UPDATE:
Wish we cold edit old posts!
Bias finally stopped creeping up. Bias was initially set pretty high on this sample. Not sure why that would be, unless component aging has significantly changed the setpoint. I wonder if that was the root cause of the failure of that one output device. Really touchy adjustment, I see why some folks wish for multiturns! Over a period of about half a day, I periodically kept taking bias down so that the amp would draw about 2.75A from the mains. It finally stabilized with heat sinks at 47.5 and 48-degrees C. Close enough.
Now it's time for a bit of a spiff-up of an already gorgeous sample. Yay!
Wish we cold edit old posts!
Bias finally stopped creeping up. Bias was initially set pretty high on this sample. Not sure why that would be, unless component aging has significantly changed the setpoint. I wonder if that was the root cause of the failure of that one output device. Really touchy adjustment, I see why some folks wish for multiturns! Over a period of about half a day, I periodically kept taking bias down so that the amp would draw about 2.75A from the mains. It finally stabilized with heat sinks at 47.5 and 48-degrees C. Close enough.
Now it's time for a bit of a spiff-up of an already gorgeous sample. Yay!
I just had to laugh. While perusing some other threads, I ran across this tidbit from Mr. Pass:
"Progress is our most important product.
"
I'll bet not one in a 1000 people today recognize the reference. A great slogan from a once-great company, the venerable General Electric. Dates to the late 50's/early 60's.
Thanks for reminding me of that!
"Progress is our most important product.
"I'll bet not one in a 1000 people today recognize the reference. A great slogan from a once-great company, the venerable General Electric. Dates to the late 50's/early 60's.
Thanks for reminding me of that!
Hi Pacific,
The SA/4 is biased for 100 Watts Class A per channel. That means approx. 2.5 Amps bias per channel. You can remove one of the railfuses on the back attach a MM with the highest Amperesetting between the two poles of the now empty fuseholder and rearrange the leads so you can put the lid back on.
With a cold amp it will rise very fast to get a warmed up more quickly and after a while id will gradually descent to a value you want to be 2.5 Amps (two hours max). If the heatsinks won't become any hotter then 56 degrees Celcius in the process you're basically okay.
The SA/4 has a depth 5 heatsinksegments and every segment is good enough to "waist" away 40 Watts of energy.
A mono SA/12 has even 6 segments and can be biased safely for 240 Watts Class A.
Happy listening!
The SA/4 is biased for 100 Watts Class A per channel. That means approx. 2.5 Amps bias per channel. You can remove one of the railfuses on the back attach a MM with the highest Amperesetting between the two poles of the now empty fuseholder and rearrange the leads so you can put the lid back on.
With a cold amp it will rise very fast to get a warmed up more quickly and after a while id will gradually descent to a value you want to be 2.5 Amps (two hours max). If the heatsinks won't become any hotter then 56 degrees Celcius in the process you're basically okay.
The SA/4 has a depth 5 heatsinksegments and every segment is good enough to "waist" away 40 Watts of energy.
A mono SA/12 has even 6 segments and can be biased safely for 240 Watts Class A.
Happy listening!
"Progress is our most important product.
I still say it all the time, along with
"The quality goes in before the name falls off"
I still say it all the time, along with
"The quality goes in before the name falls off"
Original from Zeniths is of course: "The quality goes in before the name goes on" That I understand.
Now it's a shame I'm not a native English speaker so I do not fully understand the meaning of the alteration "...drops off" and how that relates to Apogee according to ZenMod. I suppose it means something like still using a brandname like Infinity but making crap products with that badge???
I'm ashamed...., please explain.
Now for something completely different:
I'm buying a Adcom GFA-585 that must act as a reserve when my Thresholds SA/1 must be serviced to drive my Acouistats 2 + 2. When I've solved the leaking cap problem (leaking Elna's on the driverboards and ruining the PCB's) is it suitable an good enough to drive my speakers?
Never had an Adcom before I hope it's made in the USA and not only designed there and made somewhere in Asia.
To Pacific:
How's the SA/4 performing now?
"The mistakes go in before the name goes on"
regarding Zenith and Apogee , there isn't more than initial analogy between physical properties of Zenith and Apogee in nature ......
Well, in regards to Zenith, I always said, "The problems go in before the name goes on."
And of course, to the original... "Progress is our most important problem."
As far as bias on the 4 goes... it seems 2.5A is a smidge high. The heatsinks seem to want to hang around that 56 degree point with a 70 (F) or so ambient room temperature. That seems high to me. I've got the amp set at 2.2A (or thereabouts- man, breathe on those pots and it moves half an amp...!) after stabilization, which seems to put the sinks at about 52-degrees C.
Thoughts?
And of course, to the original... "Progress is our most important problem."
