Hi,
I'm building a preamp that will use a single 12ax7 tube. The power transformer I'm using is built for this purpose and rated at 5ma on the HV secondary. However, it's only 200 volts and to get the voltage I want on the tube's plates (250ish) I'll need to feed at least 400 to the 100k plate load resistors on the tubes, so I'm using a full wave voltage doubler. Under load, it lands right in that range. However, without a load (i.e. while the tube is warming up) the "doubler" is actually putting out 600v. Should I make sure my caps can handle this and, if they can, is that too much over-voltage for the 400ish volts they'll be seeing under load (right now the most expedient option is 350v caps in series). Would 500v caps be sufficient given how quick the 600v drops down into the 400s?
Thanks!
Joe
I'm building a preamp that will use a single 12ax7 tube. The power transformer I'm using is built for this purpose and rated at 5ma on the HV secondary. However, it's only 200 volts and to get the voltage I want on the tube's plates (250ish) I'll need to feed at least 400 to the 100k plate load resistors on the tubes, so I'm using a full wave voltage doubler. Under load, it lands right in that range. However, without a load (i.e. while the tube is warming up) the "doubler" is actually putting out 600v. Should I make sure my caps can handle this and, if they can, is that too much over-voltage for the 400ish volts they'll be seeing under load (right now the most expedient option is 350v caps in series). Would 500v caps be sufficient given how quick the 600v drops down into the 400s?
Thanks!
Joe
Applying 600 volts across a 500 volt rated capacitor is a no no, even if the duration is limited. Its a significant overload that will stress the very thin insulating layer of the dielectric film.
If you go with series connected caps then be sure to use balancing resistors across each, although if your transformer secondary is only rated for a few milliamps you haven't much to play with current wise.
If you go with series connected caps then be sure to use balancing resistors across each, although if your transformer secondary is only rated for a few milliamps you haven't much to play with current wise.
Yes. If you use a proper specced transformer, the difference between no-load and load will also be (much) less and 500V caps may be enough.
And no, you can't underrate the voltage rating on a cap, there is a max voltage but no minimum. A 500V cap will work fine at 350V.
Also, keep the current rating generously. With a low (5mA) current rating, you will lose a lot of voltage because the current pulses to charge the rectifier caps are much higher than those 5mA meaning you get a lot of drop across the thin wire. 50mA or 100mA secondary rating is not unreasonable.
Jan
And no, you can't underrate the voltage rating on a cap, there is a max voltage but no minimum. A 500V cap will work fine at 350V.
Also, keep the current rating generously. With a low (5mA) current rating, you will lose a lot of voltage because the current pulses to charge the rectifier caps are much higher than those 5mA meaning you get a lot of drop across the thin wire. 50mA or 100mA secondary rating is not unreasonable.
Jan
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Cool. Truth be told I'll probably use the 115, but 500 & 450 volt caps are cheap and available. It's that 600v surge on the doubler circuit that was an issue (well, one of them at least). And with proper circuitry, I'm thinking I should be able to help it out with a thermistor on the primary.
Thanks!
Joe
Thanks!
Joe
Maybe you've had experience in the past where a transformer wasn't as expected, and then that makes you question every time.
The VA ratings aren't what I would expect
normally the secondary voltage multiplied by the rated current gives the VA rating... but not here. The data sheet has examples where this holds true, but for most its way off
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