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Old 12th Jun 2021, 11:05 am   #1
Petedox
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Default Quad II Transformer woes!

I bought a couple of Quad II's a few years back with the idea of fully restoring.

I've finally got round to it, and realised (after I'd stripped and painted the metalwork) that it features 100v line OP Tx's. There's no version number on the TX, but I assume it's 1019.

These have clear Quad badges and not the metal Redifusion plate, so I guess that Quad also made 100v line models for other customers?

The strange thing is that when I purchased them I had listened to them on small 8 ohm speakers and they didn't sound too bad, and seeing as I was going to rebuild them I wasn't too bothered about the sound quality at that stage.

It looks as though the connections to the transformer have been bodged to make it work, please see attachments. I'm at a loss to see how it could work, the speaker is connected to tags Q and T, with Q & R and S & T shorted together, so there doesn't appear to be a winding for the speaker! Maybe I'm missing something....

The correct 100v line circuit suggests S & R should be shorted.

So, I'd be grateful for any observations on this, and also (a bit of a longshot) is there any way these could be rewound? Does anyone have a pair for sale? If not I guess I'm at the mercy of Majestic Transformers which would blow this project right out of the water.

One other option I guess would be to use line matching transformers, but I'm not sure of the sound quality, and I'd rather not go down this route, but has anyone got experience of their use in Hifi applications?
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Old 12th Jun 2021, 12:10 pm   #2
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Default Re: Quad II Transformer woes!

Keeping the value of R11 at 470 Ohms for both versions, suggests that the P-Q and P-flying lead windings have the same number of turns. I wonder if someone bodged the wiring to use it for low impedance speakers?

Plenty of people have damaged Quad II transformers before now, so I expect both Ed and Mike will have the recipe for them.

The different winding sections are artfully interleaved to minimise leakage inductance and get best bandwidth, so you really do want to be using the full secondary structure as intended in the Quad II design.

The need for high bandwidth is to keep the feedback loop stable up to a high frequency where the loop gain has dropped to less than unity. This is substantially above the audio range.

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Old 12th Jun 2021, 5:50 pm   #3
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Default Re: Quad II Transformer woes!

Many years ago I hand rewound a Quad 2 output transformer
each Anode was 900 turns the Cathode windings were 90 turns the output winding for 8 ohms was 80 turns
I used a RS 100 va transformer kit as the basis
After sorting the phasing this worked out ok
Shortly after the amplifiers were given away
To be honest I could not tell the repaired unit from the origional
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Old 12th Jun 2021, 7:34 pm   #4
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Default Re: Quad II Transformer woes!

Hi, they are a total pain to rewind, being impregnated in pitch. There is also the mystery component that was previously discussed here and I believe is a low value resistor to assist in stability, not normally shown on circuit drawings. I wonder if it is in the 100v line version?

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Old 12th Jun 2021, 7:44 pm   #5
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Default Re: Quad II Transformer woes!

Yes - I'd keep the amps as standard [ think of it as 'preserving an unique bit of history'] and get decent-quality outboard 100V-line transformers to interpose between the Quads and your speakers.

Outboard 100V-to-4-or-8-Ohm transformers wouldn't need to be designed to handle any DC component so there's no need to worry about core-saturation issues.
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Old 13th Jun 2021, 9:02 am   #6
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Default Re: Quad II Transformer woes!

Ah yes the impregnation I got myself into trouble with the wife for putting mine in the oven
This reminds me there was a n article in a 1950-60 practical wireless re winding Quad type output transformers
I remember some small Rf type chokes being included
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Old 13th Jun 2021, 11:04 am   #7
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Default Re: Quad II Transformer woes!

Thanks all, I was willing to have a go at rewinding - having survived similar 'pitch battles' with my wife over oven usage, Trevor!

However, as Ed says, I'm not sure if the mystery resistor (0.36 Ohm according to Snook) is incorporated, but more importantly I don't know if this transformer can be successfully reconfigured as a normal output tx. The winding resistances are of course wildly different, so without firm details I don't want to start something I can't complete.

