Noob Capacitor question

[flood]

hi all,

i'm putting a couple of projects together - the firefly amp, El_Martin's Lil wreck and have a couple in the pipeline. since i moved to india,, sourcing some parts has presented some difficulties - axial capacitors are difficult to find, and axial 450v are pretty much unobtainium. i need to remind you that i'm on a budget here - still fresh out of college and looking to pay off CC debt.

in any case, i went to the electronics market and picked up the only 47/400V caps i could find. they are radial - what however worries me is their relatively small size, compared to the pictures i've seen and the monster facon dual caps i have in my old italian amp.

in any case, i noticed at some point of time that they were manufactured by a company called L-Tec - a korean company that makes passable caps for computer power supplies.

like i said, this worries me a bit - can HV caps used in SMPS supplies stand up to 50Hz? i did test my power supply for the firefly, and i got 300V+ at all B+ (no load). or should i scour the market to look for alternatives, which are hard to find? thanks!

[Stanz]

A cap is a cap.

As I understand it, axial caps have one of the leads strung across the inside of the cap so that there is a lead on both ends. Radial caps are a "more natural"' design as both leads have a short run to the outside of the casing, thus the smaller size for same ratings.

In the end, if that is all you can obtain, then you really don't have much choice. The main issue would be reliability, and I am not familiar with L-Tec.

As for the multi-caps, having individual caps is the same, so it really comes down to a design issue. Both work fine. Which do you prefer taking up space in your chassis? If a multi-cap fails, you are replacing two (or more) caps, even if only one failed.

[flood]

hi stanz, thanks for the explanation. i did a bit of research on Ltec, and their reputation isn't all that good after all - most people seem to sub the caps for other brands like sanyo.

i think the caps should suffice for the 12AU7 projects on hand, for larger projects i will look for more reliable makes. i'm going to the market today; hope i find something.

the reason for my concern was the explanation i got off some SMPS site, saying something about how the cap size could be significantly reduced due to the fact that the switching frequency was so high. i suppose i could give it a try.

[flood]

i didn't want to being another thread for yet another silly noob question - but i must ask this: i've all but assembled my lil wreck (2X12au7 SE designed by El_Martin, built from whatever i have lying around), am in a bit of a quandry now though - i picked up a couple of inexpensive trannies (12-0-12 500mA) that i wanted to connect back to back, ignoring the 0V tap.

in any case, i get 28V on no-load measured across trannies, and 189 VAC when connected back to back - after rectification and filtering, i get about 289.

i figured these trannies would suffice for a 2x 12AU7 design, using 24V to power the filaments in series. the SE OT is 7k primary with 4/8/16 secondaries.

however, when i hook the transformer up to power the filaments in series, i get only 16V after rectification (DC heaters) or even less unrectified?

why does this happen? is there some fundamental of electronics that i've missed? is the 0V tap creating the problem?

thanks for any info!

[Andy Le Blanc]

it depends on the rectifier, how it loaded and what your measuring...
a bridge with no centertap will give you a peak dc v of 1.41 x sec. ac
and an avg dc v of .9 x sec. ac in to a capacitor input load
theres good info at www.hammondmfg.com in the transformer selection guide.

[flood]

hi andy, thanks for the assistance... that doesn't seem to solve my problem though. if a transofrmer's secondary is rated at 12VAC, shouldn't this suffice to heat up the filaments (4-5) of a 12AX7? for some reason, the voltage drops to 7V when 12VAC is connected to pins 4-5.

is it the transformer that i am using?

EDIT: is it possible that i am being overtly optimistic about the load that the heaters present - perhaps the transformer impedance is too high?

[Andy Le Blanc]

by back to back you mean youve connected the secondary to the secondary?

If so id use trannies with a larger rating ..... 1 to 3A

[flood]

ah ok, i will give that a try.

i feel kind of silly asking ultra-noob questions on a forum with advanced builders - but am quite impressed with the friendliness and attitude ointhese boards. thanks a lot!