And what about a inrush current limiter thermistor on the mains live, before the tranfo?
Any remarks about the fusing?
Langford Smith in his well known Radiotron Books, suggests that using fuses with vacuum tubes is very complicated, because the first ten seconds after turning on, the current is too high o the fuse, and false blown will result, as a result of the heaters consumption.
Let me find the page of the book. Is is free in the net. Wait few minutes.
Let me find the page of the book. Is is free in the net. Wait few minutes.
http://www.ax84.com/static/rdh4/chapte35.pdf
page 54 paragraph IV. Here is an interesting description.
page 54 paragraph IV. Here is an interesting description.
It is difficult to give an answer. As this book says, the normal initial current in usually higher than a severe overload. And in case of short circuit condition, I really don't know what to say.
Ah, don't worry. All we here are learning always. This is the idea, to share experiences and knowledge.
Ah, don't worry. All we here are learning always. This is the idea, to share experiences and knowledge.
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Well, I may just try it the empiric way; now my amp is running just fine (even if the psu can be tweaked to be more friendly on the rectifier), so I may just try some 2.3A Slow blow and see if it burns in normal operation condition, if no, trying a lower slow blow and test again, until I find a value that blows, then I may use the last value that worked fine.
The input Power to an amplifier is expressed in Watts. But the VA (Volt Amps) of an amplifier
is more than the Watts. This is due to the different phase of the Volts and the Amps at the transformer primary (and phase of the loads on the secondaries). The inductances and capacitances cause this phase shift. This effect is called PF (Power Factor).
If you take the Cosine of the phase (0 degrees to 90 degrees) you get the PF.
For example, an amplifier with a phase of 36.9 degrees will have a PF of 0.8
Watts / PF = VA.
An amplifier that is rated at 100 Watt power draw, and a Power Factor of 0.8 will have 125 VA.
125VA / 230V = 0.54 Amps.
If the phase is 53 degrees, the PF is 0.6. 100W / 0.6 = 167VA 167VA / 230v = 0.73 Amps.
Another factor is the large inrush current at turn-on, and soon after turn-on.
It can be several times the steady state current. It depends on the power supply,
cold filaments, warm up of rectifiers, output tubes, input tubes (especially with RC or DC coupling from that stage to the output stage), regulators, etc.
The inrush current is also dependent on the exact moment you turn on the amplifier.
Turn it on at the power line sine wave zero crossing, you get less inrush.
Turn it on at the power line sine wave peak, you get much more inrush.
AS ALWAYS, WORKING ON ANY PRODUCT THAT IS POWERED OFF THE POWER MAINS,
YOU MUST BE KNOWLEDGEABLE, AND TAKE CAUTION. THE MAINS, AND THE PRODUCT CIRCUITS CAN KILL.
So, what are we to do?
First of all, we must have the amplifier working properly, and without any failure of any circuit.
Then you can use a smaller fuse and see if it does not fail.
If it fails at first turn on, it is too small. But don’t stop there if it does not fail at first. Let the amp warm up, and see if it still works. Then power it down and let it cool. Later when it is cool, turn it on again (we are trying to turn it on cold, and when the power sine wave peaks). How many times do you have to do this … it is by chance.
If it does fail, use the next larger amperage fuse. And test again.
Sometimes, a Hot start will cause the fuse to blow. This happens when the amp is warm
and a power line goes out briefly, say 1/2 second. The lights flash off/on. But the amp
is warm, and the fuse blows.
Sometimes a fuse just fatigues. This happens when the Amp draw is almost equal to the fuse Amp rating, or many many turn-ons at the power line (Mains) sine wave peak. I have had fuses fatigue after 1 or 2 months. Then the new fuse may not blow again, even after several months (fuse ratings have a tolerance, they are not perfect or exact).
IN NO CASE SHOULD YOU USE A FUSE WITH A LARGER AMP RATING THAN WHAT THE MANUFACTURER SPECIFIES.
PAY ATTENTION TO THE TYPE OF FUSE, YOU MUST USE THAT TYPE (FAST BLOW FOR FAST BLOW; SLOW BLOW FOR SLOW BLOW).
I believe you are making your power supply more friendly to the rectifier, the high voltage secondary, and are not going to bias more current in the output stages than in the original design. So, I expect the amp to draw less Watts and VA (with less capacitance immediately after the rectifier and before the choke), and to have as good or better PF.
I do not subscribe to using fuses in the high voltage secondary (suppose one fuse fatigues
or correctly blows; but now the other rectifier plate and 1/2 secondary are struggling).