As far as bias on the 4 goes... it seems 2.5A is a smidge high. The heatsinks seem to want to hang around that 56 degree point with a 70 (F) or so ambient room temperature. That seems high to me. I've got the amp set at 2.2A (or thereabouts- man, breathe on those pots and it moves half an amp...!) after stabilization, which seems to put the sinks at about 52-degrees C.
Thoughts?
Well, here's my dilemma of conflicting advice:
Originally, per previous posts, bias was set so that heat sink temperature at the rear sinks is about 50 degrees C after stabilization. The trouble is that this isn't 2.5 amps of current through the rail fuses.
If I set bias at 2.5 amps after stabilization, the heat sinks get to perhaps 56 degrees C, considerably hotter than I've been told is correct.
I'm perfectly willing to set this amp to 2.5 amps, if someone can tell me that there's plenty of safety margin left at 56 degrees. I don't like seeing devices or heat sinks this hot, but then, this is the only sample I've ever seen. I'd feel a lot better if I had serviced seven or eight of these and I knew the intrinsic characteristics more intimately, but I don't.
Originally, per previous posts, bias was set so that heat sink temperature at the rear sinks is about 50 degrees C after stabilization. The trouble is that this isn't 2.5 amps of current through the rail fuses.
If I set bias at 2.5 amps after stabilization, the heat sinks get to perhaps 56 degrees C, considerably hotter than I've been told is correct.
I'm perfectly willing to set this amp to 2.5 amps, if someone can tell me that there's plenty of safety margin left at 56 degrees. I don't like seeing devices or heat sinks this hot, but then, this is the only sample I've ever seen. I'd feel a lot better if I had serviced seven or eight of these and I knew the intrinsic characteristics more intimately, but I don't.
As I recollect the SA/4 is a 2 x 100 Watt/8 Ohm Class A amp.
You need therefore 2.5 Amps per channel to reach that spec.(although Martin Colloms would not agree here).
I'll drive my SA/1 at 56 degrees Celcius and Nelson Pass viewed that as a safe value. The thermal safequard will kick in at 70 degrees Celcius.
If you do not feel comfortably at 56 degrees stick with the 52 degrees Celcius.
Remember also that you have a optical biasversion that is probably less sensitive for variations in temperature, voltage and current.
Nelson gave me two of those temperature sensitive adhesives (the round ones that you have at the back of the last coolingfin) but I attached them at the front of the coolingfin were the last srew for the topplate goes in because that's also the place were you have to put your temperaturerod in for the biasprocedure according to Threshold. Now I can see immediately what temperature of the amp is and so long at stays around 55 degrees I'm happy.
You need therefore 2.5 Amps per channel to reach that spec.(although Martin Colloms would not agree here
).I'll drive my SA/1 at 56 degrees Celcius and Nelson Pass viewed that as a safe value. The thermal safequard will kick in at 70 degrees Celcius.
If you do not feel comfortably at 56 degrees stick with the 52 degrees Celcius.
Remember also that you have a optical biasversion that is probably less sensitive for variations in temperature, voltage and current.
Nelson gave me two of those temperature sensitive adhesives (the round ones that you have at the back of the last coolingfin) but I attached them at the front of the coolingfin were the last srew for the topplate goes in because that's also the place were you have to put your temperaturerod in for the biasprocedure according to Threshold. Now I can see immediately what temperature of the amp is and so long at stays around 55 degrees I'm happy.
Well, I've figured out what's going on here. It's rail voltage honking me up.
I'm measuring current through one rail fuse, on each side. In one channel it's the negative rail fuse, and the other one is positive (convenience- those are the ones at the top of the amp). My rail voltages measure 43.6 on the positive side of one channel, and -43.8 volts on the negative side of the other channel. 2.5 amps at that voltage means we're dissipating about 109 watts per polarity, for a total of 218 watts per channel.
At my rail voltage, if I want to dissipate 100 watts per polarity, I need a current of about 2.28 amps, not 2.5. Make sense?
Something that I think is interesting is the large difference in heat-sink temperature in the middle vs. the rear. If the rear is at around 52C, the middle might be as high as 63C. I would expect some difference, but not this much. Then again, the middle has devices on each side making heat, whereas the closer you get to the edge, the fewer devices there are.
This is fun!
I'm measuring current through one rail fuse, on each side. In one channel it's the negative rail fuse, and the other one is positive (convenience- those are the ones at the top of the amp). My rail voltages measure 43.6 on the positive side of one channel, and -43.8 volts on the negative side of the other channel. 2.5 amps at that voltage means we're dissipating about 109 watts per polarity, for a total of 218 watts per channel.
At my rail voltage, if I want to dissipate 100 watts per polarity, I need a current of about 2.28 amps, not 2.5. Make sense?
Something that I think is interesting is the large difference in heat-sink temperature in the middle vs. the rear. If the rear is at around 52C, the middle might be as high as 63C. I would expect some difference, but not this much. Then again, the middle has devices on each side making heat, whereas the closer you get to the edge, the fewer devices there are.
This is fun!
