[Hallicrafters] Re: [R-390] OT: Pass Transistor Question

[rbethman]

Jon,

Building a homebrew pair of 813s in grounded grid - An amplifier.  
Needed to rectify the plate transformer HV output.

Bob - N0DGN

ews265 wrote:
> 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
>>>>
>>
>
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-- 
Bob - NØDGN