Perhaps I have had a bit of brainstrain as I really have not come across this term much...some background😕
I've dumped my 813 SE project in favor of a 2A3 SE project & I'm putting the chassis together, sizing up components, etc. & I come across the mains 240/240 isolation TX rating of 500VA.......how does that convert to amps???
I would be driving two 2A3s' & two 6SL7s' HT.....I will run 3000uF(Or more), SS rectification............do I have enough headroom with the current?
________________________________________Rick........
I've dumped my 813 SE project in favor of a 2A3 SE project & I'm putting the chassis together, sizing up components, etc. & I come across the mains 240/240 isolation TX rating of 500VA.......how does that convert to amps???
I would be driving two 2A3s' & two 6SL7s' HT.....I will run 3000uF(Or more), SS rectification............do I have enough headroom with the current?
________________________________________Rick........
Current...
OK....got 480 milliamps to work with, thats' about four times the current draw of all my tubes combined so I have a good safety margin........Did I read that right?😎
___________________________________Rick..........
OK....got 480 milliamps to work with, thats' about four times the current draw of all my tubes combined so I have a good safety margin........Did I read that right?😎
___________________________________Rick..........
VA is another way of saying WATTS or Volts x Amps(current)....
The reason the Volts and the Amps are considered seperate is due to the POWER FACTOR....
When hooking up this transformer to a bridge rectifier to make DC...You will have a phase shift in the current due to the conduction angle when the diodes forward conducts, therefore voltage angle and current angle are offset..making for the effective watts...Volts x Amps x (offset factor)...
Chris
The reason the Volts and the Amps are considered seperate is due to the POWER FACTOR....
When hooking up this transformer to a bridge rectifier to make DC...You will have a phase shift in the current due to the conduction angle when the diodes forward conducts, therefore voltage angle and current angle are offset..making for the effective watts...Volts x Amps x (offset factor)...
Chris
Nope! Your 500VA 240V transformer could supply 500/240 =2.08 Amps of current. In reality this is not true due to the Power Factor mentioned above. With a pure resistive load the power factor is 1. It drops below 1 with either a capacitive or inductive load. In the case of a bridge rectifier and a big capacitor load it could drop to as low as 0.6. This still leaves you at least an AMP to play with, enough for a whole box full of 2A3's.
I have used 50 VA transformers to power small P-P amps. I have a 500VA 120 to 480 volt industrial transformer feeding a voltage doubler that runs 2 X 845's at 1100 volts @ 200 mA. It is the coolest running transformer on the amp. The Hammond 274BX that I have powering 2 X KT88's (B+, filaments, and rectifier tube) is only rated at 138VA. 500 is plenty for even the biggest of tube amps.
PSU overkill.......
OK...heres' what I found for a HT transformer, A Thordarson 23V604...quite unique as it can use 120 VAC or 220VAC for the primary.www.alliedelec.com ...Page 922.
For the caps I found some 1500uF @450V, 6"X3".www.amazing1.com.
Output TX using Edcor CXSE25-8-5K.
The design is based on the JE Labs "Simple 45/2A3".
I like the HT transformer as I will be running the caps directly from the SS rectification,.......The startup pulse will be rather high so I want some headroom (haven't done these calculations yet)...
Not off the subject but I have a Sony 32" Trinitron and upon "start-up" this supposed high tech TV has an immense start-up pulse which flickers the lights as it powers up....not something to be expected from such an expensive unit....as some consolation it does have a delay relay before sending the HV down thru, but it is annoying. This pulse is something that our decidedly "low-tech" gear might do...& not the newer SS stuff.
_____________________________________Rick........
OK...heres' what I found for a HT transformer, A Thordarson 23V604...quite unique as it can use 120 VAC or 220VAC for the primary.www.alliedelec.com ...Page 922.
For the caps I found some 1500uF @450V, 6"X3".www.amazing1.com.
Output TX using Edcor CXSE25-8-5K.
The design is based on the JE Labs "Simple 45/2A3".
I like the HT transformer as I will be running the caps directly from the SS rectification,.......The startup pulse will be rather high so I want some headroom (haven't done these calculations yet)...
