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Rectifier rolling


michaelwjones
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I found this on the internet, so it must be true...

 

Typical Rectifier Voltage Drops

 

• 5Y3 – 60 volts @ 125 mA

• 5U4GB – 50 volts @ 275 mA

• 5U4 – 44 volts @ 225 mA

• 5V4 – 25 volts @ 175 mA

• 5AR4 – 17 volts @ 225 mA

 

As a safety note – you can generally go “down” but not always “up”. This means you can generally put a 5U4 or 5Y3 in an amp that had a 5AR4, but it is not recommended to put a 5AR4 or 5U4 in an amp that had a stock 5Y3. The increase in voltages may be too much for the other components in the amplifier.

 

Actually it came from a gear forum of guitar amps.

 

I have a variety of rectifiers, some great others no so great, that would be interesting to roll. Let's assume the numbers in the above blurb have some basis in reality. Am I correct in reading you could (all other things being equal) run a rectifier of greater voltage drop in place of one with a lesser voltage drop? Thus, "down" means more voltage drop?  Yes, I understand that is pretty much exactly what it says, but I'd have a greater comfort level if some here chimed in.

 

Thanks in advance.

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You need to be careful.  The 5U4 filament draws 3 amps while the 5Y3 draws 2.  If the filament winding of the power xfmr can’t handle the extra current you could have a problem.  Also, equipment designed for a cathode type rectifier may use electrolytic caps which cannot handle the peak voltage they would see at initial turn on with a filament type.  Best to stay with the recommended type unless you are sure about specs.

 

Maynard

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I wouldn't mix and match recrtumfriers as it could go south quickly. For scratch built power supples I like the tv damper diodes ike the 6DM4 as they have a slow start of about 30 seconds. There are so many great damper diodes and pretty cheap. Then there are ancient rectifiers like the 80 81 82 83. Since I have 5 81's I might do a full bridge for a DIY PP2a3 amp. For MV I really like the little 816's.

 

Its all fun till something catches fire.

 

1625T.thumb.jpg.0634b29e543eb2172b87b0a56a3f00d1.jpg

 

 

 

 

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5 minutes ago, michaelwjones said:

 

 

😂😂😂

 

How true. I can't begin to describe my expression when 4 caps in a power supply let go at once. Eyes like saucers would be apt.

Jebus..

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I know this chart was pulled from another website but it would have been a more useful comparison if the data used the same load current. But math is a very useful tool and we can use this data presented to us to convert it over into something more useful like what the internal resistance of the tube is. That's basically what will dictate how much of a voltage loss you will get from the rectifier for a given load current.

 

 

To determine resistance:

 

∆V / ∆I

 

Now we have the tools lets compare the rectifier internal resistances

 

5Y3

 

60 / .125 = 480 ohms

 

5U4GB

 

50 / .275 = 180

 

5U4

 

44 / .225 = 195

 

5V4

 

25 / .175 = 142

 

5AR4

 

17 / .225 = 75

 

 

5Y3 - 480 ohms

5U4GB - 180 ohms

5U4 - 195 ohms

5V4 - 142 ohms

5AR4 - 75 ohms

 

The tube wizards reading this will know that the internal resistance of a tube rectifier is not a constant, if it was it would be a straight line when comparing DC voltage out vs DC current. It's not, there is a slight curve to it meaning it's nonlinear. As current increases through the tube the internal resistance decreases. For the 5Y3 it would be 480 ohms at 125mA but if there were only 20mA of current through the rectifier the resistance will be more like ~700 ohms. Although the resistance of the rectifier is higher at lower currents you still only get 14v drop across it with 20mA where compared to 125mA of current and 480 ohms you end up with 60v across the rectifier.

 

 

For Class A amps and changing rectifier types, since the load current is fairly constant in Class A the change in sound from the different internal resistance will be very low. Since you are increasing/decreasing rectifier resistance voltage out will go either lower or higher respectively. This changes the tubes operating point and may or may not change the sound of the amplifier depending on the circuit and how big of a change in voltage is. For Class AB or B amps where the circuit experiences current demands the internal resistance of the rectifier chosen can certainly have an impact on the sound of the amplifier. As the amplifier increases current demand a higher resistance rectifier will drop the voltage down effectively lowering the output power. It's a compression effect where the resistance will "sag" the B+ down which not only reduced output power but also changes the operating point and bias current, the plate curves end up squishing together. So Class AB amp users will see the largest difference in sound when changing rectifiers.

