Look for help reviving Foreplay
- Thread starter slomatt
- Start date
Success! Tonight I wired up a 12vdc wall wart and a LM317 adjustable regulator to form a 6.3v supply (ok so it actually put out 5.8v due to resistor variances but it was close enough for an experiment). Using this to power the heaters drastically reduced my hum, all I hear now is a soft hissing that is only audible within 1-2 feet of the speakers. The speakers I'm using are an older set so I have no impression of any sound quality changes yet, but my initial listening seems to show a quieter sound stage which you would suspect and possibly tighter bass than I remember from the last time the preamp worked.
Unfortunately regulating a 12v supply down to 6v and then drawing 300ma results in ~1.8w dissipated by the LM317 which leads to extremely high temperatures even with a heat sink. I'm going to look for a more appropriate wall wart.
Again, thanks to everyone here for their input and help during this project.
- Matt
Unfortunately regulating a 12v supply down to 6v and then drawing 300ma results in ~1.8w dissipated by the LM317 which leads to extremely high temperatures even with a heat sink. I'm going to look for a more appropriate wall wart.
Again, thanks to everyone here for their input and help during this project.
- Matt
Grainger49
New member
Matt,
The soft hiss might even go away with some tube rolling. Tube rolling is one of the most fun things you can do by yourself. I bought all kinds of 12AU7 variants just to play. Toshiba and Mazda branded tubes were interesting but not the best I heard.
You can use the straight 12V DC if you use pins 4 to 5. But that takes rewiring. Otherwise you can find a 6V wall wart.
The soft hiss might even go away with some tube rolling. Tube rolling is one of the most fun things you can do by yourself. I bought all kinds of 12AU7 variants just to play. Toshiba and Mazda branded tubes were interesting but not the best I heard.
You can use the straight 12V DC if you use pins 4 to 5. But that takes rewiring. Otherwise you can find a 6V wall wart.
The tube was designed to run on either Voltage, and I certainly have never heard of any advantage/disadvantage to running it on either.
You will need to rewire, of course, because 12 Volts will need to be applied across pins 4 & 5, with pin 9 left unconnected. At 12 Volts, the heater draws 150 mA, so two of them will need 300 mA. If this is an older FP with a DC bias Voltage applied to the heaters, then it will be good to maintain that. Simply leave the heater supply floating; in other words, do not connect the heater supply to any ground. Then, you can apply the bias Voltage to Pin 9.
You will need to rewire, of course, because 12 Volts will need to be applied across pins 4 & 5, with pin 9 left unconnected. At 12 Volts, the heater draws 150 mA, so two of them will need 300 mA. If this is an older FP with a DC bias Voltage applied to the heaters, then it will be good to maintain that. Simply leave the heater supply floating; in other words, do not connect the heater supply to any ground. Then, you can apply the bias Voltage to Pin 9.
Small update... I bought a 6vdc 800ma wall wart at Radio Shack. They are usually $20 but luckily they are clearing out the Radio Shack branded ones for $5 so I bought one of those. Step 1 was to cut off the prongs so it would fit in the case.
I'm using a terminal strip so I can go back and forth between AC and DC heaters. The wall wart fits nicely into the corner of the enclosure and is wired to the stock fuse and power switch.
Under load it puts out 5.8v which is within the 6.3v +/- 10% rating of the tube, and in fact the slightly lower voltage could prolong tube life. The hum is decreased to a level where I can live with it. Next up I need to lower the gain on my chipamp and I might try out some more tubes.
Here's the chipamp in case anybody is interested.
Thanks again for the input on this project!
- Matt
I'm using a terminal strip so I can go back and forth between AC and DC heaters. The wall wart fits nicely into the corner of the enclosure and is wired to the stock fuse and power switch.
Under load it puts out 5.8v which is within the 6.3v +/- 10% rating of the tube, and in fact the slightly lower voltage could prolong tube life. The hum is decreased to a level where I can live with it. Next up I need to lower the gain on my chipamp and I might try out some more tubes.
Here's the chipamp in case anybody is interested.
Thanks again for the input on this project!
- Matt
