PSUD Warning Message - Important?

[FlaCharlie]

I've been using PSUD to sim some possible PS designs for a preamp. When calculating some configurations I get a warning message.

In case you can't read it, it says: "A current sink has pulled the voltage below zero for more than 5 mains cycles, at time 0.0834375".

How serious is this situation? Does this mean I shouldn't use this configuration?

The value of the current sink is based on the estimated current draw. When I change the values of C1 to 15uf and C2 to 33uf the warning goes away.

Also, I've never used a 6AX4GT, which is a TV damper diode, and I can't find any spec for the maximum value of the first cap. Anyone know?

 

[macaltec]

Watching for the answer and to learn along with you.

 

[Palustris]

That's kind of curious since it doesn't happen to me when I replicate your supply. I have used the 6AX4 many times since it is a good reliable rectifier. But I always use them full wave. Try changing the half wave to a full wave and see what happens. Notice that you will also get a warning message if the current is too great at power on. However, since the tubes don't conduct immediately it is a false "fault." I use the "stepped" function on the current tap to simulate the tubes warming up.

 

[Bob]

its because a capacitor (both caps and inductors have phase attributes) provides a short while its
charging. this is what blows tube rectifiers, fuses, and can damage transformers. then the voltage
rises and then steady-states.

by lowering the value of the first cap, you are "lessening" that effect.

there are ways to mitigate this issue, add a small resistor, change to choke input, lower
the first cap value to maintain eventual and necessary to the design final voltage, then
add caps for the reservoir capacity.

then look at the datasheet for the tube rectifier, and look for maximum capacitor value.
It is unlikely to be 47uf but even if it is, it is not necessary to use this largest maximum.
lowers life of the tube unless you need maximum voltage (best with caps than inductors).

and if you need a "robust" tube, switch to a 5u4GB, they are cheap and better for up
to push pull el34s running high/hot bias.

and try a full-wave. your half wave will not give you the higher voltages and will
require more PS stages and capacitance to achieve as low a ripple as a full wave.

 

[FlaCharlie]

That's kind of curious since it doesn't happen to me when I replicate your supply. I have used the 6AX4 many times since it is a good reliable rectifier. But I always use them full wave. Try changing the half wave to a full wave and see what happens.

I'm using different settings for the calculation - 10,000ms after a delay of 8 sec. This was at the suggestion of @kward since I wanted to see the voltage and how much ripple the supply had once it was up and running.

I'm trying to build with some scrounged PTs. The PT I want to use for this particular circuit won't supply enough voltage using a FW rectifier.
It also originally used a half wave so doesn't have a center tap. That's not a major issue as I've built hybrid PS before. But using half wave with the 6AX4 gets me the voltage plus a slow start.

 

[kward]

The thing that's different between the FlaCharlie's sim and palustris' sim is the parameters on the simulation time. But yes, check the data sheet size on max cap size, you may be exceeding it, and it might need to be half the size as shown in the sim since you are simulating half wave rectification so all current to charge the first cap must come from the one rectifier.

 

[FlaCharlie]

its because a capacitor (both caps and inductors have phase attributes) provides a short while its
charging. this is what blows tube rectifiers, fuses, and can damage transformers. then the voltage
rises and then steady-states. . . . by lowering the value of the first cap, you are "lessening" that effect.

I'm still trying to learn PSUD but I figured that since the calculation is "after a reporting delay of 8 sec" that the cap would be charged by then. Maybe I'm misinterpreting that setting.

then look at the datasheet for the tube rectifier, and look for maximum capacitor value.
It is unlikely to be 47uf but even if it is, it is not necessary to use this largest maximum.
lowers life of the tube unless you need maximum voltage (best with caps than inductors).

Apparently, since the 6AX4 was not originally used as a rectifier, the data sheets don't give a max first cap value.

and if you need a "robust" tube, switch to a 5u4GB, they are cheap and better for up
to push pull el34s running high/hot bias. . . . and try a full-wave. your half wave will not give you the higher voltages and will
require more PS stages and capacitance to achieve as low a ripple as a full wave.

This is a preamp circuit, not an amp, and I'm estimating a total current draw of only 6 mA so a "robust" tube would be overkill. As I mentioned above, the PT I want to use with this particular preamp tube is not center tapped. I could create an artificial center tap and use a hybrid supply but the B+ wouldn't be high enough.

