Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: W5FH on November 02, 2022, 09:58:40 am
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Hello-
Wanted to see if my assessment of a power transformer requirement was in line with more experienced builders here. Planning a build using pair old US stock TungSol 6550 push-pull, about 5 preamp tubes (6.3 volt fil). Not desiring really high B+, somewhere around 450 to 475 VDC for 6550's. Looking for about 60 clean watts, not wanting to push it. I was considering the Edcor XPWR127-120 (690 VCT @400 mA.) plus this one has 60 VAC bias tap and plenty of fil power. Other one looking at was Hammond 290FX (640 VCT @ 518 mA.) Do I really need the 400 to 500 mA range capacity? I do want to avoid B+ sag as much as possible, however.
Have searched for other manufacturers. Have not found any except Heyboeor APD8004H (640 VCT @ 450 mA.) which appears nearly identical to Hammond 290FX. I do desire a 50 to 60 volt bias tap. Thanks
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I like to use this calculator to get a general idea of B+ voltage under load and current draw:
https://thesubjectmatter.com/calcptcurrent.html
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Here are the results with two 6550's and five preamp tubes. A 660V (330-0-330) TX rated around 225mA with at least 5A for the 6.3 secondary looks adequate.
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Thank you for the help and the handy transformer link. I can see that I may be a little too conservative on transformer current ratings. I have a spare Hammond 290DX and it appears it would serve my needs for a pair of 6650's using 4.2K OT.