Just something that makes me wonder. Why do some amps run cold on idle and some hot? For example I can run Pioneer SA-508 (and others) whole day long on idle (no music) and it's cold. On the other hand my Quad 303 is always hot. I mean it's ok to touch the heatsink, maybe 40C or so. Aren't these both AB class? What makes the difference?
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Your Quad should run cold. Check the midpoint voltage and the quiescent current should be between 5 and 10mA.
Differences in the amount of quiescent current ( the amount of current used by the output stage transistors to keep them " open " with no signal ) . Possibly age and or quality of the bias pots also .
Are you sure? It's been running the same temperature for nearly 30 years since I own it. It does get warmer when driven hard off course.
I've just taken a measurement of the heat sink. It's 48C, the Quad has been idle for one hour but stays that way for ever.
So maybe wrong bias? I would ask is that bad? But than we have the 30 years of the same temperature. (Yes I've changed the main caps few months ego).
They say 5-10mA bias but don't mention any heat-sink temperatures. I'm not quiet sure how the check bias on SS amp or even which are the right pots.
Is it bad if it's over biased? I know it can wear out tubes quickly but this is SS.
I've found this site explaining how to do it so I think I'll do the measurements at least for now, hope it's a same model.
http://www.net-audio.co.uk/quad303setup.html
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Two different types of output stage:
Darlington pairs (pairs of the same polarity, forming compound emitter follower) need highish quiescent current (100-200mA)
Complementary pairs need quite low quiescent current (5-20mA)
Two different schools of thought:
Minimum distortion - see above.
'First watt' - high quiescent current so 'small signals operate in Class A' - essentially move any crossover problems away from small signals to mid-level signals, at the likely cost of a bigger transition when it happens.
Darlington pairs (pairs of the same polarity, forming compound emitter follower) need highish quiescent current (100-200mA)
Complementary pairs need quite low quiescent current (5-20mA)
Two different schools of thought:
Minimum distortion - see above.
'First watt' - high quiescent current so 'small signals operate in Class A' - essentially move any crossover problems away from small signals to mid-level signals, at the likely cost of a bigger transition when it happens.
I would say, 5 = 10mA will run cold and in my professional experience of over 40years repairing equipment for major manufacturers, it is cool if not cold with no signal.
Still not convinced?
P = V * I in Watts
Work out the power in no signal mode and then take the heatsink, 0.65 Degrees / Watt if I remember correctly and add it to your ambient temperature.
See ... cold!
Perhaps the heat in the Quad comes from the transformer. Some can run hot when lightly loaded.
Measuring the bias current will tell you all you need to know for this. Simply measure the volt drop across either of the 0.3 ohm emitter resistors and calculate the current using ohms law. 10 milliamps should see a voltage of 3 millivolts DC (so quite small).
Measuring the bias current will tell you all you need to know for this. Simply measure the volt drop across either of the 0.3 ohm emitter resistors and calculate the current using ohms law. 10 milliamps should see a voltage of 3 millivolts DC (so quite small).
Our climate in Oz is around 8°C higher temperature than the UK and this leads to problems with many power amplifiers but not with the conservatively designed 303, even if the paint does restrict heatsink dissipation. I have one and can't say that the heatsink ever rises more that 10° above ambient even with music playing. The paint on it is still as new, so I doubt it ever gets really hot in domestic use. The transformer doesn't get warm until about 30 mins from power-up but that could differ between suppliers. My speakers are not Quad ESLs but average sensitivity KEF monitors, so I guess the average power dissipation is quite low too - perhaps only 10W total maximum (neighbours!).
Not many manufacturers use the front panel as a heatsink so we don't expect it to get warm and perhaps think there is a problem even though it's quite normal for older Quads, including 405, 306 and professional models. If you describe the heatsink as hot rather than warm even if well ventilated, I'd agree there must have always been a problem and I would expect the paint to have cracked and lifted long ago.