 

[Squidward]

I bet a 6AX4 is more "robust" than most purpose specific rectifiers.

 

[Bob]

you don't need a center tap. you could use a 4-diode bridge where the AC feeds two of the four
junctures, and the other two are + and -. and a dual diode (aka 5u4 which is the other reason
but not mentioned) will allow a full wave, and if two additional SS diodes used, then you
have a graetz bridge.

all will give you a higher voltage than a half-wave. you could also use a second diode
to do a voltage doubler with higher voltage than half wave.

6ma total? how many and what tubes? the 12ax7 is about 1ma draw on the plate and three
of them makes 6ma. anything else needs a little bit more.

 

[sKiZo]

If it's a false warning due to the readings on the start up voltages, maybe add a thermistor or two in series to step down the initial surge?

 

[Bob]

more input. 6 cycles on US AC is less than 100ms. not noticeable except in
wear and tear on tube, transformer secondary. hence my input on a more
"robust" tube, and these are cheap. ham fests sell these for $1-2 each
and I bought a lifetime's worth.

 

[maxhifi]

I bet a 6AX4 is more "robust" than most purpose specific rectifiers.

I built an amp with 2 x 19AU4 as a full wave rectifier, and it's been running with the same tubes since 1998. That's a stereo amp, with 50W/ch of power, running two pairs of 6L6GC.
Have changed the output tubed about five times since I built the amp - it gets lots and lots of use.

The dampers have very little voltage drop too, kinda like a super duper 5AR4, but at a very low price. Very long lasting tubes. In TV service the damper takes a lot of abuse, so these tubes need to be very robust.

The only real downside is they use a lot of heater power compared with other rectifiers.

 

[kward]

I figured that since the calculation is "after a reporting delay of 8 sec" that the cap would be charged by then. Maybe I'm misinterpreting that setting.

Except for what PSUD determines as error conditions, which it will report anytime during the simulation.

 

[FlaCharlie]

I bet a 6AX4 is more "robust" than most purpose specific rectifiers.

I imagine pretty much any damper diode / tube rectifier would be robust enough given that the current draw will only be ~6 mA. Even the 6x4 would be overkill, right?

you don't need a center tap. you could use a 4-diode bridge where the AC feeds two of the four
junctures, and the other two are + and -. and a dual diode (aka 5u4 which is the other reason
but not mentioned) will allow a full wave, and if two additional SS diodes used, then you
have a graetz bridge. . . . all will give you a higher voltage than a half-wave. you could also use a second diode
to do a voltage doubler with higher voltage than half wave.

6ma total? how many and what tubes? the 12ax7 is about 1ma draw on the plate and three
of them makes 6ma. anything else needs a little bit more.

You lost me a bit there.

But, here's the thing, I don't need more voltage and as for the 5u4, this PT doesn't have a 5v winding and I want a slow start. So 5v directly heated rectifiers won't work. I've got one 6.3v winding rated for 2.1A.

To fill out the picture a bit, this is one of the preamp designs I want to try using somewhat atypical tube choices.

This one will use a pair of 1LE3s. How's that for atypical? It's a Loctal DHT typically used for battery powered radios. I plan on using rechargeable batteries to heat it. The filament is 1.4v and draws only 50mA. I'll probably use a 1.5v lithium to supply neg voltage to the grid for the bias.

I want to load the plate with Hammond 156C chokes. They should drop ~11v or so and the plates need to be right around 90v or a bit less so B+ needs to be near 100v.

Given those requirements, I'm open to other possibilities. I've got plenty of different rectifiers.

I just wondered why I was getting the Warning from PSUD. The sim which gave the warning (above in the first post) shows ripple of only ~412 uV. That's micro-volts, not millivolts. The altered version, which doesn't generate a warning, has ripple at 1.7 mV.

So the half wave with the 6AX4 doesn't look bad but it would be better - on paper at least - if I could use the version that gave the warning. I wonder if the difference in ripple would even be audible, though. Obviously, I don't want to damage anything, hence my question about "how much does it matter?"


For comparison, I ran one using a SS bridge with larger caps and it has the ripple at 4.3 mV.

