? How to spec a transformer correctly

[acheld]

Greetings everyone.   This has probably been answered before, but I can't seem to find a generic answer, so . . .   And all of my assumptions are suspect . . .

I need to build a standalone preamplifier with 5 12AX7 and one 12AU7 tubes.   

The filament spec is simple, 6.3 VAC@2.1A.

The 12AX7s should be run at about 250 VDC, and the 12AU7 lower at 200VDC.   

It's not so clear to me the anode current -- somewhere in the range of 3-5mA for the 12AX7s and about 20-25mA for the 12AU7 (?).  So the VA spec for the HT side should be about 10VA (250VDCx.04A)?

Using the above, I should be looking for a transformer with 10VA on the HT secondary, but at what AC voltage?
 

[shooter]

what type rectifier?
choke or CAP/resistor after rectifier?
these each add their own math to the equation

[PRR]

You want buzz? Then you want filtering. Large resistors are far cheaper than large capacitors. So it is economic to allow large voltage drop in your filtering. RDH suggests 20%. But you are actually going to use the transformer you can buy, which is likely even higher. So find a transformer and then figure the filter drop to make your "magic voltages".

250VDC plus 20% drop is 300VDC, which with solid-state rectifier needs at least 212V AC. (Not a common part, so expect to be blowing much more than 20% in filter.)

> about 10VA

The AC VA will be higher than DC Watts because cap-input rectification is abusive. But this will probably vanish in the overall budget and nearest-available-part process. (If you then want DC heaters the AC-to-DC correction may matter.)

AND-- you can buy 110V-240VAC to 12V DC regulated supplies VERY inexpensively. Also 12VDC to 300V DC step-up units.

> choke...

It would be unlikely to use a choke for this small load. It would need a LOT of Henries to do any good, and today fat Aluminum (caps) is cheaper than fat iron (chokes).

[HotBluePlates]

I need to build a standalone preamplifier with 5 12AX7 and one 12AU7 tubes.   
...
It's not so clear to me the anode current -- somewhere in the range of 3-5mA for the 12AX7s and about 20-25mA for the 12AU7 (?).  ...

One 12AX7 stage will pass around 1mA:
   -  Think about your probable supply volts, your desired plate voltage, and your planned plate load resistor
   -  250v supply, 150v on the plate?  With a 100kΩ resistor?  250v - 150v = 100v,  100v / 100kΩ = 1mA

5x 12AX7s at 1mA per triode is 10mA.


Your 12AU7 will probably pass about 1mA as well.
   -  Yeah, the data sheet shows 10-20mA is possible.
   -  But you have a 100kΩ resistor between the plate & that 250v supply:  250v / 100kΩ = 2.5mA
   -  The supply voltage & plate load resistor dictate that you can't reach 3mA, much less something higher.

[PRR]

This is very typical of g-amp design. This is 410V supply and you aim lower.

[acheld]

Great replies.

This pre-amp will be SS rectified for sure.  (KISS !).   And no choke for this . . .

The truth is that my main design philosophy has been to lift bits and pieces of amp designs from here and there, and there is always a PT to fit the scenario.

For this project, I went out and bought Merlin's "HiFi" preamp book -- but while there is a lot of great information there, it is not as practical as his guitar amp or power supply books. 

LOL, I'm sure there is a tranny out there that meets my need -- I just don't know which one it is.

Thank you!  Each one of your answers was helpful.    I'll post a schematic once I've actually done it, though not sure how interested folks are in this forum about hifi gear (which I'm building for daughter and SIL).

[Williamblake]

Any powertransformer from a radio with single ended audio power stage would very probably work and give leeway to your filtering, too. They are good and cheap.

[kagliostro]

Only be caute and verify your donor radio didn't used an autotransformer instead of a safe transformer with separated primary and secondary windings


Franco

[sluckey]

This Weber PT is perfect for your project. Cheap too! I used it in my Revibe. Simply juggle the dropping resistors until you are happy.