Power Transformer Current

[dscottguitars]

Hi,


I am having a difficult time understanding how to choose a power transformer given the specs. I read another post here about adding up the max current draw for the power tubes plates and screens. But, that doesn't make sense because it's much higher than the current ratings listed for most PTs.


Ex: 2- 6L6GC tubes in PP require 210mA (plates) and 22mA(screens) for a total of 232mA. Normal transformers are in the 150mA range such as the MPT50 Heyboer listed in the catalog.


Ex: 2- EL84 in PP require 92mA (plates, 300v) and 8mA (screens) for a total of 100mA. The transformer listed here for this #759 is 75mA.


There is a calculator I found online that gives the specs based on a certain configuration. http://thesubjectmatter.com/calcptcurrent.html


Thanks,


Daniel

[shooter]

for a total of 232mA

If you were building a PSE class A, that would be ~~~~, but with PP, you're normally biased AB(1,2), so one tube is cut off for ~~1/2 the duty cycle so it's not drawing current.  Filament specs however need to count all the tubes all the time

[dscottguitars]

for a total of 232mA

If you were building a PSE class A, that would be ~~~~, but with PP, you're normally biased AB(1,2), so one tube is cut off for ~~1/2 the duty cycle so it's not drawing current.  Filament specs however need to count all the tubes all the time

Thanks! That makes sense now. I didn't think about the PP-half situation.

[jjasilli]

Yes, but let's use the 6L6 exmple.  At -0- signal they draw only about half that, 100mA.  A musical amplifier won't be at max signal all the time, because musical notes are transient.  (By contrast, a constant square wave will stress the tubes, etc., beyond the spec sheets which assume a constant sine wave which has a "resting period" in ea. cycle.)  (N.B.: It's the sine wave itself that has the resting period, independent of the amplifier, and this is built into the PP specs)


Also, if you want sag and use a quality PT, you can choose to run it up to 50% under spec.  I.e., you can use a PT in the 120mA range.  Trannies have poor voltage regulation, so if you use a PT rated for 450V @ 120mA and pull 232mA through it, due to Ohm's Law it will be unable to supply its rated voltage, so voltage must sag.  If you use a PT rated for 450V @ 232mA, and pull no more than 232mA, it will never sag.  But, at less than 232mA, which is almost always, it will supply more than 450V!


So, 450V @ about 150mA is a good compromise, unless pronounced sag is your goal.


@shooter:  I think you're double dipping -- i.e., the duty cycle of PP is already built into the PP specs for current draw.



 

[dscottguitars]

Yes, but let's use the 6L6 exmple.  At -0- signal they draw only about half that, 100mA.  A musical amplifier won't be at max signal all the time, because musical notes are transient.  (By contrast, a constant square wave will stress the tubes, etc., beyond the spec sheets which assume a constant sine wave which has a "resting period" in ea. cycle.)  (N.B.: It's the sine wave itself that has the resting period, independent of the amplifier, and this is built into the PP specs)


Also, if you want sag and use a quality PT, you can choose to run it up to 50% under spec.  I.e., you can use a PT in the 120mA range.  Trannies have poor voltage regulation, so if you use a PT rated for 450V @ 120mA and pull 232mA through it, due to Ohm's Law it will be unable to supply its rated voltage, so voltage must sag.  If you use a PT rated for 450V @ 232mA, and pull no more than 232mA, it will never sag.  But, at less than 232mA, which is almost always, it will supply more than 450V!


So, 450V @ about 150mA is a good compromise, unless pronounced sag is your goal.


@shooter:  I think you're double dipping -- i.e., the duty cycle of PP is already built into the PP specs for current draw.

I have a 275-0-275 PT @150mA. I was planning on running four EL84s. So it sounds like that would be a good match to get a little sag as you described.

[shooter]

I think you're double dipping

transformers are in the 150mA range such as the MPT50

I was just pointing out why 150mA is normal PP spec, vs his 232mA quandary, but I do like the taste of dip more than chip sooo......

[jjasilli]

I have a 275-0-275 PT @150mA. I was planning on running four EL84s. So it sounds like that would be a good match to get a little sag as you described.

That's a secondary of 550VAC + rectification factor.  I suggest that the voltage is too high for EL84's.  Sounds like a good match for a pair of big bottle power tubes.

[dscottguitars]

I have a 275-0-275 PT @150mA. I was planning on running four EL84s. So it sounds like that would be a good match to get a little sag as you described.

That's a secondary of 550VAC + rectification factor.  I suggest that the voltage is too high for EL84's.  Sounds like a good match for a pair of big bottle power tubes.

I used a similar size for two EL84s just less current rating and got 350v on the plates. Not worried about the voltage, I can step that down if needed. Thanks


BTW: The Marshall 18W here is 580v

[jojokeo]

350 to 375 isn't the true tube voltage seen...you subtract the cathode's voltage if measured to ground. OR measure across the plates to cathodes pins. Also, most "working voltage" is lower after the circuit's load is present, not the standard B+ x 1.41 calculation. I find appx 1.3 or so usually close for initial calcs. If worried use Sovtek EL84s or similar that can definitely handle higher voltages with less worry and expense than NOS tubes.

[DummyLoad]

350 to 375 isn't the true tube voltage seen...you subtract the cathode's voltage if measured to ground. OR measure across the plates to cathodes pins. Also, most "working voltage" is lower after the circuit's load is present, not the standard B+ x 1.41 calculation. I find appx 1.3 or so usually close for initial calcs. If worried use Sovtek EL84s or similar that can definitely handle higher voltages with less worry and expense than NOS tubes.

about 15V-18V Vk with 350V B+.


simulation with that PT shows about about 370V B+ with 80mA idle load using SS rectifiers. NOS 5Y3GT yields about 310V B+ at idle.


use the valve rectifier if you use EL84 and don't want amp to leak smoke. better still, use 5881 output stage with SS rect.


--pete

[dscottguitars]

I currently have some Russian equivalents. Other tubes I've used are new Electro Harmonix.


Also, I have seen on other schematics like Trainwreck, similar B+ voltages. When I get this put together I will post pictures and a schematic with all the voltages.