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?
Thanks,
Will
Thanks,
Will
Will,
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.
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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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
SCompRacer,
I see you have some Sanders (Magtech?) amps too...care to elaborate on the merits of each? I just got a KSA-250 tonight, and so far, so good...
I see you have some Sanders (Magtech?) amps too...care to elaborate on the merits of each? I just got a KSA-250 tonight, and so far, so good...
Hi John! That was a Sanders ESL amp. I was running it on the panels of a pair of InnerSound Eros MK III hybrid ESL's. The bass modules were run by an InnerSound MK II active crossover bass amp (600WPC @ 4 Ohm) and it sounded really good.
I had tried the Krell KSA-250 on the panels but it didn't work out so well. There was a gain difference where the panels were softer than the bass. The active allowed bass/mid bass adjustments but I couldn't balance it out. I tried a Behringer digital active xover where I could boost the panel amp slightly and that worked to balance the amps but the digital active introduced a nasty hum where none existed before. Since then I sold the ESL speakers and Sanders amp.
Anyway, the Sanders A/B amp can be very neutral sounding with some speakers depending on pre amp used. Best paired with a tube pre or jfet type SS pre. Class A seems to make anything sound richer. I took that KSA-250 to a friends house and he tried it on his Revel F52's. He had BAT tube mono block amps with a BAT SS pre amp. The Sanders sounded too neutral with the BAT SS pre on his speakers, but that old Krell did not and it stayed there; better bass, mids and extended highs. He sold the BAT mono blocks.
Fortunately the friend I got the first one from had another KSA-250. (No worries for him, he still has a pair of Krell MDA 500 mono blocks). The weather may prevent delivery of it Monday. Can't wait to try it on my new speakers.
Rich
I had tried the Krell KSA-250 on the panels but it didn't work out so well. There was a gain difference where the panels were softer than the bass. The active allowed bass/mid bass adjustments but I couldn't balance it out. I tried a Behringer digital active xover where I could boost the panel amp slightly and that worked to balance the amps but the digital active introduced a nasty hum where none existed before. Since then I sold the ESL speakers and Sanders amp.
Anyway, the Sanders A/B amp can be very neutral sounding with some speakers depending on pre amp used. Best paired with a tube pre or jfet type SS pre. Class A seems to make anything sound richer. I took that KSA-250 to a friends house and he tried it on his Revel F52's. He had BAT tube mono block amps with a BAT SS pre amp. The Sanders sounded too neutral with the BAT SS pre on his speakers, but that old Krell did not and it stayed there; better bass, mids and extended highs. He sold the BAT mono blocks.
Fortunately the friend I got the first one from had another KSA-250. (No worries for him, he still has a pair of Krell MDA 500 mono blocks). The weather may prevent delivery of it Monday. Can't wait to try it on my new speakers.
Rich
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Yes, I had the Sanders ESL amp with a pair of 10C. It was very nice, but couldn't get past the narrow sweet spot. The amp was impressive.
I am listening to the KSA250 now on my Apogee Duetta Sigs, Very nice...a bit darker than the KSA-50 Clone I built. But need to reserve my judgement until I hear them on the Scintillas I have coming. Plan an interesting Scintilla shootout with the amps I have...
I am listening to the KSA250 now on my Apogee Duetta Sigs, Very nice...a bit darker than the KSA-50 Clone I built. But need to reserve my judgement until I hear them on the Scintillas I have coming. Plan an interesting Scintilla shootout with the amps I have...
Did you have it re capped , it will need it at it's age and have the bias checked, they will sound dark if not done. The true value of Krell amplifiers show up when driving difficult loads , as you will see on the Scinnies......
Wayne, I have not recapped, but as long as no hum/noise, I will leave as is for now...as far as setting the bias, these are not adjustable pots on the board (there is a bias circuit?)...have to say I am a little disappointed in the KSA-250...but I really have to reserve judgement again until I try the Scinnies...
Anyway, I have a Krell KSA-300S chassis that I have been thinking of putting two KSA-50 Clone amps in (4 channels), one pair driving the bass and the other pair driving the mid/tweets. But then saw the KSA-250 showed up locally...
Also, I biased my KSA-50 clone as high as i could and pulls 6.5 amps from the wall (just 2 channels) and gets to around 140F on the sinks, while the ksa-250 pulls 12 amps from the wall and gets warm, not hot...maybe I built a KSA-125 clone?
Decisions...
SCompRacer, are you in Illinois? You are in Plainfield? Holy crap, so am I...
Anyway, I have a Krell KSA-300S chassis that I have been thinking of putting two KSA-50 Clone amps in (4 channels), one pair driving the bass and the other pair driving the mid/tweets. But then saw the KSA-250 showed up locally...
Also, I biased my KSA-50 clone as high as i could and pulls 6.5 amps from the wall (just 2 channels) and gets to around 140F on the sinks, while the ksa-250 pulls 12 amps from the wall and gets warm, not hot...maybe I built a KSA-125 clone?
Decisions...
SCompRacer, are you in Illinois? You are in Plainfield? Holy crap, so am I...
