Quad II test run.

[Radio Wrangler]

Could this be a simple confusion?

Changing where the wires take the U and W signals to will not change the phase of the signals on the U and W turrets.

If you want to reverse the phase, then you have to swap which valves are connected to those turrets.

If you want to swap the phases of the signals on the turrets, then you have to swap the winding lead-out wires inside the transformer which go to the turrets.

David

[Herald1360]

Talking of confusion, I'm quite lost. Any chance of a circuit diagram of the transformer showing the windings, their connections and their relative phasing- the dot at the end style of thing?

[snowman_al]

Like this?

And I am confused too...

[sparkymike]

http://www.turneraudio.com.au/quad2powerampmods.html

Diagram here of the OPT.

[Radio Wrangler]

Perhaps, if the phasing is in doubt, we should switch to talking of cathode of V3, anode of V2 and so on.

The turret tags and thewindings they're connected to are what's in doubt.

The valve electrode identities are what we want to get right.

David

[GrimJosef]

Originally Posted by sparkymike

I tried your last test and V W and YZ are out of phase.
Reversed U W and result the same, still out of phase.
Mike.

Apply the usual 8V AC drive signal to PT. Then with a multimeter on its AC Volts range measure the voltage you get between V and W, then between V and U, then between U and W. Check the AC volts between P and T too.

Cheers,

GJ

[sparkymike]

In my mind, I feel sure it is the OPT . There are so many combinations that come into play here, for example, U-V and V-W windings. The two coils are linked internally on the inner V turret. Now we know that both of the coil's resistance is correct, but either ends of one coil could be on V-W or W-V turret and the same on coil UV. It would not be too much trouble to change internally as this OPT has not been potted yet.
Mike.

[Herald1360]

Originally Posted by snowman_al

Like this?

And I am confused too...

Like that but with the dots added to the appropriate end of each winding to show phasing.

As in if you apply a signal to the dot end of a winding, ground to the other end then monitor another winding probe to dot end, ground to other end, the applied signal and the monitored signal are in phase with each other.

If you're intimate with the workings of a Quad II you should be able to infer where the dots should go for all of the windings as shown in the circuit diagram. At that point it becomes easy to check that the transformer internal wiring is correct then to decide where each of the lettered tags should be connected if for any reason the TX is wrongly wired to the tags.

And to follow all the descriptions of what should be in phase or out of phase with what!

[PJL]

Post #94 phasing matches the circuit diagram in trader 1427 with U/Z (V5) in phase and W/X (V6) out of phase - but the trader may be wrong!

Post #50 scope pictures are showing W as in phase and Z as in in phase so that differs from the trader sheet. The connections to turrets U and W need to be swapped as suggested by GJ.

I wouldn't bother rewiring the transformer connections as the amp can be tested simply by changing the connections to the turrets so that U connects to cathode V6 and W to cathode V5, then try the amp again.

[sparkymike]

Hi Chris,
Be handy if someone has such a drawing of the winding orientation.That may well be needed quite soon.
Mike.

[GrimJosef]

Ah.

I may have been talking at cross-purposes for rather a long time.

When suspicion arose about the connections between the transformer and its turrets, I assumed that what we were aiming to do was to change those connections so that the windings would end up wired to the correct turrets. I've been encouraging Mike to make those changes but it suddenly occurs to me that he might have been changing the wires between the turrets and the valves, which would have very different effects on the measured phasing (usually none, unless one of the AC grounded centre-tap connections was involved).

An electrical analogy would be if we were to find a 13A outlet in our kitchen which had live on its left terminal and neutral on its right, viewed from the front of the socket. The way to sort this out would be to get into the back box behind the socket and swap the blue and brown wires from the twin-and-earth mains supply cable so that they were connected to the correct screw-connectors on the rear of the socket. If, for any reason, we couldn't do that, say because the back box (=transformer can) had been filled with potting compound, then the next best thing would be to open up the 13A plug to the appliance and swap the live and neutral wires in there too - blue to the right, brown to the left. That would ensure that the appliance was receiving the correct live and neutral supply. But really it wouldn't be right. The right thing to do would be to fix the problem where the mistake had been made, not to correct for it in a two-wrongs-make-a-right sort of way.

Cheers,

GJ

[Radio Wrangler]

That's what I think has been happening. Mike must have been swapping the wires around between the turrets and the valves. As it's the output transformer driving the turrets, I wasn't surprised that the swapping produced no changes in the phase when the turrets were probed.

If the turret to valve wires were done to sort a cross-over in the transformer, it could create some real confusion for whoever is next into that amplifier!

David

[sparkymike]

Looks like my next step is to remove the OPT from the chassis then ?
Mike.

[PJL]

Taking the transformer apart is not necessary as all we need to do is move the connections so the correct phased windings are attached to the correct valves. If it works then the transformer can be sorted out.

