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Old 17th Jan 2023, 11:57 am   #1
cathoderay57
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Default Quad II ringing

Morning all. Based on the comments to my post in this thread https://www.vintage-radio.net/forum/...d.php?t=197146 following my statement concerning replacement of the KT66 grid coupling capacitors with yellow polys, I have just fitted 22pF filter caps between grid and chassis, as per Keith Snook's advice. Just for fun I then connected up a sig gen and fed in a 0.6v pk-pk 100 Hz square wave and monitored the speaker outputs (connected to a 10R resistor) with my scope. Both units showed ringing. The first unit is Quad II Ser No. 71237 fitted with a pair of Groove Tube KT66. The outputs from this amp are the first 2 (left and centre) attachments below, which show ringing on the rising and falling edges. Peak oscillation is 0.7v pk-pk at 56 kHz, against a 100Hz square wave output of 4.5v pk-pk. The second Quad II, a Rediffusion unit Ser No. 491 (originally had a 100v line transformer but replaced with a Quad 1003A) is fitted with one Marconi KT66 and one Osram. As you can see, this one is slightly better with only about 0.5v pk-pk HF oscillation decaying more quickly. I tried replacing the 22pF grid filter caps with 30pF but it didn't make any difference. Is this level of parasitic oscillation acceptable? Cheers, Jerry
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Old 17th Jan 2023, 12:17 pm   #2
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Default Re: Quad II ringing

Hi Jerry, I followed the very excellent advice from Patrick Turner and fitted the zobell (or shelf) like network he had between the anodes of the driver tubes.

Lots of interesting reading here ...
Patrick's Quad IIs

The change I did was for the VERY BASIC UPGRADE, as described some way down the page, (separate cathode bias resistor, etc).
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Old 17th Jan 2023, 12:30 pm   #3
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Default Re: Quad II ringing

Just so we have all the info, can you a) confirm that the sig gen has no ringing on its output, b) tell us whether the output transformers are jumpered for an 8ohm load or for a 15ohm one and c) tell us whether the other two small capacitors (the 25uF electrolytic bypassing the KT66s' common cathode resistor and the 0.1uF connecting the EF86s' screen grids) have been replaced and, if they have, what with ?

Cheers,

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Old 17th Jan 2023, 1:06 pm   #4
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Default Re: Quad II ringing

The ringing is decaying slowly so this is not a loop stability issue. If you are looking for a perfect waveform from a fast rise time square wave you need to get rid of that output transformer.
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Old 17th Jan 2023, 1:14 pm   #5
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Default Re: Quad II ringing

The third photo shows a damped ring, with modest damping. The amplifier is stable, but not as stable as you'd want it.

The second photo shows an amplifier with very little damping and may well be capable of sustained oscillation. The ring/oscillation frequency looks a lot higher than that in the third photo. Knowing the frequencies may prove useful.

Tried swapping valves over?

David
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Old 17th Jan 2023, 1:21 pm   #6
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Default Re: Quad II ringing

Cross-posted.

The middle photo shows a slow decay but will it really go to a full stop?

Not necessarily. Oscillation amplitudes are gverned ultimately by something being driven into compression, reducing the loop gain and making further growth impossible. A triggered oscillation from say a squarewave edge can drive the thing into a larger oscillation than it would sustain long term, and you see a decrease that may be to an oscillation level it never goes below.

However, such things with so little damping showing might just as well be fully self-sustaining. The phase/gain margins are going to be trivial, and when you consider the tolerances there will be in what creates the gain and frequency response up at the top end of the frequency range, the thing is in trouble come what may.