Found this reply from Nelson Pass at this forum on the biassettings for a SA/4
"Exact value of the bias is heat sinks at 50 deg C - good for a
5-10 second touch."
So stick with the value you got now.
Here's the Service manual also good for your SA/4.
This one recommends a setting of "only" 2 Amps per channel after an initial setting of 1.6A.
And another one recommending 2.1A as a final setting.
My SA/1's have some lower temperatures at the front because the huge frontplate also helps to cool down the amp.
As said before the official biasprocedure indicates to measure the temperature in the last screwhole. 63 degrees is definitely to high and the difference of 11 degrees is strange. After the amp is settled a temperaturedifference of a few degrees is acceptable.
I think you could have a problem with your Stasissection. These are the first two sets of TO3's on every side on the back then there's some space (where the driverboards are) and there are 12 sets of currentdumping devices all the way to the frontplate.
The SA/1 has also two sets of Stasis devices (divided over two heatsinks though) but they become just as hot as the rest.
Maybe Nelson can comment on this huge temperaturedifference between the middle and backsite of your SA\4. Maybe it is intended
"Exact value of the bias is heat sinks at 50 deg C - good for a
5-10 second touch."
So stick with the value you got now.
Here's the Service manual also good for your SA/4.
This one recommends a setting of "only" 2 Amps per channel after an initial setting of 1.6A.
And another one recommending 2.1A as a final setting.
My SA/1's have some lower temperatures at the front because the huge frontplate also helps to cool down the amp.
As said before the official biasprocedure indicates to measure the temperature in the last screwhole. 63 degrees is definitely to high and the difference of 11 degrees is strange. After the amp is settled a temperaturedifference of a few degrees is acceptable.
I think you could have a problem with your Stasissection. These are the first two sets of TO3's on every side on the back then there's some space (where the driverboards are) and there are 12 sets of currentdumping devices all the way to the frontplate.
The SA/1 has also two sets of Stasis devices (divided over two heatsinks though) but they become just as hot as the rest.
Maybe Nelson can comment on this huge temperaturedifference between the middle and backsite of your SA\4. Maybe it is intended
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Wow, this is great stuff, but the uploads seem to be incomplete. Can you take a look at what's here and put up the rest?
You know, I'm still thinking about this. Should we not be dissipating 50 watts per polarity, for a total of 100 watts per side? That would take current through each rail fuse down to 1.14 amps, not 2.28 as I have it now.
The current values shown in the document are confusing (though this may be because some sections are not complete). For example, the cold start value shown for the 4e is 2.1A. I presume that's current draw from the line, not at the rail fuse.
Thanks for this!
Oh- this "locking in" behavior has been driving me crazy. Now I see it's normal and how to double check.
You know, I'm still thinking about this. Should we not be dissipating 50 watts per polarity, for a total of 100 watts per side? That would take current through each rail fuse down to 1.14 amps, not 2.28 as I have it now.
The current values shown in the document are confusing (though this may be because some sections are not complete). For example, the cold start value shown for the 4e is 2.1A. I presume that's current draw from the line, not at the rail fuse.
Thanks for this!
Oh- this "locking in" behavior has been driving me crazy. Now I see it's normal and how to double check.
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If your amplifier shows a 2 amp bias, then it will enter
cutoff at 4 amps peak, which into 8 ohms is 4 X 4 X 8 =
128 watts peak which becomes 64 watts rms with a
sine wave.
So you need 2.5 to get the 100 Watts Class A RMS:
2.5 amp bias, then it will enter
cutoff at 5 amps peak, which into 8 ohms is 5 X 5 X 8 =
200 watts peak which becomes 100 watts rms.
This is along the lines Nelson explained it to me.
I would not bother to much with the math or values in this manual.
Just see them as a guideline how to bias your amp properly.
If you keep your amp beneath 55 degrees Celcius it's okay.
What worries me more are the temperature differences of more then 10 degrees.
I'm not familiar with the 'E' series amps (the last series supervised/designed by Nelson Pass) but I know that the outputstages of all the Stasis amps starting from 1983 till 1992 are pretty the same.
Succes
cutoff at 4 amps peak, which into 8 ohms is 4 X 4 X 8 =
128 watts peak which becomes 64 watts rms with a
sine wave.
So you need 2.5 to get the 100 Watts Class A RMS:
2.5 amp bias, then it will enter
cutoff at 5 amps peak, which into 8 ohms is 5 X 5 X 8 =
200 watts peak which becomes 100 watts rms.
This is along the lines Nelson explained it to me.
I would not bother to much with the math or values in this manual.
Just see them as a guideline how to bias your amp properly.
If you keep your amp beneath 55 degrees Celcius it's okay.
What worries me more are the temperature differences of more then 10 degrees.
I'm not familiar with the 'E' series amps (the last series supervised/designed by Nelson Pass) but I know that the outputstages of all the Stasis amps starting from 1983 till 1992 are pretty the same.
Succes
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