From: Jim Brain <>
Date: Fri, 17 Feb 2012 00:38:03 -0600
Message-ID: <>
On 2/17/2012 12:30 AM, Bil Herd wrote:
> Jim Brain asked:
> Questions:
>    * Why was the darlington pair needed with the 2n3904/2n2222 setups but
>      not with the 2sc1815/7406/7416 setups?  Is the TIP29 that hard to
>      toggle?
> The TIP29 has the gain(hFE) of a small brick typically, as low as 15 at
> current so I assume if I run the numbers that the 4401 is used to source
> more current when the TIP29 was on and to keep it in saturation at full
> current. If a small value resistor (higher current) was used to bias the
> TIP when on then the 2N3904 would have to sink too much current when the
> TIP29 was off to be practical. So it is hard to drive because it doesn't
> help that much by adding DC gain.
I can understand that if the 2n3904 was used to drive the TIP29, but the 
TIP29 was biased on with a 1.5K resistor.  The 2n3904 merely drove the 
base to ground (.6V or so), thus removing current from the base.  
Obviously, it would need to sink the 7.5mA through the resistor 
(11.4/1500), but where was the resit of the current required?
>    * The 2n3904/2n2222 and TIP29 are commodity.  Why did the VIC go with
>      more specific transistors?
> They were overseas sourced, if they used US transistors they would have
> had to ship them from US to Japan for stuffing, plus more expensive in
> general.  This was back when the Japanese were kings of the .5 cent
> transistor.
Ah, that makes sense.
> I didn't like the interaction of two transistors by the time you run
> min/max gains on both it felt a little sloppy, I designed it out on
> TED/Plus4 to use my favorite transistor... the output stage of an '06/'07
Yep, I noticed that, and when I saw it first on the C128, I went to look 
at the 264 series, as I assumed you brought that over from the +4 
design.  Can you expand on your "sloppy" note, though?

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Received on 2012-02-17 07:00:22

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