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Old 17th Jan 2023, 2:00 pm   #7
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Default Re: Quad II ringing

I think the frequency of the ringing works out as the number of peaks *2 *the nominal frequency of the square wave. What's the Mullard like?
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Old 17th Jan 2023, 2:06 pm   #8
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Default Re: Quad II ringing

Hi folks, thanks for the replies.Richardgr -'I appreciate the link. I'll have a read and will consider such mods if a simple fix isn't successful. GJ - there is no ringing on the input signal. I'm using an ancient but serviceable Farnell LFM. OPTs are jumpered for 8 Ohm outputs. The 25uF cathode decoupler on the first unit (71237) is original. The 0.1uF on the EF86 board is a new yellow poly. On the second Quad II (491) the cathode decoupler is a Rifa 25uF 64v replacement and the 0.1uF a new yellow poly. David - the first Quad oscillates an initial 2 cycles at about 100 kHz then flips to 56 kHz and slowly decays, whereas the second amp (only a couple of cycles) is about 105 KHz. Swapping the output valves over makes no difference. Worth replacing the remaining original cathode decoupler and if no improvement adding grid stoppers? If so what value grid stoppers do you suggest? Cheers, Jerry
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Old 17th Jan 2023, 2:10 pm   #9
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Default Re: Quad II ringing

Hi Gabriel, not measured my homebrew 5-10 yet. I think I have enough on my plate with the Quads at present.... Jerry
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Old 17th Jan 2023, 2:42 pm   #10
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Default Re: Quad II ringing

One other thing. Mindful of Keith Snook's warning that ground bonding of the KT66 sub-chassis might be bad, negating the function of the grid filter capacitors, I checked before I fitted them. The first Quad II mentioned above showed a resistance between the KT66 sub-chassis and the earth terminal of the HT cap was about 25 Ohms so I added an earth bonding lead between one of the KT66 mounting screws (to which one of the grid filter caps earth lead was connected via a solder tag) and the earth tag for the 25uF cathode capacitor. Continuity between the earth wires of both filter caps and chassis earth was then short circuit. On the second Quad amp the KT66 sub-chassis was already bonded short circuit via its mounting screws. Jerry
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Old 17th Jan 2023, 2:44 pm   #11
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Default Re: Quad II ringing

Patrick, who unfortunately passed away last year', was a very entertaining writer, with Australian wry wit. He often berates the old colonial power, but considers the quads to be a miracle of post war technology born out of austerity. They were even painted in surplus 'battleship grey' paint left over from the war effort.

So, they are not going to be able to compete with the finest amplifiers, but they can be coaxed to give a bit more than originally conceived. Also, they were prinarily designed as partners for the ESL57 speakers, which Patrick really enthused about, and he goes to some lengths to explain impedance matching.
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Old 17th Jan 2023, 3:22 pm   #12
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Default Re: Quad II ringing

Just fitted a new RS components 22uF 63v cathode filter cap - no difference. Jerry
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Old 17th Jan 2023, 3:30 pm   #13
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Default Re: Quad II ringing

When jumpered for an 8ohm load the voltage gain in the amp should be 15W out = 10.95V RMS out for 1.4V RMS in i.e. a gain of 7.8 or just under 18dB. You measure a gain of 4.5Vp-p out for 0.6Vp-p in, which is 7.5. This is near enough to 7.8, especially since the Quad II gain typically measures slightly below the spec value in any case. This, in turn, suggests that the feedback loop is working as it should, at least at 100Hz.

You could try replacing the original cathode bypass cap to see if it helps reduce the first amp's ringing. As long as the existing cap doesn't have any substantial electrical leakage it would probably be enough, for a quick test, to croc-clip a new one across the wires of the existing one.

Before changing the circuit by adding grid stoppers I'd concentrate on establishing whether this level of ringing is actually a fault, or whether it's just a thing that all Quad IIs do. In any case I'm not sure grid stoppers will necessarily help. I think of them as mostly used to isolate the internal capacitances of a valve from external stray inductances with which they might otherwise form an under-damped oscillatory circuit. Given that the KT66 grid wiring is physically quite extended I suppose there might be some significant stray inductance there but I don't think it would be where I first looked if I was looking for an inherent tendency to ring. I'd be with PJL who's suspicious of the output transformer. If you wanted to try stoppers though then I'd start with a value of 1k or so.