Go through the Official Quad service data too: Quad 303 - Manual - Stereo Power Amplifier - HiFi Engine
Not many manufacturers use the front panel as a heatsink so we don't expect it to get warm and perhaps think there is a problem even though it's quite normal for older Quads, including 405, 306 and professional models. If you describe the heatsink as hot rather than warm even if well ventilated, I'd agree there must have always been a problem and I would expect the paint to have cracked and lifted long ago.
Go through the Official Quad service data too: Quad 303 - Manual - Stereo Power Amplifier - HiFi Engine
Well...it does run hot, the heat sink, not the trafo. I've not used it properly for couple of years but did change the original 2200uf caps for 5600uf few months ego because one old one vented. I don't think the higher value is a problem, at least that's what I've found people use when re-caping these.
Anyway now that I think of it, the heat sink might be actually slightly hotter than it always was. It's either that or it's because we have cold weather now and it it "feels" hotter. I wish I would have taken the temperature years ego.
But the heat does scare me a bit now because not long ego I was using an old amp that was also running hot, but hotter and than it blew up one day.
I can check the voltages but I don't understand why would it would drift unless there is something else causing it. I need to open it up when I get time, guessing won't find the problem. Best not to use it till than. I'm glad I've mentioned it and got some responses otherwise I'd just keep using it, thanks.
Anyway now that I think of it, the heat sink might be actually slightly hotter than it always was. It's either that or it's because we have cold weather now and it it "feels" hotter. I wish I would have taken the temperature years ego.
But the heat does scare me a bit now because not long ego I was using an old amp that was also running hot, but hotter and than it blew up one day.
I can check the voltages but I don't understand why would it would drift unless there is something else causing it. I need to open it up when I get time, guessing won't find the problem. Best not to use it till than. I'm glad I've mentioned it and got some responses otherwise I'd just keep using it, thanks.
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Bias current can drift because the trimpots controlling it, the power supply regulation and other voltages maintained with voltage reference circuits can drift. This may be due to component deterioration or even failure. Trimpots are not the most reliable component to use either. A lifespan of >35 years with only breakdown maintenance is at the extreme of expectations for any commercial power electronics and a full refurb. might be advisable. Also, I sometimes encounter amplifiers that have been altered by previous owners to have higher bias current intentionally and so will run hotter - sometimes destroying the output stage in the process but in any case, it's a headache for the next owner and any repairer involved.
These aren't the only faults that could lead to higher dissipation but you have the test details and voltages to check so hopefully, all is still functional and can still be adjusted to factory settings if need be. Consider replacing any pots that need adjustment though.
These aren't the only faults that could lead to higher dissipation but you have the test details and voltages to check so hopefully, all is still functional and can still be adjusted to factory settings if need be. Consider replacing any pots that need adjustment though.
Is the 303 set on 240v? (it has 4 positions)
Also, depending on where you live in Australia, you could have well over 240v. Combine that with poor thermal design and our higher ambient temperature and you get warm running Quads.
All the 405s I've ever worked on, run warm here too.
Bear in mind, pretty much all the vintage UK made stuff used their casework as heatsinks (if you were lucky) or some inadequate heatsinks, often buried inside a sealed case with zero ventilation. At least your 303 has a bit of a heatsink hanging off the end.
Also, depending on where you live in Australia, you could have well over 240v. Combine that with poor thermal design and our higher ambient temperature and you get warm running Quads.
All the 405s I've ever worked on, run warm here too.
Bear in mind, pretty much all the vintage UK made stuff used their casework as heatsinks (if you were lucky) or some inadequate heatsinks, often buried inside a sealed case with zero ventilation. At least your 303 has a bit of a heatsink hanging off the end.
Yes it's on 240V. To put it in perspective the heat sink runs at about same temperature as an average tube amp power trafo on touch, so fairly warm. Since I use tube amps the most, the temperature of the heat sink never crossed my mind, I just thought it was a different design than general AB SS amp (being a Quad), maybe tuned more to A class operation (if that's possible).
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