12 volt AC Wall Wart + Backwards Filament Transformer to Power One 12ax7?

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JoeTele

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#1
I'm finalizing plans to build a guitar preamp pedal that uses a single 12ax7 tube and was thinking of doing the following. 12v/500ma (or perhaps 1a for extra safety) AC wall wart into a 115v/6.3v filament transformer run in reverse (xfmr rated for 300ma on the secondary and total power of 1 volt amp). The 12v in would power the filaments directly (wired in DC) and the filament transformer would only supply the high voltage. Is this a sound option? Perhaps it's a bit unorthodox but I have the transformer just sitting around and it would be great if I could use it. If so, with no 3-prong set-up should the chassis still serve as ground for the circuit?

Thanks!

Joe
 
#2
No, putting 12Vac into a 6.3V end of a 115/6.3V transformer will saturate the core of the 115/6.3V transformer causing excessive heat and most likely the destruction of the 115/6.3V transformer and your 12Vac wallwart. Just like hooking up your 115/6.3V transformer to a 230V primary voltage.
 
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#5
Now that I think about it, a filament transformer with a 12 volt secondary run backwards (so 12v wall wart into 12v) seems like a potentially desirable option, and the Hammond I'm looking (350ma secondary, 4.2VA) at has 117 and 230v primaries so the 234 as secondary would still give me the output voltage I'm after. Assuming that's ok from a transformer perspective, is this sound from a safety perspective? I've only worked with 3-prong power setups before and want to be sure that 200+ volts without the mains earth connection is ok. There are several US pedals that us a 12v power supply and have high voltage (i.e. Friedman Motor City Drive w/220 volts on the plates). Would I just ground to the pedal chassis?

Joe
 
#6
JoeTele said:
I'm finalizing plans to build a guitar preamp pedal that uses a single 12ax7 tube and was thinking of doing the following. 12v/500ma (or perhaps 1a for extra safety) AC wall wart into a 115v/6.3v filament transformer run in reverse (xfmr rated for 300ma on the secondary and total power of 1 volt amp). The 12v in would power the filaments directly (wired in DC) and the filament transformer would only supply the high voltage. Is this a sound option? Perhaps it's a bit unorthodox but I have the transformer just sitting around and it would be great if I could use it. If so, with no 3-prong set-up should the chassis still serve as ground for the circuit?

Thanks!

Joe
Click to expand...

I have been doing the reverse transformer trick for years ! works very well.
 
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