AC or DC for 300b Heaters

[jarthel]

if DC is quieter than AC and you have a spare transformer winding (or separate transformer) and the necessary caps and diodes (and other parts as required), why use AC?

 

[Evenharmonics]

Ok, you have an 8.5V, 4A center tapped transformer. If you are not using the center tap and taking the output from the ends of the transformer then, in effect, you have an 19V, 2A transformer. It will be current limited. If you want to use this kit, you need to rewire the diodes so that it is a full wave rectifier. That way, you can get adequate current from your transformer.

I measured it with the tube alone and it's still stuck at 1.7 VDC. You may be right Sheldon.

What if I just use another transformer (I have couple laying around) with higher current?

 

[Tweeker]

Biasing oxide cathode (indirect heated) type DC fed heaters for less noise coupling.

 

[jarthel]

any reply to my question? thank you

 

[cotdt]

Biasing oxide cathode (indirect heated) type DC fed heaters for less noise coupling.

Ah I see what you mean now, Tweeker. So 100K would give 30V? Why the capacitor?

 

[Evenharmonics]

if DC is quieter than AC and you have a spare transformer winding (or separate transformer) and the necessary caps and diodes (and other parts as required), why use AC?

For me, it's because of my 105 db horn tweeter. It lets you hear all sorts of things including unwanted. Keeping it quiet is possible but not an easy task.

 

[Tweeker]

The cap keeps B+ ripple from appearing on the heater.

Actual voltage with 100k and 1M would be 1/11th of B+.

 

[jarthel]

For me, it's because of my 105 db horn tweeter. It lets you hear all sorts of things including unwanted. Keeping it quiet is possible but not an easy task.

I assume you're using AC.

I thought using DC heaters is quiter. like John Broskie has suggested to use DC heaters 'cause they are quieter. and I somehow believe other people have said that DC are quieter.

 

[cotdt]

Thanks very much, Tweeter.

 

[Evenharmonics]

I assume you're using AC.

I thought using DC heaters is quiter. like John Broskie has suggested to use DC heaters 'cause they are quieter. and I somehow believe other people have said that DC are quieter.

I am at this moment.
My plan to switch to DC is mainly for noise issue. Will there be sonic benefits? Some say yes and others say it looses something. I’ll find out how it is for a high efficiency horn.

 

[sgerus]

For what it’s worth…
I have built 2 amps, the first a 2A3 with AC heaters, and the second a 300B with DC heaters.

For my needs (high sensitivity horn speakers and listening ay low volume) on the 300B I would go with DC heaters no questions asked.

For the 2A3 AC heaters worked fine for me. I had the 2A3 cathode bypass resistor and Cap right off the 2.5vac transformer center tap…. This is as simple as it gets, no hum pot required.

You may want to look at your grounding if you still have hum problems.

In general, this is how I do the ground buss.
One 14ga bare copper wire that starts at the input jack and ends at the T1 center tap.
So as you move along the ground buss you have:
Input ground
Input cathode R & C to ground
2A3 grid to ground
2A3 cathode R & C to ground
PCU Caps to ground
T1 ct to ground

NOTE: THE GROUND BUSS CONECTS TO THE CHASSIS ONLY AT ONE LOCATION, USUALLY AT THE INPUT JACK

 

[jarthel]

Is the generally accepted idea is to use AC first and see if it's quiet and if not, move to DC?

but the assumption I have mentioned is that the parts to make a DC heater are already available. no need to buy them. why not go straight to DC?

 

[zarniwoop]

because many people say DC sounds inferior. I don't know if this is true or not. Generally, the worst is CRC filtered/unregulated DC. Then voltage regulated,, then current regulated. And LC filtered is in there somewhere.

 

[nafunga]

Hi,
I've just 'breadboarded' a clone of the AudioNote Quest amp. (6SN7 & 300B per. channel) It uses voltage regulated supplies for the 5v 300B filaments. As I did'nt have a suitable 6.3v transformer for the 6SN7's, I fed these with DC from the unregulated 300B circuitry. (via 2 x suitable dropping resistors to supply the 6.3V). This effectively raises the heaters of the 6SN7's to around 50v (the 300B filament voltage) which is required for the second stage of the 6SN7 input Circuit - DC coupled stage) It doesn't hum! Anyone else tried this? Or, if I've missed the discussion, can you point me to it please?

Regards,

David.