outer foil orientation on micro bassman ltp

[Jason McG]

hi folks,

wondering if someone could please help me with my caps, does this look correct for outer foil end (marked ends)?

thanks very much for the help

edit:
update, see below for fixed version

 

[Tom Kamphuys]

The foil should be connected with the lowest impedance to ground (or B+).

A schematic would be easier to check that imho.

 

[Phrygian77]

Ask yourself, which way does the signal flow through the cap? Which side has the lowest impedance? For a coupling cap, that will/should always be the input side. Gain stages have a low output impedance relative to the input impedance.

 

[King Fan]

@Jason McG , I can tell you know this, but for folks who aren’t familiar with the idea, Mr. Aiken has a good summary of why and how.

“Why do the capacitor manufacturers go to the trouble of marking the outside foil with a band? Aren't electrolytic capacitors the only ones where polarity matters? While it it true that polarity on a non-electrolytic capacitor doesn't matter, signal-wise, the outer foil is marked because it can be used as a shield against electric field coupling into the capacitor. In order to take advantage of the shielding properties of the outside foil, the capacitor must be connected in the circuit in a particular orientation.

The proper way to connect the outside foil is to the low impedance side of the circuit, which, in the case of coupling caps, will normally be the plate of the previous stage. If it is a bypass cap to ground, connect the outside foil to the grounded side. If it is a bypass cap from a signal to B+, connect the outside foil to B+. The outside foil will act as a shield against electric field coupling into the capacitor, so you want it to have the lowest impedance return path to ground.”

For shunt caps, ie, those that connect to ground, the ground end (of course) gets the outer foil. For your layout above, I’m gonna venture you’ve got it right.

 

[Tom Kamphuys]

Ask yourself, which way does the signal flow through the cap? Which side has the lowest impedance? For a coupling cap, that will/should always be the input side. Gain stages have a low output impedance relative to the input impedance.

Great insight! Why didn't I think of this myself?

 

[Phrygian77]

Great insight! Why didn't I think of this myself?

It's kind of confusing with the PI layout because those coupling caps are upside down. They're arranged at the input, instead to how they're normal arranged at the output of a gain stage.

Caps used for low pass filters, or cathode bypass, as @King Fan noted, the 'output' of the cap will have the lowest impedance.

 

[robrob]

The only thing I'd change is the Middle tone cap. The upper terminal has <25k ohms to ground through the Middle pot and 200k to ground on the bottom terminal through the 100k Tone Slope and 100k Cathode resistors.

 

[Jason McG]

thanks very much everyone, will leave the update here for others future reference

 

[Hyakuya]

I'm just about to start building a Bassman Micro LTP EF80 so this has saved me some time..... thank you.