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Old 5th November 2012, 11:30 AM   #1
Clintec is offline Clintec  United Kingdom
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Default My QUAD 2 Restoration

Hi I am fairly new to the forum and about to start my first little project which should keep me busy for around a month.

I have purchased a pair of Quad 2 Amps, I decided to go for a pair that needed restoration as at least I will know whats inside them rather than buy suposidly workign ones only to find they need lots of replacement parts inside anyway.

So I decided to start on them this morning, first thing I did was take a lot of detailed picture of them so I could see what they looked like when they were together.

My 2 amps are not that close in serial numbers and I understand that this does not matter, but I will be wanting to test this and make sure they perform within QUAD's specification.

I will not be restoring these 100% original, I will be including a few safer features like chassis earth, and some better plugs and sockets on the exterior, I would also like to move all connections to the rear so I havnt got cables coming in from the front and back.

Anyway to start the post off here are some pics of what I have now:

Amp 1:
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Amp 2
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Old 5th November 2012, 02:11 PM   #2
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It's a long time since I had Quads - believe it or not I once bought a job lot of 26 with line outputs that came from a factory. Yes, they're all sold long ago..... Quad in Huntingdon were rather bemused at a request for 26 output transformers but they did provide them.

I agree with updating the connections. You'll want better resistors - I'm sure the tweakers can advise. But keep all the old parts and remove them carefully in case you sell one day to a collector, so you could include them in the sale. The first thing I'd do is buy 6 Russian FT-3 teflon caps to replace the 0.1 caps. These teflon caps are superb - ultra clean and detailed. They are bigger, so will need to be elevated in some way. I'm assuming they will fit somehow or somewhere! I use a lot of hexagonal nylon spacers, male-female M3 with a solder tag and nyloc nut (important to stay tight when you solder). You can mount these instead of nuts on M3 bolts in existing holes, for example the bolts holding in the tube sockets. And you can stack them so you get a few solder tags on one pillar. Very useful for discreet mods of old equipment.

There are many better input sections you could use, but that's for a later day when you have cleaned up all the parts that need it.

Last edited by andyjevans; 5th November 2012 at 02:16 PM.
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Old 5th November 2012, 05:11 PM   #3
Clintec is offline Clintec  United Kingdom
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Hi Andy thanks for the reply

I was going to replace all resistors with 3W Metal film except the R12 whichI was going to use a 5W for as I had read this one gets hot and had already been replaced with a much larger one in Amp 2.

Funny you mentioned the Russian capacitors, I didnt know about them ones but had already ordered the K40Y variant which I think are god quality around the same size and look of the originals and still metal screened. Do you think they would be a lot to be gained between the 2?

Anyway I have stripped Amp 2 down to the metal work today and will build this one up first, still wondering if I like the original grey paint, and if I do what the closest match would be, mine are both original and look a bit like grey hammerite paint but with smaller 'hammer' effect. I do quite like the idea of doign them in the latest colours for some reason, so they would look like the modern classic 2 - so choice is Original | Modern | somthing very shiney so the glow refects off them in the dark.
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Old 5th November 2012, 07:00 PM   #4
oshifis is offline oshifis  Hungary
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You can also consider using MILLS MRA-5 or even MRA-12 wirewound resistors. They are inductionless, I have good experience with them.
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Old 5th November 2012, 08:04 PM   #5
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Quote:
Originally Posted by Clintec View Post
Hi Andy thanks for the reply

I was going to replace all resistors with 3W Metal film except the R12 whichI was going to use a 5W for as I had read this one gets hot and had already been replaced with a much larger one in Amp 2.

Funny you mentioned the Russian capacitors, I didnt know about them ones but had already ordered the K40Y variant which I think are god quality around the same size and look of the originals and still metal screened. Do you think they would be a lot to be gained between the 2?
Resistors that get hot should be rated at twice the wattage or even more for comfort. Look at the Welwyn green porcelain wirewound ones. If a 14W fits, you could use that (W24) or otherwise a 10W (W23) or 7W (W22)

There's a world of difference between K40 which is PIO and K72 and FT-3 which are teflon. The FT-3 are ever so slightly better than the K72, but the main thing is they are slightly smaller. Some people find the teflon "sterile" and prefer PIO K40s, but frankly I think they are much inferior. Teflon is the ultimate in clean and detailed. I don't get all this "warm sound is nice" stuff.
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Old 5th November 2012, 09:39 PM   #6
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I found the biggest improvment was some big pp supply smoothing caps. tightens the bass no end, but it doesnt sound like a quad then!
having bulit a few amps of my own design now, i'm full of admiration for quad in getting a valve rectified push pull amp into such a small chassis.
enzo
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Old 5th November 2012, 10:19 PM   #7
Clintec is offline Clintec  United Kingdom
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Originally Posted by enzoastro View Post
I found the biggest improvment was some big pp supply smoothing caps. tightens the bass no end, but it doesnt sound like a quad then!
having bulit a few amps of my own design now, i'm full of admiration for quad in getting a valve rectified push pull amp into such a small chassis.
enzo
Hi Enzo, when you say big what values are we talking about ?