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Ok , so you are both frozen solid ...... 🙂
John,
From experience they don't sound right until after a recap, dull and dark to me is what i remember, so I'm not suprised to hear you say so, 12 amps sounds higher than what i recalled for a ksa250, thats pretty close to the Ksa200 (14amps) and the 250 was biased a lot less if memory serves me correctly ...
John,
From experience they don't sound right until after a recap, dull and dark to me is what i remember, so I'm not suprised to hear you say so, 12 amps sounds higher than what i recalled for a ksa250, thats pretty close to the Ksa200 (14amps) and the 250 was biased a lot less if memory serves me correctly ...
John, that's a 10 4 on Plainfield and being frozen a.wayne. Howdy neighbor!
The KSA I showed in pics had been to Krell for a refurb. The one on the way has not, but had light use. I know the previous owner of that KSA 250. This KSA will be the one that was paired with the KSL pre I have. I re-capped the KSL along with the phono board myself. Many caps measured 25% - 30% out of tolerance. I'll still pop the top on the KSA and check for toasted caps. It is either second or third generation. My friend told me that a Krell tech told him that they still have new Sprague caps for doing repairs.
Currently I am loaning a pair of Wyred4Sound SX-1000 mono blocks for my Salk SoundScape 8's (Raal tweeter, Accutron mid range). Class D sounds pretty good, but I hear it all depends on the pre used. I paired them with a Krell KRC HR pre and it gets fatiguing after a while. Not so bad with the KSL. I hear you about the narrow sweet spot. I had the Eros MK III's for 7 years.
The KSA I showed in pics had been to Krell for a refurb. The one on the way has not, but had light use. I know the previous owner of that KSA 250. This KSA will be the one that was paired with the KSL pre I have. I re-capped the KSL along with the phono board myself. Many caps measured 25% - 30% out of tolerance. I'll still pop the top on the KSA and check for toasted caps. It is either second or third generation. My friend told me that a Krell tech told him that they still have new Sprague caps for doing repairs.
Currently I am loaning a pair of Wyred4Sound SX-1000 mono blocks for my Salk SoundScape 8's (Raal tweeter, Accutron mid range). Class D sounds pretty good, but I hear it all depends on the pre used. I paired them with a Krell KRC HR pre and it gets fatiguing after a while. Not so bad with the KSL. I hear you about the narrow sweet spot. I had the Eros MK III's for 7 years.
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Ha! Unreal!...I am in Dunmoor estates.
I also have the ICEPower 1000ASP Amp Modules with a pair of 500A for four channels of Class D, with an Aikido All In One Noval Tube Preamp driving the Apogees.
I have a number of Acoustat panels / interfaces and ML CLS Panels that I experiment on too...
Dude, tell me you also like to drink Guinness, smoke a good Cuban Cigar and dream of Jessica Alba!
I also have the ICEPower 1000ASP Amp Modules with a pair of 500A for four channels of Class D, with an Aikido All In One Noval Tube Preamp driving the Apogees.
I have a number of Acoustat panels / interfaces and ML CLS Panels that I experiment on too...
Dude, tell me you also like to drink Guinness, smoke a good Cuban Cigar and dream of Jessica Alba!
Well Wayne, you gotta move here now!!
OK, confused. I read this about the ksa-250...
Krell KSA-250 power amplifier When is Class-A Class-A? | Stereophile.com
So I decided to look at my amp a little closer. Popped the hood and took some readings.
KSA-250
Rails - +/- 80VDC
Voltage across emitter resistors - 100mv
Emitter resistors - 1 ohm
12 pairs output devices
Amps from wall - 8.3 amp (actually a lot closer to my Krell KSA-50 Clone than I thought...)
Anyway, had to verify if the article is true, is this not a full class A amp like the KSA-50?
Maybe that's why its a bit dull and not as vibrant as my KSA-50 Clone? I have that bias up the hilt
KSA-50 Clone
3 pairs Toshiba 1943/5200
+/- 35VDC rails,
.648 ohm emitter resistors,
650ma across them giving 1A per output device
Am I actually getting more Class A watts out of my Clone? Is this just a great Class AB amp with monster heatsinks?
OK, confused. I read this about the ksa-250...
Krell KSA-250 power amplifier When is Class-A Class-A? | Stereophile.com
So I decided to look at my amp a little closer. Popped the hood and took some readings.
KSA-250
Rails - +/- 80VDC
Voltage across emitter resistors - 100mv
Emitter resistors - 1 ohm
12 pairs output devices
Amps from wall - 8.3 amp (actually a lot closer to my Krell KSA-50 Clone than I thought...)
Anyway, had to verify if the article is true, is this not a full class A amp like the KSA-50?
Maybe that's why its a bit dull and not as vibrant as my KSA-50 Clone? I have that bias up the hilt
KSA-50 Clone
3 pairs Toshiba 1943/5200
+/- 35VDC rails,
.648 ohm emitter resistors,
650ma across them giving 1A per output device
Am I actually getting more Class A watts out of my Clone? Is this just a great Class AB amp with monster heatsinks?
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