Not off the subject but I have a Sony 32" Trinitron and upon "start-up" this supposed high tech TV has an immense start-up pulse which flickers the lights as it powers up....not something to be expected from such an expensive unit....as some consolation it does have a delay relay before sending the HV down thru, but it is annoying. This pulse is something that our decidedly "low-tech" gear might do...& not the newer SS stuff.
_____________________________________Rick........
As a general rule, the above statements are correct, but the actual operating conditions are not. If you scope the waveform of volts versus amps on a transformer with rectifier and capacitive input filter, you will find that the current is drawn in somewhat high peaks. These peaks are basically lined up with the peaks of the voltage waveform.
Net result is that the power factor of a capacitive input filter is quite good, not anywhere close to 0.6. If I were dealing with ac loads, such as motors and power factor correction capacitors, these assumptions would be true. But not with rectification.
The peak current pulses give rise to two issues: rms measurements and harmonics. Basically, you need to measure the rms current on the AC side to size for correct VA. Using the DC voltage and current values will NOT work properly, as they do not reflect the high peak currents. There are straightforward equations to do this. Some Hammond transformers save you this effort, and rate their small signal transformers based on the DC output, and don't give you a VA rating.
For instance, if you power your filaments off AC, you have a very straightforward 6.3V * 1.5A = 9.5VA rating per filament. However, take this same filament and power it off DC, and you will need significantly more VA from your transformer (I would expect at least 15VA per filament). Even if one does not regulate, wasting voltage across the pass element. Simply due to the peak currents, but not at all related to power factor.
On the more esoteric side, harmonics affect a transformer's operation, and there are further derating factors to use based on the spectrum. The high crest factors give rise to the harmonics, and affect a transformer's operation. Core steel doesn't like harmonics, and eddy currents increase. Skin effect becomes more prominent, so conductor size needs to increase. For the small loads we are dealing with, this is not much of an issue, but with larger transformers it becomes significant (Google K-rated transformers).
I would highly recommend Motorola's Rectifier Applications Handbook. An excellent read, and is available for free download from On Semiconductors website, as it explains many of these concepts in easy to understand terms with excellent figures and equations.
Note: industrial control transformers are the best choices for brute force rectification. Their core steel is overrated in order to handle high inrush inductive loads, and their impedance is much lower than similar transformers. You would not need to derate for harmonics, certainly.
If you find yourself very near the VA rating on a transformer, pick the next size up. But tubelab's statement is certainly true; 500VA is big enough for all but the most ridiculous designs.
Net result is that the power factor of a capacitive input filter is quite good, not anywhere close to 0.6. If I were dealing with ac loads, such as motors and power factor correction capacitors, these assumptions would be true. But not with rectification.
The peak current pulses give rise to two issues: rms measurements and harmonics. Basically, you need to measure the rms current on the AC side to size for correct VA. Using the DC voltage and current values will NOT work properly, as they do not reflect the high peak currents. There are straightforward equations to do this. Some Hammond transformers save you this effort, and rate their small signal transformers based on the DC output, and don't give you a VA rating.
For instance, if you power your filaments off AC, you have a very straightforward 6.3V * 1.5A = 9.5VA rating per filament. However, take this same filament and power it off DC, and you will need significantly more VA from your transformer (I would expect at least 15VA per filament). Even if one does not regulate, wasting voltage across the pass element. Simply due to the peak currents, but not at all related to power factor.
On the more esoteric side, harmonics affect a transformer's operation, and there are further derating factors to use based on the spectrum. The high crest factors give rise to the harmonics, and affect a transformer's operation. Core steel doesn't like harmonics, and eddy currents increase. Skin effect becomes more prominent, so conductor size needs to increase. For the small loads we are dealing with, this is not much of an issue, but with larger transformers it becomes significant (Google K-rated transformers).
I would highly recommend Motorola's Rectifier Applications Handbook. An excellent read, and is available for free download from On Semiconductors website, as it explains many of these concepts in easy to understand terms with excellent figures and equations.
Note: industrial control transformers are the best choices for brute force rectification. Their core steel is overrated in order to handle high inrush inductive loads, and their impedance is much lower than similar transformers. You would not need to derate for harmonics, certainly.
If you find yourself very near the VA rating on a transformer, pick the next size up. But tubelab's statement is certainly true; 500VA is big enough for all but the most ridiculous designs.