I was considering moving from 16 to 47 or maybe 68uF.
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Old 5th November 2012, 10:22 PM   #8
Clintec is offline Clintec  United Kingdom
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Originally Posted by andyjevans View Post
There's a world of difference between K40 which is PIO and K72 and FT-3 which are teflon.
I will purchase some FT-3's and see if I can get them in there nicely, worst case is they will sit and wait for somthing else.
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Old 5th November 2012, 11:27 PM   #9
Zen Mod is offline Zen Mod  Serbia
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oh boy , what a treat of a project

whatever - regarding first cap after tube rectifier - stay as low as possible , it will increase rectifier longevity

if you want tighter bass ( in fact - amp faster overall ) - put RC after first C , so that second C can be bigger

I think 47R to 68R/5W will suffice , without sacrificing too much of Ub

150mA x 47R will give ~ 7V of drop
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Old 5th November 2012, 11:49 PM   #10
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Mmmm .....

I should stay away - but ever since I heard the first Quad II down here south in 1956 (with EL loudspeaker) and befriended Peter Walker, this little engineering marvel got close to my heart. If I may then advise (kindly excuse the length):

I could start with the dozens I have refurbished thus far, but let us leave the 'promotion' there. Firstly, my respects to those promoting super-caps .... but in audio there is no reason not to use normal polyester or polyprop. capacitors for coupling. Yes, there are far better caps out there (at a price, naturally), but define 'better'. Those are made for medical purposes or measuring instruments where the 'betterness' is of value - not here*

Also, I feel you are overdoing things with 3W resistors. For most of them 0,25W MF is ample, but they look rather silly small. I replace with 0,5W or 0,6W MF where obtainable. If you like the 'same size' look of 1W MFs better, fine - but no technical requirement for such.

I usually make R12 10W, but then rather to distribute heat more than necessity. Anyway, keep in mind that component ratings are usually for 25 degrees ambient; it is higher underneath a Quad chassis.

If desired by the owner I remove C4 and C6 from the tin - there is a twin can capacitor inside. One can unsolder the bottom with a gas flame and do a neat job of it. Regarding values, using a GZ34 rectifier as all later Quads did, the filter input capacitor may be 60F maximum. Modern types are quite small in size, and C6 = 47F and C4= 100F, both 450V, can fit in quite comfortably. I use some polyurethane foam inside to keep the caps from dangling around and the can lid is easy to replace. A re-paint job might be needed.

C5 is the one other component I change - quite drastically. Though the class-A design is supposed to draw constant current, there is a small rise in KT66 current at full output thus back-biasing. The effect of fixed bias is simulated for music by making C5 quite large. I use 2200F 35V or at least 1000F 50V, whatever is available. Again small physical size these days. Up to 20W of peak output is possible this way.

And Clintek! Please keep the hot R12 away from right next to C5! It is the only stupid thing I ever saw Peter Walker do. Heat is the enemy of especially electrolytic capacitors! I have not had a single Quad II where C5 was not completely dried out. One can glue C5 to the chassis just next to V5, but move R12 away to the component side, mounted somewhat higher on its leads.

Semiconductor rectifiers are attractive, but somehow a Quad with an empty valve socket does not appeal. Some leave the valve in non-operative .... There is the slight advantage of a tube rectifier bringing in a soft start.

Then, the equal voltages showed on the electrodes of the EF86s are incorrect; they cannot be equal as V2 takes its G1 potential from a slightly positive point in the circuit, thus making V2 potentials about 15% lower than those for V1.

Replacing C2, C3 by capacitors without the outer earthed metal body removes about 22pF from KT66 grids to common. As a result you will find a slight overshoot at about 100kHz on the output looking at a square wave. One can restore by placing external 22pFs there, but I prefer a phase lead correction. This is accomplished by the addition of a 1500nF capacitor in parallel with R11. Not really essential, but just to make matters neat ....

Finally! One finds shape resembling KT66s these days, but at a cost. Otherwise keep the following in mind. The 6L6GC equals the KT66 to about 10% (this is < than the tolerance on tube production). It draws lower heater current but is rated at 30W compared to 25W for a quite more expensive KT66. (You will find that some Russian KT66s have the exact same innards as 6L6GC - at higher cost!) And please keep EL34s out of the mix! They are quite different from the KT66 and draws more heater current.

Thanks for your patience.


*I am always puzzled by the paranoia regarding quality of coupling caps. Yes, of course they must not leak; hardly a concern these days. But consider all the talk about dielectric absorbtion, smearing and what else .... These capacitors are shorts at all audio frequencies, they do not charge/discharge except initially; those 'shortcomings' if of meaning are not operational during signal passing. Just thought I would mention in passing. Don't waste your money on boutique-ism.

Last edited by Johan Potgieter; 5th November 2012 at 11:52 PM. Reason: Typo
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