 

 

 

 

 

 

 

 

 

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2 hours ago, John Warren said:

If you want to hear an improvement swap the tube for a plug-in solid-state rectifier.   Rail sag is, for all intents and purposes, eliminated.  It's harder on the tubes but that's why we have in-rush current limiters.  


I can’t agree with this.  The extra voltage on the tubes may exceed the rated limits and, in addition, could shift the operating point of the tubes resulting in different distortion characteristics which are not desirable.  
 

Maynard

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6 hours ago, John Warren said:

It's harder on the tubes but that's why we have in-rush current limiters.  

 

Can you elaborate?

 

I don't see how an in-rush current limiter is making the life of tubes any easier, well unless you mean the rectifier tube.

 

I think you may be alluding to "cathode stripping" which is when there is a high potential at the plate before the cathode has emissions. This is a myth mostly as the only times I have ever come across this is with transmitter tubes where you have a few thousand volts at the plate. With ordinary receiving valves running <600v there is no concern for this phenomena.

 

Cathode poisoning on the other hand is a real issue, that's the opposite though where you have zero plate voltage and the cathode is hot creating emissions. This will lead to higher internal impedance within the tube, it will behave as though there is a large value external cathode resistor with no AC bypass cap. Lower gain, higher output impedance etc...

 

 

In-rush current limiters are installed along with silicon rectifiers when there are high amounts of filter capacitance, the current limiter will reduce stress on the power transformer winding and rectifiers by reducing the surge current present in charging the capacitance during power up cycle.

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On 10/24/2022 at 8:52 AM, tube fanatic said:


I can’t agree with this.  The extra voltage on the tubes may exceed the rated limits and, in addition, could shift the operating point of the tubes resulting in different distortion characteristics which are not desirable.  
 

Maynard

I'll get back to you on this.

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Not to get involved too much in this conversation but I would re-bias the tubes or lower the voltage somewhat with some resistance when installing a SS device where a circuit is designed for a tube rectifier. Most conservatively designed amplifiers could safely handle the extra voltage you get from SS rectification but there are probably many that would not like it. In this day and age I see no benefit designing a tube amplifier with tube rectification when SS is better and cheaper. Call me a heretic. 

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2 hours ago, henry4841 said:

In this day and age I see no benefit designing a tube amplifier with tube rectification when SS is better and cheaper. Call me a heretic. 

 

Thanks, Henry, for this insight, I've wondered about this point.

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I have a massive collection of tube rectifiers so that is what I use. Honestly I love the look of the tubes as well. I like the small MV rectifiers and the GZ34's. I even have some old ST16 bottle shape 81's. If your on a budget or space constraint I could see using SS rectifiers or if ya want dc filaments.

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I'm pretty certain most people that say SS rectification is "better", really are saying that from a purely hypothetical efficiency perspective. Of course this only holds true for silicon rectifiers due to the forward voltage drop of .6v no matter the load current compared to selenium.

 

There are pros and cons to everything, including something like a silicon diode which appears to be "better".  For example a "con" with SS diodes is they will not current limit the surge current during power on cycle. Without the addition of a soft start circuit or an in-rush current limiter at the very least large current surges will stress the transformer and if you think about it the reservoir capacitors. Limiting the current passing  through an electrolytic capacitor will extend it's service life.

 

Why do I pick tube rectifier when I do? Because I like them. Class A circuits their internal resistance is not going to be a deal breaker. What about Class AB amps? We can be smart and use a regulated supply. Whether it's a simple choke input filter which has darn good regulation in itself or an active regulator circuit, then the tube rectifier is moot.

 

Of course if you are on a budget or don't want complexity in the circuit sure go for SS rectification. It's all fun and if you are a good engineer you can deal with the pros and cons of each decision because they all have trade offs. If someone comes to me and wants a Class AB amp with a tube rectifier I'll just explain that if they want it to be a stiff and unwavering power amp then it's probably best to regulate the supply with the tube rectifier or at least use a choke input filter power supply.