Cheers,

GJ

EDIT Posts crossed (again !).
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Old 17th Jan 2023, 4:07 pm   #14
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Default Re: Quad II ringing

It might not be unusual:-

https://www.stereophile.com/content/...r-measurements

"Quad II Classic monoblock power amplifier Measurements"

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Old 17th Jan 2023, 4:19 pm   #15
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Default Re: Quad II ringing

Many thanks GJ. I have refitted the original cathode decoupler to the first amp. Next I looked up Patrick's Quad IIs (Post #2) and noted that in one of the simpler modifications instead of fitting grid-to-ground filter caps on the KT66 grid1s, a 22k resistor in series with a 33pF cap was used between the KT66 grid1s. I tried this and, while not perfect, it has made a significant improvement as you can see from the pics below. The HF oscillation now peaks at about 0.5v pk-pk (whereas it was 0.7v) and decays much more rapidly than before. I'm afraid I can't afford the cost of new transformers and to be honest I don't think it is justified. After all, I've been using both amps for over 3 years with yellow waxies for C2 and C3 without any additional grid filter caps, with no audible instability and no damage to my tweeters. Above all, they sound good so I think it's best if I leave them alone at this point. Thanks for all the tips and comments! Great tip Trigon, many thanks. Jerry
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Old 17th Jan 2023, 5:54 pm   #16
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Default Re: Quad II ringing

Morgan Jones describes using a radio variable tuning cap and a pot to find the optimum values for lead and lag compensation caps/resistors - leading to a square wave with minimum ringing. However, you seem to have improved things quite a bit with Patrick's values. Strange how one performs significantly better than the other though
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Old 17th Jan 2023, 6:49 pm   #17
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Default Re: Quad II ringing

The 22k + 33pF between the KT66 grids will, presumably, reduce the ringing by rolling off the open-loop HF gain (Patrick says, at the para numbered 13, about 40% of the way down his page, that this happens above 20kHz). If the ringing is a feature of all Quad IIs then we're perfectly entitled to dial it down by changing the circuit this way. But if the ringing is actually the result of some still-to-be-located fault then we shouldn't really conceal it like this. It would be the same as dealing with the smell of gas in your kitchen by investing in a cooker hood.

The Stereophile review is interesting, and they do see some ringing at 50+kHz. Crude measurements off my screen with a plastic ruler suggest their first peak is 25% higher than the square wave's flat top which is comparable in amplitude with the OP's scope traces in the first post. However the Stereophile ringing decays very quickly - perhaps in a few tens of microseconds - whereas the OP's is still clearly visible after 25 cycles, which is ~450us.

It's worth pointing out though that the Stereophile review is of Quad's modern production "Quad II Classic" amp. They claim that this is a very close copy of the original Quad II. But the new output transformer secondary seems to bring no tappings out. Whatever speaker you might have, it will be connected across the whole of the secondary, and the negative feedback signal will be taken from the whole of the secondary too. So not that close a copy then, even if we disregard that fact that the new tranny will use a modern grade of lamination steel and possibly a different core size and copper wire gauge. And who knows how the stray reactances of, and between, the winding sections relate ?

Cheers,

GJ
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Old 17th Jan 2023, 7:02 pm   #18
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Default Re: Quad II ringing

The frequency plots in the stereophile article rather nicely show peaking into higher impedance loads, reducing into lower impedance loads.

It also shows the gain reducing into the lower Z loads, so whether the reduction in peaking is a result of lower Z loading reducing the OLG contribution of the output stage or not is an interesting result. You can't just say that the feedback control is dominating CLG, because the CLG can be seen to drop.

David
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Old 17th Jan 2023, 8:18 pm   #19
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Default Re: Quad II ringing

I am sceptical about the slowly decaying high frequency resonance having anything to do with open loop gain, if it was as bad as shown in the earlier scope image, the slightest change would have it bursting into oscillation. My suspicion is the ringing is local to the output transformer, keep in mind that the KT66 anodes look relatively high impedance to the transformer.
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Old 17th Jan 2023, 10:01 pm   #20
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Default Re: Quad II ringing

Any other Quad II owners willing to repeat the test? J. P.S. in post #15 for "yellow waxies" read "yellow polys" but I'm sure you knew what I meant.....
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