Hi,
where does the filament power come from? That's more VA.
Have you considered what happens to the valves if you turn them on suddenly when they are cold?
Slow start? or tube rectifier? or both?
where does the filament power come from? That's more VA.
Have you considered what happens to the valves if you turn them on suddenly when they are cold?
Slow start? or tube rectifier? or both?
Filament???
I was thinking three more TXs' one for the 6SL7s' one each for the 2A3s'.....
The filaments I want to run with "Power cord plug in" at 63% with a simple series PWR resistor to drop them down....I am concerned with these resistors & filaments running 24/7...upon "start" the HT would come up & the filaments would go 100%, "Run" would connect the HT. I know I know this is lotsa iron on board but?? Trying to keep it simple.
___Standby_Start_On____
___________________________________________Rick.......
I was thinking three more TXs' one for the 6SL7s' one each for the 2A3s'.....
The filaments I want to run with "Power cord plug in" at 63% with a simple series PWR resistor to drop them down....I am concerned with these resistors & filaments running 24/7...upon "start" the HT would come up & the filaments would go 100%, "Run" would connect the HT. I know I know this is lotsa iron on board but?? Trying to keep it simple.
___Standby_Start_On____
___________________________________________Rick.......
This is technically true. However as stated above the current is drawn from the transformer in short pulses. The transformer "sees" the capacitor for a short period of time near the peak of the sine wave. This "conduction angle" varies with the size of the capacitor, load current, DC resistance, and type of rectifier used. The worst case is a bridge rectifier connected to a large cap fed directly off of the power line as in the SMPS in your computer (and possibly your TV). This short pulse of current draw puts large amounts of noise on the power lines and can lead to inacurate readings on electrical power meters. Needless to say as these type of devices became popular the power companies got upset. There is legislation coming in the EU and the US to require "power factor controllers" on devices that consume a lot of power. There are now PFC chips available for this purpose (ON semiconductor and others).
Whether the term "power factor" is textbook correct or not it is becoming common in this meaning. What does this mean to us? It means that if the load on the transformer is a box full of resistors, you can use 100% of the transformers rated capacity. If the load is a solid state bridge rectifier followed by a zillion microfarads of capacitor, the transformer has a low DC resistance, and was not intended for this purpose (this is the case here), you need to derate the transformer. I have used "industrial control transformers" in large tube amps with SS rectifiers and large caps, and 0.6 is a conservative but safe number.
There are transformers rated for use into a rectifier and are often rated in DC current. Most of the Hammond tube type transformers are specified for a certain DC load current on the HV winding. the 300 series is available with a 240 volt primary, and could be considered for your amp.
The large peak currents of a setup like this can lead to noise coupling that can be hard to fix. I assumed from the initial post that the writer already had the transformer and possibly the big caps. I built amps (mostly high power amps) this way for a while until I realized there was a better way. A power supply to feed 2 X 2A3 and 2 X 6SL7 needs about 300 volts at 120 to 150 mA max. That is 45 watts worst case for the HV (seperate transformers for filaments). If you wanted to use the isolation transformer approach, I would use the 100VA unit or the 250 VA unit. Too big of a transformer just drives the peak currents up. Use a bridge rectifier made with FRED diodes, to reduce "switching noise" and follow it with a reasonably sized cap. I use 100 uF with SS rectifiers and 47uF with tubes. Then go through a choke to your monster cap. This will further reduce the ripple currents flowing in the design and make fighting noise easier. While you have that Allied catalog in your hand (all 10 pounds of it) look up the Triad C-14X choke (page 927) it is cheap and works well.
On the subject of monster caps, I used to use some surplus 2400uF 450V "computer grade" caps the size of a Fosters beer can. I have since found better sonics by using a 120 to 270 uF quality electrolytic (like a Panasonic or Cornell Dublier) in parallel with a polypropelyne motor RUN (not start) cap of 50 to 100 uF. This combination has a low ESR (effective series resistance) and low inductance. Many surplus monster caps are highly inductive and may not behave much like a capacitor in the upper audio range. This is very important on a SE amp. I use some 100 uF 370 VAC ASC motor run caps that I got from Ebay. They work well and I have fed them up to 520 volts DC.
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