten the response

Feedback from point Q on the output transformer via R11 and R10 reduces the open loop gain of the QUAD II by about 18 to 20dB {≈10 x} and with the stray capacitance of the original C1 and C2 plus the wiring the open loop response is often around 8dB lower at 20kHz {ref: 1kHz} and -3dB @ 7~9kHz which is typical of many valve amplifiers with a single "dominant" pole

Are C2 and C3 okay ?

The simplest way to check C2 and C3 on an otherwise working amplifier is to measure the voltage on the grids of the KT66 valves with a high (>10MΩ) impedance meter ~ If you measure from each KT66 grid {pin 5 see drawing} to ground the voltage should be about 200mV ~ 400mV ~ Not 0V ~ due to the voltage across R10//R11 and the high value grid resistors

If the voltage is higher than 1V this may be due to insulation breakdown in C2 or C3 or the KT66 may be "gassy" ~ A faulty KT66 can be confirmed by removing the valve {after switching off} and making the measurement again without the valve fitted ** If the reading is now okay the valve is most likely on the way out

Measurements of grid voltage will be higher when the capacitors are hot and in very humid areas may be due to dirt on the valve bases ~ With very dirty valve bases a voltage can even be measured by placing the probe of a high (>10MΩ) voltmeter on the insulation between pins 4 and 5 at the point marked "4" on the base material

Pin 4 is at about 340V and its proximity to the grid pin 5 requires clean insulation ~ I have seen 2 cases where household pets have "marked" a QUAD amplifier as their territory ~ I can only assume not powered at the time as the faults were unknown to the owners until they smelt the odour when warm and the flashes from the KT66s ~ In each case replacement of the burnt valve bases was the only cure

** this is perfectly okay ~ In fact the BBC liked the QUAD II because it could operate safely with only one output valve fitted

Resistor R12 is under-rated

The R12 resistor fitted by QUAD is rated at 3W but under the best conditions will dissipate about 3.75W and in some circumstances even more ~ Even if it looks okay it is best to replace R12 with a 5W or a 7W resistor like the TT electronics W22 type shown opposite and at the same time replace C5 with a capacitor adequately rated at a.c. and as well as d.c. (see bad examples in buying guide)

C5 Replacement

As a rule C5 should be at least 10mm diameter and >20mm long ~ The capacitors fitted to early models s/n <8,000 pictured on the left were 47mm long and the 4BA bolts used to mount the choke were made round head so their edges did not prevent the capacitor fitting the clamp neatly

The voltage rating is only 25V although in practice they had low leakage at 30V or more and the original OSRAM KT66 pair gave lower voltage across R12 than some later types

A 22µF to 68µF 63V–100V with good a.c. rating should be fine ~ There is no need for a very high value ~ The –3dB response of C5/R12 with 22µF would appear about 40Hz but this only applies to the "out of balance" voltage across R12 ~ The common a.c. current through R12 should balance out so C5 does little work until there is an a.c. imbalance

If separate bias resistors are used for each KT66 then the value of C5 could be made higher as each half of R12 will have "full" a.c. voltage deteremined by the C5/R12 time constant ~ Keeping the time constant around 40Hz will have some interesting results as the output transformer will be working more in its "comfort zone" and the separated C5s can be made 10µF across each 360Ω R12

My preference for C5 was the Philips 22µF 160V axial with blue plastic insulation which are no longer made so I now use RIFA PEG124RD2220Q 22µF 200V or PEG124PB222VQ 22µF 100V for "back to original" repairs and higher values for some modifications but never higher than 220µF

Sprauge made some good vintage replacements and you may find some new old stock Hunts or TCC capacitors but make sure these have low leakage at 30V

Output Transformer

Most concerns from DIY enthusiasts about the output transformer are that one primary winding resistance is 150% different ~ not balanced ~ This is correct because the windings have the same number turns for each respective section but those wound on the outside of the bobbin use more wire and so have higher resistance ~ Typical resistances measured @ 20°C are shown below ~ At higher temperatures the resistance will be higher

Picture: QUAD II power amp output transformer winding diagram
The winding scheme on the left appears to be accurate for the QUAD II output transformer ~ spec 1003A ~ and has been determined from my own dissections and from information generously supplied by others

