Tube Power transformers Current Rating Question (DC or AC?)

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May be a stupid/rookie question, but i'm kind of confused at some brands transformer ratings. In that i don't know if the Secondary current rating is AC (peak) or DC.

As i gather there is some ambiguity. If i read correctly supposedly the Current rating listed on HV tube transformers are DC due to them being purely used for that, but few brands be it Hammond, Antek or Edcor really specifically mention if what they call is. Hammond mentions that it is in a older listing (assuming cap-input full-wave rectification), but that doesn't seem up-to-date with some discrepancy in numbers here and there. So need some verification.

I'm looking to build/experiment with some small valve amplifier circuits. Calculated to consume max 60-80mA DC. Want to know if that means that i can go for say a Hammond 370DAX (520C.T, 102mA) or a Antek AS-05T240 (240, 100ma) or that i need something quite a bit bigger.

Just hoping for some clarity as ive ran into a number of conflicting sources and hope to get it straight. NOT that i end up with a smoking transformer or an Overkill one.


Thanks to anyone reading and/or if they can clarify it a bit.
 
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Some transformers are rated in terms of the DC current their secondaries can provide with a specific rectifier and a specified capacitance.

Most transformers sold to hobbyists are rated in terms of their continuous secondary AC current. For a capacitor input (CRC or CLC) it is a good idea to choose a transformer with at least 50% greater capacity than the anticipated maximum average DC load, many will even double it for more margin.

There is almost never a transformer that is too large except in terms of size and cost if the output voltage is suitable over your load current range, but I learned to my chagrin that even a slightly undersized transformer is bad news. In an effort to save a few $$ I undersized a power transformer in an amplifier I built commercially several decades ago and the plate power transformers all failed in those amplifiers after a couple of years - that was an expensive mistake. Replacing them cost much more than building the right ones in the first place. (Still under warranty when they failed)
 
rayma, the concern stems from some PT's having secondary winding specs that provide an AC voltage rating and a DC supply rating. Some PT manufacturers do that to make it simpler for DIYers to choose an appropriate PT for a project, but some manufacturers also don't clarify whether that is for capacitor input filtering or not.

Nighter3D, the Hammond website has a doc that clarifies/confirms their secondary current rating.
https://www.hammfg.com/files/literature/5c.pdf

The Antek data shows secondary Vac for secondary Iac.
http://www.antekinc.com/content/AS-05T240.pdf

Using PSUD2 is a convenient way for confirming Iac(rms) for a particular rectifier-filter-load.
 
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rayma, the concern stems from some PT's having secondary winding specs that provide an AC voltage rating and a DC supply rating. Some PT manufacturers do that to make it simpler for DIYers to choose an appropriate PT for a project, but some manufacturers also don't clarify whether that is for capacitor input filtering or not.

Nighter3D, the Hammond website has a doc that clarifies/confirms their secondary current rating.
https://www.hammfg.com/files/literature/5c.pdf

The Antek data shows secondary Vac for secondary Iac.
http://www.antekinc.com/content/AS-05T240.pdf

Using PSUD2 is a convenient way for confirming Iac(rms) for a particular rectifier-filter-load.


Alright so Antek uses traditional Iac and Hammond indeed uses the DC supply rating. Good to know. It is quite a hassle when some don't just stick to standard format and then NOT mention it in current documentation. The old brochure list clarifies it. The newer specs/pages don't. It is that i ran into a post mentioning the ordeal today else i might have ended up with a gigantic hammond transformer down the line or something.



But yeah. Thanks!



Also yeah i am using PSUD2. Has proven super handy in quickly checking simple rectification/filtering setups.
 
No. Hammond provide secondary winding Iac(rms) rating. They also provide selection information that includes equations to relate the Iac(rms) rating to a DC supply mA rating for a variety of common rectifier and filter configurations.


But i'm looking at the Tube power transformers. There the Hammond document you listed mentions DC current for the series like the 260,263,300,etc. is this not correct?
I also notice that the DC current listed there is rounded down compared to the values on their current sheets. So idk which one to trust. Inclined for the more conservative one.


See this is why i got a bit confused by all that stuff. So many discrepancies.
 
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Hammond provide secondary winding Iac(rms) rating. ....

Hammond has Old Designs (some from the 1930s) and New Designs (general modern replacements).

"If it has a 5V winding" (was made for tube rectifier) it is likely rated in DC current.

BUT it could be rated choke-input or cap-input.

And my impression is that nobody working there today knows which is which.

The saving grace is that Hammond's Old Designs, made in modern materials, are incredibly conservative. I'd have little fear running the choke-input mA with a cap-input filter.
 
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If in doubt, the easiest way is to look at the VA rating of the coil and go from that... All of the Hammond tube power transformers are centre tapped. My 278CX is a 454VA transformer that is rated at 800VCT@535mA They are usually meant to use full wave cap input meaning 1A AC == 1A DC. If you use a center tap with two diodes with a capacitor input filter, the AC rating == the DC rating.

See http://www.hammondmfg.com/pdf/5c007.pdf for other options.

When using a bridge input, I take the VA rating multiplied by 0.6 == DC current with cap input. In other words, a 100VA coil is good for 60W of DC.
 
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