~ Quad II C4 & C6 Block Capacitor Replacement ~

QUAD II replacement block capacitor


October 2023 Batch of New Replacement Capacitors for the QUAD II PSU block capacitor C4 & C6 ~ Shown right are supplied in Admiralty grey painted aluminium covers to replicate the look of the original part [which varied a lot]

These drop in replacements fit directly in place of the original TCC 715525 16uF + 16uF 450V capacitor without cutting or drilling or soldering and as shown fitted below they don't look out of place


Fitting is a simple DIY job requiring only a 1⁄4" or 6mm flat blade screwdriver and 4BA or M3.5 nut spinner or socket and a small 4BA or 6mm M3.5 spanner ~ To fit the replacement using the original solder lugs remove only the top brass nuts and washers from each terminal and fit the solder lugs then washers and nuts

Take care not to overtighten the nameplate screws as the plastic may crack ~ Often the chrome plated raised countersunk 4BA screws that held the block capacitor with the name plate rust and no longer look good ~ 4BA replacements are available but if you fit M3.5 which is a similar more common size you can use M3.5 nylon lock nuts and tighten the name plate just enough without the nuts coming loose over time which happens as we get older

All three connections to the capacitor should come from the QUAD II wire loom in line but also check the continuity from the terminal posts to the points E P and GND marked around the choke as shown in the picture below ~ If you need solder lugs to fit the new parts please let me know

Inside the QUADII replacement capacitor block assembly before the cover is glued in place ~ Selected Low ESR 68µF 450V 105˚C electrolytic capacitors and polypropylene capacitors: 68µF for the P section C6 bypassed with 4.7µF 630V polypropylene capacitors and 680kΩ discharge resistor and 2 x 68µF (136µF) for the E section C4 also bypassed with 4.7µF 630V polypropylene capacitor and 680KΩ discharge resistor

The terminals are 4BA brass studs which are soldered to the PCB to provide a good connection and to prevent them turning loose


A pair (x2) of replacement block capacitors are £100 plus postage

Please contact me to check availability and how you can pay as they are hand made in small batches and there may be a lead time until the next batch is made


Output power of the QUAD II amplifier can be increased >20W by fitting larger power supply capacitors with increased peak power for better bass transients ~ Hum and residual noise can improve to >100dB ~ The polypropylene bypass to both sections and the PCB mounting method tend to give an improved sound reproduction from what is already a very good valve amplifier


Below ~ The QUAD II C4–C6 block capacitor replacement and also a replacement Tag board PCB update a QUAD II power amplifier giving it many more years of active service and improved performance ~ The cover of the replacement block capacitor supports the components on the PCB while providing good electrical and mechanical isolation

You may have read that there are limits to the value for C6 set by Mullard in their specification for the GZ32 ~ Most comments on this subject do not take into account that the current drawn by the QUAD II should be no more than 150mA (200mA maximum with QUAD 22 and tuners) ~ The GZ32 data is for 300mA @ 300~0~300 Vrms plus there is the impedance of the HT secondary the reflected primary load impedance in series with each GZ32 section

Although I was concerned about the rectifier dissipation I was more concerned about any increased loading on the more expensive Mains transformer but as it happens neither increases with C6 as high as 100µF and possibly greater ~ The GZ32 current waveforms have both an a.c. and d.c. component and it is the a.c. or the ripple current that increases as the value of C6 is increased but with new modern capacitors the d.c. leakage current is lower

When C6 value is increased the d.c. HT voltage and current may increase slightly provided the QUAD II is otherwise okay ~ Any increase in GZ32 dissipation will be mainly due to a.c. current and should be seen as an increase in the mains input current which will place more load on the mains transformer ~ Measuring the mains input current is a good indicator but don't be surprised to find the input power is more than the 90W quoted in the handbook

For many years I have fitted higher values for C6 and seen no significant increases in mains input current ~ In Europe the mains voltage can be as high as 253 Vrms and this simultaneously increases both the HT and the heater supplies which have a far greater effect reducing valve and the mains transformer life ~ Check the heater supply is not more than 6.7 Vrms and if it is change the mains taps and consider increasing the value of R12 to say 220Ω

If fitting my C4–C6 replacement increases the HT more than about 5% then the original block capacitor may have been faulty or there is another fault on the amplifier ~ With a good condition original C6 the there will be about 15–17 Vrms ripple on the HT supply to the KT66 anodes with no signal ~ If higher than about 20 Vrms then C6 may be low value or the KT66s are drawing to much current which can be checked by measuring voltage across R12

The other capacitor C4 can be made almost any value ~ My replacement block capacitors are supplied with 2 x 68µF 450V 105˚C electrolytics bypassed with 4.7µF 630V Polypropylene for C4 ~ If the QUAD II HT is more than 350V or R12 voltage (between V and GND at the choke) is more than 28V then consider increasing R12 to 220Ω to reduce the dissipation in the KT66s but also check for other faults before doing this

A 'leaky' C2 and or C3 will increase the KT66 current due to positive grid voltage ~ This may be seen as a larger than normal d.c. voltage across R12 ~ With the original C2 C3 or other aged paper in oil capacitors this effect may appear 'weather dependant' as it varies with humidity and temperature and with time in use again as we do as we get older

Leakage can be checked in the amplifier by disconnecting the lead of C2 or C3 that is connected to pin 5 of the KT66 base and looking for a reduction in R12 d.c. voltage but take care not to disconnect the wire to pin 5 ~ If you measure a voltage between 0V GND and the open lead of C2 or C3 and you know the resistance of the meter you can calculate the leakage resistance and current

If you have fitted modern 'poly' capacitors for C2 and C3 ~ or very rare non–leaky paper in oil capacitors ~ but still have a voltage from each KT66 control grid pin 5 to ground then it is likely that the KT66s are 'gassy' and producing grid current ~ The voltage on each KT66 grid should be no more than about 1V when the amplifier is hot

Testing Capaciotor Leakage

With perfect C2 and C3 and the best KT66s the voltage on the KT66 control grids should be about 0.25V with a few mV low frequency fluctuations

The KT66 grid bias resistors R7 and R9 are connected to the junction of R4 R10 and R11 as explained here rather than the conventional 0V

Measuring between the junction and each KT66 grid as shown pictured ~ the ideal voltage should be 0V but in practice it fluctuates 10s of mV but should be no higher than about 1V

If you want to test for KT66 grid current you might just turn the amp upside down and poke around with the probes of your 10MΩ input multimeter but this will not give the full story ~ The KT66s should not be operated upside down for a long time but the grid voltage measurment idealy needs to be made over a period of several hours

Placing the QUAD II on its side raised by blocks under the transformers to keep the rectifier heat away from the work surface you can measure the KT66 grid voltages over a period of time ~ Checking that the voltage across R12 is less than 28V should also be done regularly ~ R12 greater than 30V or KT66 grids higher than 1V indicates a problem


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