• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Are rectifier tubes still relevant? Why would you use one, or avoid using one?

Rectifier tubes still relevant in NEW designs?


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The difference in B+ between 6AU4 to 6DA4 to 6AX4 is about -5V volts per step when you are drawing around 150mA.
In other words, the same effect as a 33 ohm sand-cast power resistor?

But I am most interested in anyone's cheap, reliable and easy to implement slow-start circuit for silicon rectification. :)
A MOSFET capacitance-multiplier would do the trick if you really want a slowly rising B+. Feed the MOSFET gate through a 1 meg resistor from the main filter cap positive, wire the drain direct to the same location, wire a 47 uF cap to ground, and you have a nominal 47 second turn-on time constant.

One MOSFET, one resistor, one capacitor, one heatsink: that's certainly not more complex than a rectifier valve, a valve socket, and an entire additional heater winding on a transformer.

Perhaps I'm wrong, but it seems to me the place where a slow start would be most useful is in the heater power supply, because as we all know, cold filaments have much lower resistance, and there is a huge and destructive current surge at turn-on. And that's one place where a hollow-state rectifier won't help at all.

-Gnobuddy
 
Do we need to have a standing PSUD2 help thread?

I took a technology elective in the 7th grade. Textbook had a diagram of ohm's law on the cover. That was 1986. It was only 3 days a week for one semester. That's the sum total of my formal training on the subject of electricity (as well as metalworking) but i understand PSUD2 just fine, and excepting times when i misunderstood the markings on a transformer, it has predicted reality pretty reliably for me.

From this i infer that it is Not Hard To Use. But it sounds like some people could use a hand.
 
In other words, the same effect as a 33 ohm sand-cast power resistor?

Not far off! yes, the difference in place resistances varies, but you can also look at it that way. Of course no soldering required, just swapping out.


A MOSFET capacitance-multiplier would do the trick if you really want a slowly rising B+. Feed the MOSFET gate through a 1 meg resistor from the main filter cap positive, wire the drain direct to the same location, wire a 47 uF cap to ground, and you have a nominal 47 second turn-on time constant.

One MOSFET, one resistor, one capacitor, one heatsink: that's certainly not more complex than a rectifier valve, a valve socket, and an entire additional heater winding on a transformer.

Sounds like a very nice idea. I wonder if anyone has done it before. I guess if the NPN 'FET fails then it will simply not conduct?

Perhaps I'm wrong, but it seems to me the place where a slow start would be most useful is in the heater power supply, because as we all know, cold filaments have much lower resistance, and there is a huge and destructive current surge at turn-on. And that's one place where a hollow-state rectifier won't help at all.

-Gnobuddy

Actually, I like the heaters nice and warm as my B+ rises slowly. Don't forget that the surge you talk about applies mostly to silicon rectification. Hollow state damper diodes do not exhibit this "huge and destructive current at turn-on".

Of course.. if you were a fool, you could also turn it on and off and on and off and on and off until something fails. We are surrounded by fools.


Perhaps you have simulated silicon devices in PSU II? Then you will know incredibly quick and to what lovely hights your B+ can rise with silicon diode rectification.

I just don't want my proposed 320V B+ supply breaching 550V on turn-on, stripping the cold cathode on my output tubes..... I have old tubes. I don't want to kill them. Damper diodes have never let me down. However silicon components.....

Ian
 
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Good for everyone that finds power supply design easy.

What I find most difficult in designing power supplies with PSUD2 is: 1) estimating the true secondary output voltage when running the recycled transformers I use under partial loading and 2) correctly adding multiple current taps for different stages. I'm certain that it is simple for lots of people, and I'm sure that if I had more time to invest in the problem, I would get better results.

However, the reality for me, and how this post relates to the thread topic, is that by swapping between (5V) 5AR4 and 5R4, you get an ~50V difference. That's a lot. 5U4 are in between. Same with 5Y3 and 5V4. I have tons of these old valves. For me, it is very cheap and effective to wire in a socket and swap valves and adjust power supply resistors in an ugly, idiotic way, until I get a multi-tap supply that I think I can work with.
 
Though, in my most recent case, I knew i needed to burn off many volts to use the transformer I was determined to use. I decided to use a 7Z4 - which isn't in PSUD2's rectifier list but it turns out the EZ81 is Close Enough. 7Z4 datasheet says to put a 75ohm resistor on each plate, so i did. Told PSUD2 that the source impedance of the transformer was 150ohm. Then had a CL140 inrush limiter into 33uf, and so on. I entered the cold DCR of the CL140 of 75 ohms as a resistor. Added an additional R-C stage for further smoothing. Used a 400v 470uf cap i had spare from another project because cheap and lazy.

PSUD2 predicted 340vdc at 65ma. I put a 5K 50w load on the assembled circuit and got a slow comfortable rise to about 343 volts. Done and done.
 
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Do we need to have a standing PSUD2 help thread?

I took a technology elective in the 7th grade. Textbook had a diagram of ohm's law on the cover. That was 1986. It was only 3 days a week for one semester. That's the sum total of my formal training on the subject of electricity (as well as metalworking) but i understand PSUD2 just fine, and excepting times when i misunderstood the markings on a transformer, it has predicted reality pretty reliably for me.

