do you think this toroidal PT is enough for a SS rect ac15?

[pietro_moog]

hi guys.
i was looking to this transformer.
do you think it's enough for a sort of AC15 clone?
i mean (2) el84s, (4) 12ax7, diode rectifier
i never used toroids before..

Tube-Town Online-Store - Ringkern Netztrafo tt-t50

[greekie]

No, you'd want this: Tube-Town Online-Store - Ringkern Netztrafo 106VA / Marshall-18Watt-Style tt-t106

[pietro_moog]

cool. but i don't understand why.
i need .3A for each 12ax7, so 1.2A. 0.75 for each el84, so 1.5.
The total is 2.7A and that is 3A so i should be covered for heaters.
for B+ 45mA each el84 and 1.5mA each triode so about 0.1A total. that is 0.12A. i should be in too.

are this math good? am i missing something?

[imaradiostar]

Depends on how overbuilt the toroids are- I've regularly used Antek Inc toroids above their rated wattage but that isn't universally true. The 18 watt marshall TF should have very similar requirements to that of an AC15 so I'd stick with that one.

jamie

[greekie]

Yes, it's important to keep that in mind what the gentleman above me wrote.

And also, what you are calculating is an amplifier that is near 100% efficient. That is not the case with our glass bottled friends.

My rule of thumb is that you roughly needs a ~100VA transformer for a <25W amp, a 140VA for a <35W, a 180VA for a <45-50W amp, and double that for a 100W tube amplifier.

I like to build my stuff out of parts stronger than needed as you can see. But it's a good rule of thumb, give and take +/- 10% depending on the application. Of course Marshall gets away with using a 260VA power transformer for their new JVM410.

FWIW I'm from Denmark and buy from TT regularly. I have their 139VA toroid hooked up to a testing rig for new single channel amp that I am building. 2x6V6 is about what it can handle and still have some muscle without being too stuff. 2x6L6, even biased light would make it sag too much for my taste, and the tone sounded weak to my ears.

Jake