filter cap value for tube rectifier

[rdrgtr]

I am building a 50watt marshall type amp but have decided to used a tube rectifier to bring down my B+ a little.  I read that the resevoir cap (first cap 50 uf + 50uf) needs to be around 22uf to reduce the in-rush current a bit.  On the typical Marshall schematic the first stage caps are wired in parallel (100uf total).  Since I have the chassis punched and the dual 50uf cap in place, what would be the down side of wiring the dual 50uf caps in series (25uf total)?  Logic would say yes but i just thought I would check.  

[eleventeen]

A bridge rectifier is going to RAISE your B+...sometimes considerably (30-50v) over a tube rectifier...if that's the comparison you're talking about. You'll completely lose the tube voltage drop across the 5Y3/5U4/GZ34, meaning...the output B+ will INCREASE. No question about it.

It is true that the first cap the rectifier diodes "see" is supposed to be on the lowish side--about 20 uf...because you want to reduce the effect of massive inrush current when the amp is first turned on.

However, what you are proposing 1: may be impossible, and 2: will reduce your total ufds quite a bit. You may want to rethink that. 

You're not going to be able to series-connect two sections of an electrolytic cap that share a common ground, that should be clear.

If what you are after is a B+ reduction, the usual reco is to place the choke before any of the filter caps. That could drop B+ by 10-30 volts. It also pretty much eliminates having to think about surge current at all. If this is what you want--reduce B+ and keep your caps as installed, simply changing the position of the choke in the filtering circuit would seem like the simple answer. If you don't want to do that, then I'd say you'd want to throw in a separate 20 ufd, whether by chassi-mounted can or under-chassis radial/axial type. Electrolytic cans = not flexible!

[rdrgtr]

Sorry, I meant to say tube rectifier.

[rdrgtr]

I am using a tube rectifier in my build.  I was thinking about using this config like this old 50watt fender on my first stage of filtering.  I have the dual 50uf cap in place.  Curious about the resistors.  In theory this would work right? 

http://www.ampage.org/schematics/blkpwr50.gif

[FYL]

I am using a tube rectifier in my build.

 

Which build?

I was thinking about using this config like this old 50watt fender on my first stage of filtering.  I have the dual 50uf cap in place. 

What transformer are you going to use?

Curious about the resistors.

Balancing Rs for the caps in series.

[rdrgtr]

I am building a 50 watt marshall type amp.  The pt I  want to use is an older type that would put a little bit to much b+ on my plates (el34). So, I decided to use a tube rectifier since I had 5vac taps.  This would give me enough b+ drop to give my plates and screens some relief.  My question is... Why can I not series my dual 50uf cap can to bring my total value down to 25uf?  This would reduce the amount of inrush current created by my first filter stage.  The dual cap is there why not use it?
This illustration shows what I would like to do.  Curious about resistor values.

http://www.ampage.org/schematics/blkpwr50.gif

[sluckey]

Why can I not series my dual 50uf cap can to bring my total value down to 25uf?

Because the two caps inside that can have their negative terminals connected together. There's no way to undo that.

[rdrgtr]

duh...of course, my bad.

[eleventeen]

Schematically, you can certainly do that...UNLESS the two-caps-in-one-can share a common -minus- terminal, which is the state of affairs 98% of the time. How are you planning to connect the minus terminal of the upper series-cap to the plus terminal of the lower series-cap? Can't be done. Well, OK, it *can* be done, but not without completely shorting out the lower cap. 

The methods I would suggest would be either to

1: use a completely independent single section 50 ufd or 47 ufd cap, with the same type of 220K balancing resistor around it, in series with section 1 of your dual-50 ufd can-cap.

2: I'd prefer though, to use an external 20 ufd cap, whether choke or capacitor input filter is your schema.

The reason I prefer #2 is that under #1, the two positive terminals of the first electrolytic would be roughly 220 volts apart, as one section would be taking the midpoint of the series-connected cap (70/70 in your dwgs) and the other would be taking nominal amplifier B+, less a few volts dropped by a resistor or two. While that's certainly within the capability of the single can, it is "asking for it", IMHO, and you are probably looking at an external, independent electrolytic cap in *either* case.