Quad II classic - help with tips, valve rolling and what to look for?

[vishalk]

Hi All

Hoping some of you who are owners of these lovely legendary amps can help me with a few queries.

I took the plunge and bought a pair, having been searching for the right pair at the right price. I found these and couldn't say no.

As we all know there are many out there for sale which for the money are molested, in bad condition etc. I thought i was very lucky to find these:

Pictures don't really do these justice as they really are in a beautiful condition.
Whether they are original or been refurbished, i am not sure. Looking at the internals it looks good?


Would love your thoughts?

It came with Chinese matched KT66 Electron valves, the other's a Mullard.

They do sound wonderful, i had them running today for 4 hours.

Can I have tips on what to look out for and what valves you guys recommend?
Any help would be appreciated to help improve the efficiency of these lovely amps and if I can choose some lovely valves to roll to get the sound I like.

Keith-Snook.info. — QUAD II Buying —

This was a great source of information.

Thanks!

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Looks like two resistors and a cap have been changed here. As you can see they are in really good condition and they sound beautiful, that with the chinese valves. Do you think i need to check the other resistors? Or is it a case that if they sound good and not giving you trouble leave and enjoy them. I hear that capacitors and resistors over years degrade and need changing.

Thanks!

Vish

[Nuvistor]

Valve rolling, sorry I don't know that term.

They look lovely amplifiers, components can change value but just replacing without testing them will cause more harm than good. There cannot be much if anything wrong with it if after 4 hours use it is working fine.
There are some knowledgeable members who know much more about Quad amps than I do so I will leave it to them.

Frank

Valve rolling, sorry I don't know that term.

Swapping valves to get a "better" sound", this doesn't work with a Quad II as the circuit is well/properly designed and will take any make of valve without changing its "sound". Which it doesn't have, like any good amplifier.


There used to be a sign in the repair department at Quad saying "Here we believe in ohms law"

[M0FYA Andy]

I can well imagine they believed in all the laws of physics...........

[RojDW48]

There is a whole Facebook community dedicated to the curious practice of 'Valve Rolling'. They seem to be a happy, even incurably optimistic bunch. I, in common I suspect with most members of this forum, prefer to assume that designers, especially people like Peter Walker, probably chose the best valves for the job in the first place.

[RojDW48]

I have heard it rumoured that Chinese KT66s sound very nice but don't last very long - so it might be worth looking for NOS British made examples....but they won't be cheap!

[Nuvistor]

Thanks for the clarification, I have obviously led a sheltered life

Frank

[paulsherwin]

Vish, as you are discovering, most members here aren't very sympathetic to the preoccupations of the modern valve hifi community. The advice members will give is good, but it may not be what you want to hear.

[Michael Maurice]

The two capacitors arrowed are the grid coupling capacitors, if they go leaky (electrically) they can wreck good valves and transformers, you should have them replaced.

[frankmcvey]

Hi, Vish,

Nice pair of amps, congratulations. Yes, there has been a little restoration, but it looks to have been well done. Two of the Erie Type 8 carbon composition resistors have been replaced with what look like carbon film resistors and the original Hunts Mouldseal capacitor has been replaced with a modern poly cap (although whether -ester or -propylene I can't see)

The output stage coupling capacitors (the two paper-in-oil metal capacitors mounted on the octal valve bases) look to be shorter and fatter than the originals and may well be replacements. These last two items are perhaps the most safety critical items and their failure can cause very expensive damage to the output valves and/or the output tranformer and should be checked for leakiness.

The 180 ohm cathode bias resistor (the green vitreous one mounted on the choke base) looks like the original 3W, which is not quite man enough for the job when you're running at full chat and may get a bit hot under the collar.

I think it's likely that a number of the remaining original Erie resistors will have gone high in value, but they're not so crucial, safety-wise.

I'd say that it's important that you check on the two metal coupling caps, C2 and C3 in the Snook schematic, check the value of the mains fuses are correct (one in the plug and the other in the chassis fuseholder). Other than that, if they sound good, then I'd leave them be!

Cheers,

Frank

Edit - crossed with Michael!

[nutteronthebus]

Hi these are the user and service manual for the Quad

[vishalk]

Thanks Nutteronthebus very helpful information.

[vishalk]

Originally Posted by frankmcvey

Hi, Vish,

Nice pair of amps, congratulations. Yes, there has been a little restoration, but it looks to have been well done. Two of the Erie Type 8 carbon composition resistors have been replaced with what look like carbon film resistors and the original Hunts Mouldseal capacitor has been replaced with a modern poly cap (although whether -ester or -propylene I can't see)

The output stage coupling capacitors (the two paper-in-oil metal capacitors mounted on the octal valve bases) look to be shorter and fatter than the originals and may well be replacements. These last two items are perhaps the most safety critical items and their failure can cause very expensive damage to the output valves and/or the output tranformer and should be checked for leakiness.

