Need advice on capacitor substitution in tube preamp design

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dgatwood

dgatwood

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I'm considering building a tube preamp (non-starved plate). Here's the schematic I'm basing it on:

http://www.tubecad.com/october99/page15.html

The design is kind of nice in that I can use a common 110/220V transformer with center taps ($12) and a full wave bridge to get a close approximation of the correct voltage (about 156VDC, by my math).

It's also nice because for under $30, I can get all the parts I need except for the tubes, the tube sockets, a chassis, and connectors, so the entire preamp should come in under $50 (and with enough of the small parts left over to build several more, transformer/rectifier/jacks/chassis/tubes notwithstanding).

I'm running into one really nasty snag, though. It specifies a 0.1uF polarized capacitor. I can't find any evidence that such a thing exists in nature, much less at the 100V level or higher.

Does anyone on this board understand tube circuit design enough to tell me what the effect on the overall circuit will be if I substitute a .47uF electrolytic? Is this thing just being used as a DC block?

That's not the only substitution, but it's the only one that's far enough that I'm worried about it. All the others fall well within the tolerances you'd get for an ideally-rated part if I could easily get them. That said, the complete substitution list follows:

.1 uF electrolytic -> 4.7 uF electrolytic
500 ohm resistor -> 510 ohm
12.5k resistor -> 12.51k (12k + 510 ohm)
2.5k resistor -> 2.51k (2k + 510 ohm)

Thoughts?
 
I can't for the life of me think of one reason why a cap would need to be polar in any application. Electrolytics are polar as a function of economy, not necessity.
If you used a non-polar cap there, all it means is that it wouldn't explode if you put it in backwards :confused:

EDIT: I looked at the schematic and I think all it is, is that this guy only has one symbol for "capacitor" and it happens to be the polarized symbol. Don't worry about it. The .1uf, .47uf, 1uf should all be non-polar. You will get worse performance even if you did manage to find polarized electrolytics for all those. Use ceramic or metal film or (preferably) poly caps.
 
In general, you might get questions like this answered here, as there are a few knowledgeable sorts here (not me!), but there are many, many more people here:

http://www.prodigy-pro.com/forum/viewforum.php?f=2

That will be happy to help. Don't be afraid to ask stupid questions there either, they are exceedingly nice to newbies :)
 
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