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26th Mar 2023, 2:53 pm | #1 |
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Quad II mains transformer 'no load' current
I have a pair of Quad II mono amplifiers (GZ32 rectifier, push-pull KT66s driven by a pair of EF86s, rated for 15W) in for service. After a few hours soak-testing I noticed that the mains transformer of one of them was running distinctly hotter to the touch than the other. This might have been due, in some part, to their positioning on my living room floor, with each amp getting slightly different ventilation and seeing different radiative heating from the other amp's valves. But I was concerned that there might be some fault in the hotter transformer so I tried some more quick-and-dirty tests.
I knew the 'hot' amp also had higher-current KT66s in, so I swapped the complete valve sets between one amp and the other. This reduced the temperature difference between the amps but didn't eliminate it. So I let the amps cool completely and then powered them with all of the valves removed. This should have left all three windings (315V-0V-315V HT, 3.15V-0V-3.15V LT and 0V-5V rectifier heater) with no load. The mains transformer on the cool amp didn't warm significantly, but after a couple of hours the one on the hot amp, while not what you'd call 'warm', was clearly 'less cold' than the other. I rounded up all the other Quad IIs I could conveniently access (some mine, some here 'visiting'), put an AC milliammeter (Fluke 89 IV) in the mains neutral lead and measured the no load currents into the amps' primaries. I also used a hand-held inductance meter (Tenma 72-8155) to try to measure the primary inductances in case a partially shorted turn might reveal itself in an unusually low inductance. The results (amp serial number, steady-state no-load current, inductance on Tenma 20H range) were CUSTOMER'S AMPS 443xx, 36-37mA, 1.7H (the cooler amp) 901xx, 50mA, 0.9H (the hotter amp) OTHER AMPS 453xx, 52mA, 1.9H 512xx, 56mA, 1.4H 624xx, 43-44mA, 2.3H 66913, 47-48mA, 1.7H (one of my own 'living room' pair of amps) 66960, 39-40mA, 1.4H (the other one) When I measured my customer's amps I thought that the differences in current and inductance might be significant and that I was going to have to suggest that the hot transformer might be on its way out. But the results from the other amps show that actually there's quite a spread of no load currents and the apparent correlation between high current and low inductance doesn't always hold. In case it helps the primary current when there are valves in the amp (measured on the 'hot' amp) settles around 490mA. So I'm not sure quite what to make of this. I don't know how much consistency I should expect between the transformers under no load conditions, nor whether the causes of the variations might include some inconsequential things (different insulation levels between the core laminations leading to eddy current heating ? voids in the potting bitumen leading to different transformer can temperatures ?) as well as possible faults (partially shorted turn in the windings ? some minor current leakage in the secondary circuits but outside the transformer ?). I'd appreciate any thoughts. Cheers, GJ
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26th Mar 2023, 3:03 pm | #2 |
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Re: Quad II mains transformer 'no load' current
Hi GJ, is there any evidence that one of the mains transformers been rewound, or replaced with one from a different manufacturer, and repotted into an old enclosure? The difference in inductance maybe pointing in this direction. Jerry
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26th Mar 2023, 3:11 pm | #3 |
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Re: Quad II mains transformer 'no load' current
Not that I can see Jerry, but to be honest I haven't dismounted the hot transformer from its chassis and wiring loom and without doing that I can't really see much of its underside (particularly the holes through which the bitumen is poured in).
One point that might be significant is that Quad made the Quad II over a continuous period from 1953 to 1967/8 during which time the perspex serial number plate carried numbers from 2000 to about 83500. Then there was about a year's gap after which Quad produced perhaps a few hundred more amps, starting at serial number 90000. The hot amp is one of these. They did use some different components e.g. the IO valveholders were of a different type, the cathode bypass electrolytic was different too, as was the serial number plate itself (now metal, not perspex). So I suppose it's possible that they sourced the mains transformer from a different supplier. But I've no way of knowing. Cheers, GJ
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26th Mar 2023, 3:55 pm | #4 |
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Re: Quad II mains transformer 'no load' current
Have you tried a primary/secondaries insulation test? It would have to be at a suitably high voltage of course.
Craig
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26th Mar 2023, 4:43 pm | #5 |
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Re: Quad II mains transformer 'no load' current
I haven't Craig, but two of the secondaries have centre-taps to the chassis/ground and I'm testing with the chassis safety-earthed so I know there isn't gross leakage there (then again the RCD on the consumer unit here is a 30mA one I think, so it wouldn't alert me to 10mA going astray). There's also a grounded interwinding screen in the transformer so any leak to a secondary would have to be bypassing that somehow. But a leak to the interwinding screen itself might be a possibility.
