PP 6B4G stereo amp filament question

[BHD]

First post!

I'm considering building a PP stereo amp using DHT's. I was going to use 2a3's, but my One Electron BFT-1B power transformer has a beefy (9.6A) 6.3 volt secondary for output tubes and it would be a shame to waste it, so I think it'll be 6B4G's instead.

My question is: with directly heated power tubes, do you need separate filament supplies for each channel of a stereo amp? And if so, why? (I'm new at this, so I'm trying to learn).

Thanks!

🙂

 

[fdegrove]

Hi,

My question is: with directly heated power tubes, do you need separate filament supplies for each channel of a stereo amp?

Need, no. Not really but...

And if so, why?

To avoid crosstalk between channels.

Cheers,😉


P.S....Hadn't noticed this is your first post straightaway...
Welcome to the forum.🙂

 

[dhaen]

No, but life is much easier if they are separate 😉

Welcome to diyAudio 🙂

 

[fdegrove]

Hi,

Considering that 2*2A3s only need 2.5A when run at 5VDC and you have 9A available it could probably still be done so that each channel runs from its own dedicated heater supply.

Cheers, 😉

 

[BHD]

Thanks for the welcome guys. 🙂

I was hoping to keep it relatively simple by using 6V versions of the 2A3 and using AC, since PP operation should cancel out hum. It probably shouldn't be too difficult to come up with DC cathode supplies, though.

Thanks again!

 

[Eli Duttman]

BHD,

The scheme I outline below will take care of your 6B4G filaments. A cautionary note, there is plenty of talk about "washed out" sonics when "ordinary" DC is used to energize DHT filaments.

Use the 9.6 A. filament winding to feed a full wave voltage doubler made from 2X Schottky diodes. 50 A. rated parts are not overkill, as the turn on surge will be massive. The doubler cap. stack consists of 2X 22000 muF. 'lytics. The doubler feeds a pair (1/channel) of 7812 3 terminal regulators. The regulators are being used at their limit; so, BIG heatsinks, insulating washers, and thermal compound are a must. Follow the regulators with 20 muF. 'lytics in close physical proximity. Wire the 6B4G filaments in series. The junction point of the 2 filaments is where the RC bias network (1/channel) gets connected. An enhancement is to add a 4PDT switch to periodically (once a month) reverse the polarity the filaments "see". Reversing the filament's DC polarity compensates for the effects of the voltage gradient on them.

 

[SHiFTY]

Or you could just use your 6V winding to power one pair plus all the preamp tubes, and a small second 6V transformer (2A or so) to power the other pair.

Just watch the preamp tubes cathode to heater voltages...