Valve Rectifier Question

[surfsup]

Regarding the rectifier tube 5Y3 on this layout

[img1067]http://ceriatone.com/images/layoutPic/f ... Layout.jpg[/img]

For a 5Y3 the pins used are 2, 4, 6, and 8 (2 and 8 heaters) and 4 adn 6 for the plates.

http://tdsl.duncanamps.com/show.php?des=5Y3

I assume pins 5 and 7 are being used as "terminal strips" where the diodes to pins 4 and 6, respectively, are in series with the leads so if the tube fails the amp is okay, yes?

One question:
1) whats the cap for?
2) shouldn't something go to ground? (EDIT- I see the PT has a centertap)

[Structo]

The cap on the diodes is to quiet any transients from the diode switching.
Yeah, it's kind of a safety for the rectifier tube but tube purists hate those things.

Many agree that current production rectifier tubes aren't that great and don't have a long life so you see mods like this one.

[tubeswell]

I assume pins 5 and 7 are being used as "terminal strips" where the diodes to pins 4 and 6, respectively, are in series with the leads so if the tube fails the amp is okay, yes?

Yep. And I'm a 'tube purist' and I always put those 'safety diodes' in my amps . They only make 0.6V voltage drop difference in the rectification, and you don't notice that against the tens of volts dropped across the tube rectifier. What's more, they don't adversely affect the functioning of the tube rectifier, and they not only act as a backup if the tube rectifier shorts, but they also prevent destructive high VAC being dumped onto the filter cap(s) if the tube rectifier does short (which prevents the filter caps shorting to ground, which prevents the PT B+ winding being shorted to ground, which prevents the PT going up in smoke.