Quad II ringing

[Valvepower]

Hello,

Again, out of total curiosity, I had a look at the AC waveforms directly on the anodes of the EF86 valves (V1 and 2). The feedback is closed loop.

There will be some loading effects of the probe. Mind you, the loading wouldn’t be that dissimilar to the additional capacitor discussed earlier in the thread.

There was some ringing present on the anode of V1 but not on V2.

Terry

[Radio Wrangler]

look at the slow edge speeds of the anode not showing ringing. So the lack of ringing might not be surprising.

If a loop is unstable, the ring or oscillation is likely to be found anywhere within the loop. In this case it seems to be more on one side than the other of the push-pull stages.

Your scope picture shows there is a significant difference in the bandwidth to the two anodes of the phase-splitter circuit. One of them is rather slow. This is all well above audible frequencies, but in the zone which affects ultrasonic rings and tweets.

The valve industry is to blame, they never did come out with electron/positron complementary pairs. Anyway, this is where the Bailey-Radford phase splitter shows its merit.

David

[GrimJosef]

As you pointed out back at post #48 Terry, the feedback is negative to one EF86 and positive to the other.

Cheers,

GJ

[Valvepower]

Originally Posted by GrimJosef

As you pointed out back at post #48 Terry, the feedback is negative to one EF86 and positive to the other.

Cheers,

GJ

GJ,

I had to double check I'd got the waveforms correct to the respective valves

Regards the ringing its present on V1 anode, the first EF86, which is directly connected to the input, whereas there is no ringing present on V2 the anode, the second ‘inverter’ EF86, the cathode of which I think is being subjected to some positive feedback.

It does make me think about the conditions of the inverter stage comprising of V2.

I did a bit of quick back of a fag packet rough and ready gain calculations and the 2K7 and 680K attenuates the signal by -48 dB and V2 has a gain of roughly 44dB... I can only estimate the gain from the EF86 data sheet as i want to go and make a coffee

Terry

[cathoderay57]

Originally Posted by Valvepower

Out of curiosity what is the history of the suspected rouge output transformer?

Terry

Hi Terry,

I bought this amp (71237) about 15 years ago. I was told it had belonged to somebody who had worked for ITV, but whether it was ever used commercially or as a private asset I don't know. When I got it one of the 0.1uF KT66 grid couplers was very leaky so I replaced it but the connected valve had already been damaged since, although the amp worked, one of the KT66 anodes started to glow red by virtue of the common cathode resistor being unable to deliver correct biasing to both valves simultaneously. The amp had also been stored for some time in damp conditions as the baseplate had some corrosion. I guess neither of these aspects of the amp's history would have done the OPT much good. I stripped and re-sprayed the chassis, changed the out of tolerance resistors and leaky caps and fitted a new matched pair of KT66. It has (apparently, at least) worked fine since! Jerry

[cathoderay57]

Yesterday, I attempted to do a Bode gain/phase plot along the lines described in the link in Gabriel's Post #35. First I removed the zobel filter that I fitted earlier between the KT66 grid1s, and removed the negative feedback wire from the OPT tag Q. Then I connected the Farnell LFM sig gen 600 Ohm sine wave output to the open end of the 470 Ohm NFB resistor and also to the scope Y input. I connected the scope X input to the speaker output, using a 10 Ohm dummy load. My scope is a Tektronix T912 and although it has an X-Y function it doesn't allow you to turn off the timebase and so even though I managed to get the 6cm ellipse Lissajou figure floating slowly across the screen using the slowest timebase (0.5 seconds) I could not get the trigger to lock so as to take accurate measurements to determine relative phase. I looked at the scope manual and it recommended using the dual trace instead to compare and measure phase differences. I've tried this but I'm struggling and could do with some advice. At frequencies around 100 Hz it's clear that the output is 180 degrees out of phase with the input, which is what you would expect. Going up in frequency the phase shift begins to change with the output trace moving right relative to the input trace. For example, I've attached a sketch of the traces at 1 kHz which show the output lagging the input by 268 degrees. The article in Gabriel's link talks about a first breakpoint at 135 degrees and a second at 45 degrees but I can't get my head around whereabouts on the input and output signals these phase differences are referenced, as compared to the way I have done it. I'm probably missing something obvious so please be kind. It's a long time since I did this sort of circuit theory..... I cannot do the image at present so I'll post it separately. J

[cathoderay57]

Here's the sketch

[trobbins]

Normally the amp input signal is taken as the 'input' for gain-phase assessment of an amp. The dummy load should be pretty close to the defined output impedance, and should be resistive out to beyond 100kHz (ie. made from say 2W metal film resistors, rather than say 10W ceramic box resistors).

That T912 has some options for X-Y in the manual - hopefully you can achieve a stable straightline X-Y figure for say 1kHz mid-band response, with an ellipse forming at LF and HF ends of the audio band.

