Power Transformer question about current output

[Lectroid]

 
As I understand it, an electrical  component or circuit in good repair will only attempt to draw a certain amount of current.  Amp circuits all have an internal resistance that limits their total current draw. Correct me if that's wrong.

Power transformers typically have secondary windings to put out various voltages, each with its rated max current output.  For example, the secondaries in a Hammond 290BX are rated to put out:  660Vct @ 138mA for the B+, 5V @ 3A for the rectifier, and 6.4V @ 3A for the heaters.

Thinking about these two ideas
1.  Are these posted current levels a ballpark region, or an absolute do-not-cross limit? 
2.  Do PT manufacturers build some padding into their specs?  If so, how much?

For example, imagine an amp with three preamp tubes (12A_7s), plus 2 output tubes (6V6) being heated by the 6.4V @ 1.5A winding in my imaginary PT.  The tubes' heaters need exactly 1.5A. 

In that scenario, what would happen if we replace the rectifier tube with diodes?  That would reduce the current drawn from the 5V secondary down to zero.  (Yes, the potential voltage is still being generated by the PT, and it's a bad design choice, but bear with me.)

My question is this: By not using the rectifier and its allotted current, would that make more current available on the 6.4 secondary?  Or could the HT deliver a higher voltage or more current to the B+ draws? 

For example, what happens if we add another preamp tube?  Would the heater current available on the 6.4V secondary just increase to satisfy the circuit's demands?  Or would it cause an overload in that secondary winding?

Thanks.

[shooter]

6.4V @ 1.5A winding in my imaginary PT.  The tubes' heaters need exactly 1.5A.

heat is the missing component, the more heat over time tends to breakdown the stuff needed to keep all those copper winding "separated" from each other

  Do PT manufacturers build some padding into their specs?

IF a company like EDCOR built it, I'd assume they care about the product AND the customer, IF CCP built it, i wouldn't make that assumption

[triode]

You asked a number of questions, let me try to answer them in order.

1) They are design center values. To use your example, hammond winds the B+ (HT) secondary winding to give you 660Vct at 138ma while living within their own specs (e.g. the transformer temperature rise will not go above some safe value, and you can run it like that all day long. It also considers the size of the wire to not overheat due to current draw, but I am leaving out things like magnetic saturation of the core of the iron that I will not get into)  Just a side note, if you pull less current than 138ma, the voltage of the secondary will rise slightly. So, unloaded the 660vct might spit out 675 or 680 or some such number, which will in turn increase your DC voltage after rectification. This is one of the reasons you will see transformers with the same secondary voltage but specced for different currents (e.g. 50ma, 100ma 138ma, 180ma), and also one of the reasons solid state amplifiers use voltage regulation (tubes are not so finicky)

2) It depends. The best way would be to ask them. The tech-way would be to keep an eye on the temperature rise of the power transformer as you increase the secondary load.

>> In that scenario, what would happen if we replace the rectifier tube with diodes? The secondary voltage would be higher as tube rectifiers have a much larger voltage drop (there is a chart somewhere) compared to diodes (typically 0.7v drop per solid state diode where for example a 5U4GB will drop 35V at certain loads)

>> By not using the rectifier and its allotted current, would that make more current available on the 6.4 secondary?  In absolute terms, absolutely not. The 6.3V winding is a separate winding. In theory, we could argue again about magnetic saturation of the core, BUT, I digress and say the 6.3V secondary was wound for a rated current, and going over that current will cause the temperature of the coil to increase, you will eventually reach the limit of the core and wire. The separate secondaries (660Vct, 6.3V, 5V) do not "share" current in the way you are thinking, but again, some people "cheat" and just pay attention to how much current they are passing based on the size of the wire and the temperature rise of the power transformer. I will state again, this is a good way to possibly burn up a transformer, though. :)

>> Or could the HT deliver a higher voltage or more current to the B+ draws?  No, BUT, again, on any secondary, you can always pull more current than specified... the question remains: How long will the power transformer last and will it proverbially catch fire? I know I have had conversations with Hammond (and in fact in some of their literature they used to say "conservatively rated"), and they mentioned they were designed to be cool running, and that you could easily pull more current from any and all secondaries. If you wanted an exact number for the 290BX, why not just call or email them? I bet one of their engineers might email you back and say "yes, you can pull X more current on the 660vct line".

[Lectroid]

@shooter,

So PTs, especially the higher quality ones, do have some level of current 'flexibility' in what they can deliver to the various secondaries.  If the 6.3V circuit demands more current than the winding's rated output, it degrades the secondary winding by heat.   The PT will try to supply it, but that flexibility comes with a penalty.  All good to know, thanks.

I'm reading about transformers but it's thick reading.  I don't think my mind groks magnetism very well.   

[Lectroid]

@triode,

Very detailed answer, thank you.  I can't say that I understand the physics of the actual conversion process but you've given me a good understanding of how they're rated, and how to use them effectively, and safely.  Thanks.   
 

[shooter]

I don't think my mind groks magnetism very well.

I'm what happens when you do magnetism for a living 

[Merlin]

1.  Are these posted current levels a ballpark region, or an absolute do-not-cross limit? 
2.  Do PT manufacturers build some padding into their specs?  If so, how much?

The short answer is: those are the maximum allowable current limits of the transformer, long term. Short term overloads are allowable, it's the long term average heating of the transformer than counts. More current = more heat. Most power transformers are designed to cope with an internal core temperature somewhere between 70-100 celsius depending on the manufacturer's own design choices (the surface temperarture will be lower since it is further away from the centre of the core).
Why do you ask? Are you hoping to run a transformer beyond its rating?

*For the avoidance of doubt, excessive load current does not cause magnetic saturation, it just causes more heating.