2 x el34 power transformer specs

[Pierre]

I'm looking to get parts together to build a new 50 watt amp. I was looking at the Marshall 50 watt transformers from classictone and the 150ma secondary seems a bit under powered. Looking at the el34 data sheet, I see plate and screen max values adding up to 235ma. Do they just rate their transformers conservatively? Edcor has a lot of transformers in the 200ma range. Would this be safe? I'm curious to hear what others have used. Thanks

[ToneMerc]

I'm looking to get parts together to build a new 50 watt amp. I was looking at the Marshall 50 watt transformers from classictone and the 150ma secondary seems a bit under powered. Looking at the el34 data sheet, I see plate and screen max values adding up to 235ma. Do they just rate their transformers conservatively? Edcor has a lot of transformers in the 200ma range. Would this be safe? I'm curious to hear what others have used. Thanks

The original 50W Marshall PT is right around 150mA, so it's correct.

TM

[pdf64]

The lowish current rating just results in the the voltage sagging; the PTs have to be able to cope with it, eg in terms of heat.
HT sag is a key part of those amp's characteristics. It also makes the operating point a lot easier on the EL34 than the voltages at idle suggest, especially so when overdriven.
Bear in mind that the 3k4 p-p load would probably overstress the EL34 if the HT stayed solidly up ~450V at full drive.
So it would be a good idea to keep the target HT lower than normal if a stiff HT is preferred, eg the Mullard info suggests 400V and 3k5 load for 50W.
http://www.mif.pg.gda.pl/homepages/fran ... e/EL34.pdf

[Stevem]

If you want max punch I would go with something that 285/285 with some 300 ma.

[M Fowler]

I've used those Classictone Marshall transformers 150mA in 10 or more builds and no problems at all.

I had looked at that 150mA rating too but after checking out other manufacturer's specs for the same amplifier it was correct so never looked back.

Mark

[Pierre]

Thanks for all the great info. That gives me good food for thought!

[Littlewyan]

I thought as the Marshall PTs are centre tapped that you had to account for this by multiplying the current rating by 1.4? Or by dividing the current required by 1.4.

Quote below taken from the topic I linked at the bottom

It is also necessary to recalculate current rating to account for center tapped HT winding, as each half of the winding will be supplying current to charge the filter cap only every 2nd half cycle.
To not confuse you any further I'll ask you to trust me when I say that the current rating of the center tapped HT will be 0.5A/1.4=.35A

Now, WTF? Center tapped means we 640V (520V) end to end and with .35A it makes 224VA (182VA) !?!? Yes, but remember that each half of the winding delivers current (and thus power) every 2nd half cycle. There's the magic trick and the reason why center tapped HT winding will account for a more expensive transformer. More copper & a little more iron.

https://tubeamparchive.com/viewtopic.ph ... ransformer