Some HH Scott 250 questions

[Selmerdave]

Working on the power supply for a Scott 250, it has an unusual can cap coming off of the 5U4GB. Schematic calls for two 40uF/300V sections in series, my can is actually two 40uF/350V sections with a tab for the negative of one of the sections for the series connection. There are also a pair of 270k balancing resistors. Am I right in thinking it would be reasonable to replace this arrangement with a single 20uF/630V cap and no balancing resistors? I'm assuming the explanation for the design was the lack of availability or perhaps prohibitive cost of 600V+ caps? Further, is there much reason not to make it 40/630? Schematic detail attached, B+ output is 455V at the plates of the EL34s.

 

[Shadowdog]

I agree with you! Why not a single 40/ 630 as the 5U4 should handle it and a little extra capacitance shouldn't hurt. You could also strap on small bypass poly caps with the same voltage for a sonic improvement.

The resistors may be important for the circuit to the the neon lamp etc.

However I'm no expert so maybe one will chime in!

 

[Dave451]

I think you are both correct. The series-connected capacitors are a practice you seen in tube ham transmitters to get 1000V-capable capacitance and the 270K resistors are indeed balance resistors for the series-connected caps. Looking at the schematic, I don't think the 270K resistors have anything to do with the operation of the neon B+ indicator lamp; they are for the series-connected caps and the voltage drop across them is minimal. Balancing resistors wouldn't be required for a single cap. The higher voltage rating would be good to cover any voltage surge on startup (though it is minimized with a tube rectifier).
The 5U4G/5U4GB datasheet allows for up to 40 uF exit the tube (some older datasheets say 32 uF). Haven't used the bypass poly caps, so can't really comment on those, but I know that many do use them.

 

[kvflyer]

As you probably know, back when this amplifier was designed and built, high voltage electrolytic capacitors were either very expensive or non-existent. Connecting two in series of the same value yields a capacitor of one half the value of the capacitor and double the voltage. There is an ongoing debate on another electronics forum that says you need (or you don't need) balance resistors. For that matter, some 500 volt electrolytic capacitors had two lower voltage capacitors in series inside of the aluminum can.

Capacitor manufacturing processes today can produce higher voltage products. But duplicating what HH Scott did "Back in the day" certainly isn't wrong.

 

[Selmerdave]

As you probably know, back when this amplifier was designed and built, high voltage electrolytic capacitors were either very expensive or non-existent. Connecting two in series of the same value yields a capacitor of one half the value of the capacitor and double the voltage. There is an ongoing debate on another electronics forum that says you need (or you don't need) balance resistors. For that matter, some 500 volt electrolytic capacitors had two lower voltage capacitors in series inside of the aluminum can.

Capacitor manufacturing processes today can produce higher voltage products. But duplicating what HH Scott did "Back in the day" certainly isn't wrong.

Yeah, the problem is that nobody makes a can cap like that. I'd imagine it's possible to get one made from a place like Hayseed Hamfest, but when 630V caps are readily available (and on hand...) I just wanted to confirm that it's an electrically safe way to go.