Cathode bypass cap orientation question

[Tapsnap]

OK, I’m confused now. I’m sure I read here on this forum that electrolytic cathode bypass caps are soldered into the circuit upside down; that is, with the positive side connected to ground and the negative side connected to the cathode. But  I just read a whole forum discussion claiming this is wrong. Can someone here definitively settle this one way or the other

[jjasilli]

A picture is worth a thousand words:  https://el34world.com/charts/Schematics/files/Fender/Fender_champ_aa764_schematic.pdf

[sluckey]

I’m sure I read here on this forum that electrolytic cathode bypass caps are soldered into the circuit upside down; that is, with the positive side connected to ground and the negative side connected to the cathode.

Could you post a link to that discussion?

[Tapsnap]

Here’s the link to that article.
https://www.thegearpage.net/board/index.php?threads/backwards-cathode-bypass-cap-is-this-okay.477313/

[sluckey]

I want to see a link to the discussion on the HOFFMAN forum! You know, the one you were sure you read here, not on Gear Page.

[glass54]

+1 
The internet is a wonderful thing, connects us guys and we learn BUT there is the dark side ie everyone has an opinion, fact or otherwise.
You really can't go wrong with the Hoffman pages, especially with Sluckey and others guiding you.
See attached fixed bias circuit and cap orientation (by Sluckey of course). Note C17 positive terminal goes to Power Ground. I think its clear.
Kind regards
Mirek 

[Joel in Texas]

My best guess, Tapsnap, is that you have confused two terms that sound somewhat alike, and start with the same letter.

Specifically, my best guess is that you saw discussion here at Hoffman forums about the counter-intuitive, positive-terminal-to-ground orientation of the electrolytic cap in the bias supply of fixed-bias circuits, and it was NOT a discussion about cathode bypass caps.

In a fixed-bias power supply circuit, the bias supply is a negative voltage, relative to ground (-40 volts, for example).  In turn, you orient the electrolytic capacitor in the bias supply circuit with the positive terminal to ground and the negative terminal to the negative supply line.  This makes sense, because ground is "more positive" than the negative voltage of the bias supply.

[PRR]

Build it. Wear safety glasses. Connect voltmeter to cathode network, checking your red and black.

Most simple circuits, the *cathode bias* cap is "-" on ground.

(The rare back-bias scheme is the other way.)

On fixed grid bias supplies the "+" is on ground, because the live end must be NEGative. (I suspect there is an exception somewhere.)

If the black lead is more Positive than the red lead, the cap will blow-up in a minute or two. Shut down fast and fix it.

[kagliostro]

Isn't it that you've also read something like this and a bit of confusion has happened?

https://el34world.com/Forum/index.php?topic=11427.0

Franco

[Tapsnap]

Joel in Texas, you might be right. So just to be clear; the positive side goes to the cathode in a regular bypass cap situation, but the are flipped in a fixed bias circuit, right? Yes or no answers please.

[Tapsnap]

Sluckey, I think I read it in the construction notes. But I can’t be sure. Anyway, I was confused.

[sluckey]

The important thing is you're straight now, right? 

[pdf64]

It’s worth noting that reverse biased ecap will only ‘blow up’ if the circuit they’re in can provide sufficient energy (current). An ecap that’s connected backwards on a preamp cathode won’t explode. Rather it just won’t work as intended / expected, eg high leakage current, non linear performance.