Everything was going so well. The sound was the best ever, and there had been no shutdown since I put the right speaker back together with both transformers connected, with the new electrolytic capacitor.
Then, after 4 hours playing, it was waay past my bedtime, but I decided to measure the temperature of the amp.
Things were not well after all. The right side of the Krell, powering the newly "restored" Acoustat, had heated up to almost as high a temperature I've ever measured on either side of the Krell, 194F. And the right side has reliably run 10-15F cooler, not hotter, for reasons I'd always assumed were due to airflow caused by the air inertia of circulation of my always circulating HVAC system.
194F was what I measured on the right side, and a mere 178F on the left side (typically for 4 hours or more). This was not at all normal for my system. (It appears, btw, that the Krell FPB 300 runs plateau bias at the highest plateau demanded, UNTIL the temperature reaches 180F. Then it shuts off all but the two highest bias levels, but it can still go higher if there's enough excess current draw, at the lower bias levels. I observed this behavior a few years ago when I always had the Krell running through a AC wattmeter. I believe the ultimate thermal shutdown of the entire amp occurs at 200F. All these numbers are way beyond most power amplifiers, but the Krell can do these temperature levels because it was designed to, and the transistors are special high temperarture parts. Inverters typically run at temperatures like these and higher. The inside of the amp is cooler than the heat sinks too, by some clever design. When the right channel was ultimately "broken" in November 2016, before I shipped it in for "Capacitor Service", I recall seeing temperatures on the left side approaching 200F just before shut down. But that was the ONLY time I had ever seen that, or gotten thermal shut downs. Even when the amp was "normally malfunctioning" as it had been, actually, since the day I got it, I didn't see temperatures above 189F on the left (while the right was 150F or so). Since the 3/2017 capacitor service, all day L/R temperatures of 169F/159F were typical, an enormous improvement.
Because it was easy to do, I disconnected the LF transformer first. For some reason I also temporarily disconnected AC power on the Krell (I still wonder if this forces it to "recalibrate" on the next startup). When it started up, power went up to 1200W fairly quickly. I was immediately convinced this proved the HF transformer was the one causing the excess current draw. However, after awhile, the power settled down around 650W, and all seemed normal. The next day the startup was normal, simply going from the standby of 70W up to about 650W and holding there. And both days the right side temperature did not exceed 160F, even after the apparently excess current draw during the first day.
I decided I'd better check the LF transformer after all. I hate to do this because this test requires cutting a wire, and then resoldering it. Or I supposed I could "resolder" the wire, but fortunately there's enough slack I can just cut the wire to start, which greatly simplifies the start of testing.
And then, it was clear by the first day of operation that the LF transfomer on the right side was causing the excess current draw, probably at some peak voltage, forcing a higher bias plateau in the Krell. And sure enough, power use was more like 830W, and the right side of the amp would begin exceeding the right side in less than an hour, reaching as high as 170F in 60 minutes, and 194F (just as I had seen before) in less than 3 hours, when the right side was a typical (for it) 168F.
To be sure nothing has happened to the amplifier, I hooked the right channel of the amp to the left speaker (I will shortly be moving the right interface away to the test/rebuild bench anyway). And I'm seeing normal operation--for the right channel of the amp--with about 145F at one hour, not 170F.
So...I've got a transformer replacement job to do. When I complete this soon, which I must, it will be the biggest job I've actually done (and not just contemplated...like my idea for an external Acoustat crossover box for the right channel, where I'd put the Solen capacitor and fuse) since about 1982, when I built an Audio Research inspired phono preamp from scratch. Well, almost scratch, as I gutted a Scott preamp, which I should not have done, in retrospect. However, my phono preamp was stellar, easily besting an Audible Illusions, for example. I never compared to a real Audio Research, but I had incorporated many of the ideas we had gleaned from that and other preamps at Audio Directions.
Actually, building the Preamp was the biggest audio project I've ever done by far and way, and it's almost inconceivable to me now that I did it. (I no longer have the power supply for that preamp, which was crucial as it provided 6 seperate 400V regulated B+ supplies, plus 2 regulated 12vdc supplies for the filaments.
To be fair, the Acoustat transformer replacement is a much smaller job that building a preamp from scratch. I should be able to finish this by the end of the month, if not the end of this 3 day weekend.