I guess I should stick to the current arrangement and keep my eye out for replacements, at least then I will be able to return them to their original '100v Line' configuration if sold in the future.
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Old 13th Jun 2021, 1:42 pm   #8
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Default Re: Quad II Transformer woes!

I suspect that the primary windings and the cathode windings must be the same as the original domestic version because they are to suit the HT voltage, the power level and the local feedback ratio and the KT66. Onlt the output windings need to change for the 100v line application. I'd be very surprised if Quad did more than that.

Also, the overall-feedback resistor going back into the amplifier is the same as the domestic version, so I expect that the feedback winding has the same number of turns (wire gauge might be different)

These are valuable amplifiers, or at least the ones with the domestic output transformers are. So I don't suppose you want to do anything outlandish or irreversible to them. Have a look at completed bids for the domestic and 100v line versions and the difference might help you justify things to yourself

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Old 13th Jun 2021, 4:35 pm   #9
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Default Re: Quad II Transformer woes!

Someone who wasn't concerned about preserving the amps might well disassemble them carefully and sell the parts individually. Typical Quad IIs are worth more as spares than they are complete. You'd need to check current prices for bits and pieces on eBay, and the repainting (unless it's a very good copy indeed) won't help with selling the chassis parts, but you might still find you could raise enough to buy yourself another pair of doer-uppers.

Cheers,

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Old 13th Jun 2021, 5:16 pm   #10
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Default Re: Quad II Transformer woes!

Thanks GJ, I've done this myself in the past with Quad and Leak parts when the amps required too much work to get going. I note that someone has broken up a pair of II's on a well known auction site at the moment. Prices as follows:

Mains TX: 199
OP TX: 189
Choke: 65
Chassis: 110
2 x Original 16uf: 85
Name Plate: 50
Tag Board (pair): 70
Base Plate (pair): 55
Plus a few other bits and pieces.

Interesting! So well over 700 with no valves or or cabling etc, which is well above the average going price for a single working unit.

I like these amps, but they seem to have created their own little niche among audiophiles (and phools). Supply and demand - I can't see an end to the price escalation.
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Old 13th Jun 2021, 5:33 pm   #11
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Default Re: Quad II Transformer woes!

As Quad recently reprised the Quad II, do they have any output transformers in stock as spares? Won't be cheap but would be the real thing, which is exceedingly important to those who listen with their belief-systems rather than with their ears.

Sold as parts, you wouldn't get much for the output transformers, except from someone seeing them as raw mateeial for re-winding.

Anyway, they're your amps, so you get to decide. We've all chipped in our thoughts and suggestions.

David
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Old 13th Jun 2021, 6:03 pm   #12
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Default Re: Quad II Transformer woes!

There's the Chinese transformer option of course, which have generally attracted rather dim views on this forum in the past. Cloned Quad OPTX's for $189 a pair on Aliexpress....

However I've decided to bite the bullet and go for the Majestic option which is pricey but according to Snook is better sonically, better made than the original and fits in the can.

Thanks, for your inputs!
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Old 13th Jun 2021, 10:01 pm   #13
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Default Re: Quad II Transformer woes!

I have rewound quite a lot of the 100v Quad output transformers for a number of the Hi-Fi dealers/enthusiasts here in the UK and abroad.

The primary windings between the two types are different.
There are thirteen separate windings which are interleaved in a specific manner.
There is the hidden resistor which is made up from a couple of loops of winding wire in a specific place.

The 'parts' currently available from an auction site as second hand and used make my prices seem cheap! I do them in the traditional way and they look totally original.

The Chinese replacements are... let's just say I would put them in the scrap bin, but I suppose it depends on your hearing and if you can still hear anything over 10KHz.

If you want your quad II's to sound as they were designed to be, then the output transformers must be made to the original design which will require the re-use of the original laminations due to the required winding window for all of the wire.
Anything else will be a compromise and can be expensive and disappointing.

