Tube Preamp PSU XFMR Selection

[heatwalk]

Hi all,

I've been planning my first tube mic pre build with the intent on experimenting with the altec 1566a design. Regarding the power supply I have questions regarding power transformer selection.

12ax7 heaters in series draw 150mA, so the two -> 300mA. Is this current pull a constant, or is this max spec? Looking at options from Hammond in the 262 Series there are a handful of XFMR options that have 120V and 12.6V secondary windings, the filament winding of the smaller ones rated at 300mA. Are these ok to use? The preamp section current draw I believe shouldn't be an issue with any of the options, but the heaters is where I'm unsure. I'd have to insert an artificial center tap that will add to the current draw, so do I need to use a higher rated transformer? or is 300mA rating still relatively safe.

 

[moamps]

Yes, it is a constant heater current. If you choose a transformer that has that winding rated at exactly 300mA and if you connect that winding directly to the 12AX7 heaters (AC heating), then it will probably be ok, but your transformer will be maximally loaded all the time and will heat up quite a bit. This will certainly affect its working life.
If you are going to use DC voltage for heating (rectifier + capacitors) as it was done in the original version, then you still need a slightly more potent heater winding and in that case I would choose a 262E12 transformer with 0.6A. Check the dimensions.
I always try to have a redundancy of at least 50% of mains transformers in my designs, because you never know exactly the working conditions that can change due to fluctuations in the mains voltage, operating temperature, etc.

 

[ruffrecords]

Here is a link to a thread that lists some alternative HT transformers:

HT Transformers for Vacuum Tube Preamps

Cheers

ian

 

[heatwalk]

I see. Plan was to go with DC heaters. I was hoping to just barely squeeze a transformer into a 1U chassis like the originals but the bigger ones would mean 2U.

Thanks for the link Ian, great info and glad to read about other options. SMPS for heaters isn't something I had considered before

The dual flipped transformer idea is pretty neat too, avoids the limitations I'm running into with off the shelf single supplies. US based so no need for universal. Altec used a doubler to get to their B+ so could try that as well