DC Heater Question

[bal704]

I've got a hum issue on an amp that I've tried about everything I can find to fix. My latest attempt is to install DC heaters on the pre-amp/PI tubes. To see if it really got rid of the hum, I used a 6V lantern battery to power the heaters. The hum basically disappeared, so I'm trying to convert my 6.3VAC heaters into 6.3VDC heaters.

The PT I'm using has Center tapped heaters, so I'm pulling off one of the 3.15-0-315 taps. This is pretty much like my HT side, so I was going to use a similar rectification circuit. I'm using 1 diode (1N4007) per leg.

Here's the circuit I'm starting out with:

[IMG:460:285]http://i755.photobucket.com/albums/xx19 ... eater1.jpg[/img]

The heater voltage is a bit high, around 3.7VAC with respect to ground on each leg.

Since it has a CT, I'm assuming the DC heater voltage should be roughly 1.4*7.4. However, I'm only seeing about 3.7V DC, which is too low.

I'm sure I'm missing something obvious. Can anybody point it out for me?

[Firestorm]

You're only rectifying 3.7VAC that way. Need to use a FWB bridge to rectify the full 7.4.

[bal704]

Got it. Thanks.

[JazzGuitarGimp]

Also, you don't multiply by 1.4 until you've added the reservoir cap. Your meter is showing you the RMS voltage, but when added, the cap will charge up to the peak AC voltage, which is 1.414 times the RMS voltage.

[loctal]

I think it is a good idea to use a 6.3 lantern battery to run your preamp tubes. I have been thinking about that solution for heater hum with octal tubes like the 6SL7s. You could keep the battery and use your circuit to charge the battery.

[Tillydog]

You're only rectifying 3.7VAC that way. Need to use a FWB bridge to rectify the full 7.4.

Trouble is, you lose 2 x diode drops, so you actually end up with a bit less than 6V. (T-shirt available)

[bal704]

I found a Fairchild KBL04 bridge rectifier that I wired up. This is a 400V/4A rectifier. I hooked up a 220uf/50V cap across the + and - DC outputs. This has 2 inputs and 2 outputs (+,~,~,-). When the rectifier isn't hooked up to the tubes, I'm getting about 9.2V DC across the cap.

Using a 6V lantern battery, I'm getting about 5V DC across my heaters, and the amp sounds great. Practically no noise.

Using the bridge rectifier, I'm getting about 5V DC across the heaters, and there is more noise than the battery, but less than the AC heaters. I'd like to decrease this noise.

Using the AC heaters, I'm getting about 7V AC across the heaters, and there's quite a bit of noise.

What's causing the added noise from the DC supplied by the rectifier, and how can I get rid of it?

[Tillydog]

Your smoothing cap is very small IMHO - try going up 10x - 20x (so 2200uF or 4700uF, or even bigger) - you will be getting AC ripple on the DC supply and also switching noise from the rectifier. It may also help to put a low value (100nF ??) cap across the electrolytic to get rid of HF noise.

Have you tried elevating the heater CT to ~50V DC? It might avoid all this.

[bal704]

I moved the heater CT to the cathode on one of the power tubes (about 25V DC). Didn't seem to help at all. I'll put in the caps you suggested and see what happens.

[soma_hero]

depending on the amp (high gain, low gain) you may get away with a big filter cap. I use a bridge followed by 22,000uF and only have issues with high gain amps. In those cases, its regulated with a LM1085.

Poorly designed DC heaters can be noisier than AC heaters done right. You're introducing 120hz ripple that you have to smooth out.

[Phil_S]

I'm late to the party. Have you tried a humdinger? That is much simpler than going DC if it works.

[rp]

I might try something like this in my 5C8 when I have some extra money and fewer bills:

http://www.tentlabs.com/Products/Tubeam ... index.html

note limit is 1 amp, I emailed and they said I'll be ok with 1.2A.

http://www.pmillett.com/DC_filament_supply.htm

millet has the boards on ebay.

[bal704]

I tried putting a 2200uf and 0.1uf cap across the +/- on the rectifier, and it didn't help. This is a really high gain amp by the way.

I've ordered the LM1085 to try.

I've never heard of a humdinger. Got a link to more info?

[soma_hero]

The humdinger is a pot connected across the 6.3vac winding with the pot wiper going to ground (or an elevated voltage).

Even if a heater winding has a CT it may not be balanced as well as possible. The Humdinger allows you to sweep the pot back and forth in order to find the most quiet position for AC heaters.

[shoggoth]

You're not going to be able to use a regulator with that supply unfortunately. They need to drop 2 or 2.5 volts, check the data sheets.

You'll need at least a 8v transformer, full-wave rectified. You've also got to worry about heat dissipation, because all that "waste" voltage will get converted to heat by the regulator, so you'll have to drop voltage before it gets to the regulator, and heatsink the regulator. And that 8v is a bare minimum btw, it leaves little wiggle room. I'd personally got for a 12.6 w/ a bridge rectifier, drop a lot of voltage with more diodes and/or resistors to get around 9.5v, and then regulate that down to 6.22v using a 6v regulator and few diodes as Merlin does here: http://www.valvewizard.co.uk/heater.html

Also remember if you use a bridge, you halve the potential amps the transformer can output.