Also, of course, I'm seeing somewhat raised current in the mains neutral line, where my meter is, and I'm not sure that leakage out of the primary would register there, would it ? Cheers, GJ
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26th Mar 2023, 4:56 pm | #6 | |
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Re: Quad II mains transformer 'no load' current
Quote:
Might I suggest that you re-measure the inductances with a proper bridge type instrument and balance not only the inductance but also D factor. D might shed some light. Best at a realistic 50Hz as well. We could simply be looking at not-so-well coated laminations giving eddy losses. David
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26th Mar 2023, 5:47 pm | #7 |
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Re: Quad II mains transformer 'no load' current
I don't have a full-blown inductance bridge here David, much though it might be the best thing for the job. Tenma say that measurements on the 20H range are taken at 100Hz. I've found that when I can get a measurement on the 2H range the inductance can read as much as 30% higher than on the 20H, but the 2H measurements are at 1kHz and perhaps the effects of strays will be different then ?
I've just checked for any AC flowing down the mains earth lead. There seems to be about 40uA, so a negligible amount compared with the primary current. At 50Hz 1H presents a reactance of 314ohms or so (not sure if ohms are the right units, but at least they're dimensionally correct - volts/amps). So if the primary inductance was the only thing limiting the current then I'd expect 240V/314ohm=0.75A to be flowing, albeit 90 degrees out of phase with the voltage. Instead I'm seeing typically 5% of that in all the transformers. The primary resistance is about 12ohms, so that won't be limiting the current either. I have to say I'm suspicious of the inductance measurements. Cheers, GJ
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26th Mar 2023, 7:15 pm | #8 |
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Re: Quad II mains transformer 'no load' current
40uA down the interwinding screen implies a winding to screen capacitance of somewhere around 500-600pF.
That sounds like a reasonable sort of number. Craig
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26th Mar 2023, 7:51 pm | #9 |
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Re: Quad II mains transformer 'no load' current
Could well be. I don't know if there's any convention about putting the live end of the mains primary closest to the interwinding screen or furthest from it. If it were me I might put it furthest from it to minimise the capacitatively coupled earth current. That might also put the highest voltage wiring close to the centre of the bobbin, which might be the best insulated place. But those arguments might be trumped by something like the routing of the tappings' tails.
Cheers, GJ
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26th Mar 2023, 8:02 pm | #10 |
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Re: Quad II mains transformer 'no load' current
Hi GJ,
I have a Quad II with a metal plate Ser# 90427 I’ll endeavour to get it on the bench and pull the valves to see what it’s quiescent current consumption measures. I'll use a Fluke 197 True RMS Multimeter. I naturally assume the mains it set at 240V. The bench is still warm from finishing off the last of the the bits a this and bits a that clone amps and the Marshall JTM50, Fender 5F6 mash-up using sound City 50Watt Partridge transformers - I'd forgotten just how hard those EL34's bottles were being driven in those 50Watt amps! I'm getting 60Watts at clip! Regards Terry |
26th Mar 2023, 8:57 pm | #11 |
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Re: Quad II mains transformer 'no load' current
And the results from the Norwegian Jury…
240V (set with an isolating Variac) No valves. Primary quiescent current = 40.5mA 5V = 5.35V 6.3V = 6.91V HT = 335.8/335.5V Inductance @1K using a Peak Atlas LCR40 = 940.6mH (this is out of Calibration by a year!) Terry |
26th Mar 2023, 9:03 pm | #12 |
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Re: Quad II mains transformer 'no load' current
Thanks Terry, yes the mains setting was 240V (who knows what the voltage actually was here though ...). I suspect it makes little difference but I stuck with the convention that looking into the Bulgin chassis plug from outside, with the amp upright obviously, live goes to the right pin and thus via the fuseholder and voltage selector while neutral goes to the left pin and direct to the transformer. I grounded the amps via the lower speaker socket. I also wired a plug-and-socket short across the meter so I didn't blow one of Fluke's expensive fuses with any inrush current at switch-on. Once the amp was powered I opened up the short to take the reading, then closed it again before switch-off.
EDIT: Posts crossed, I think. Your quiescent current is 10mA less than mine, but comfortably within the range of currents I measured. Your 0.94H inductance is very close to mine. Cheers, GJ
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26th Mar 2023, 9:44 pm | #13 |
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Re: Quad II mains transformer 'no load' current
More results from the Norwegian Jury…
245V = 42.7mA Quiescent with no signal with valves at 240V = 0.48A At clip (full power) at 240V = 0.513A Terry |
26th Mar 2023, 10:09 pm | #14 |
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Re: Quad II mains transformer 'no load' current
Close agreement with the current I measured with valves in too .
Cheers, GJ
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26th Mar 2023, 10:13 pm | #15 |
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Re: Quad II mains transformer 'no load' current
Some thoughts in random order:
It's interesting that the 'hot' transformer is from the late production run. I saw a similar effect where a commercial audio device I used to build into a system received a 'face-lift' which included re-sourcing some parts. It was clearly an attempt to reduce manufacturing cost and one of the parts affected was the mains transformer. Visibly less well finished, acoustically noisier, and ran hotter at the same voltage compared to the original (the load was negligible). How do they compare at lower line voltage? A test at 220V might prove that the newer transformer is slighly less generously built and starting to run out of iron at the prevailing mains voltage of your previous test. It might be informative to resolve the line current into the active and reactive components, and hence separate the magnetising VARs from the loss power. A few shorted turns won't necessarily manifest as significantly different inductance. As an alternative to using a bridge you could try ring-testing it, taking care not to zap the HT secondary insulation. What I don't know, in the case of iron-cored transformers, is whether high eddy current loss due to poorly insulated lams, gives equal damping on the ring test as copper loss due to shorted turns that dissipates the same power at normal flux density. Shorted turns dissipate high power in a small volume, resulting in hot-spots that progressively destroy the winding. Eddy-current loss from shorted lams in a small transformer core is less concentrated, has much lower thermal resistance to ambient, and might persist harmlessly forever. (This is not true of a turbo-alternator stator; high iron loss is dangerous and can result in a meltdown. Preventative diagnostic thermal imaging is used during overhauls to map the loss density throughout the core.) |
26th Mar 2023, 10:57 pm | #16 |
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Re: Quad II mains transformer 'no load' current
Wise to be so.