[audiomm]

Originally Posted by sparkymike

In my mind, I feel sure it is the OPT . There are so many combinations that come into play here, for example, U-V and V-W windings. The two coils are linked internally on the inner V turret. Now we know that both of the coil's resistance is correct, but either ends of one coil could be on V-W or W-V turret and the same on coil UV. It would not be too much trouble to change internally as this OPT has not been potted yet.
Mike.

Whoa! That last sentence raises a red flag. If the OPT isn't potted, then it'll have been rewound because , as far as I am aware from servicing these amps for the past 30 yrs or so,all the original Quad transformers and chokes are potted.
That being the likely case(rewound), then there is a very high probability that the wiring to the turrets has been transposed. I have the winding data but it shouldnt be necessary to sort this - just swap the wires as already mentioned and that will most likely fix the out of phase issue. Mike

[PJL]

I would recommend repeating the phase and amplitude tests of #93 but this time putting the channel 2 probe on the valve base:
V5 anode (pin 3)
V5 cathode (pin 8)
V6 anode (pin 3)
V6 cathode (pin 8)

[Radio Wrangler]

There's a little niggle in the back of my mind that the burst of oscillation was shown on only one half-cycle when the amp was playing a sinewave signal.

I wonder if one of the cathode windings is wired reversed between the cathode turret and the centre tap turret. This would put positive feedback on one valve only, the other would still have negative feedback.

Swapping the wires to the cathode turrets would put positive feedback on both valves and I'd expect it to hoot over a full cycle.

Could be wrong, just guessing, but something to bear in mind if things don't go to plan with the cathode connections swapped.

David

[PJL]

Originally Posted by GrimJosef

If we think this through then we ought to be able to convince ourselves that these phases are correct. Let's consider the KT66 V3. This has ZY connected to its anode and UV to its cathode. We've measured the phase of WV and found it to be in phase with TP and therefore with ZY. So since UVW is a centre-tapped winding we know that UV will be out of phase with ZY. If V3's grid voltage rises, causing the valve to pass more anode current, this will pull Z more negative of Y. The out of phase relationship means that U will go more positive of V and will push V3's cathode positive, lowering the V3 current and so providing the negative feedback that it's supposed to.GJ

It is probably me as it is late but I am having trouble understanding this explanation. The current path is Y -> Z -> anode -> cathode -> U -> V so they are already wired 180deg out of phase so for -ve feedback I would expect Z wrt Y to be in phase with U wrt V?

[sparkymike]

I have decided to partially remove the OPT can if possible but leave the external wiring as it is, so this will enable me to change the windings internally to the turrets. I seem to remember that the turret paxolin board was held in to the can with two screws, so not sure yet if I will be able to get to those.
I did a drawing showing some (not all) of the combinations that are possible, just on two of the windings, U-V and W-V. Some of these would cancel out and have same effect as previous ones.
All this exercise takes me back to the late 50's when I decided to build a transistor amplifier as these new fangled things were then coming into vogue.
I had sent away to Mullard for the circuit which I believe was a 50 watt PA amplifier. My father helped me build it. He was much better in the subject than me having built a superhet radiogram. I remember there were two thermistors with a VA number and we had a heck of a job to get them as Mullard had used pre-production parts, and none were available on the open market.
Well to cut a long story short, when it was done, the amp squealed like mad and I shelved it for several years. I went back to it and reversed one pair of the OPT output windings and it worked.!! History is now repeating itself.

[Radio Wrangler]

50W transistor amp in the late 50's was a bold thing to make. Thermistor bias compensation and output transformers (probably a driver transformer as well) make it an interesting early design. Interesting from the historical point of view, but problematical for anyone building one. It was well before effective techniques for taming bias instability (thermal) were evolved. It would have had a ready market in 12V car/van mounted PA systems driving horn speakers on the roof. I wonder how many of them ran around shouting Vote Wilson or Vote MacMillan? Their benefit would be in avoiding vibrators.

The evolved 50W audio amplifier from Mullard is in the second edition of their blue book. It's house-tamed, quite reliable and properly stable both thermally and in avoiding oscillation. I've built some and found them fine.

As the next step for your Quad, I'd stop trying to find a quick fix... it already hasn't been quick! and I'd opt for 'certain' instead. How about analysing your transformer, All windings, to find out just what you have as a first step. Then what to do with it can be planned. If one winding has been reversed, or two winding ends swapped, then there could be other reversals. Let's find them all. It may save time in the long run.

I stopped building valve amps decades ago. I'm lucky that my religion allows me to listen to transistor amplified music... just good transistor amplifiers, mind. All I have to contend with now is a rational impulse to choke the devils making fake audio transistors... Grrr!

David