From this i infer that it is Not Hard To Use. But it sounds like some people could use a hand.
Good idea. I never had any electronics training at all, find computers difficult to use, and yet I am trying to build a DHT pre. At the age of 67 I am no longer a quick learner, and although I have no problems with the mechanical side of building an amplifier, my brain just doesn't retain electronics information.
I am of course grateful to all those who are helping me, but it would be nice to work things out for myself. And don't even mention LTspice! :confused:
 
LTspice is not necessary if you know how to design the old-school way. Understanding ohm's law thoroughly is the most important 1st step, just like ericj notes.

PSUD II is a very useful tool as well. I remember my first designs.. 20+ years ago.. ugh.. terrible stuff working out the power supply! Lots of trial and error back then.. of course I was younger and less experienced too... ;)

In any case, if you are in the UK, I am sure you are aware of Ale Moglia's blog. I tend to use older and less common DHT's than Ale. It might simply be worth copying one of his ideas if design seems too heavy. Its not a bad thing to let someone else help with the heavy lifting.

Ian

p.s. this board (and others) seems to have no shortage of people who will simulate your circuit in LTspice anyway.. no idea if they have actually built anything... but they are happy to help! :)
 
Good for everyone that finds power supply design easy.

What I find most difficult in designing power supplies with PSUD2 is: 1) estimating the true secondary output voltage when running the recycled transformers I use under partial loading

Look here

https://www.youtube.com/watch?v=Yzo3A-NywSs

seriously. its pretty much exactly how I've been doing it. don't be foolish with mains voltages...
 
Sounds like a very nice idea. I wonder if anyone has done it before. I guess if the NPN 'FET fails then it will simply not conduct?
I've seen BJTs fail both open-circuit and dead-short. I haven't ever blown a MOSFET (yet - fingers crossed!), so I don't know how they fail!

Don't forget that the surge you talk about applies mostly to silicon rectification. Hollow state damper diodes do not exhibit this "huge and destructive current at turn-on".
I'm afraid you misunderstood me: cold valve heaters have low resistance. When you apply old-school 6.3V AC from the transformer - instantaneously - to cold heaters, you get a huge and destructive current surge through those thin, delicate heaters.

The problem isn't caused by rectifiers, either silicon or vacuum-state. The problem is caused by the fact that metals have a positive temperature coefficient: cold metal has a lot less electrical resistance than glowing, red-hot metal.

With a solid-state, DC heater power supply, it's possible to eliminate that current surge. You can either have a DC voltage that rises slowly, so the heaters have time to warm up, or you can have a circuit that behaves like a current source when the heaters are cold, and transitions to a voltage source after a few seconds, when the heaters are fully up to temperature.

As I said before, this is one area where hollow-state rectifiers cannot be used at all: they don't work at 6.3 volts plate voltage, and they can't flow enough current to power half a dozen valve heaters.

How important is this? I guess that depends on whether you want to stretch out the lives of your valves. Some of you are using very precious, very expensive valves that are no longer in production - under those circumstances, it might be worth trying out.

Of course.. if you were a fool, you could also turn it on and off and on and off and on and off until something fails. We are surrounded by fools.
I have a favourite valve guitar amp. I turn it on, almost every day, then I turn it off. Some days, if I'm not working, I turn it on, and then off in the morning, and then on, and then off, again in the evening.

Yes, I actually use that amp, almost every day. Turning it on and off is part of using it.

I have the usual complement of human failings and frailities, but I don't believe being a fool is one of them. :D

I just don't want my proposed 320V B+ supply breaching 550V on turn-on
So use a nominally 227 volts RMS transformer and silicon diodes, and you will get your 320 DC volts. There will be no overshoots to 550V.

Obviously, if you use a transformer that puts out far too much voltage, and then rectify that using silicon diodes that actually work efficiently, you will get too much DC voltage. That's not the fault of the good diodes, that's what happens when you use the wrong transformer for the B+ voltage you want! To get the 550V you mention, you'd have to use a 390V RMS transformer. That's ridiculous, if your intention is to have a 320 volt B+.

That's one of the nice things about using silicon diodes and adequately sized filter caps: your DC voltage will be close to 1.4 times the AC RMS voltage from your transformer. No need for complex simulation software.

Your last post was verging on being a bit personal, so let me add here that there's no skin off my nose if you prefer valve rectifiers. This is supposed to be a hobby, for fun; if you have fun with big glass rectifier valves, hey, go for it!

-Gnobuddy
 
LTspice is not necessary if you know how to design the old-school way. Understanding ohm's law thoroughly is the most important 1st step, just like ericj notes.

PSUD II is a very useful tool as well. I remember my first designs.. 20+ years ago.. ugh.. terrible stuff working out the power supply! Lots of trial and error back then.. of course I was younger and less experienced too... ;)

In any case, if you are in the UK, I am sure you are aware of Ale Moglia's blog. I tend to use older and less common DHT's than Ale. It might simply be worth copying one of his ideas if design seems too heavy. Its not a bad thing to let someone else help with the heavy lifting.

Ian

p.s. this board (and others) seems to have no shortage of people who will simulate your circuit in LTspice anyway.. no idea if they have actually built anything... but they are happy to help! :)
Well, if I knew what "designing the old school way" was .........:D
My type 26 DHT is based on Rui Lourenco's version of Ale's 26 design. And Ale, among others, is one of those who has offered help.
But I have built kits and clones for years; actually understanding what I am doing is important if I want to go beyond that, and my brain resists theoretical knowledge! :(
 
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