[HAM] CV Preamp rebuild

[David Anderson]

Hi Andrew,

Actually, I wrote without checking the schematic to see what  
capacitors C5 and C6 do and forgot that this preamp was built in the  
age before fairly reliable electrolytic capacitors were widely  
available, so they are probably, as OF said, oil-filled units, but  
they are there simply for filtering the 6SJ7 screen supplies, so you  
could actually increase their value with no problem. Many radios,  
etc... from this era got by with the bare minimum of power supply  
filtering capacitors because they had to use these oil-filled units,  
which are larger per unit of capacitance than electrolytics. These  
oil filled capacitors are essentially like motor-run capacitors. In  
the 1940s, it was probably even before they started adding PCBs to  
the oil, though I'd have to check. With mainly DC across them, there  
shouldn't be much heating of the capacitors like you'd see in a motor- 
run application.

 From the point of view of modern best practices, I've always been a  
little horrified at the idea of having the power supply so far from  
the preamplifier and then having no B+ capacitor inside the  
preamplifier to couple to ground any noise the B+ might have picked  
up along the way, but I guess if it works, it works..... I also don't  
like the idea of having 300VDC running through a long, multi- 
conductor cable and think an internal B+ supply is a very good idea.

I'd still like to see those original capacitors tested under voltage,  
however, to find out if there's anything wrong with them. Resistors,  
of course, are probably all over the place in values.

DAA

On Jan 17, 2007, at 4:31 PM, Andrew Young wrote:

> OF wrote:
>> I'd be pretty surprised if its normal position is outside
>> the chassis.
> Oh, it is. Its a large (2"x2") metal box attached by two screws.   
> The chassis has two holes that the
> each terminal post goes through.
>
> I don't know if it is failing or not.  I guess I'm just shot- 
> gunning the rebuild and replacing
> everything since its 60 years old and I'm in it anyway.  Maybe I'll  
> just skip it for now though and
> install the amp again and listen to how it sounds.
>
> I was also considering building a B+ power supply for it.  I've  
> seen Bob S.'s 3 designs and was
> wondering if anyone has built them and which.
>
> -Andy