Diodes on a rec socket?

[azatplayer]

Just looking at some layouts over at ceriatone, and notice he uses this idea a lot.
Whats the deal with running the HT thru diodes before hitting the rectifier?

Ive seen this before, but never stopped to think about it, i think first time i figured there was a switch somewheres to flip between tube a SS rectifier.

Not so here.

As im typing im thinking im understanding...
Your standard marshall has 4 diodes, only needs 2, but double for safety.
So.. tube acts as 2 diodes of rectification, does the xtra diodes in front of it take some of the strain off the tube? That makes it allready rectified tho?

I thought i was understanding..

[paulster]

It's for a couple of reasons.

One is that the rectifier tube doesn't have to deal with blocking the negative-going waveforms so it makes for an easier life for the tube, with less chance of arcing over internally.

Secondly is that if the rectifier tube fails short then rather than pass unrectified AC it's already passing rectified AC courtesy of the diodes.

The downside, in my opinion, is that although you still get the sag and compression that tube rectifiers create, one of the nice things about them is the lack of switching noise that diodes suffer. Put a diode in from that's already switching then you've potentially already got that switching noise to deal with coming through your tube rectifier.

So, if I were going to adopt this, I wouldn't be using 1N4007s in front and would be using something like a UF series diode to minimise the switching noise.

[azatplayer]

Thanks Paulster. That i understand!
Cheers Don

[Cliff Schecht]

Could you not put caps in parallel with the diodes to reduce switching noise?

[tubeswell]

if the rectifier tube fails short then rather than pass unrectified AC it's already passing rectified AC courtesy of the diodes.
.

And the reason you don't want AC going through a shorted rectifier tube, is that AC will destroy the filter caps* and thereby cause them to short, and then you would have AC directly from the HT winding shorting to ground, and that would blow the PT. So those diodes are an insurance/protection for the PT in the event of the rectifier tube shorting.

The diodes themselves only drop about 0.6V each, which is hardly noticeable compared to the (much greater) voltage drop through your rectifier tube.

I've built this mod into all my home-brew amps that use rectifier tubes, and there is no switching noise that I can discern.

*electrolytic filter caps can't take AC.

[Andy Le Blanc]

There's an impedance relation ship in there. The HV sec., the rectifier, and the first filter cap.

Seems like there might be more to it.