Mike...
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Old 13th Jun 2021, 10:23 pm   #14
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Default Re: Quad II Transformer woes!

H!!

I've seen this circuit before, and it is referred to as "Cathode Loaded Push–Pull" – the anode and cathode windings for each output valve are specially designed & phased to allow a large amount of negative feedback without the need for a very large amount of peak–to–peak grid drive voltage for the push–pull output valves!

Don't confuse it with a very similar looking arrangement called "Unity Coupling" – in the Unity Coupled arrangement, one anode connects direct to the h.t.+ line along with the screen–grid of the second valve, and one half of the bifilar primaries act as an anode load for the first valve, the second half of the bifilar primaries is both anode load for the second valve and screen–grid load for the first valve, the screen–grid of the first connects directly to the anode on the second valve, an ingenious arrangement that uses the inner–amplification factor of the second valve to help reduce the overall grid–voltage drive needed for the stage.

The circuit descriptions in the book I got my reference to these circuits were very brief to say the least –"High Fidelity Sound Engineering" by Norman Crowhurst, Newnes, 1961, it's very scarce and the last copy I saw for sale was 78!

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Old 15th Jun 2021, 8:27 am   #15
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Default Re: Quad II Transformer woes!

Quote:
Originally Posted by Radio Wrangler View Post
As Quad recently reprised the Quad II, do they have any output transformers in stock as spares? Won't be cheap but would be the real thing ...
The last time I was in touch - several years ago now - they could supply replacement output transformers, and they were reckoned to be made to the drawings for the original Quad IIs (I don't know if that's the case for the re-released 'Classic'). The story goes that they were then being sourced from a Swiss company who had been persuaded to make them by Manfred Stein at Quad Musikwiedergabe in Germany. To say that they were 'not cheap' would be an understatement. And they were supplied, I think, painted grey but not very closely colour-matched to the original.

Cheers,

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Old 15th Jun 2021, 10:08 am   #16
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Default Re: Quad II Transformer woes!

Re-wound to the normal design by Mike or Ed might be rather competitive on price with anything from Quad, and with Switzerland involved....

If you go the 100v line speaker transformer route, then most of these things were done for PA systems and their bandwidth and distortion requirements were substantially relaxed compared to domestic hifi. You would really be looking for a special quality transformer to be made, and you're into expensive territory again. You also have to deal with your own curiosity about whether the normal Quad II would have sounded any better.

The Quad II is just an amplifier. It's job is to make a signal passing down a wire get bigger as a perfectly scaled-up replica. Otherwise the amplifier should vanish and have no other contribution to the sound. Peter Walker himself used the slogan "A piece of wire with gain" Quad moved on to their 303 amplifier, which they saw as an improvement, and then their 405 which they saw as a further improvement. Were they so deaf that they couldn't hear what all sorts of pundits claim? Or were they and their listening panels right, just not believers in invisible fairies at the bottoms of their gardens? Anyway, you could buy an amplifier that Quad themselves, back in the overlap period thought was better, for less cost than sorting out your Quad IIs.

So one valid way to progress would be to see your 100v amplifiers as fine examples for display purposes, and buy yourself a newer amplifier with a choice of makes and models, and less financial outlay. Sell the Quads if need be, someone else might be interested in converting them to the mainstream variety.

Sometimes you can get hit by too much choice

David
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Old 15th Jun 2021, 10:36 am   #17
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Default Re: Quad II Transformer woes!

Quote:
Originally Posted by Chris55000 View Post
H!!


The circuit descriptions in the book I got my reference to these circuits were very brief to say the least "High Fidelity Sound Engineering" by Norman Crowhurst, Newnes, 1961, it's very scarce and the last copy I saw for sale was 78!

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Old 15th Jun 2021, 11:05 am   #18
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Default Re: Quad II Transformer woes!

