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 Post subject: Vacuum Tube Rectifiers
PostPosted: 02 Jan 2011, 15:10 
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:cop: UPDATE - 6 February 2012 - full details on the Power Supply Design for Vacuum Tube Amplifiers article. :cop:

In light of the new year's season and in the spirit of reflection and resolution I have another of my design philosophy questions for people to ponder. And again, I am sincerely interested in what people think on the subject. This time it's about choosing vacuum tube rectifiers. Personally I break the design decision into two parts, octal base tubes and miniature (7 and 9 pin) tubes. I freely admit that this separation is little more than cosmetic. I like big octal rectifiers where I'm using larger octal base tubes for the rest of the project and I like miniature rectifiers where the preponderance of the other tubes are miniatures.

After reviewing the specs sheets, I have to say that I find the 5U4GB (or the older 5U4 if the design can stand the higher losses) is at the top of the heap. I say this for several reasons. This tube can handle a 40µF first filter cap due to its impressive 4.6A inrush rating giving a much lower ripple at the entrance to the PS filter, it has current ratings which will allow a 275mA dc load at over 450Vdc, and it is relatively cheap and readily available. Personally I find the outer big octal rectifiers mostly wanting either because of ratings, price, or availability. The one exception might be the 5V3/5AU4. This tube is a real beast; its only drawback being availability.

In the realm of miniature tubes I have settled almost exclusively on the 6CA4. This tube not only gives the best operating range of the miniature rectifiers, but it can handle up to a 50µf first filter cap which seriously reduces the requirements on the power supply smoothing filter. I have been thinking about giving the 6X4 a try in a little media amp project (one 6C4 and one 6005 per channel), but the low current capability would mean that I'd need a one 6X4 per channel as well. I like the symmetry of having all 7-pin miniature tubes, but one rectifier per channel in an integrated amp may be too much. Maybe if I could build them as really small mono-blocks.

In any event, I am interested in hearing how other designers choose the rectifier tubes (or make the decision to go with solid state rectification) for their projects. Am I over thinking this? What do people think?

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PostPosted: 04 Jan 2011, 12:08 
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I used to like the GZ34, but when I got less voltage drop and better bass from a pair of cheap damper diodes, I never looked back, I've plenty of 5U4, 5Y3, and 5V4 tubes that I'll never use.


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PostPosted: 04 Jan 2011, 13:06 
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Send them to me! Got my address?? :idea:

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PostPosted: 04 Jan 2011, 16:16 
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I'd be more than happy to take those 5U4s off your hands to help clean up your storage area. ;)

So you believe that the added real estate necessary for two damper diodes is a good trade for the lower voltage drop? When you say "better bass", do you normally use a decent size choke (5H-20H) in your PS designs?

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PostPosted: 04 Jan 2011, 20:25 
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The 5U4 is great for higher current power supplies below 450, but I like the 5R4 more in some ways because it can handle really high voltages and only has a 2 amp heater current over the 3 amps required for the 5U4. With two 5R4's in parallel you can get a fairly beefy high voltage rectifier at only 4 amps heater current. :thumbsup:

Can't remember the actual voltage ratings for the 5R4, but I think it has a lower voltage drop than the 5U4.

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PostPosted: 04 Jan 2011, 23:17 
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Oh my, my, my.... to choose a rectifier.

I base the rectifier on what I'm building. Assuming the current is in spec. and the power transformer has the ability, I'll take a 5U4 over anything else to begin with. They can handle regular 47uF caps just fine.

For an 845 or other high voltage amps, I go with 5R4.

Power amps and preamps are where I have done the most testing of rectifiers with, especially on the ST-70 design. Of all the rectifiers, I liked the 5U4 for the mids, but it totally bagged out for bass power. The GZ34 was great for bass, but made the mids sound "solid-state". I ended up with a compromise with a SS GZ34 sub creation of all things that had the best of both worlds!

For preamps, the 5U4 again, or 6/12X4 or octal 6X5. The EZ80 isn't bad either.

The EZ81 is wicked on guitar amps.

And THIS from a guy who laughed at other geeks as fools for believing rectifiers sound different :blush: ;)

Now I believe they ALL sound different and each has their own place :D

Cheers!

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PostPosted: 05 Jan 2011, 11:31 
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All other things being equal(choke and cap size), the reason SS dioes have a reputation for great bass is due to their low impedance.
Damper diodes are as close as you can get in the thermionic realm. As for real estate the best sounding designs I've heard use a seperate chassis for the PS.
Suncalc wrote:
I'd be more than happy to take those 5U4s off your hands to help clean up your storage area. ;)

So you believe that the added real estate necessary for two damper diodes is a good trade for the lower voltage drop? When you say "better bass", do you normally use a decent size choke (5H-20H) in your PS designs?


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PostPosted: 05 Jan 2011, 20:51 
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This is all great information.

How about SE vs. PP? PP amps have a fairly significant peak to quiescent PS current ratio (PQCR) whereas in SE amps this ratio is usually fairly small (usually pretty close to 1). So, if as Rock4016 says...
Quote:
... the reason SS diodes have a reputation for great bass is due to their low impedance.
then in amps where PQCR is highest (i.e. where PS impedance is going to have the most pronounced effect) shouldn't we look for the lowest loss rectifiers?

Here is a plot of rectifier plate characteristics (albeit a fairly old one) showing plate current vs voltage for various tubes.
Attachment:
Rectifier PLate Curves.jpg
The tubes corresponding to lines on the left have lower losses then those to the right. Should we endeavor to use these tubes when the PQCR is higher?

What do people think?


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PostPosted: 09 Jan 2011, 16:07 
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Well, obviously not a lot of interest here. But before I drop it, I decided to generate a more up-to-date version of the plot above. So, using GE data sheets for consistency, I generated the following plot.
Attachment:
Plate Voltage Curves.png

The thing that really surprised me is the behavior of the 5AR4/GZ34. I had always considered the 6CA4 a low loss regulator but the 5AR4 really blows it away. In cases where I can't stand the voltage drop of the 5U4 I'll definitely be looking at the 5AR4.


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PostPosted: 09 Jan 2011, 21:51 
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Thank you for the plot! :thumbsup:

Cheers!

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