The first winding {closest to the core} L1 and all other odd numbered sections forming the primary are about 477 turns of ~37 SWG enamelled copper wire in 3 layers of about 159 turns ~ Between each layer are 2 layers of 0.05mm oiled paper ~ The tertiary windings L7a and L7b are each a single layer of 159 turns of ~37SWG giving the the 1:9 cathode feedback

Even numbered winding sections forming the secondary are each single layers of about 51 turns of ~23 SWG separated from the adjacent odd numbered windings by 2 layers of 0.16mm oiled paper on each side ~ The outside L14 is wound with a few layers of 0.16mm paper to finish

R? is a small ~0.36Ω wire wound resistor as seen in picture below ~ presumably to prevent instability by lowering the Q of L2 which has the lowest resistance ~ Stability with feedback is helped by the fact that one third of the output winding for the 8Ω setting and one half for the 15Ω setting are outside the negative feedback loop so a short circuit or high capacitance on the output cannot completely shunt the feedback

Although the winding resistances are unequal (Y–Z 50% higher than X–Y) the turns ratios should be balanced ~ The approximate resistances and ratios of an unloaded output transformer normalised to 100V rms at 100Hz across X-Y are:

Primary V4 ~ 115Ω to 120Ω ~ X~Y = 100V

Primary V3 ~ 170Ω to 180Ω ~ Y~Z = 100V

Cathode V4~ 16.8Ω to 16.9Ω ~ U~V = 11V

Cathode V3 ~ 16.5Ω to 16.6Ω ~ V~W = 11V

Output ~ 0.55Ω to 0.61Ω ~ P~Q = 7.15V

Output ~ 0.55Ω to 0.61Ω ~ Q~S = 3.58V

Output ~ 0.43Ω to 0.47Ω ~ R~T = 3.58V

Majestic Transformer Co. in the UK make an excellent replacement core that fits the original can in the correct orientation to the mains transformer ~ It has a better frequency response and overall lower distortion and losses than original OPTs and is better d.c. balanced ~ They also supply a complete replacement finished in gunmetal grey metalic paint ~ They also make mains transformer and choke replacements for QUAD and many other makes

There are comments elsewhere on the internet about QUAD making some "experimental" transformers for their early models of the QUAD 11 ~ I think these refer to the variants made for broadcasters and professional installations with Spec No 1010 or higher

Early versions of the QUAD II prior to serial number 3787 had transformers marked ABCDEFGHIJ The output impedance or speaker loading was changed by moving a a flying lead connected to terminal D for 15Ω or C for 7Ω ~ These early transformers can be replaced {ideally as pairs for stereo} with the spec 1003A version where J=Z I=Y H=X G=W F=V E=U A=P B=Q and Output=T ~ R2 and R3 are 680KΩ and R4 is 1kΩ and should also be changed to match the schematic 11175 iss.1

One obvious sign of a non standard transformer is an additional wire from a hole in the base plate near the X post and a number higher than 1003 written by hand ~ These variants for 100V line Public Address or AM Modulator Drive outputs have no connection between P and Q and have isolated windings Q~S and R~T which have resistance in the region 20Ω to 30Ω be careful DO NOT connect these directly to loudspeakers

I have made measurements on a typical unmodified QUAD II using a HP35670A FFT analyser and the results in HP/Agilent SDF format can be found here

The link will open a directory where you can download the HP35639A SDF data program "DataView.exe" and the pdf manual ~ When opened in HP35639A the files can be viewed in several ways with active markers TRAC1.DAT ~ Frequency response of QUAD II from 50Hz to 50kHz at ≈10W output ~ Bode plot with gain and phase

TRAC2.DAT ~ FFT spectrum analysis of QUAD II with 1kHz tone at ≈10W output ~ THD ≈0.14%

TRAC3.DAT ~ Open loop Frequency response of QUAD II from 50Hz to 50kHz at ≈10W output ~ No feedback

TRAC2.DAT ~ FFT spectrum analysis of QUAD II without feedback ~ 1kHz tone at ≈10W output ~ THD ≈1%


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