Hello All,
Every so often I check the heat sink temperature of my J with an infrared thermometer to make sure things “seem ok”. I’ve noticed that after about an hour or two at idle without music playing or with lower music volumes that the temperature at the sinks is 51 to 52 degrees Celsius. If I play music at higher levels for a while the temperature drops a little to anout 49 degrees, but will then again return to 51/52 without music playing. Is this normal? I thought I read once that class A amps get a little cooler at rated output but have since been unable to find that again despite many Google searches. Is there any truth to this, or is there an issue with my amp that I should be concerned about?
Every so often I check the heat sink temperature of my J with an infrared thermometer to make sure things “seem ok”. I’ve noticed that after about an hour or two at idle without music playing or with lower music volumes that the temperature at the sinks is 51 to 52 degrees Celsius. If I play music at higher levels for a while the temperature drops a little to anout 49 degrees, but will then again return to 51/52 without music playing. Is this normal? I thought I read once that class A amps get a little cooler at rated output but have since been unable to find that again despite many Google searches. Is there any truth to this, or is there an issue with my amp that I should be concerned about?
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I built a class A tube amp and BEAT it. The idle dissipation was 39 Watts, and severe overdrive would put 23W to dummy load leaving around 15 Watts in plate.
With one old 6550 I could *hear* the tube creak as it cooled-down and heated-up as signal was applied and removed. (I could also measure the IR degrees fall off but don't have those numbers now. Probably from near rated max to about half that.)
That's gross abuse. With LOUD but un-clipped speech/music, a few degrees down from 50C seems quite likely.
With one old 6550 I could *hear* the tube creak as it cooled-down and heated-up as signal was applied and removed. (I could also measure the IR degrees fall off but don't have those numbers now. Probably from near rated max to about half that.)
That's gross abuse. With LOUD but un-clipped speech/music, a few degrees down from 50C seems quite likely.
There is a LOT of crap out there.
I just hit a curve of class A dissipation which has the right trend, BUT states that max efficiency of A is 25% (for Sine it is 50%), and plots to 33%. Of course they are trying to make a class D amp look good, but they don't have to use wrong numbers to do that.
Say the peak/average ratio of speech/music is 16dB. The A efficiency will fall-off as-if we interpret dB as Voltage (because current is fixed but the voltage divides between amp and load). So the average reduction in amplifier heat is 0.158. 52C in a 25C room is 27C rise. 27C times 0.158 is 4 degrees C drop for just-clipped speech/music. Your observed 3 deg C drop means your average may be more like 19dB down from clipping. Nothing wrong with that. Indeed a strict headroom spec is 18dB (and won't clip in a year). Or you maybe could have built an amp with 3dB less power, HALF the heat, and still hardly-ever clipped. OTOH 3dB is not much safety margin for user-controlled audio-- what if you really dig it and turn it up a few dB?
Overall I say your numbers are happy.
BTW you can put a thermometer on the sink and a display on the faceplate, no need to poke around with an IR thermometer, build it in. Many old broadcast boxes had internal voltage monitoring to watch for failing tubes.
DROK LED -55 to 125℃ Temperature Panel DC 7-30V 12V/24V
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