Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: TIMBO on May 15, 2012, 03:41:22 am

Title: HEATER PSU
Post by: TIMBO on May 15, 2012, 03:41:22 am

HI guys, Trying to sort out a little project. I'm trying to heat a 6AU6 tube using a wallwart (12v DC @ 1A)  and i used this PSU in a pedal build. it gives me just over 6v great, but when connected to the tube NOTHING, doesn't heat or even a slight glow, checked the right pins (3&4) checked all connections (breadboard) and even tried direct connection of 12v and heater glows, used a 1k dropping resistor in sieres and still nothing.

What am i missing  :w2: Thanks

Title: Re: HEATER PSU
Post by: darryl on May 15, 2012, 05:05:37 am

All you need is an 18Ω dropping resistor in series with the valve heater.

The 6AU6 heater requires 6.3 volts at 300mA, so its nominal resistance is: 6.3/0.3 = 21Ω

If the supply voltage is 12 volts, and the required current is 300mA, then the combined resistance of the heater and the series resistor is: 12/0.3 = 40Ω

The dropping resistor must then be: 40 - 21 = 19Ω    The nearest standard value is 18Ω

The heat dissipated in the dropping resistor will be: 0.3² x 18 = 1.6 watts    A 5-watt, 18Ω resistor would be a very safe choice.

Title: Re: HEATER PSU
Post by: PRR on May 15, 2012, 10:51:32 pm

FWIW: there is a 12AU6.

Price plus shipping on 12AU6 versus the 5W 18r resistor Darryl kindly computed may not be very different, and it means half the heat in the box.

Title: Re: HEATER PSU
Post by: HotBluePlates on May 15, 2012, 10:57:18 pm

I'm surprised I saw AES charging almost $5 for a 12AU6. For that matter, I'm surprised how much 6AU6's are going for.

I say that because if you're lucky, there will be some kind of ham radio fest near you eventually. 6AU6's are dead-common in radios, and you could likely find 12AU6's from tube vendors for $1 (US) for as many as you're willing to carry away. I know I've got a bunch of 6AU6's that I bought for a buck a piece or less.

Now I just need to build something to use them! Probably oughta look into doing PRR's old 1/3w amp project...

Title: Re: HEATER PSU
Post by: TIMBO on May 16, 2012, 01:21:11 am

I bought four 6au6 from tubes and more four a few bucks each awhile back with Zambo's build in mined. But at the moment i'm looking at a fuzz type fx that looks like PRR may have had a hand in.