Quad II transformer rewind

[Diabolical Artificer]

Re insulation paper, I use Nomex & DMD, see attached. There's are different density's of Nomex - 410 to 994 - https://www.ukinsulations.co.uk/materials/papers-fibres/nomex-papers/ can't remember which one I got last time. I used the 0.05 mm for inter layer insulation & the 0.76mm for covering the outer. I have a big sheet of this if any one wants a bit to try.

I've used fine sand to pot tfmr's in cans, well tamped down. Here in the UK you can get fine dry sand which is used for filling in gaps in block paving. I used modeling/self firing clay to seal it in before fitting the lid, just a thin layer about 3/8" thick.

Andy.

[Richardgr]

There is a lot of interesting information about the foibles of Quad IIs on Patrick Turner's website.

He discusses the taps and possible outputs about halfway down the page.

[kalee20]

I use Nomex 410 as my go-to transformer insulation, too - it looks like paper, cuts like paper, but being synthetic aramid-based rather than wood or cotton fibre, it is tough and good for 200°C-plus. My preferred thickness is 0.05mm almost every time between layers.

And in greater thickness, it makes a good former for if I don't have a bobbin. (Though an accurate winding mandrel is then needed).

Good work to Retailer for this rewind - and sharing the results with us!

As to how to hold in the can - I'd honestly be thinking bake the transformer for 4 hours, 105°C to remove moisture, followed by wax impregnation and potting. Wou can at least melt it and remove if you ever need to. Epoxy would have been my choice but it does have the disadvantage of being almost impossible to remove. Wax does set as a solid, as opposed to pitch which just gets more viscous as temperature drops without ever becoming truly solid - witness the pitch drop experiment.

[Richardgr]

Is varnish an option? I have a lovely old Gresham transformer, and it has this rich, reddish, thick varnish coating.

[kalee20]

That sounds like a tough protective enveloping varnish. It would be excellent on an exposed transformer - but doesn't solve Retailer's problem of how to fix the transformer in the can.

The can of course provides protection itself (as well as screening) - you wouldn't do both.

[Richardgr]

Would a high temperature silicon mastic work? This stuff is used for sealing a rocker cover on an engine.

[GrimJosef]

Originally Posted by kalee20

... The can of course provides protection itself (as well as screening) ...

The original potting compound (bitumen) also damps/absorbs any electromechanical vibration. This probably doesn't matter for the output transformer but there's no point in getting the amp's electronic hum and noise down to 80dB below the music output if you then have to live with irritating buzzes and hums from the mains transformer, potentially amplified by resonances in the hollow undamped metal can. I've come across more than one hi-fi amp (not Quad IIs though) rendered literally unusable by a buzzy mains transformer.

The potting compound also plays a thermal role. I don't know for sure but I suspect that the bitumen causes the transformer to run hotter than it would if it were mounted 'free' inside the pretty much sealed can.

Quad used wax (beeswax in fact) to pot the few-kV voltage multiplier in their ESL57 electrostatic speaker. Again that has the very useful feature that it can be melted away if you need to work on the electronics. I fear it would liquefy at the normal running temperature of the Quad II transformers though - they're heated both by the currents flowing in the windings and also by the very close proximity of the KT66s and GZ32. If the OP does choose wax then I'd advise picking one with a suitably high melting point.

Cheers,

GJ

[retailer]

When trying to run some basic tests on the transformer I found one of the secondhand EF86 9 pin sockets caused a bad crackle through the amp - my fault for not even cleaning the socket, I have a really fancy ex dental practise ultrasonic cleaner but didn't bother to use it - the EF86 has to be in just the right position or the volume drops by half - I still went ahead with the basic tests though - the amp is my cobbled up version of the Quad II, HT voltage is a bit down at 310V but it still managed a fraction over 11volts into 8 ohms - almost 15watts at the point where clipping started.

Frequency response with sinewave at just on 2 watts into 8 ohms is more or less flat from 20Hz up to 50Khz the scope screen shots show output voltage 4.18V rms (approx 2 watts) at 20Hz and 4.07V rms at 52Khz - I didn't try for any higher frequency.

I also did some square wave testing - the 1Khz scope screen shot was badly out of focus so I haven't included it.

[kalee20]

Originally Posted by GrimJosef

Quad used wax (beeswax in fact) to pot the few-kV voltage multiplier... I fear it would liquefy at the normal running temperature of the Quad II transformers though - they're heated both by the currents flowing in the windings and also by the very close proximity of the KT66s and GZ32. If the OP does choose wax then I'd advise picking one with a suitably high melting point.

Yes. I was musing over what I'd put - I would not advise wax for the mains transformer, but the output transformer would have been sized according to the constraints of inductance required and tolerable losses - temperature rise would not have been a concern. (Granted, the proximity to KT66 would cause heat by external means). But waxes are available - Google will find suppliers.

Originally Posted by retailer

Frequency response with sinewave at just on 2 watts into 8 ohms is more or less flat from 20Hz up to 50Khz the scope screen shots show output voltage 4.18V rms (approx 2 watts) at 20Hz and 4.07V rms at 52Khz - I didn't try for any higher frequency.

I also did some square wave testing - the 1Khz scope screen shot was badly out of focus so I haven't included it.

The performance you see is credible and satisfying! It would be interesting to compare with an original Quad, particularly for the square-wave response - but in the meantime, I reckon you can give yourself a pat on the back!