Putting it in the high voltage secondary center tap is not a good idea either. Number 1, Safety … you still can get surprised and nailed; and what protection is there if the rectifier plates short, or the wiring shorts.
I also do not recommend using MOVs in the amplifier.
Using a MOV from the rectifier plate to the B+ will destroy the filter capacitor if there is a transient that activates the MOV. (Protect the rectifier, blow the capacitor, and maybe the high voltage secondary too).
If you have power line problems, transients, etc., then you should be using a transient protection rated multi output power strip. (guess what, they use MOVs). Put the protection in the hands of the professionals (hopefully) that designed and built the transient protection power strip.
I take old amplifiers and use the power transformers. I design a completely different circuit.
But I always am careful to use less power from the transformer secondaries than was in the original amplifier. So sometimes I can use a fuse of Lesser Amp rating than the original.
If the manufacturer specifies a fast blow fuse, I use a fast blow.
If the manufacturer specifies a slow blow fuse, I use a slow blow.
I even double protect by double fusing (series fuses, one fast blow, one slow blow).
THIS IS FOR VERY KNOWLEDGEABLE AND EXPERIENCED AMP DESIGNERS.
I use solid state diodes in the B+. When possible, I use the more friendly choke input B+, since there is no rectifier resistance in series to the first filter cap, just the DC resistance of the choke.
And sometimes I use solid state diodes to get DC filaments.
Rectifying and CRC filters draw 1.8 times the filament current (1 Amp filament requires 1.8 Amps from the filament secondary).
For the primary fuse:
Suppose I am using the original 1.25 Amp fast blow fuse, and suppose that works fine, and a lower 1 amp fast blow fuse does not (I can not use a fuse of lower rating).
But suppose that amp only draws 0.5 Amp after it warms up. I find that I can also wire a 0.6 Amp slow blow fuse in series with the original 1.25 Amp fast blow fuse.
If the 0.6 Amp does not blow, I am double protected:
Now if the inrush exceeds 1.25 Amp the 1.25 Amp fast blow fuse blows open.
Now, if the warmed up amp has a failure that causes a draw that exceeds 0.6 Amp, but does not exceed 1.25 Amp, the 0.6 Amp slow blow fuse blows open. Double protection.
You need to purchase, rent, or borrow a DC current probe for your oscilloscope.
They are very expensive. But they clamp over one of the power wires in the amp. They do not come in electrical contact with the wire. Note: I am talking about a DC current probe, not an AC current probe.
Also, remember that the inrush current is only maximum if you close the power
switch at Exactly when the power sine wave is at peak voltage.
You have set the scope trigger for Normal, Single. Auto trigger, Line trigger, etc.
will not capture the inrush current waveform. Start with maybe 250 milli seconds / division. It takes practice to get it right. Set the trigger level to zero volts.
You must use the DC probe conversion to calculate the Amps. A probe factor
of 1V / 10 Amps will produce 3V if the inrush current is 3 Amps.
The amp (hopefully) will not put out any sound during the inrush time. The inrush current is one thing to measure.
The warmed up amp current is another thing to measure.
A warmed up Single Ended (Class A) amplifier does not draw significantly more current as the volume is turned up until it is near clipping. It might be a good idea to measure that current draw, but I don't recommend listening at those levels ... get a more powerful amplifier.
A Push Pull amplifier Will draw significantly more current as soon as it comes out of Class A (when it goes into Class AB). The further into class AB (louder and louder), the more the current.
The warmed up amp current is another thing to measure.
A warmed up Single Ended (Class A) amplifier does not draw significantly more current as the volume is turned up until it is near clipping. It might be a good idea to measure that current draw, but I don't recommend listening at those levels ... get a more powerful amplifier.
A Push Pull amplifier Will draw significantly more current as soon as it comes out of Class A (when it goes into Class AB). The further into class AB (louder and louder), the more the current.
I am reminded by others in this thread that inrushes with vacuum tube rectifiers is much different than inrushes with solid state rectifiers. Solid state rectifier inrushes are Fast. Tube rectifier inrushes are slower.
So, getting the peak and total inrush current may take 3 seconds or more.
So, getting the peak and total inrush current may take 3 seconds or more.
Hi, thisndently switchable with the 3 black switches at the bottom left corner, it may be paralleled to give 100, 200, 300, 400 500 and 600W in series. The outlets are below the work bench. Note here the supply is 220VAC (Under normal conditions).
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