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Old 2nd May 2007, 03:31 PM   #1
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Default Transistors for Blameless

I've recently built a Doug Self Blameless amp using a PCB from a mentor friend. My first set of transistors I'm using are MJL21195 and MJL21196. I'm trying to get it biased up to run Class A for the first few watts. So far the sound is very nice. I also have a second PCB I'm building up to try some other parts for comparisons sake.

One suggestions I've gotten is to try faster transistors (which I realize may require some compensation for stable operation). What I'd like to get input on is what constitutes a faster transistor. From the MJL21195 datasheet the only parameter that seems to match this is called "current gain BW product" and this is rated at 4MHz min. No typical or max is listed. Is this the parameter that is related to speed?

So please suggest other transistors that might be interesting to try that are faster. I appreciate any input you have.

thx,
Tom
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Old 3rd May 2007, 02:05 AM   #2
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Toshiba's 2SA1943/2SC5200 or Sanken's 2SA1216/2SC2922.
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Old 3rd May 2007, 08:10 AM   #3
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Default Re: Transistors for Blameless

Quote:
Originally posted by tommytube
Is this the parameter that is related to speed?
Yes...if you had read the data sheet, you can see that they run 1A at a Vce of 10V, and feed it a 1MHz signal. Whatever current gain they get at that frequency is multiplied by the test frequency (1MHz), and that equals the 'ft' parameter on the data sheet. So a faster transistor would have higher gain at that frequency, and thus a higher ft rating.

If you're using TO-3's, you've likely painted yourself into a corner. Besides NOS transistors, there's a bazillion counterfeits out there for the faster TO-3's, which are all out of production. Faster than these On-Semi TO-3's nowadays means a plastic case, such as the TO-3P or TO-264.

Nothing wrong with the MJ21195 & '96. Faster isn't always better, and it sometimes makes little sense to toss in a 30MHz transistor, only to compensate it so much that you might as well have stayed with the slower one to begin with.
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