Is this transformer wrong for this purpose ?

[kagliostro]

I'm planning an SE amp with 2 x 6SL7 + 1 x 6SJ7 + 1 x 1625 (@ 350V - plate 50mA - G2 5mA max)

at the first I was thinking to use this transformer (270V @ 80mA - 14V @ 2A)

http://www.musikding.de/Toroidal-transformer-230V-270V-14V

then I've think it will be better to use this transformer (250V @ 100mA - 6.3V @ 1.5A - 12.6V @ 1A)

https://www.tube-town.net/ttstore/Transformer/Power-Transformer/Toroidal/Toroidal-47VA::5391.html

But ..... but this morning I remembered a recent post from PRR

(this is an extract)




so, of those 100mA @ 250V I can obtain only 62mA @ 350V DC

and the other transformer (80mA @ 270V) obviously is less, 49mA @ 378V DC

Always when thinking to buy a transformer I look to the availability of current declared by the seller

but I think they declare the AC rating, not the DC rating

Must I use a bigger transformer like this ? (250V @ 120mA - 6.3V @ 3A - 12.6V @ 1A)

https://www.tube-town.net/ttstore/Transformer/Power-Transformer/Toroidal/Toroidal-62VA::5322.html

Thanks

Franco

[HotBluePlates]

... But ..... but this morning I remembered a recent post from PRR
...
so, of those 100mA @ 250V I can obtain only 62mA @ 350V DC
... I think they declare the AC rating, not the DC rating

The "AC Rating" of current is directly related to the VA rating of the transformer.  However, most often transformers for tube amps do not specify the high voltage secondary in terms of that "AC Current".  Instead, they often provide a d.c. current rating of what can be drawn after rectification.

You can only verify one way or the other if you have the total VA rating of the transformer, along with the individual secondary ratings.

It would seem you can use any of the transformersa you mentioned (within constraints of heater voltage/current)

[kagliostro]

Ciao HotBluePlates, Thanks

About this transformer, nothing is told about VA, only output voltages and currents

http://www.musikding.de/Toroidal-transformer-230V-270V-14V

but those other transformers has a VA rating

This is rated 47VA

https://www.tube-town.net/ttstore/Transformer/Power-Transformer/Toroidal/Toroidal-47VA::5391.html

250V @ 100mA - 6.3V @ 1.5A - 12.6V @ 1A

so

250 x 0.1 = 25 (VA or something like that)

6.3 x 1.5 = 9.45 (VA)

12.6 x 1 = 12.6 (VA)

25VA + 9.45VA + 12.6VA = 47.05VA (close to the spec of the seller)

the other is rated for 62VA

250V @ 120mA - 6.3V @ 3A - 12.6V @ 1A

250 x 0.12 = 30 VA

6.3 x 3 = 18.9 VA

12.6 x 1 = 12.6 VA

30VA + 18.9VA + 12.6VA = 61.5VA

---

For the other transformer I think it is the same thing also if they didn't specify the VA rating

---

so I think I can only use the 62VA transformer (250V @ 120mA - 6.3V @ 3A - 12.6V @ 1A)

Am I wrong ? Can the 47VA transformer be used ?

Franco

[HotBluePlates]

Can the 47VA transformer be used ?

Your first post stated you only needed ~55mA (plus a few for the preamp).  You've got plenty of margin, even with Hammond's derating to 0.62 * Current.

[kagliostro]

Thanks HBP

so the 47VA seems to have enough juice

this is  the schematic (near definitive I hope)



I know the power tube presumed consumption, and I can think about few mA for the preamp tubes

but I've no idea about the consumption of the tube used as oscillator (??)

100 x 0.62 = 62

62 - 55 = 7mA

Franco

[HotBluePlates]

It's not that critical.  Go a few milliamps over and you might get a few volts less on that secondary, which will mean nothing compared to the overall B+ voltage.

[kagliostro]

Yes, that has sense also because the filament windings will not be used at max


Thanks again

Franco

[kagliostro]

The "AC Rating" of current is directly related to the VA rating of the transformer.  However, most often transformers for tube amps do not specify the high voltage secondary in terms of that "AC Current".  Instead, they often provide a d.c. current rating of what can be drawn after rectification.

http://www.hammondmfg.com/pdf/EDB290CAX.pdf

This is the universal version of the 290AEX
Fender Powertransformer for Champ, Vibro Champ, Princeton, Princeton Reverb, Tweed Deluxe
Fender Part Number 125P1B and 022772

is correct to say that Hammond indicate the AC disposable current ?

Franco

[sluckey]

Absolutely. Specifically says so right in the electrical data table.

[kagliostro]

Thanks Steve

it was not so clare to me (that they refer to AC and not to DC)



Franco

[sluckey]

"115mA rms" is the clue.

[kagliostro]

rms .... OK

Thanks

Franco