Hello, I have got a Line Magnetic Mini 218ia, single ended triode, 3+3 watt. The room is not small by italian standards, 46 square meters, but t is a nearfield listening set up, I'm about 2.5 meters away from the speakers.
I have been driving with it a couple of Triangle Plaisir Lymna bookshelf speakers, rated at 90 db/1w/1m, nominal impedance 6 ohm, minimum impedance 4.1 ohm. These are Triangle data. The 90 db of sensitivity may be a bit overrated but it worked well. I used the 4 ohm tap and listened with the volume knob at 9 or 10 o clock.
Line Magnetic specify that the amp can drive 6 ohm speakers, they say to try both the 8 ohm and 4 ohm taps to get a better result.
I now got a couple of Kef Coda 7 from the late nineties. Rated by Kef at 91 db sensitivity. Nominal impedance 6 ohm. I found an old online review where they measured a minimum impedance of 4.6 ohm and a slightly lower sensitivity, 90 db 1w/1m. They didn't talk about phase angle.
These Kef are more efficient than the Triangle, I use the 4 ohm tap and listen with the volume knob at only 7 o clock.
I prefer these, they have got higher sensitivity and a more british, relaxed sound. But the amplifier power transformer gets hotter with these. It is not scorching hot to the point of getting burned, just hotter. On paper these shouldn't be a very difficult load for a tube amp, should they? I can't hear problems with the bass or signs of distortion. I have tried 4 and 8 ohm taps but they seem to work better with the 4 ohm.
Is the power transformer higher temperature worrying? Should I stop using the Kef?
Thanks in advance
I have been driving with it a couple of Triangle Plaisir Lymna bookshelf speakers, rated at 90 db/1w/1m, nominal impedance 6 ohm, minimum impedance 4.1 ohm. These are Triangle data. The 90 db of sensitivity may be a bit overrated but it worked well. I used the 4 ohm tap and listened with the volume knob at 9 or 10 o clock.
Line Magnetic specify that the amp can drive 6 ohm speakers, they say to try both the 8 ohm and 4 ohm taps to get a better result.
I now got a couple of Kef Coda 7 from the late nineties. Rated by Kef at 91 db sensitivity. Nominal impedance 6 ohm. I found an old online review where they measured a minimum impedance of 4.6 ohm and a slightly lower sensitivity, 90 db 1w/1m. They didn't talk about phase angle.
These Kef are more efficient than the Triangle, I use the 4 ohm tap and listen with the volume knob at only 7 o clock.
I prefer these, they have got higher sensitivity and a more british, relaxed sound. But the amplifier power transformer gets hotter with these. It is not scorching hot to the point of getting burned, just hotter. On paper these shouldn't be a very difficult load for a tube amp, should they? I can't hear problems with the bass or signs of distortion. I have tried 4 and 8 ohm taps but they seem to work better with the 4 ohm.
Is the power transformer higher temperature worrying? Should I stop using the Kef?
Thanks in advance
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If you can touch it without suffering, it is about 40°C that is quite good.
Class A amplifiers if not overloaded, mantains its power drain between small ranges. So in case of overtemperature, there must be negligible difference with and without audio programm. If not, thete is other issue.
Class A amplifiers if not overloaded, mantains its power drain between small ranges. So in case of overtemperature, there must be negligible difference with and without audio programm. If not, thete is other issue.
Thanks.
I knew that in a single ended triode the output tubes work even without signal. But I'm afraid I didn't understand your answer.
I'll try to reformulate the question. I recently purchased a couple of second hand Kef speakers for this SET amp. They are nearly 30 years old. I was previosly driving with this amp some Triangle speakers. They are both 6 ohm speakers nominal. The Kef are more efficient than the Triangle, the volume is only at 7 o clock versus 9 or 10 o clock with the Triangle.
But the power trasformer with the Kef gets hotter after just one hour of listening. It's pretty hot. I put back the Triangle and after one hour the power trasformer is cooler than with the Kef.
Since these Kef Coda 7 are 30 years old and I purchased them used, could it be that the capacitors on the crossovers went out of spec and present now a difficult load? Or that the previous owner tampered with the crossover or the speakers?
On paper these Kef shouldn't be a heavy load. Impedance always between 6 and 4.6 ohm, sensitivity 90 db measured.
To make a long story short:
I'm using now the Triangle speakers and everything is ok, the transformer is way cooler, but:
In a single ended triode amp, if the speakers are for some reason a difficult load, the power trasformer may get very hot after one hour of listening?
Bad capacitors in a 30 years old speakers which I purchased used may cause that? Is it worth having them replaced on the crossover? What else could cause this problem in a 30 years old speaker?
Thanks
I knew that in a single ended triode the output tubes work even without signal. But I'm afraid I didn't understand your answer.
I'll try to reformulate the question. I recently purchased a couple of second hand Kef speakers for this SET amp. They are nearly 30 years old. I was previosly driving with this amp some Triangle speakers. They are both 6 ohm speakers nominal. The Kef are more efficient than the Triangle, the volume is only at 7 o clock versus 9 or 10 o clock with the Triangle.
But the power trasformer with the Kef gets hotter after just one hour of listening. It's pretty hot. I put back the Triangle and after one hour the power trasformer is cooler than with the Kef.
Since these Kef Coda 7 are 30 years old and I purchased them used, could it be that the capacitors on the crossovers went out of spec and present now a difficult load? Or that the previous owner tampered with the crossover or the speakers?
On paper these Kef shouldn't be a heavy load. Impedance always between 6 and 4.6 ohm, sensitivity 90 db measured.