Then, after 4 hours playing, it was waay past my bedtime, but I decided to measure the temperature of the amp.
Things were not well after all. The right side of the Krell, powering the newly "restored" Acoustat, had heated up to almost as high a temperature I've ever measured on either side of the Krell, 194F. And the right side has reliably run 10-15F cooler, not hotter, for reasons I'd always assumed were due to airflow caused by the air inertia of circulation of my always circulating HVAC system.
194F was what I measured on the right side, and a mere 178F on the left side (typically for 4 hours or more). This was not at all normal for my system. (It appears, btw, that the Krell FPB 300 runs plateau bias at the highest plateau demanded, UNTIL the temperature reaches 180F. Then it shuts off all but the two highest bias levels, but it can still go higher if there's enough excess current draw, at the lower bias levels. I observed this behavior a few years ago when I always had the Krell running through a AC wattmeter. I believe the ultimate thermal shutdown of the entire amp occurs at 200F. All these numbers are way beyond most power amplifiers, but the Krell can do these temperature levels because it was designed to, and the transistors are special high temperarture parts. Inverters typically run at temperatures like these and higher. The inside of the amp is cooler than the heat sinks too, by some clever design. When the right channel was ultimately "broken" in November 2016, before I shipped it in for "Capacitor Service", I recall seeing temperatures on the left side approaching 200F just before shut down. But that was the ONLY time I had ever seen that, or gotten thermal shut downs. Even when the amp was "normally malfunctioning" as it had been, actually, since the day I got it, I didn't see temperatures above 189F on the left (while the right was 150F or so). Since the 3/2017 capacitor service, all day L/R temperatures of 169F/159F were typical, an enormous improvement.
Because it was easy to do, I disconnected the LF transformer first. For some reason I also temporarily disconnected AC power on the Krell (I still wonder if this forces it to "recalibrate" on the next startup). When it started up, power went up to 1200W fairly quickly. I was immediately convinced this proved the HF transformer was the one causing the excess current draw. However, after awhile, the power settled down around 650W, and all seemed normal. The next day the startup was normal, simply going from the standby of 70W up to about 650W and holding there. And both days the right side temperature did not exceed 160F, even after the apparently excess current draw during the first day.
I decided I'd better check the LF transformer after all. I hate to do this because this test requires cutting a wire, and then resoldering it. Or I supposed I could "resolder" the wire, but fortunately there's enough slack I can just cut the wire to start, which greatly simplifies the start of testing.
And then, it was clear by the first day of operation that the LF transfomer on the right side was causing the excess current draw, probably at some peak voltage, forcing a higher bias plateau in the Krell. And sure enough, power use was more like 830W, and the right side of the amp would begin exceeding the right side in less than an hour, reaching as high as 170F in 60 minutes, and 194F (just as I had seen before) in less than 3 hours, when the right side was a typical (for it) 168F.
To be sure nothing has happened to the amplifier, I hooked the right channel of the amp to the left speaker (I will shortly be moving the right interface away to the test/rebuild bench anyway). And I'm seeing normal operation--for the right channel of the amp--with about 145F at one hour, not 170F.
So...I've got a transformer replacement job to do. When I complete this soon, which I must, it will be the biggest job I've actually done (and not just contemplated...like my idea for an external Acoustat crossover box for the right channel, where I'd put the Solen capacitor and fuse) since about 1982, when I built an Audio Research inspired phono preamp from scratch. Well, almost scratch, as I gutted a Scott preamp, which I should not have done, in retrospect. However, my phono preamp was stellar, easily besting an Audible Illusions, for example. I never compared to a real Audio Research, but I had incorporated many of the ideas we had gleaned from that and other preamps at Audio Directions.
Actually, building the Preamp was the biggest audio project I've ever done by far and way, and it's almost inconceivable to me now that I did it. (I no longer have the power supply for that preamp, which was crucial as it provided 6 seperate 400V regulated B+ supplies, plus 2 regulated 12vdc supplies for the filaments.
To be fair, the Acoustat transformer replacement is a much smaller job that building a preamp from scratch. I should be able to finish this by the end of the month, if not the end of this 3 day weekend.
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