I've been using AC heating for all my amps, until now. I am working on a 2A3 with DC heaters. Got the 2.5V DC via 4x1N5402 in bridge configuration with 4,700uF x 2 filters.
I was shocked
to hear that the noise was louder 
than my AC heated 2A3 (currently at 2.5mV hum).
What have I done wrong?
I thought my PSU was crappy (first time I used an SS PSU) which is 2x1N5408 -> 470uF -> 1K -> 470uF so I replaced the 1K with a choke. But I still get the same level of noise.
Any thoughts will be highly appreciated.
I was shocked
to hear that the noise was louder than my AC heated 2A3 (currently at 2.5mV hum).What have I done wrong?
I thought my PSU was crappy (first time I used an SS PSU) which is 2x1N5408 -> 470uF -> 1K -> 470uF so I replaced the 1K with a choke. But I still get the same level of noise.
Any thoughts will be highly appreciated.
@ arnoldc
That's quite possible since each 2A3 draws 2.5A of filament current. You'll need a-lot more filter capacitance, I'd say around 22,000uF x2 or more and maybe a choke. How's your GND reference?
Reminds me of my younger days of hooking up a car radio and mobile CB radios to a battery charger. Burrrrrr! @120Hz or 100Hz depending where you live, plus switching noise. Had to use a big cap/big choke/big cap. More current draw = more filtering. l\l\l\l\l\ Get it!?
Wayne
That's quite possible since each 2A3 draws 2.5A of filament current. You'll need a-lot more filter capacitance, I'd say around 22,000uF x2 or more and maybe a choke. How's your GND reference?
Reminds me of my younger days of hooking up a car radio and mobile CB radios to a battery charger. Burrrrrr! @120Hz or 100Hz depending where you live, plus switching noise. Had to use a big cap/big choke/big cap. More current draw = more filtering. l\l\l\l\l\ Get it!?
Wayne
Hi,
If you can, filter as well as you possibly can using the best caps you can and lots of them.
Chokes are certainly welcome too and come in pretty handy when you face the situation where you started from an AC winding meant to heat your DHT straight.
The DCR of the chokes can be put to good use to lower the filament voltage to the appropriate value and should be put straight after the rectifiers.
Try a snubber cap, preferably MKP (1 to 5µF) right across the heater pins + an AC shunt to ground as well.
In order not to pollute the mains, Shottky diodes are clearly the best best and have a little higher voltage drop than the usual suspects.
Again this can be put to your advantage.
I normally use the same method Thorsten uses and can recommend it without reservations, it does what it's supposed to.
My 2A3 PP amp (PP has a natural advantage here, I'm aware of that) is dead quiet on even 100dB+ horns bar a little hiss.
If you design from scratch you can regulate but be aware of the high amperages involved.
The best method I've seen so far is Guido Tent's well thought out idea taking temperature gradients into consideration as well.
Just like Guido Tent pointed out, I too notice an audible improvement depending on how I hook up the heater so you could experiment with this as well.
GUIDO'S HEATER SUPPLY FOR DHTS.
Cheers,
If you can, filter as well as you possibly can using the best caps you can and lots of them.
Chokes are certainly welcome too and come in pretty handy when you face the situation where you started from an AC winding meant to heat your DHT straight.
The DCR of the chokes can be put to good use to lower the filament voltage to the appropriate value and should be put straight after the rectifiers.
Try a snubber cap, preferably MKP (1 to 5µF) right across the heater pins + an AC shunt to ground as well.
In order not to pollute the mains, Shottky diodes are clearly the best best and have a little higher voltage drop than the usual suspects.
Again this can be put to your advantage.
I normally use the same method Thorsten uses and can recommend it without reservations, it does what it's supposed to.
My 2A3 PP amp (PP has a natural advantage here, I'm aware of that) is dead quiet on even 100dB+ horns bar a little hiss.
If you design from scratch you can regulate but be aware of the high amperages involved.
The best method I've seen so far is Guido Tent's well thought out idea taking temperature gradients into consideration as well.
Just like Guido Tent pointed out, I too notice an audible improvement depending on how I hook up the heater so you could experiment with this as well.
GUIDO'S HEATER SUPPLY FOR DHTS.
Cheers,
arnoldc said:I've been using AC heating for all my amps, until now. I am working on a 2A3 with DC heaters. Got the 2.5V DC via 4x1N5402 in bridge configuration with 4,700uF x 2 filters.
I was shocked
What have I done wrong?
You used an inapropriate circuit. Running what you have described through Duncan Amps PSU Designer siggests a voltage that falsl to near zero during a mains cycle. So you have instead of 2.5V AC with 50Hz which has a nice sinewave shape and generates 100Hz as main hum component around 1.5V AC with 100Hz which has a nasty waveshape. Or in other words, you are still heating your valves with AC, only instead of "nice" AC you managed to generate very nasty AC.
You need much more filter Capacitance, think 10,000uF per Ampere current drawn unless you use LC filtering.... But even that will be noisy.
I would fit a seperate 6V + 6V Mains transformer with around 4A rating, add a 10,000uF main reservoir capacitor and clean up the supply to the heaters with a second 10,000uF filter capacitor and a pair of 0R68 Ohm 9/10 Watt filter resistors in the + connection and in the - connection (adding a suitable Common Mode Choke in series with the resistors, accounting for the Chokes DCR is another good idea).
Then add higher value (starting with maybe 22R) in parallel to get the 2.5V to nominal if the supply is low. You should still retain a pair of balance resistiors (a pair of 10R Resistors will do) and not just ground the negative supply with this circuit.
The resultant supply should have 2.5V DC with around 50 - 100mV ripple. As a Bonus, you can plug in a 300B and get almost the right DC Heater Voltage, just adjust the filter resistors a little in value (can be made switchable).
Sayonara
I'm not sure if I still have these boards lying around (cleaned out my closet just this last weekend), but here's what I did:
http://www.diyaudio.com/forums/showthread.php?s=&threadid=21409&highlight=
I got it pretty quiet, I had to go back to AC because the regulators were creating too much heat for my already cramped chassis. Take a look through the thread, that might give you some idea of what could work. You can do something similar, I would think. Also if you search here for "2A3 regulated heater" or something like that with my name, you should find the thread where I asked for help on this.
Hope that helps.
Saurav
http://www.diyaudio.com/forums/showthread.php?s=&threadid=21409&highlight=
I got it pretty quiet, I had to go back to AC because the regulators were creating too much heat for my already cramped chassis. Take a look through the thread, that might give you some idea of what could work. You can do something similar, I would think. Also if you search here for "2A3 regulated heater" or something like that with my name, you should find the thread where I asked for help on this.
Hope that helps.
Saurav
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