To make a long story short:
I'm using now the Triangle speakers and everything is ok, the transformer is way cooler, but:
In a single ended triode amp, if the speakers are for some reason a difficult load, the power trasformer may get very hot after one hour of listening?
Bad capacitors in a 30 years old speakers which I purchased used may cause that? Is it worth having them replaced on the crossover? What else could cause this problem in a 30 years old speaker?
Thanks
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Single ended amplifiers (with tubes or transistors) are always in A class and have a constant load on the mains transformer regardless of the signal level. According to the construction of the output stage, they cannot leave the A class, as push pull amplifiers in the A class can. Maximum quiescent current constantly flows through the output stage, and part of that current is fed into the speakers as the signal increases. So this is a very strange problem. 🤔
One consideration is about the possibility that this amp has a Feedback so a instability with a particular load can happen and more current will flow
Walter
Walter
Yes, but if he doesn't have an oscilloscope, he won't know. In any case, if it oscillates, the output tubes are also at risk. I would not use those speakers until it is checked what is going on with a scope. Since the speakers have variable impedance and phase, oscillation is probably possible if the amplifier has NFB.
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If NFB is negative feedback, I couldn't find out if it has it or not. It is not specified on the user manual. I couldn't find schematics either, otherwise I would have showed it to you. I will stop using those speakers. The amp works fine with the Triangle.
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Yes, NFB is a negative feedback loop. It may or may not be there, it is more likely that it has not been applied.
Obviously there is a problem, find someone with measuring equipment. I looked for the schematic too, it's nowhere to be found.
These are pretty simple schematics, but then again, when it's not there, you can't know how they did it, unless someone does the reverse engineering and draws the schematics.
Obviously there is a problem, find someone with measuring equipment. I looked for the schematic too, it's nowhere to be found.
These are pretty simple schematics, but then again, when it's not there, you can't know how they did it, unless someone does the reverse engineering and draws the schematics.
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The more expensive single ended Line Magnetic models have a switch to adjust negative feedback, so it is possible that they applied some to this amp.
I'm not an expert and don't have a tube amp specialist in my town. For the time being I will see if it keeps working well with the Triangle speakers.
But there's always the possibility that the Kef speakers were tampered with by the previous owner.
I'm not an expert and don't have a tube amp specialist in my town. For the time being I will see if it keeps working well with the Triangle speakers.
But there's always the possibility that the Kef speakers were tampered with by the previous owner.
Not sure there is overheating depending on load, can you measure it?
Even a plain glass kitchen thermometer will do.
Also remember we are not worried about absolute temperature here but differential: how much it raises above ambient temperature.
Even a plain glass kitchen thermometer will do.
Also remember we are not worried about absolute temperature here but differential: how much it raises above ambient temperature.
Might be oscillating. If it is single ended class A running stable, then it actually should run hottest with nothing playing -- constant power into the circuit, but none of that going out the the speaker -- even with low load impedances. (Though with music, the amount of actual average power going out to speakers is usually pretty small except in P.A. situations or frat parties). So, if it is running hotter then I'd suspect that it is not actually running stable. Or, possibly, the bias point has just by coincidence drifted upwards -- SE class A isn't known for bias stability over time.
Thanks anyone
With the Triangle speakers I have been using the temperature is nothing to worry about, it's always been the same kind of heat after one hour. Yes, even without signal, when i warm up the amp before listening, the temperature grows but I have never left it one hour without signal, so I can't tell the difference.
In any case, the transformer is in a metal case, so it takes about 40 minutes to get really warm to the touch. Then, after about 40 minutes the temperature stabilizes.
With the Kef it seems that the transformer runs hotter. I don't know...
This is an image of the inside of the amplifier. Perhaps you can understand something about the circuit.
This is not my amp, I found the image
online. It is a 120 V, american version. Mine is 220 V.
The thermometer I have is in a plastic case, without point of contact. I will get another and measure the tranformer temperature, thanks.
As for the bias, I suppose the amplifier has got autobias but I don't know. I will try to get some information.
With the Triangle speakers I have been using the temperature is nothing to worry about, it's always been the same kind of heat after one hour. Yes, even without signal, when i warm up the amp before listening, the temperature grows but I have never left it one hour without signal, so I can't tell the difference.
In any case, the transformer is in a metal case, so it takes about 40 minutes to get really warm to the touch. Then, after about 40 minutes the temperature stabilizes.
With the Kef it seems that the transformer runs hotter. I don't know...
This is an image of the inside of the amplifier. Perhaps you can understand something about the circuit.
This is not my amp, I found the image
The thermometer I have is in a plastic case, without point of contact. I will get another and measure the tranformer temperature, thanks.
As for the bias, I suppose the amplifier has got autobias but I don't know. I will try to get some information.
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The white thing at the bottom of the photo? That's not a transformer, that's a choke, possibly the feed inductor for an SE output device. Or, more likely if this is a tube amp, a power supply filter. Either way, getting hotter would mean it is drawing more current, which would happen if it were oscillating. If you had an oscilloscope, that would let you check.
Connect your old speakers, turn the amp on. Measure the room temperature and the transformer temperature after one hour. Next day, connect the KEFs turn amp on. Measure the room temperature and the transformer temperature after one hour. Compare the temperature rise on transformer between day one and day two. Is it excessive, i.e. more than 10 degrees? If not then you are okay. Fingers cannot measure temperature, not even close.
It seems that is UL configuration; there are two wires, one white and one gray that go from trafo to board on noval EL84
And it seems no other connection from output to input socket of amp, as FB.
It is evident that in this way is not possible to say others consideration.
Walter
And it seems no other connection from output to input socket of amp, as FB.
It is evident that in this way is not possible to say others consideration.
Walter