 

Even a 5AR4 is like having between 75 and 100 ohm resistor in series with a Silicon diode. If the output stage demands a large ∆current, lets say 200mA, that's a 20v drop in B+. The output tube will shift it's operating point and squish together the plate curves giving a different sound along with reduced power output. If you don't want this to happen then don't use a tube rectifier or regulate the B+

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On 10/24/2022 at 1:19 PM, captainbeefheart said:

 

Can you elaborate?

 

I don't see how an in-rush current limiter is making the life of tubes any easier, well unless you mean the rectifier tube.

Before the days of the internet, I measured filament draw at start with tube, ss and ss + various in-rush limiters, AC, DC power on both a few preamp and power tubes.  I don't have the data, so I'll have to replicate the measurements.  Differences are significant.    

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I say better being SS does the job as good if not better at a much cheaper cost. I probably favor SS rectification being involved in SS amplification for most of my life and not tubes. Just started working with tubes recently, 10 years ago so I see no need to go to the expense and trouble designing an amplifier using tube rectification. Face it guys tube rectification is old school. This is not to criticize the diehard tube heads who still favor tube rectification. I respect and admire them for stating there opinions and have to admit an extra tube sure looks nice in an amplifier. I would say half of my dozen or so tube amplifiers use tube rectification but I would not design a new amplifier using a tube for rectification. Why bother. Sure a tube might bring some flavor into the mix but it was not designed to do so. It was just all the old timers had for rectification way back when. I went a step further in my last build and used a SS PS section along with a SS constant current source for my input tube.  The problems associated with SS over tubes for rectification can be easily worked around with working solutions. George Anderson in his SSE amplifier saw no problem offering the option of building either way. Anderson is a very respected guy among tube fanatics. 

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1 hour ago, henry4841 said:

...and have to admit an extra tube sure looks nice in an amplifier.

 

 It was just all the old timers had for rectification way back when. I went a step further in my last build and used a SS PS section along with a SS constant current source for my input tube.  The problems associated with SS over tubes for rectification can be easily worked around with working solutions. 

 

All this discussion actually involves a last generation Cary SLI-80. The current iteration uses SS rectification. I think its looks a bit naked without its two rear tubes...

 

 

1 hour ago, henry4841 said:

Its seems the key is the best results occur when the amp is designed for SS rectifiers.

 

The article I took the "chart" from also tested a SS rectifier. They noted its voltage sag/recovery was .01 sec while the 5Y3 was almost .5 sec. Their point was in a guitar amp that difference can be audible. Such a replacement also could place a strain on the transformers and tubes. To my non-technical/engineering mind, that makes sense just looking at the numbers. One is bringing your hands together, the other snapping your fingers.

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Aside from having an extra tube filament to enjoy looking at I don’t see any need to add the expense of another tube which will eventually need replacement, and another socket.  Modern SS diodes can typically handle a 100-200 amp surge so that is a non-issue, and I find that the power xfmr impedance is sufficiently high to serve as a peak current limiter.  I have never had a power xfmr fail due to peak current issues.  The component which takes the hit is the fuse and often needs to be a slow blow type.  Another advantage of SS rectifiers is that they allow the use of very large value input filter caps so that ripple can be brought down to negligible levels very inexpensively.  Two caps and a resistor can create ripple values which can only be duplicated by using a more costly choke with a tube rectifier (the value of the input filter cap is generally very low to protect the tube).  When designing new equipment it comes down to a matter of taste.  

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5 hours ago, John Warren said:

Before the days of the internet, I measured filament draw at start with tube, ss and ss + various in-rush limiters, AC, DC power on both a few preamp and power tubes.  I don't have the data, so I'll have to replicate the measurements.  Differences are significant.    

 

Of course filaments when cold have a surge current. If said it time and time again that the best design for heaters is a current source, not a voltage source.

 

With a current source supply the tube service life is far better since there is no surge current to stress weak spots in the filament and less stress on the transformer.

 

Of course if you eliminate the tube rectifier you are removing part of the  large surge current  being the tube rectifier filament.

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