~ The QUAD II Valve Amplifier information ~

Dimensions ~ 13" x 4 3⁄4" x 6 1⁄2"

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

KT66

KT66

The exposed unused tap positions are live at dangerous mains potential ~ These 4 holes can be plugged with 4BA Nylon bolts or other insulated parts that fit

Replacement Resistors

R2
R3
R4
R5
R6
R7
R8
R9
R10
R11
C1

Almost any modern resistors can be used to replace the ERIE resistors in the QUAD II because they DO NOT affect the sound of the amplifier unless faulty due to age or the wrong value is fitted ~ There is however one subtle point often missed when replacing resistors ~ They have a Voltage rating as well as the 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 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.

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

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 original output transformer also 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'

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) nearer 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 stabilise the amplifier but if you cannot measure this maybe use 33pF

Feedback from point Q on the output transformer via R11 and R10 reduces the open loop gain of the QUAD II by about 18dB 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 but 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 faulty

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 saw 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

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)

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

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 determined 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

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

Most concerns from DIY enthusiasts about the output transformer are that one primary winding resistances are 150% out of 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

It follows the original QUAD 1 Main amplifier design style ~ see QUAD 1 schematic ~ and also indicates the relative phase of each winding section unlike many later transformer icons

The first winding L1 {closest to the core} and all other odd numbered sections forming the primary are ≈477 turns of 37 SWG enamelled copper wire in 3 layers of ≈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 37 SWG giving the the 1:9 cathode feedback

Even numbered winding sections forming the secondary are each single layers of ≈51 turns of 23 SWG wire separated from the adjacent odd numbered windings by 2 layers of 0.16mm oiled paper on each side ~ The outside of 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 also helped by the fact that only one third of the output winding for the 8Ω setting or one half for the 15Ω setting are outside the negative feedback loop so a short circuit or capacitance on the output will not completely shunt the feedback path

The majestic QUAD II OPT replacement core without can or potting weighs more than the original QUAD potted OPT ~ mainly due to more iron in the space efficient core which helps it achieve at least 1 octave lower frequency response due to double the primary inductance but no more leakage inductance than the originals at their best

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 and many were made toward the end of production

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 and R4 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 my HP35670A FFT analyser and the results in HP/Agilent SDF format can be found here

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%