Question about AC voltage supply for project (tube preamp)

[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]

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?

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.

[spec]

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?

Hi valley001

1) Most probably be OK, but not worth the risk.
2) Yes, but it would require a bit of messing around and the transformer effective secondary output impedance would be a touch on the high side.

But I have a cunning plan, not elegant though: connect a string of eight 1N400x diodes (you must use 1N400x diodes) in series, with all diodes facing the same way. Then make an identical string of eight 1N400x diodes in series. Connect the two strings of diodes in parallel, except with the diodes in the strings facing in opposite directions.  Call this circuit a voltage dropping diode assembly (VDDA).

Connect the VDDA in series with the secondary winding of the transformer.

As I say, not elegant, but effective, easy to construct, and cheap: the 1N400x diodes should cost under £2.0US. You may be wondering why not use two back-to-back 7V5 Zener diodes. The reason is the peak current demand of the valve amp is not defined: the input could be a diode capacitor rectifier, which would take huge gulps of current and would destroy a Zener diode (rectifier diodes are designed to handle huge peak currents).

The 1N400x diodes may get hot, so mount them in free air for cooling.

[ArthurDent]

There are many variations of these amps. Some have an LED under each tube for light to make it look cool. Some of them only use the tube heaters for a soft glow and the tubes aren't even functional. There are some that use a DC-DC converter to supply HV to the tube plates and these kind of work.

I personally wouldn't buy one because I view them as a cheap poor quality amp.