[R-390] OT: Pass Transistor Question

[ews265]

Bob,

Can you fill me in by what you mean  when you say "build a HB pair of 
813s"?  Sounds like something to do with silicon diode stacks to replace 
mercury vapor rectifiers but not really sure.

Regards,
Jon WA3MVM



rbethman wrote:
> 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
>>>
>