The 180 ohm cathode bias resistor (the green vitreous one mounted on the choke base) looks like the original 3W, which is not quite man enough for the job when you're running at full chat and may get a bit hot under the collar.

It's not often I get to have an amplifier on full song living in a flat! But certainly loud enough that I feel the music, yes I have read these are under rated and that the Quads dissipate about 3.75W. Snook recommends replacing this with a 5W - 7W resistor.

I think it's likely that a number of the remaining original Erie resistors will have gone high in value, but they're not so crucial, safety-wise.

Can I check each resistor connected on the board? Or do I have to take each one off?

I'd say that it's important that you check on the two metal coupling caps, C2 and C3 in the Snook schematic, check the value of the mains fuses are correct (one in the plug and the other in the chassis fuseholder). Other than that, if they sound good, then I'd leave them be!

Again can I check while it's connected, I realise I have to disconnect the power supply, use a suitable resistor to discharge the cap then test it. I'm still reading Snook's info, I read you can test these capacitors:-

Are C2 and C3 okay?

The simplest way to check C2 and C3 on an otherwise working amplifier is to measure the voltage on the grids of the KT66 valves with a high (>10MΩ) impedance meter ~ If you measure from each KT66 grid {pin 5 see drawing} to ground the voltage should be about 200mV ~ 400mV ~ Not 0V ~ due to the voltage across R10//R11 and the high value grid resistors

If the voltage is higher than 1V this may be due to insulation breakdown in C2 or C3 or the KT66 may be "gassy" ~ A faulty KT66 can be confirmed by removing the valve {after switching off} and making the measurement again without the valve fitted ** If the reading is now okay the valve is most likely on the way out

Measurements of grid voltage will be higher when the capacitors are hot and in very humid areas may be due to dirt on the valve bases ~ With very dirty valve bases a voltage can even be measured by placing the probe of a high (>10MΩ) voltmeter on the insulation between pins 4 and 5 at the point marked "4" on the base material

Pin 4 is at about 340V and its proximity to the grid pin 5 requires clean insulation ~ I have seen 2 cases where household pets have "marked" a QUAD amplifier as their territory ~ I can only assume not powered at the time as the faults were unknown to the owners until they smelt the odour when warm and the flashes from the KT66s ~ In each case replacement of the burnt valve bases was the only cure.

Good to know and something I will check! Thanks Frank good info to get me going learning more each day.

[frankmcvey]

Hi, Vish,

The good news is that the majority of resistors in the Quad II can be checked without disconnecting them.

1. Disconnect the monobloc from the mains and the preamp. (6-way Jones plug and 2-way Bulgin)

2. Ensure that the main power supply reservoir/smoothing capacitors (C4/C6) are discharged before working on the amp.

3. Check each resistor on the board in turn and calculate whether it's in tolerance - the majority are +/- 10%, with the exceptions of R5 and R6. These are the anode loads for V1 and V2, the phase-splitter stage. These resistors ensure that the output valves both receive the same amount of input, so they need to be close to the design value and, more importantly, close to each other. They have a 5% tolerance.

The only resistors you can't check in circuit are R10 and R11, the feedback resistors, since you'll have a circuit through the feedback winding of the output transformer which complicates things. Disconnect one end of R11, that being the easiest one to access, carry out your checks of R10 and R11, then reconnect. These are also 5% tolerance.

The method that Snook suggests for checking the coupling caps is the simplest way. Basically the coupling caps are there to allow the AC music signal from the anodes of V1 and V2 through to the grid inputs of the power valves, V3 and V4, but at the same time they need to block the high +ve DC power supply which is also present on the anodes of V1 and V2.

So what you're going to do is to check for any DC voltage on the grids of V3 and V4 - if you find a +ve DC voltage there, then some of the HT is sneaking through C2 and/or C3, which would imply that C2 and/or C3 is leaky. We can usually live with a slight leak, but if it is too much and we apply too much +ve voltage to the grids, it has the effect of opening the throttle on the valves, allowing too much current to pass through them with a possibility of the valves red-plating and the internals melting. This overcurrent also passes through the primary of the output transformer and can easily burn this out as well. You really, really do not want +ve DC getting through the coupling caps!

Obviously, this test needs to be done with power applied to the amp. The power supply is capable of delivering around 350 volts at over 150mA, which is easily enough to kill you. If you're not confident about doing this, then get a friend who is!

If you haven't any electronically-minded friends, then I'm about 1 1/2 hours up the A1 from you - come and see me and I'll run through the checks with you.