[cathoderay57]

In post #66 I said the sketch was made of the phase difference at 1 kHz. This should have said 10 kHz. Thanks trobbins. I have tried as many options as possible with the scope but I was unable to get the Lissajou ellipse to lock. Pressing the positive/negative slope button gets the ellipse to sweep once from left to right then the screen goes blank. Maybe there's a fault on the scope but I don't want to divert this thread down that rabbit hole; my main aim is to get a clearer understanding of how the 135 and 45 degree break points are related to the end-to-end open loop phase difference as shown by the input and output signals displayed on a scope. J

[PYE 405]

Just for interest, I tried one of my amp's to see any ringing. Firstly, I applied 10khz and then 1khz for 1 watt output into a 15 ohm resistive load.
See first pic below, second is 1khz and then the third is the same but with the 'scope timebase on X10. Feel free to ask for any further details or tests, but nothing too elaborate as I have only simple test equipment.

The amp is as near to original as possible, but with replaced cathode bypass electrolytic and TCC 0.1uf coupling capacitors. These have zero DC leakage and have ceramic end caps as opposed to rubber as in the original Hunts type.

[cathoderay57]

I've tried increasing the capacitance of the zobel filter across the KT66 grids. All that happens is the initial overshoot decreases but the ringing stays the same. Increasing the cap value still further results in an increase in risetime and a slope-shouldered square wave. So before I go totally crackers I'm buttoning it up and will continue just to use it with the existing transformer and the zobel filter comprising a 22k resistor and 30pF cap across the grids. Thanks for all the inputs and feel free to continue to contribute but I think I've got to the end of this particular road. From a bit of reading I think the 135 and 45 degree points are maybe referring to phase margin which is 180 degrees minus the end-to-end phase shift of the open loop amp but that still doesn't help me make much sense of my measurements.... J

[GrimJosef]

Originally Posted by PYE 405

... Firstly, I applied 10khz and then 1khz for 1 watt output into a 15 ohm resistive load ...

Do you know what the risetime of the input square wave was, or at least an upper limit on it ?

Cheers,

GJ

[PYE 405]

Hi GJ,
I used a TTI TG502 and the rise and fall times are specified at less than 45ns.

[GrimJosef]

Thanks, nice and quick then. And with 50ohm output.

Your amp seems to have practically no overshoot. I wonder if there's something subtly different about the 15ohm output configuration ?

Cheers,

GJ

[PYE 405]

Originally Posted by GrimJosef

Thanks, nice and quick then. And with 50ohm output.

Your amp seems to have practically no overshoot. I wonder if there's something subtly different about the 15ohm output configuration ?

Cheers,

GJ

Yes, a 50ohm output on the gen, not sure about the 15ohm tapping on the amp making any difference.
I have repeated the 1khz test, but this time in the first photo, have adjusted the 'scope to display the ringing to maximum effect. Second pic is a normal setting.
I am using original Mullard EF86's and GEC KT66, but can't see this making a difference?

[PYE 405]

It might surprise poeple to see in my previous photo's of the under chassis that I have left the HUNTS mouldseal 0.1uf 150v cap in place. This, along with the one in the other amp, is still in good electrical condition and I see no need to replace it. Unusual for such a type of capacitor I know. The resistors are still sensibly in spec too.

[Valvepower]

Hello,

I had a crack at measuring the phase and amplitude etc. And I’ll post some results in the next day or two.

Off to the workshop to do a bit of wiring of the current guitar amp project... I feel some heater wring coming on

Terry

[audiomm]

I've been following this thread with great interest. Many moons ago I took some photos of the waveforms at various frequncies on some Quad II's I had in for repair at that time. I finally found the pix today but I cannot find the accompanying notes I wrote at the time (c.2004). I have changed residences and computers 4 times since then so was surprised I still had the 'scope shots. Well that's my excuse So my memory is a bit vague but for what it is worth, I got the same/similar results for both amps and subsequent repairs to others I have done since then -albeit quite a few- have all shown similar responses,so I conclude that these may be 'normal' for Quad II's?
Here are the photos.. Mike

[PaulM]

As the long-term owner and user of a pair of Quad IIs I've followed this thread with interest.

I just wonder whether this is 'real world'? Using a resistor as a dummy load is all well and good, but a loudspeaker is anything but a pure resistor. Are these observations of ringing something that happens in reality? Has anybody tried doing this with a real loudspeaker load? It's a complex impedance (in the fullest sense of the word) and how it reacts with the phase/gain response of the amplifier will not be simple.

OK, park the loudspeaker in another room, but do give the amplifier a real world load!

Best regards,

Paul M

[Gabe001]

Originally Posted by PaulM

As the long-term owner and user of a pair of Quad IIs I've followed this thread with interest.

I just wonder whether this is 'real world'? Using a resistor as a dummy load is all well and good, but a loudspeaker is anything but a pure resistor. Are these observations of ringing something that happens in reality? Has anybody tried doing this with a real loudspeaker load? It's a complex impedance (in the fullest sense of the word) and how it reacts with the phase/gain response of the amplifier will not be simple.

OK, park the loudspeaker in another room, but do give the amplifier a real world load!

Best regards,

Paul M

There are many variations of the sqaure wave test, including using a capacitor across the output (220nf), to try and mimic the effect of electrostatic tweeter loads, loudspeakers crossovers and even to some extent, long cables. Feel free to try it. It will make the square wave worse, but not much worse if the amp is decent.

With regards to the OPs ringing, I wonder if a few nanofarads across the feedback resistor would help.