PS decoupling cap placement

[Klimon]

How important is it to place the PS decoupling cap near the stage it decouples (e.g. near driver tube load resistor or power stage opt)? Diversifying the question:

1) Would a smaller bypass cap placed closer to the stage make up for the 'main' cap placement say 5" away?

2) What variables influence the necessity of minimal physical distance? Is a phono stage more crucial than a driver stage than an output stage / do capacitor specs like capacitance or esr make a difference?

You probably get my drift by now; any elaboration on the underlying mechanism or practical input is most welcome

Simon

 

[Miles Prower]

How important is it to place the PS decoupling cap near the stage it decouples (e.g. near driver tube load resistor or power stage opt)? Diversifying the question:

1) Would a smaller bypass cap placed closer to the stage make up for the 'main' cap placement say 5" away?

Depends. If you're talking a PP topology, then you don't need such a capacitor in the first place since AC cancels at the primary center tap. For something like an Ultrapath, it would be best to mount it close. A distance of 5" isn't going to make any difference at audio frequencies. For RF, it's a whole 'nother story.

2) What variables influence the necessity of minimal physical distance? Is a phono stage more crucial than a driver stage than an output stage / do capacitor specs like capacitance or esr make a difference?

The variables that necissitate minimal distances are operating frequency and gain. If the frequency is high enough, 6" of wire might be an appreciable percentage of a wavelength. A phono stage would be more critical since you'd expect that to have much higher gain. Remember: hollow state isn't like solid state where, especially with BJTs, it's at its best at DC and it's all downhill from there. Any VT, even the "audio" types like 12AX7s, 6SL7s, etc. will all work as well at DC as they do well into the VHF range. Combine high gain with a sloppy layout, and you just made a QRP oscillator.

Therefore, when using pentodes and cascodes, keep all leads as short as possible. If your tube sockets have a large, central pin, be sure to ground it as it's there to make an electrostatic shield. When installing screen bypasses, connect the outside foil to ground, and mount them across the socket so that they'll do double duty as an electrostatic shield. Install grid and screen stoppers as close to the socket pin as possible. O'scope and look for RF parasitics.

With low gain stages, it's not so critical since small signal, medium gain triodes are low gain devices, as compared to transistors and some small signal pentodes. Still a good construction practice to keep lead lengths short.

 

[Klimon]

That's very useful Miles, thanks a bunch

Simon