Voltage rating for AC coupling caps between stages.

[armstrom]

I have what may seem to be a bit of a newbie question... but what voltage rating should I use on AC coupling capacitors in between preamp stages? At first glance it would seem you need caps at least rated to handle your +B voltage since they're tied directly to the plate (after the plate resistor). For example, in this schematic: http://www.ax84.com/static/corepream..._Schematic.pdf
The coupling cap C100 does not have a voltage specified while bypass caps C101 and C105 do (25V)

However, there is one case where I have seen voltages specified; The "Real McTube II" pedal. It uses a lower +B voltage but still specifies 200V coupling caps http://www.ax84.com/media/ax84_m152.pdf

Are these needed?? Can I get away without the high voltage caps?

Thanks!
-Matt

[6267]

Well, you want caps that are rated for at least the B+ voltage (before the anode resistor), and maybe even for the highest voltages in the amp (in case there is a possibility of that voltage appearing at the cap). With too low a voltage rating, the caps could break down or explode. It's better to go for safety here. 450v is probably fine for amps with 400v b+ or below, 630v for anything up to 600v, etc. If your amp's B+ is only say 250v, you should probably just use the 400v parts, they're cheap. Safety first!

BTW--If you're using grid leak bias (where the grid resistor is like 10meg and the cathode is grounded, with a cap in front of the grid & resistor) make sure the first cap is rated for the worst case voltages possible to protect the musician from tube shorts...

[armstrom]

Thanks for the info. I suspected this was the case, but I couldn't figure out why 99% of the schematics out there omit voltage specs on coupling caps yet specify them for things like the bypass caps on the cathode... Is it just assumed everyone knows? I guess this applies to tone control caps as well...

[armstrom]

Ok... nevermind, I found it. In the "General notes" section of the first schematic I linked to there's a note saying all the coupling capacitors should be 400V... Should have read the fine print It made logical sense to me that they should be able to handle the +B voltage, so that's why I was confused that voltage wasn't specified directly in the schematic.

[Enzo]

Many things are either "understood" or are specified in the notes. Especially things that would have otherwise been written over and over and over. Instead of noting all the coupling caps as 400v or 600v individually, they just mention that all caps are 400v unless otherwise noted. So the 25v bypass caps would be exceptions to the 400v rule here.

We routinely use ground symbols instead of drawn-in ground wires. The grounds are understood to be there. Now and then we see an older schematic where all the ground wires are drawn in - lots of extra lines on the page.

Some things are not mentioned because they are standard practices, or required by electrical code, or once again, just understood. For example, in Fender amps, some wires are insulated, but some wires are bare. Running from a panel control to the parts board would be insulated, but running a ground between two caps might be bare wire. The drawing never specifies insulated wire anywhere, we all know it needs to be there. And wire insulation itself has ratings - voltage and temperature for example - and you won;t see 600v wire specified on teh schematic, but we know it is there.


Like 6267 points out, your cap needs to withstand the highest possible voltage. And that means the top B+. In a typical amp, the power tubes might run on 450v, and the preamp tubes might tun on only 300v. And with a B+ of 300v, that input stage 12AX7 triode might have a plate voltage of 200 or even less. The coupling cap comes from that plate. However, the only reason that 300v B+ is down to 200v at that plate is through the tube current. Pull that tube and that plate voltage - voltage at that pin on the socket anyway - now pops right up to the full 300v. Not only that, pull ALL the tubes, and the whole row of tubes will ALL run their plate pins up to the full 450v. Except now with no load that 450v B+ will rise to close to 500v. SO if that amp were ever powered up without its tubes - something done during test or construction certainly - most of the coupling caps would be exposed to as much as 500v. In my example anyway.

SO even in the absence of mention on the schematic, I would expect any cap connected in any way to the B+ voltage circuits to have a rating at least of the highest B+ voltage in the amp.

[armstrom]

understood. Thanks for the info guys!