diyitalia forum

Come da subject, e' possibile alimentare i filamenti con un secondario 3,15-0-3,15? Se non erro mi basta collegare assieme i due tap 3,15 e utilizzare lo 0 come altro tap per ottenere 6,3V...O sono molto ignorante io?
Mi e' venuto questo dubbio amletico, e prima di far cazzate preferisco chiedere....

Devi usare i due estremi per avere i 6.3v lo zero lo puoi collegare a massa...

Ah ecco... mi sembrava bene di aver detto una cavolata prima... 
Grazie...

Invece che a massa, sollegalo a 70-80Vdc rispetto a massa, soprattutto se si tratta di preamp.

E' il trafo per la testata 50W con preamp DIYItalia... Praticamente mi consigli la mod che c'e' anche su SLOClone per gli elevated heaters?

Sì, quella configurazione. Ti ho scritto come fare nell'altro thread.

a cosa serve  mettere lo zero a 80 Vdc ?

Da "The Valve Wizard"

DC elevation: DC elevation is often used when a valve in the circuit has a high cathode voltage. The heater voltage is elevated to a higher level to avoid exceeding the maximum heater-cathode voltage rating of the valve. This is done simply by 'adding' a DC voltage to the heater supply. The heaters still operate at 6.3V (or whatever you're using), but the AC component 'floats' on top of a DC voltage.
This method is also used to reduce audible heater hum, because the leakage current between heater and cathode becomes more-or-less constant when the voltage between heater and cathode is greater than a certain threshold (which will depend on the type of valve)*. Therefore, although there is still leakage current, its is constant and therefore the noise voltage appearing at the cathode becomes DC, which is inaudible!
This effect works in both 'directions' too, so we can either elevate the heater supply (which is usual) or make it lower than the cathode voltage. Reference voltages used are typically between 10V and a few tens of volts.

A typical way to apply the DC reference is by connecting the centre tap (real or artificial) to the cathode of a power valve, providing the power valve is cathode biased of course. The bias voltage of most power valves is usually more than 5V, and this will be 'added' to the heater voltage.
The other common method is to take the DC reference from a potential divider from the HT (useful if the amp has fixed biased power valves). Typical voltage references are around 20V to 90V, placing the heater supply well above the potential of most cathodes in the amp.
The potential divider should have a fairly high resistance so there is no significant current drawn from the HT (it can also serve as the bleeder path for the HT smoothing capacitors).
The lower resistor in the divider (R2) should not be excessively high or the maximum heater-to-cathode resistance may be exceeded. Many data sheets do not quote this so it is advisable not to make it greater than 100k. A fairly large value capacitor (C1) can also be added to ensure a smooth DC reference and to prevent the 50Hz heater hum reaching the HT supply. It's actual value is not critical, anything over 10uF should be fine.