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willcycle 1st January 2013 02:48 PM

Krell KSA-250 Output Transistors
I am in the process of recapping and slightly lowering the bias (to version 2 levels) of my version 3 Krell Ksa-250 amplifier. As I have to remove all output transistors as part of the job, is there anything to gain by replacing the MJ15024/25 output transistor pairs with the perforated emitter MJ21195/96 pair?


LineSource 2nd January 2013 06:02 AM


Personally, I would stay with the original power transistors:

1) Krell "claims" Motorola custom tested production lots to get output transistors that met Krell's tighter specs, and that Krell technicians hand tested and grouped closely matched sets for each amp.
2) An all-original KSA-250 is a reference standard in both sonics and resale value.
3) The alternative output transistors you are looking at do not have significantly superior specs. Both are fT~6Mhz with hfe~50. A modern output transistor has fT~50Mhz with hfe~130.

What are you doing to change the output bias current? Are you changing the autobias Zener Diode or the comparator resistors?

My KSA250s use a 1.235V Zener Diode in the autobias circuit. Do you know if this is a Generation 2 or Generation 3 ClassA power level?

Keep the faith :-)
A high power DIY amp would cost you $800-$1000 to build with little resale value. A modern DIY amp would measure better than the KSA250 but would not get the same cult respect. An all-original KSA 250 is a time proven reference standard.

willcycle 2nd January 2013 10:51 AM

My main concern for the amp is sound quality. It looks like hfe of the 95/96 pair is slightly higher and they have a greater SOA. I suppose there would not be any noticeable sound improvement.

For the bias, I am changing four resistors in the comparator circuit.

In rebuilding my version 2 amp I increased the power supply capacitance to 400,000uF (the soft start power resistors also required upgrading to handle the increased current draw). I plan to do the same on my version 3.

Based on the photos you provided in the low frequency oscillation thread you have a version 3 amp.

SCompRacer 12th January 2013 09:13 PM

Greetings willcycle and LineSource. I'll be making a several hour trip to pick up a KSA-250 from a friend soon. It will be reunited with its old partner, a KSL pre amp. The pair were owned since new by a friend that sold them to another friend. I got the KSL pre a couple months back and solved some issues with the phono card that allow me to use it now.

How do you tell what generation the amp is? I watched a refurb video on youtube that showed some areas to look at in the amp. IIRC they also relocated some resistors on the soft start board that heated up some caps.

willcycle 13th January 2013 11:04 AM

The KSA-250 in the video you reference is a version 3. The version 2 has a very different looking soft start board with no separate low voltage power supply to activate the relays. A version 1 amplifier has the output binding posts oriented vertically rather than horizontally.

SCompRacer 13th January 2013 05:23 PM

Thanks for the info!

SCompRacer 21st May 2013 01:28 AM

4 Attachment(s)
Finally picked up my KSA-250. Looks pretty good on the inside (nothing burnt or toasted) but I'm sure I'll be recapping it in the future.

LineSource 21st May 2013 11:18 PM

Hi SCompRacer,

In this forum Heavy Metal just hums at 60Hz :-)

From the pictures, it looks like you have a very clean and lightly used KSA-250.
I'm sure you have read about the main areas to carefully look over and test.
If you can borrow or purchase a capacitor tester you will get better data than with just a resistor and multimeter setup.

Big Electrolitic power supply Caps.
Caps and relays on power up PCB.
Heat damage from soft start resistor on power up PCB.
DC offset ... DC servo.
Loose screws on output transistors.
Loose SIGNAL PATH screws between main input PCB and Output Heatsink.
Carpeting that can block bottom air flow.
Cursed Cats.

SCompRacer 22nd May 2013 05:12 PM

Hello LineSource! I knew about some of those but not all. Thanks for the shopping list! I read where some folks bash the amp but for an old school complex design it sure sounds good.

The amp has feet so the towel under it didn't block airflow. The towel made it easy to move on the wood floor. When I was in my prime I might have carried it, but I'm smarter now.....well maybe as I did lift it out of the trunk myself. I do have a sturdy amp stand for it.

Jasper is a hybrid, one kewl cat! More like a dog, follows us around, gets involved in what we do. He was looking for spiders to kill and eat. The amp had been in storage and had some webs on it.

willcycle 24th May 2013 06:48 PM

Congratulations on your KSA-250. Based on experience refurbishing my two units, I would closely check the DC offset. Do not just take one measurement but study how it varies over a two minuet interval noting peak deviations. Both my Krells required servo rebuilding due to unacceptable deviations.

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