Fan wiring

[kanderson70]

I just built an Ampeg B15N bass amp for a friend and wanted to add a 12v fan. What is the correct way to wire up a toggle switch when using the filament wires for power?

[R.G.]

If you mean the correct way to turn the fan on and off, you'd put it in series with either the diode at the input or the fan at the output. Either way is correct enough, although I personally would put it at the input.

I have this vague suspicion that the half wave rectifier circuit here might contribute a little to hum by slightly unbalancing the heaters. It's probably not a big effect, and in conditions where the amp is working hard enough to need a fan it's probably inaudible in the other racket. I thought about recommending a full wave rectifier bridge to replace the single diode, but at the low voltages in a heater supply this might not give enough DC to properly run the fan, as a bridge has two diode drops instead of one. Then I though "use schottky diodes to make a bridge" than I thought "just leave it, he has a solution he likes and it works well enough for him to want to do it". Life is hard when there is no obviously single way to do something.

[kanderson70]

If you mean the correct way to turn the fan on and off, you'd put it in series with either the diode at the input or the fan at the output. Either way is correct enough, although I personally would put it at the input.

I have this vague suspicion that the half wave rectifier circuit here might contribute a little to hum by slightly unbalancing the heaters. It's probably not a big effect, and in conditions where the amp is working hard enough to need a fan it's probably inaudible in the other racket. I thought about recommending a full wave rectifier bridge to replace the single diode, but at the low voltages in a heater supply this might not give enough DC to properly run the fan, as a bridge has two diode drops instead of one. Then I though "use schottky diodes to make a bridge" than I thought "just leave it, he has a solution he likes and it works well enough for him to want to do it". Life is hard when there is no obviously single way to do something.

Ah from the Man himself! Thanks R.G.! Yes, sorry I meant to turn the fan on and off. I think your "in series" solution with the diode works for me. I hooked it up wrong the first time (destroying the diode) so I wanted to double check with the experts

[tubeswell]

I... I thought about recommending a full wave rectifier bridge to replace the single diode, but at the low voltages in a heater supply this might not give enough DC to properly run the fan, as a bridge has two diode drops instead of one. ...

Voltage doubler?

[R.G.]

Yeah, you can do doublers. It's much the same issue as doing DC filament supplies. 6.3 is a low voltage, it makes (6.3v*1.414) minus any rectifying diode drops. At the end you may not have enough to regulate the heater voltage - if that was what you were after.

I think the OT wanted a voltage lower than 12 to run the fan slow and quiet, maybe. I suspect part of the allure of the diode/cap was how very simple and easy it was to wire up; doing a doubler would lose a great deal of that simplicity appeal. But it would certainly work. DC fans are not terribly fussy about the quality of their DC, so a single diode/cap will work. As I said, I was just quibbling.

[Phil_S]

Why fool with a 12VDC fan? Get a 120VAC fan and run it from the mains. You can just wire it parallel to the power transformer. It will turn on when you power up the amp. If you want it switched separately, you can just insert a switch on one side of the fan.
This: https://www.amazon.com/AC-Infinity-Cool ... 0660&psc=1
BTW, you want this running as an exhaust fan.

[kanderson70]

Why fool with a 12VDC fan? Get a 120VAC fan and run it from the mains. You can just wire it parallel to the power transformer. It will turn on when you power up the amp. If you want it switched separately, you can just insert a switch on one side of the fan.
This: https://www.amazon.com/AC-Infinity-Cool ... 0660&psc=1
BTW, you want this running as an exhaust fan.

That would have worked great. I already had a 12v fan so was just going to drop it in there. Honestly I wouldn't have added it if it were for me, but they wanted it since I built a head cab for it. BTW I used a Trinity Trip Top kit and I have to say I was impressed with them.

[Phil_S]

Then just put it into the 6V feed and let it run at half speed. Nuthin' fancy but it'll work and then hope it doesn't bleed down the filament voltage too low.

[R.G.]

Fan selection is a matter of objectives and aesthetics. Very often, people want the ventilation but hate the noise. Amps often get much cooler inside if even a little air moves through, so using fans that whisper air through it is a good goal. An AC fan can be very loud, even the little AC "muffin" fans. The trick we used to use was to get 240V muffin fans and run them on 120Vac. This produces a slower turning and relatively quiet fan. 12V DC fans have come a long way, especially with the rise of the quiet-computer movement. You can get 12VDC fans that are almost inaudible. You can also pull the lower-voltage trick with these, and run them at 6Vdc to 12Vdc, getting slower rotation, less noise, and of course smaller air flow. I thought this was what the OP was doing.

[kanderson70]

Fan selection is a matter of objectives and aesthetics. Very often, people want the ventilation but hate the noise. Amps often get much cooler inside if even a little air moves through, so using fans that whisper air through it is a good goal. An AC fan can be very loud, even the little AC "muffin" fans. The trick we used to use was to get 240V muffin fans and run them on 120Vac. This produces a slower turning and relatively quiet fan. 12V DC fans have come a long way, especially with the rise of the quiet-computer movement. You can get 12VDC fans that are almost inaudible. You can also pull the lower-voltage trick with these, and run them at 6Vdc to 12Vdc, getting slower rotation, less noise, and of course smaller air flow. I thought this was what the OP was doing.

Correct R.G. I just wanted a quiet fan with some airflow. The 12v fan I installed is virtually inaudible. The friend I built it for is happy with the amp and the fan is working fine so all is well. Thank you for your feedback and everyone's posts. Much appreciated.

[gingertube]

Bridge rectify the 6.3V heater supply and after diode drops etc. you end up with about 7V. That is just nice to run a 12V DC fan slow and quiet but with worthwhile amount of cooling.

Cheers,
Ian