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Re: tubes vs. transistors,

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tomservo

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In Reply to: Re: tubes vs. transistors, posted by Dan Banquer on July 17, 2006 at 07:04:01:

Hi Dan
At the point I worked at in service repair 50% and engineering 50% at Grommes Precision in the late 70�s (a company that made both Tube and Transistor amplifiers), this was in fact the case. That is what I meant by �In the beginning�.
A typical clunky power tube has an open loop high frequency corner that reaches into the 80 or 40 meter Ham bands, a good modern transistor has an ft (where current gain is zero) in the same range or lower.
One cannot say power transistors are inherently wider BW devices than power tubes.
Tubes on the other hand need grid stoppers and other tricks to keep them stable in audio circuits because of high inherent cutoff frequency.
Tom

Follow Ups:

Re: tubes vs. transistors, - Dan Banquer 09:19:58 07/17/06 (2)
"One cannot say power transistors are inherently wider BW devices than power tubes."
And just how wide do they have to be when they are used for current gain and not voltage gain?
d.b.
- Re: tubes vs. transistors, - tomservo 11:19:12 07/17/06 (1)
  Hi Dan
  For a generic complementary output transistor pair like the 2N3055a / MJ2955A the current gain is unity (base current equals collector current) at about 800KHz.
  The typical gain at DC is specified as 20 to 70.
  In this case, if the device had a DC gain of 64 and ft of 800KHz, then the open loop corner would be approximately 12.5KHz.
  For the darlington pair of MJ4035 / MJ4032, the ft is 1MHz, and has a specified DC gain of 1000.
  Here, the open loop corner would be around 1KHz.
  Don�t get me wrong, I�m not saying you can�t find better transistors but the 3055 was rugged and used in zillions of amplifiers (as well as its bigger NPN only brother the 2N3773).
  You can approximate all this if take the transistor�s ft and figure the gain here is 1 and recall it increases by 2 times each time you lower the frequency from ft an octave.
  The DC gain extends upward in frequency (approximately) to the intersection of this slope and this is the open loop corner for the device.
  Thanks for an actual technical discussion, too much of this forum is a Monty Python argument sketch.
  Tom
  - Re: tubes vs. transistors, - Dan Banquer 11:34:43 07/17/06 (0)
    Try MJ 15003 & MJ15004 and the MJ15024 & MJ15025 or even some of the newer audio power transistors. Gain bandwidth from what I see on the data sheets and in circuit is not an issue as far as I can tell. It can be when you go to higher voltage small signal transistors used in voltage gain sections.
    "Thanks for an actual technical discussion, too much of this forum is a Monty Python argument sketch"
    Your welcome.
    d.b.

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