Inductance varies an awful lot with drive level. Inductance ranges of general purpose multimeter thingies are waveform-based and get their knickers well and truly knotted with inductors. Even with a bridge, you might want to add some bias if there is DC kicking around uncancelled, like single-ended output transformers. It all gets awfully messy. I have a Wayne-Kerr ratio transformer-based bridge here which is fairly trustworthy in measuring G and C which get recalculated as reactance and loss resistance terms. Unfortunately, distance is the problem. David
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27th Mar 2023, 12:28 am | #17 |
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Re: Quad II mains transformer 'no load' current
It may be worthwhile making some simple temperature rise measurements to gauge the level of concern needed.
Assuming the PT primary DCR was even up to 50 ohm, the winding dissipation from magnetising current is going to be quite low, and likely negligible compared to total PT loss which is going to be somewhat less than the VA value of 12W. You could pass some dc current through that winding and coarsely adjust to get a temperature rise similar to what is measured with no valves in place (being cautious about the max current level and noting the final DCR at the higher operating temp). That would then indicate the total losses in the PT for magnetising operation. If you had a cheap mains power widget/meter, and it was someway accurate for such a megnatising current harmonic makeup, then it can likely also estimate real W power consumption. The measured temp rise from magnetising current should not directly translate to the same temp rise at normal amp idle, due to a change in effective thermal resistance of the PT at the different temp, but you may see a similar PT temperature difference between the two amps (when using the same KT66 pair and assuming similar bias conditions). Actually measuring the external skin temperature of the PT is also a sanity check of what really is a concern, given the PT was I believe rated for 40-80Hz operation and up to 250Vac. |
27th Mar 2023, 8:15 am | #18 |
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Re: Quad II mains transformer 'no load' current
A classic era device that specifically measures inductance under typical operating conditions is the General Radio 1633A incremental inductance bridge https://www.ietlabs.com/pdf/Manuals/...a%20Manual.pdf
I'd of course like one. But they are so darned useful the are still quite expensive, and are usually in the US with eye popping shipment. Craig
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27th Mar 2023, 12:01 pm | #19 |
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Re: Quad II mains transformer 'no load' current
Hello,
Righty-ho, I’ve taken a second earlier Quad II serial number #56xxx and I’m running that along with the 90xxx which I measured the current draw yesterday. I’m running both of them with the valves. I’ll run them for 4 hours and I’ll use a thermal camera to measure the temperatures of the transformers. Just some thought regards “Quad produced perhaps a few hundred more amps” Like GJ I also noted the different octal bases made by Cinch, this is as well as the metal ID plate which may indicate some acceptable cost engineering in certain areas seeing these amplifiers were made in the final years of the production of the Quad II in the final days of the valve era. I have read Leak carried on also making the TL50 into the final days of the valve era, however, in the final days of the production of the TL50, Leak saw this as making the last few TL amplifiers to get to the end of valve amplifiers. They used parts like the round RS ‘dropper’ resistors to complete and fulfil the final orders of the TL50 amplifier. I figure Quad wouldn’t have compromised the Quad II, as they were better than that, but in the final days, there may have been little interest in the ‘old’ valve amplifier, and certain ‘acceptable’ compromises were made to see the Quad II to the end to fulfil the final orders – so to speak. Terry |
27th Mar 2023, 12:51 pm | #20 |
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Re: Quad II mains transformer 'no load' current
As David says, inductance measurements vary with frequency, voltage used for measurement, recent previous history, phase of the moon, time of the month, hour of the day...
As long as inductance is high enough, it doesn't matter. Measuring off-load current isn't always helpful either, it could be inductive (wattless) current, or in-phase current due to core loss (hysteresis); core loss (eddy currents), winding loss (shorted turns). The temperature rise is a bit worrying, though if it runs like that for a long time, it probably isn't a shorted turn. Dissipation factor, or Q, isn't very helpful! You can do something like use a lower-permeability iron, or assemble the core with an air gap, and Q or DF will actually improve, because reactive quadrature current is now so much larger. It's in-phase current that matters... and that's difficult to assess without a power analyser, vector voltmeter, or equivalent. A shorted turn will create a hot-spot that will fail relatively quickly. A high core loss, although undesirable, won't... I'd arrange to leave running off-load and clock up as many hours as possible, either supervised, or with a low-current fuse in circuit and a thermal cut-out fixed to the transformer. |