SS rectifier & hum magic

[tubeswell]

I'd just like to confirm something as a result of trying out Merlin b's suggestion of series diodes with parallel caps - It actually makes the rectifier a lot quieter. For a FW rectifier, I used 2 x 1N4007s in series on each side of the HT, with 3kV (I know its overkill - could've used 1kV) .01uF cap in parallel with each diode (that's 4 diodes & 4 caps).

Just to satisfy my curiosity I tried the same amp (a 6G15 clone) with a standard 2-diode FW rectifier, and it was quite a bit noisier.

Needless to say the diode-cap trick is a keeper (where SS rectification is called for) - so there. JM2CW

(I realise its not a new revelation - but 'twas good for me)

[Robert M. Martinelli]

Hi tw!

BTW how is it going down there? Hope you're fine!

Yep, AFAIK those caps help to smooth each diode' s switching transient, which ( of course ) is generated at mains frequency by each diode. This kind of transient is too brief a spike to be properly filtered by the following filter caps, due to their long time constant.

A good thing to remember when SS rectifiers are to be used, so thanks you for the "heads up" ( and, of course, thanks to Merlin b as well )

Cheers

Bob

[guitician]

Those are called "Snubber" caps. If you use "Fast" diodes you don't have to use them.

[Robert M. Martinelli]

Yep, also called "Schottky" diodes ( after the name of a scientist, if memory serves me well )

[txstrat]

Well, lately I noticed a stronger hum in a DRII copy I built. The original design calls for those snubber caps, as well as double the value for the first filter cap.
My first filter is 47uF, the schematic shows a 40uF and a 80uF in parallel.
Do you guys think the hum can be notably reduced by correcting those two differences?

[Robert M. Martinelli]

Hi Matt,
120 uF is almost three times as much filtering, so I think your 47 uF cap could be too small to properly filter the ripple, especially when the current demand tends to increase ( ripple is load-dependent ). Since we're talking about SS rectifiers ( so no worries about ruining a rectifier tube with too large a filter cap ) I'd consider some over-filtering ( 200 or even 220 uF ).
Over-filtering should help the amp's dynamic behavior as well as with ripple, especially on power chords, aggressive rhythmic playing and the like, because the large filter capacitance acts as an energy reservoir ready to be released when the output tubes ask for it.

As to the snubber caps, they kill the diodes' switching transients, and they cost so little that I'd give' em a try in any case
( unless you throw in Schottky diodes ).

HTH

Best regards

Bob

[tubeswell]

Yep they definitely reduce noise where you have 1N4007s. See the attached schematic (courtesy of valve wizard).

[Sir Cuitous]

Those are called "Snubber" caps. If you use "Fast" diodes you don't have to use them.

What would be the "fast diode" equivalent to the 1N4007?

[txstrat]

What would be the "fast diode" equivalent to the 1N4007?

Yeah, I'd like to know that too.

TW
Why'd you choose 10nF for your snubber caps? In the DRII schem there are 2nF. I believe if I use a bridge recto it would make no difference, would it?
Bob
I'll definitely try a second (80uF laying around) cap in parallel.

[Enzo]

A bridge rectifier is four diodes in a box, nothing more.

There are various fast recovery diodes to choose from. 1N4007 is a common general purpose diode - 1 amp 1000v - so match those specs. Take your pick. If I recall - and that happens less and less lately - UF4007 is a popular choice. I think I have FR107 in the drawer, along with some Moto types I don;t remember.

[txstrat]

A bridge rectifier is four diodes in a box, nothing more.

Hihi, yes I know that.
I usually use four single diodes setup properly.
My question was regarding the snubber caps value. The DRII schem shows 2nF with a brigde recto.
I was wondering if the value depends on the sort of rectifier since tubeswell uses 10nF with a FW recto.
Anyway, I might go for the UF4007 (thanks BTW) but they seem hard to find in stores round here. Might order them online.

[tubeswell]

2nF, 10nF, 100nF, doesn't really matter - whatever you can get in the 1kV-3kV ish rating

[Enzo]

oh.

Tubeswell has it, the difference in value was whatever the different engineers thought would be sufficient. Has nothing to do with the bridge being four diodes or a block bridge.

[Robert M. Martinelli]

Well,
I could be wrong, and if so bear with me, ( my last two neurons just went to bed and they're peacefully sleeping at the moment ) but I think there are some limits that are not to be exceeded....if snubber caps are too large they could cause some AC to "bypass" the diodes due to their low capacitive reactance at mains frequency.

Xc=1/(2pi*F*C), so a 1 uF snubber cap at 50 Hz would behave like a 3183 Ohms resistor in parallel with the relevant diode ( 2652 Ohms at 60 Hz ), and the rectifier's functionality would be hindered, as a part of the "opposed" half wave would pass through.

Something in the 10 to 47 nF range could be in the ballpark IMHO.

FWIW in small applications like ours I've never seen snubber caps larger than 22 nF so far, though larger caps might be needed in "bigger" applications.

JM2CW

Best regards

Bob

[tubeswell]

Ok Bob that sounds scientific enough to make me a believer. Well done.