Cheers,

Frank

[ToneArm]

I've got three of these, my first one came in a Whiteley cabinet with a garrard 301 and a big wharfedale W3 speaker. It had been in regular use since 1960 up till the day I bought it in 2015 without needing any kind of service, including the valves according to the owner. I thought that was astonishing especially given how little ventilation the case allows. It sounded pretty ropey but it was the preamp (qcii) and the perished speaker foam that needed the most attention. I replaced only the faulty components - the valves are still going.

Bought two more since then that were in poor shape and had to be restored. I had a problem with the voltage selector on one of them that took an age to track down. They now work beautifully as a stereo pair despite serial numbers miles apart and one having cheap chinese KT66 the other GEC. They do get quite hot and I cluck around them a bit - should relax and remember these were built to live inside wooden cabinets!

[SteveCG]

I agree wholeheartedly with others that if the Amps were mine I'd concentrate my efforts on making sure the coupling capacitors to the output valves (C2 and C3 reading posts above) are not electrically leaky.

[vishalk]

Agreed Steve this seems to be the general consensus here and will be the first thing I do.

[vishalk]

Originally Posted by frankmcvey

2. Ensure that the main power supply reservoir/smoothing capacitors (C4/C6) are discharged before working on the amp.

This is where I would use a resistor to discharge them, correct?

3. Check each resistor on the board in turn and calculate whether it's in tolerance - the majority are +/- 10%, with the exceptions of R5 and R6. These are the anode loads for V1 and V2, the phase-splitter stage. These resistors ensure that the output valves both receive the same amount of input, so they need to be close to the design value and, more importantly, close to each other. They have a 5% tolerance.

I was thinking i might as well replace all the old ones (I'll keep them as well).

The only resistors you can't check in circuit are R10 and R11, the feedback resistors, since you'll have a circuit through the feedback winding of the output transformer which complicates things. Disconnect one end of R11, that being the easiest one to access, carry out your checks of R10 and R11, then reconnect. These are also 5% tolerance.

The method that Snook suggests for checking the coupling caps is the simplest way. Basically the coupling caps are there to allow the AC music signal from the anodes of V1 and V2 through to the grid inputs of the power valves, V3 and V4, but at the same time they need to block the high +ve DC power supply which is also present on the anodes of V1 and V2.

So what you're going to do is to check for any DC voltage on the grids of V3 and V4 - if you find a +ve DC voltage there, then some of the HT is sneaking through C2 and/or C3, which would imply that C2 and/or C3 is leaky. We can usually live with a slight leak, but if it is too much and we apply too much +ve voltage to the grids, it has the effect of opening the throttle on the valves, allowing too much current to pass through them with a possibility of the valves red-plating and the internals melting. This overcurrent also passes through the primary of the output transformer and can easily burn this out as well. You really, really do not want +ve DC getting through the coupling caps!

Obviously, this test needs to be done with power applied to the amp. The power supply is capable of delivering around 350 volts at over 150mA, which is easily enough to kill you. If you're not confident about doing this, then get a friend who is!

Well i was up for doing it, but as soon as you mention death and high voltage I get a little apprehensive!

If you haven't any electronically-minded friends, then I'm about 1 1/2 hours up the A1 from you - come and see me and I'll run through the checks with you.

Now that is very kind of you Frank and an offer I may take up! Thanks

[vishalk]

I have another query regarding using 4,6 and 8 ohm speakers. Would i have to change the resistors based on this? They were originally setup for 15ohm from what i have read? Either way i have them connected to my DIY built Planet10 mini onken builds with Mark Audio Alpair 7.3 and they sound lush.

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Some good links

http://www.valve-radio.co.uk/literat...ction-booklet/
https://www.stereophile.com/content/listening-52-page-2

Thanks

[Radio Wrangler]

No you don't have to change resistors to handle different nominal speaker impedances.

The output transformer has taps to handle them and the feedback is taken from one output winding alone. The transformer does all you need.

The actual impedance of loudspeakers wanders over quite a wide range, and exhibits large peaks at some frequencies. With a transistor amplifier, you are mostly worried about how deep the dips go and whether it can handle the currents involved. With a valve ampliifier the reverse is true, you are more worried about how high the peaks can go and whether the amplifier will generate too high voltages. The Quad II has quite complex feedback and handles things well. It has to because Quad were selling their electrostatic speaker alongside the QuadII and that speaker is a difficult load.

So don't get too worried about speaker impedance, you can never get it exactly right with real loudspeakers, and you don't need to worry about special speaker cables. Some copper of a few square mm cross section area will be fine. The fancily woven expensive specialist stuff does nothing better and its increased capacitance can make some amplifiers go unstable at RF.

David