~ QUAD II New Mains and Output Transformers ~

Some transformer manufacturers offer replacement mains transformers and chokes for the QUAD II valve amplifier but there are few that make a replacement QUAD II output transformer (OPT) and only one I know which currently (Oct 2020) makes a replacement core that fits the original case (can) and performs better than the original or the new parts fitted in the IAG "quad classic" and its modern variants

The owner of an early pair of QUAD IIs was concerned that a repair to his amplifiers was "not quite right"

Looking at the left hand side amplifier in the picture you would probably think the same thing

I had supplied replacement block capacitors to the "repairer" of these amps and I was interested to see this new mains transformer which was causing the owner some concern

The transformer was made by Majestic Transformers in Poole UK and was sized to fit the original case but it had been bolted directly to the chassis as pictured because it would not fit with the mounting cheeks in place

The 2 amplifiers pictured above are both fitted with my replacement block capacitor and other identical components including new valves ~ The amplifier with the replacement mains transformer now also had a faulty OPT which was temporarily replaced to match a pair I had just repaired for someone else so all 3 were closely matched apart from the new mains transformer and all tested and sounded similar

I had previously experimented measuring the temperature rise of various QUAD mains transformers with the original and my replacement block PSU capacitors and was keen to check this replacement transformer so on top of each mains transformer there are thermocouples held in place with copper tape

After several hours listening while the cores heated ~ the difference in temperature between the original and replacement mains transformers became clear ~ The new transformer was 5˚C to 10˚C lower than either of the originals ~ even with a small cardboard box over it to reduce radiated heat ~ so potting would not be a problem

Considering the condition of the amps and that I had always intended to make a pair of QUAD IIs with new parts for myself ~ I agreed with the owner that I would fit the new transformer cores into the cans from his and other failed transformers and give the amplifiers a face–lift at the same time I painted my own pair

The Majestic output transformer core was checked on my QUAD II test jig prior to being potted ~ Unlike the tar which these transformers were originally potted in ~ modern 2 part resins are very difficult to re–work so getting it right first time is most important when potting with these compounds

Testing was done over a long period to check the temperature rise and as with the mains transformers this was lower than an original ~ These new output transformers also tested better for leakage inductance and have a higher primary inductace giving at least an octave lower frequency response

With the mounting cheeks removed the mains transformer core fits neatly in the original can in the correct orientation ~ Majestic will supply the cores without mounting cheeks and with the lead–outs trimmed and tinned in the correct relative positions if you tell them you want the transformer for potting in the original case

You could mount the base plate to the core using the brackets as QUAD did but I find it easier to remove the base before un-potting the old core

The output transformer from Majestic also fits its can in the correct orientation ~ 90˚ to the mains transformer to prevent any stray magnetic field from it or the other channels mains transformer from coupling to the output

Unlike mains and output transformers a smoothing choke needs an air gap in its magnetic path to prevent the direct current from saturating the core and lowering the inductance

Majestic will supply new chokes with correct air gap but because I was using the original laminations I had to make an air gap with polyester tape

Assembly with clean parts and new cores forces an opportunity to give the cans and maybe the chassis a re-spray ~ Here the parts for my amp are spray painted with etch primer ready for the metallic coat when the bases are fitted

Apart from the earthed mains connector these amplifiers were going to be essentially original apart from using my replacement tag board PCB and replacement block capacitors and point to point PTFE wiring

That was the plan ~ but after a re–design of the Majestic OPT winding which allowed separate and lower resistance tertiary (cathode) windings I decided to use separate cathode bias resistors which could now be fitted from ground to each tertiary connection rather than floating between each tertiary and KT66 cathode

The cathode feedback output stage is a stable design and in most cases will not require a dominant pole for stability especially over 2 stages of gain ~ However series and parallel resonances in the transformer due to leakage inductance and winding capacitance can cause peaks and troughs in the response above about 40kHz and the correct capacitive pole 'roll off' can ensure these are no higher than 1kHz the mid band response

The output connections P Q R S T have been grouped together which is why the link R—S for 15Ω output looks odd in the picture above ~ I wanted to maintain the secondary links so a change to 8Ω could be made if required but the idea of soldering links to change speaker impedance had never appealed to me and so…

Using a double pole changeover with a middle 'off' position and the capacitors pictured gives 3 bias resistor options ~ 10µF||360Ω or 1000µF||10µF||360Ω separate bias and 25µF||180Ω common bias as in the original design but with polypropylene capacitors

The switch used was a high quality (made in Japan RS 317–011) 6A 250V which fits neatly in one of the holes where the original output sockets were once mounted

The open loop low frequency response with only 10µF in parallel with 360Ω is about –3dB at 60Hz which may appear a bit high ~ The calculation takes into account the shunt effect of the cathode output resistance and without it would be about 44Hz ~ The closed loop (with negtive feedback) low frequency response is about the same in all 3 positions but the reduced feedback factor at low frequency in the middle postion has an interesting effect

Being able to change the KT66 bias arrangement in real time without having to use a soldering iron is a nice feature and because there was a hole for another similar switch it was obvious that the speaker impedance taps should also be switched externally ~ This time I used a 4 pole change over switch and paralleled the sections to ensure a low on resistance