Quote:
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Quote:
Originally Posted by Radio Wrangler View Post
As Quad recently reprised the Quad II, do they have any output transformers in stock as spares? Won't be cheap but would be the real thing ...
The last time I was in touch - several years ago now - they could supply replacement output transformers, and they were reckoned to be made to the drawings for the original Quad IIs (I don't know if that's the case for the re-released 'Classic'). The story goes that they were then being sourced from a Swiss company who had been persuaded to make them by Manfred Stein at Quad Musikwiedergabe in Germany. To say that they were 'not cheap' would be an understatement. And they were supplied, I think, painted grey but not very closely colour-matched to the original.

Cheers,

GJ
Thanks GJ, as usual your comments are interesting and informative.

I have another set of II's in very good original condition, that I will leave untouched. The previous (now departed) owner only ever trusted Quad to fix them, and consequently they have service stickers inside.

In 2018 IAG replaced the OP TX, and yes the colour matching is poor (see pic), so I suspect this may be one of the transformers you allude to. Unfortunately I have no idea of what the cost would have been, but cost would not have been an issue to the previous owner.

So this has solved a little mystery for me.

Back to the original post: The transformers are in transit to Mike to be rewound back to original low impedance spec, so hopefully problem solved and I can get on and finish the project!
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Old 17th Jun 2021, 8:04 am   #19
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Default Re: Quad II Transformer woes!

Just a though but has anyone had a go at winding a high quality 100v line to 8/15ohms external transformer? I would guess it is a lot easier than rewinding the original and preserves originality.

When I read your first post I wondered if the loudspeaker you originally tested the amp with had a 100v line transformer inside the cabinet. I have encountered this a few times and it can catch you out.

I have a number of Quad 2 amps, tuners and preamps that were pulled off the tip about 30 years ago. They all appear to be low impedance versions and when tried all those years ago worked very well.

One was in an orange box filled with dry muck, sweet papers, fag ends and one can only guess. it's dirty coat had preserved it's original finish. It appeared to have been installed under a stage floor and must have been a fire hazard to say the least. I hope you manage to sort out your transformer problem.
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Old 17th Jun 2021, 4:27 pm   #20
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Default Re: Quad II Transformer woes!

Quote:
Originally Posted by Heatercathodeshort View Post
Just a though but has anyone had a go at winding a high quality 100v line to 8/15ohms external transformer? I would guess it is a lot easier than rewinding the original and preserves originality.

When I read your first post I wondered if the loudspeaker you originally tested the amp with had a 100v line transformer inside the cabinet. I have encountered this a few times and it can catch you out.

I have a number of Quad 2 amps, tuners and preamps that were pulled off the tip about 30 years ago. They all appear to be low impedance versions and when tried all those years ago worked very well.

One was in an orange box filled with dry muck, sweet papers, fag ends and one can only guess. it's dirty coat had preserved it's original finish. It appeared to have been installed under a stage floor and must have been a fire hazard to say the least. I hope you manage to sort out your transformer problem.
John.
Cheers John, to get back on topic....

The speakers I used were run of the mill 8 ohm Arcam bookshelfs. Not a transformer in sight!

I too also wondered if anyone would produce an external 'hifi quality' transformer, but why would anyone do that on a distributed system? The market would be vanishingly small I reckon.

The main thrust of my original point is that I couldn't understand how sound came out . I've fixed and restored many of these over the years, and this is the first time I've been 'caught' (for want of a better word). This was in pretty rough condition, although not as bad as the one you descibe!

No measurements taken, but the sound wasn't too bad, and on internal inspection the OPTX was apparently wired for an 8 ohm speaker. The only difference was that the flying lead was hidden away (it's not a feature of a 'normal' TX and difficult to see anyway), and the negative speaker terminal was wired to the transformer, not to ground as is normal. It was wired according to the diag in post #1, with S&T shorted, Q&R shorted and S&R o/c. I don't have the transformers with me now, so I'm unable to measure the resistances.

This is not a problem for me anymore, but there must be hidden windings that are not shown on Quads drawings, it would be interesting to find out!
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