 

[FlaCharlie]

Notice that you will also get a warning message if the current is too great at power on. However, since the tubes don't conduct immediately it is a false "fault." I use the "stepped" function on the current tap to simulate the tubes warming up.

Yeah, I'm wondering if that's why I get the warning. Could you explain how to use the "stepped" function.

PSUD is a great tool but I'm amazed that, considering how many people use it, that there is almost no documentation or any comprehensive tutorial for it.

 

[Palustris]

I'm using different settings for the calculation - 10,000ms after a delay of 8 sec.

The error message is at 83mS into the simulation so the time of the error message is before the actual recording of the waveform which is after 8S.

The stepped function is very useful since you will get spurious error messages if there is too much current draw during startup. Since the tubes don't actually conduct immediately, using the stepped function better simulates the power supply power on sequence.

Right click on the current tap; select "edit." At the top of the window is a check box "stepped load." Check this box and it will give you an option for a second current draw. I generally use 10mA as the first draw and then the expected full current draw minus the 10mA for the second box. I use 1S for the time to delay. This gives a nice slow ramp up that more accurately depicts the circuit.

BTW: I am using a pair of 6AX4GTB for a current build and the ramp up takes several seconds. After I switch on the amp, I instinctively start to check my volt meter connections when no voltage appears after a couple of seconds; but sure enough, it eventually powers on. The 6AX4 takes a long time to pass current.

 

[FlaCharlie]

Thanks @Palustris . . . No warning notices now with the higher value caps.

 

[trobbins]

PSUD2 is great for checking power-on levels, and steady-state levels - two important aspects of operation to gain confidence on.

Being a damper diode, the specs aren't related to power supply diode operation, so you only really have the steady-state peak current spec to work from (ie. no good confirmation on what hot turn-on peak current can be sustained at power on).

Given your simmed steady state peak is about 70mA and the spec rating is 750mA, then no issue there.

The power on peak is simmed at about 800mA for the first pulse - it could well be lower depending on timing of switch contact during a mains waveform, and transformer in-rush. So given it is not significantly more than 750mA, you don't have to make a leap of faith that that diode is going to be ok long term.

I typically use a resistor load, rather than current tap - it can avoid quirky simulation effects. Your initial warning was I believe due to you having the 'soft start' option selected, which is a special set of numerical conditions - if you remove that then you can more easily check on waveforms and levels in the first few hundred ms, as well as after many seconds reporting delay. There is also no need to simulate over a 10,000 ms range - just use say 100ms or 300ms (depending on 50 or 60Hz), and extend the reporting delay to 10 secs - you can then see that the waveform cycles in the plot have reached steady state peak levels, and all the reported values (especially rms) are valid.

 

[FlaCharlie]

PSUD2 is great for checking power-on levels, and steady-state levels - two important aspects of operation to gain confidence on.

Being a damper diode, the specs aren't related to power supply diode operation, so you only really have the steady-state peak current spec to work from (ie. no good confirmation on what hot turn-on peak current can be sustained at power on).

Given your simmed steady state peak is about 70mA and the spec rating is 750mA, then no issue there.

The power on peak is simmed at about 800mA for the first pulse - it could well be lower depending on timing of switch contact during a mains waveform, and transformer in-rush. So given it is not significantly more than 750mA, you don't have to make a leap of faith that that diode is going to be ok long term.

I typically use a resistor load, rather than current tap - it can avoid quirky simulation effects. Your initial warning was I believe due to you having the 'soft start' option selected, which is a special set of numerical conditions - if you remove that then you can more easily check on waveforms and levels in the first few hundred ms, as well as after many seconds reporting delay. There is also no need to simulate over a 10,000 ms range - just use say 100ms or 300ms (depending on 50 or 60Hz), and extend the reporting delay to 10 secs - you can then see that the waveform cycles in the plot have reached steady state peak levels, and all the reported values (especially rms) are valid.

Thanks for the tips.

Yes, I had the soft start option checked. I assumed that should be used since I was planning to use some type of indirectly heated rectifier.

When using a resistor load, how do you determine the value of the resistor?

 

[Mike Stehr]

When using a resistor load, how do you determine the value of the resistor?

It's been awhile and a wild guess, but I think you have to determine the total resistance/current that the circuit will draw from the supply.