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Question about AC voltage supply for project (tube preamp)
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valley001:
Hi all,

I have one of those 6J1/6AK5 stereo vacuum tube preamp kits on order, not realizing it will need a 12V AC supply.  Digging deep in my box of wall warts I found a 15V AC supply.  Unloaded It measures 18.5 v on the multi meter, on the scope I see a nice sine wave 60v peak to peak.  Current draw from the amplifier is supposed to be .8 amp. 

I have two questions.

1) Will I kill this this little pre amp with a 15v AC supply when it calls for 12v AC? (no +/- range indicated). 

2) Could/should I use a resistor to get down to 12V?
Tomorokoshi:
Is that the filament supply? If it is, then that transformer will be too much. A proper filament transformer would be needed. What does it power in the circuit?

Here is a typical filament transformer:
https://www.digikey.com/products/en?keywords=%20HM4950-ND
soldar:

--- Quote from: valley001 on February 02, 2019, 11:26:42 pm --- I have one of those 6J1/6AK5 stereo vacuum tube preamp kits on order, not realizing it will need a 12V AC supply.  Digging deep in my box of wall warts I found a 15V AC supply.  Unloaded It measures 18.5 v on the multi meter, on the scope I see a nice sine wave 60v peak to peak.  Current draw from the amplifier is supposed to be .8 amp. 

I have two questions.

1) Will I kill this this little pre amp with a 15v AC supply when it calls for 12v AC? (no +/- range indicated). 

2) Could/should I use a resistor to get down to 12V?

--- End quote ---


You can load the wart and draw 0.8 Amp and see what voltage it holds. If it is still too high you can put a resistor in series. It could be that the wart will not give enough current. Also check if it heats up too much.
valley001:
Yes the 12V AC supplies the whole device including the filaments.  This wall wart is rated at 15VAC 1.25 amp, its fairly large. 
amyk:
12V for the whole thing? That's a little weird... tubes usually operate with plate voltages in the 100V+ range, and the filament on a 6AK5 is designed for 6.3v.
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