Rectifier saftey

[p2pAmps]

Guys,

This is something I bet most of you have seen or know about.  Is this a good idea for all tube rectifier amps?  Just want to confirm with the smarties here...

[sluckey]

I think it's a good idea. I've never done it though.

[p2pAmps]

The theory is doing this will save your circuit from AC voltage should the rectifier tube take a dump...  I think I will do this from now unless someone has a compelling reason to avoid this mod

[shortfuse]

I often wondered about this, would you loose the sag in the tube if you install the diodes.  I guess what i am asking is would the amp sound and preform like the diodes were not there in that configuration?  To me it looks like a rec tube with ss back up?  So if the tube blows the amp will still work like it was diode rec?  Pardon my ignorance on this but I have one on the chopping block I have done a lot of research on and I was think about this same thing, thanks for the post.

[Ed_Chambley]

I often wondered about this, would you loose the sag in the tube if you install the diodes.  I guess what i am asking is would the amp sound and preform like the diodes were not there in that configuration?  To me it looks like a rec tube with ss back up?  So if the tube blows the amp will still work like it was diode rec?  Pardon my ignorance on this but I have one on the chopping block I have done a lot of research on and I was think about this same thing, thanks for the post.

You are only protecting the HT wires from the PT.  The Tube is still rectifying as the diodes are separate and before the tube.  If the tube goes, so will your B+ since the diodes are only on the AC side.

[Ed_Chambley]

I have done something similar with a switch where position 1 on the switch is diodes to pin 2 and 8 which makes SS rectification and position 2 goes to pin 3 and 5 for tube rectification.  If you add a diode to position 2 on the switch this would keep the diode between the rectifier tube and the Transformer as well, but you would them have the selection of either type of rectification.

[Willabe]

I often wondered about this, would you loose the sag in the tube if you install the diodes.  I guess what i am asking is would the amp sound and preform like the diodes were not there in that configuration?

The internal resistance of the rectifier tube is still in series with the PT B+ secondary fly leads. So any rectifier tube effects will still be in play.

Even though the SS diodes won't sag the tube rectifier still will.

[shortfuse]

Thanks Ed and Brad I have seen Sluckeys diagram before and it is there again in a recent post on switching between both but you answered my question and that makes sense.

[Fresh_Start]

If done properly, the diodes are between the PT and the rectifier tube. Shouldn't be any effect on sag.

This was the first of R.G. Keen's "Immortal Amplifier Mods"
http://www.premierguitar.com/articles/the-immortal-amplifier-mod-1

BTW links at bottom of page for subsequent mods in the series.

Seems like relatively cheap insurance to me, but I can't quantify the risk of the rectifier tube failing "closed". I did include this on my last amp build and will put it in future ones. I did not include the extra fuses in mod #2...

Chip


P.S. Had typed almost all of this then got distracted before hitting "Save" so apologies for beating near dead horse.

[VMS]

I've thought about doing this too but my question is do those diodes add noise to the amp?

I've read that solid stage rectifier is noisier than tube rectifier.   

 

[Willabe]

I've thought about doing this too but my question is do those diodes add noise to the amp?
I've read that solid stage rectifier is noisier than tube rectifier.   

Under what conditions?

Amp set clean or set dirty? On stage with a band or in quite sound proofed studio. 

You'd have to try SS diodes in an amp of yours to see if you hear a difference.

Ultra fast diodes are easy to get and very close to the same price and have less switching (turn on/turn off) noise then regular diodes.

 

[John]

I've done that on my 20 watter. Cheap, easy to do, so why not I figured?


I use plain 1N4007s for rectification in the SE amps I build (only 4 so far, so not like I'm an old hand lol) but they are all very quiet. The only amp with hum issues is my 20 watt P-P, so go figure. Although it only gets noisy past the halfway point, and past that it's just plain loud anyway.

[VMS]

You guys are probably right.

Maybe I try it someday, although there is this school of thought that tube amp sounds best just before it blows up. So making amp immortal might prevent one from hearing the sweet sound of dying amplifier. 

[p2pAmps]

All great reading and knowledge here...

[HotBluePlates]

It's probably in one of the links, but here's the bottom-line reason for the solid-state diodes:

Tube rectifiers rarely fail by becoming open-circuit; instead they fail by shorting plate to filament/cathode. When the short-circuit happens, a.c. passes through to the first filter cap without being rectified. When that incoming a.c. swings negative, it destroys the (electrolytic) filter cap due to reverse polarity.

With the solid-state diodes in place, the tube can have both sections short, but the a.c. is already pre-rectified before it hits the tube rectifier. If you're onstage, the amp will still play but you might notice the loss of sag.

Maybe I try it someday, although there is this school of thought that tube amp sounds best just before it blows up. So making amp immortal might prevent one from hearing the sweet sound of dying amplifier.

 

If the tube rectifier shorts plate-to-filament/cathode, you get that negative voltage input to the filter caps. Reverse polarity voltage on an electrolytic cap causes the oxide-layer dielectric to evaporate instantly, turning your filter cap into a short-circuit to ground. The cap will explode (due to gas build up) and your line fuse will pop due to excessive current draw with B+ shorted to ground. Hopefully, your PT doesn't get burned up. And now you have no sound, and the magic smoke escapes... 

[p2pAmps]

I learn new things daily on this forum, thanks

[HotBluePlates]

You're very welcome! I learned a lot of it on this forum, over time. So I'm just recycling it on to you... 

[Willabe]

although there is this school of thought that tube amp sounds best just before it blows up.

That's a line that Gerald Weber used to say to guy's in his Vintage Guitar Mag. articles and books.

Usually to a guy saying "you said not to use a SS rectifier in my BF Deluxe Reverb and it sounds great!"

To which he would reply "I didn't say it wouldn't work, but........." 
 

[trobbins]

The ss diodes don't exhibit reverse recovery - the valve diode forces a very soft turn-off of current with no reverse current effect.  So 1n4007 are fine for low powered amps, but you may need 2 series 1N4007 if PT secondary is above 300-0-300VAC.


If a loss of sag is noticed, or an increase in low frequency hum (mains frequency f, not 2f, if one valve diode shorts) then best to look for a failed tube sooner rather than later, as the capacitor does get stressed more.

[pompeiisneaks]

Speaking of all this rectification, It made me want to ask, is the Mesa "Dual Rectified" one that allows you to switch between diode rectification and Tube?  or does it just provide both?  And then what is triple rectified?  options between ss and two different rectifiers?  Just curious, sorry if I'm hijacking the thread :)

~Phil

[Willabe]

I thought that some of the Mesa Dual Rectified amps were SS preamp, tube rectified power amp?

[pompeiisneaks]

I guess I could not be lazy and look up a schematic :P  It looks like its literally a hybrid of what I said, its switchable between SS and TWO 5u4 rectifiers.  See snippet from schematic.  What does two rectifiers provide?  Sag but not as much or? 

[shooter]

What does two rectifiers provide

bragging rights?

[sluckey]

Two rectifier tubes connected in parallel as shown can provide twice as much current as a single rectifier tube. That means less sag. And a fancy name that doesn't mean a lot to a savy tech. But it sounds good coming from a salesman. 

[Willabe]

I think they had a few different set ups called by the same name(s).

You'll have to sort through a few schemos to find them.

[uki]

All great reading and knowledge here...

I learn new things daily on this forum, thanks

Indeed I do too learn lots of new things in this forum, it is a great place !!

Thanks !!