Krell KSA 100 MK2: bias and normal temperature?

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foxie

Member
Joined 2020
#1
Hello everyone!!

I have a Krell KSA 100 MK2 that has had a rough life.
Recently, the right channel blew up.

After replacing all the driver transistors (MJE 15030/15031), some resistors and couple of output transistors the amplifier came back to life.

Having went through the entire internet, I have come to a conclusion that most people, that own this Krell, adjust their bias to 500mV.

As a start, I adjusted the bias to 450mV on both channels, I was very happy with how it sounded. However, the heatsink temperature of the repaired channel had averaged around 95°C (203°F), while the left channel (also repaired at some point) had a temperature of around 70°C (158°F).

Because of such high temperature, I have dialed the bias down to about 375mV on both channels, and the left channel averages now around 60°C (140°F), while the right channel averages around 80°C (176°F).

I plan on doing a full restoration of this amplifier sometime in the future. They are wonderful amplifiers that deserve love. Until then, I'd like to try to get it working the way it should.


My questions are:

1. Can anyone tell me what is the normal temperature of the heatsinks and the recommended bias for the Krell KSA 100 MK2 ?

2. Could the high temperate be the result of the output transistors not being from the same batch? I have only replaced the bad ones, while left the working ones in place.

3. I have measured the bias by measuring the voltage across one of the 1ohm resistors next to one of the output transistors. Is that a correct way of doing that?

I really appreciate any advice you can give on how to approach this issue.
Thank you!! 🙂
 
#3
I would keep the stabilized sink temperature to 60C or less.

You can check the current sharing by comparing the voltages across the emitter resistors
of one side, assuming all the resistors are the same value. If you have an accurate meter,
you can check the resistor values in circuit.
 
Last edited:
#4
@foxie, heat sink should not be above 50-55C, the bias looks about right, but it is way hot. I would suggest that you either have cooling problems, is the fan running, or another issue somewhere, perhaps dodgy transistors or resistor out of range??
 
#5
I would not rely on setting the bias by only measuring across one emitter resistor on each channel. I suspect that the 1 ohm resistors are 5% tolerance, so in the worst case scenario you could have a 10% difference, also the HFE of the transistors unless carefully matched may be different.
I suggest that you determine the total bias for one channel and measure this on the supply rails, this will ensure both channels are biased equally.
Check the bias several times from cold to normal operating temperature. Also as already been mentioned check cooling fans are working correctly.
Regards
Alan
 
#6
danieljw said:
I am running 0.6 A per transistor original spec is 575mV over 1 Ohm. From memory

-Dan
Click to expand...
Do you happen to remember the temperature of the heatsinks with that spec? 🙂

rayma said:
I would keep the stabilized sink temperature to 60C or less.

You can check the current sharing by comparing the voltages across the emitter resistors
of one side, assuming all the resistors are the same value. If you have an accurate meter,
you can check the resistor values in circuit.
Click to expand...

konst said:
@foxie, heat sink should not be above 50-55C, the bias looks about right, but it is way hot. I would suggest that you either have cooling problems, is the fan running, or another issue somewhere, perhaps dodgy transistors or resistor out of range??
Click to expand...
Well, today the weirdest thing happened. The temperature on the right (repaired channel) went up to 80c again on a 375mV bias!
However, after turning on the amp again in the evening, the temperature on both channels was exactly the same (50 - 60c).
So I decided to try increasing the bias voltage again. I raised it up to 450mV once again and to my surprise, the temperature of both channels was in the normal range I'd say (50 - 65c). Strange, no? 😀

So now I am definitely leaning towards the idea that one of the resistors went out of the range.
Especially because, after the right channel blew up, I replaced 2 of visibly burnt 1k ohm metal film resistors. Once I measured those burnt resistors they were in the mega ohm range!

So there is very likely more of such resistors that went out of range over the years.

At this point, I'm thinking of replacing all the resistors and the on-board capacitors.

The original Sprague filter caps were replaced with 47000uF 100V Nippon Chemi-Con capacitors couple of years ago.

alibear said:
I would not rely on setting the bias by only measuring across one emitter resistor on each channel. I suspect that the 1 ohm resistors are 5% tolerance, so in the worst case scenario you could have a 10% difference, also the HFE of the transistors unless carefully matched may be different.
I suggest that you determine the total bias for one channel and measure this on the supply rails, this will ensure both channels are biased equally.
Check the bias several times from cold to normal operating temperature. Also as already been mentioned check cooling fans are working correctly.
Regards
Alan
Click to expand...

Yes, the 1 ohm resistors have 5% tolerance, and the voltage drop across each resistor is slightly different (+/- 10mV), so what I did was use the resistor with the highest value voltage drop as reference for adjusting the bias.
None of the output transistors are matched.

I'll check the bias from cold to the operating temperature tomorrow.

-

The fans are working and are spinning on both channels equally fast.
But whether they're spinning fast enough I do not know.
Is there a way to check?

I appreciate all of the answers. Thank you!! 😀

~Armin
 
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