[R-390] OT: Pass Transistor Question

[rbethman]

Okay - Mea Culpa!

I haven't fiddled with pass transistors since 1980 to 1981.  The same 
time frame I did all the bypassing of 1N270s to build a HB pair of 813s

Since then THAT power supply has been long ago replaced.  Bridge 
rectifiers no longer need the equalizing resistor AND capacitor.

I've also gone over to radios on the "dark side" - hollow state.  Back 
to R-390s, HT-32, BC-610, and SP-600.

So ia haven't done any solid state in a L-O-N-G time!

Bob - N0DGN

ews265 wrote:
> Hi all,
>
> Here are some thoughts that fit in with Don's.
>
> I would probably still go with emitter degeneration resistors as it's 
> considered "good engineering practice".  Also noteworthy, I just 
> replaced the output transistors on an old Crown Audio stereo 
> amplifier.  Parts list calls for a matched set of two transistors for 
> each channel.  I purchased 10 new transistors and after sorting ended 
> up with a measured Beta (current gain) RANGE of about 2:1.  I now have 
> two well matched sets installed and two lesser well matched sets as 
> spares.  The two unused outliers  have the 2:1 Beta ratio
>
> As a result, if you happened to install the two outliers of my group 
> as parallel pass transistors, you would most likely want some emitter 
> degeneration resistance to minimize the effect of the Beta variation.  
> Another issue beyond possible Beta variation is in regard to what Andy 
> Moorer pointed out regarding Vbe (Voltage, base to emitter) and it's 
> effect on collector current.  Regardless of the transistor heatsinking 
> arrangement used, it would be difficult to guarantee that the 
> transistors will see exactly the same  temperature over the power 
> supply's entire operating range.  Turns out that Vbe is quite 
> sensitive to temperature so in regard to Andy's remarks, collector 
> currents over the power supply's operating range could differ solely 
> to the transistors' temperature difference.  Emitter degeneration also 
> tends to help out  these effects.
>
> Regards,
> Jon WA3MVM
>
>
>
> rbethman wrote:
>> Barry,
>>
>> The equalizing resistor use is the result of broader mfr tolerance in 
>> days of yore.  You pointed out a reference to a 1978 handbook.  
>> Remember in that time when we built ridiculous bridge rectifiers for 
>> HB Amplifiers?  Same philosophy!
>>
>> If you are using more modern components, don't need the equalizing!  
>> If using "OLD" saved parts, equalize!
>>
>> I built a power supply about 1980 - 1981.  used 5 2N3055s on one BIG 
>> heatsink.   Go for it!
>>
>> Bob - N0DGN
>>

-- 
Bob - NØDGN