Speccing a transformer for DC heaters

[liddokun]

I'll preface this by saying I'm well aware AC heaters works fine for 99% of situations.  However, I've decided on going this route.

I'll be adding DC heaters to an Ampeg amp I've been tinkering with (a mid 70s V4). DC will only be fed to the preamp heaters, power section will be left alone. 

I have a few of Pete Millet's DC filament heater pcbs, the schematic of which I've attached.  I'm just looking for a bit of guidance on specs for the transformer to feed the circuit.

According to Pete, the regulator likes to see a minimum voltage 1.5v higher than desired output voltage.  So in this case, 7.8v. 

To find the minimum specs of the secondary, I'm using: (minimum input to regulator)+1v(diode drop)+(ripple voltage), then multiply this by .707 to get RMS, which is supposedly the minimum transformer secondary needed.  For ripple voltage I'm using I/(2*f*C), where f is the mains frequency (60hz), C is the power supply capacitance (20400uf).  Current of the collective filaments is 2700mA.

What I've come to is: (7.8v+1v+1.1v)x.707 
Pete recommends RMS current rating to be 1.8x so ultimately I'm looking for a transformer with secondary specs of minimum 6.99v, 4860mA.

Does this look correct?  Am I way off? 

Here's a link to the write up for the circuit being used, as well. 
http://www.pmillett.com/DC_filament_supply.htm

[HotBluePlates]

... According to Pete, the regulator likes to see a minimum voltage 1.5v higher than desired output voltage.  So in this case, 7.8v.  ...

Also according to Pete:  "That is the minimum voltage you need for the transformer secondary."

If you under-guess, the regulator drops out of regulation and you get all-AC output.  So if you go do all the math and see "7v RMS" then pick a 10v transformer like Hammond's 166R10.  You can easily regulate it down to what you need and not need to worry about whether you got too little voltage or current.  Its only $44 plus shipping at Hawk Electronics.

[kagliostro]

An alternative ?

https://it.rs-online.com/web/p/trasformatori-toroidali/6719097

Franco

[liddokun]

Thanks for the suggestions!

I'll look into the toroid route as well.

Luckily Hammond transformers are made not too far away from here and so my local dealer can get anything within a few days, so I'll look at this first.