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Old 17th April 2013, 11:33 AM   #1
Tesla88 is offline Tesla88  Italy
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Default Correct B+ for EL84 UL PP

I want to build an EL84 Ultralinear PushPull amp , i have a couple of power transformers .
One is 240V ac secondary , meaning about 330Vdc at idle , 320Vdc under load , a bit low voltage i think , and is a 300VA rated transformer so a little too big for this application.

The other transformer comes with tap at 290 and 310Vac secondary , meaning 400-420Vdc , is this too high ? Transformer should be a 100VA , so i think is enough.

Sorry for my bad english ,
Thanks
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Old 17th April 2013, 01:56 PM   #2
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Quote:
Originally Posted by Tesla88 View Post
I want to build an EL84 Ultralinear PushPull amp , i have a couple of power transformers .
One is 240V ac secondary , meaning about 330Vdc at idle , 320Vdc under load , a bit low voltage i think , and is a 300VA rated transformer so a little too big for this application.

The other transformer comes with tap at 290 and 310Vac secondary , meaning 400-420Vdc , is this too high ? Transformer should be a 100VA , so i think is enough.

Sorry for my bad english ,
Thanks
I would say the contrary. For the common EL84 320V is already 20V above max ratings. Anyway it should be fine if the other max ratings are not exceeded. I have found that Russian 6P14P-EB (or Sovtek EL84M which is the same) or ER work fine up to 350V. With 400V you need to use special types like E84L (max ratings of 450V for both plate and g2) or 7189A (max ratings of 440V for plate and 400V for g2).
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Old 17th April 2013, 01:57 PM   #3
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320 VDC is not too bad. Use combination bias and you'll do fine.

The bigger power trafo can be used too. Just use a CLC filter, where the 1st filter cap. is small. Start with 4.7 μF. in that position. At least 5 H. of inductance is needed and 220 μF. in the 2nd cap. position. Load the PSU down, or your measurements will be incorrect. Measure the voltage and increase the 1st position capacitance, incrementally, to bring the rail voltage up to where you want it. The cLC "trick" has been around for a very long time.
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Old 17th April 2013, 02:30 PM   #4
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