Rectifying Filaments

[drewl]

So I was making a dc supply for channel switching and fed a bridge rectifier with the filament supply.
So the ground lead of the rectifier started getting very hot, as it was pulling alot of current.
Why does the heater center tap have to be floated or the bridge not referenced to ground to prevent this, I can't remember...

[Steve Conner]

Because if the center tap isn't floated, the ground lead of the rectifier will start getting hot...

(If you think about it more deeply, you've created a FWCT rectifier, as the old tube rectifier amps used for their B+, with its + and - outputs both connected to ground.)

In my latest amp project I've used a full-wave tripler to get +12V from the heater winding while keeping the center tap grounded, details on request :P

[drewl]

Thanks,
On channel swithcing amps I build I usually float the filament center tap, (reference it to ground with a couple caps to get rid of noise) that way I can use the chassis ground for 2 different switching applications.

While I'm here, any benefits/disadvantages to using a big ole bridge rectifier for the B+ instead of a couple diodes?

I have a bunch, (obsolete stock from work).
And I'd love to see your 12v supply. PM if you'd please.

[drewl]

I'll answer my own question, the big ole bridge was drawing too much current....overkill.
This amp is an 18w Marshall clone I buiilt a little while back when everybody was getting into these
I had modded it to cascade the first preamp tube for more gain and finally made it footswitchable.
While I was there I replaced the power trans (I got the iron from an old Gibson Medalist) with one from a recent Marshall 18w reissue.
The old tanny was putting out like 450v B+, which I had lowered with a power resistor (this was before we knew about the zener dropping trick).

I figured I'd overhaul the beast while I had it apart... It was slow day at work

[Enzo]

I don't think so. Bridges and rectifiers do not draw current of their own, they merely conduct it. Your bridge being big and heavy means is gets hot instead of just burning right out.

Draw the circuit on paper, and read this description.
6VAC center tap to ground, call it 3-0-3. Now each 3 feeds a corner of the bridge. YOu grounded the negative corner of the bridge and the positive corner goes off to be some sort of supply voltage.

But wait,each 3v has a diode to ground in the bridge. That means you have two circuits - one for each 3v - that are essentially a 3v winding with a diode across it. See? Center tap is grounded, through the winding to become 3v which then runs through the rectifier diode back to ground. The bridge is shorting out the winding.

[Steve Conner]

Here we are, I've posted it in the forum so that others can comment...

[loudthud]

When I saw Steve's circuit, I had to go outside and smoke a cigarette before I figured out how it works. Simply Brilliant !!!

[hamfist]

Here we are, I've posted it in the forum so that others can comment...

Thread resurrection !!

What values for the caps and diodes would be a good starting place. I'd really like to try this out.

[martin manning]

Any reason this wouldn't work with a 2x 100R virtual CT?

[Steve Conner]

Hi all, sorry for the long delay but I've been mad busy at work lately.

Something like 1000uF, 16V would do for the caps, and the diodes can just be regular 1N400x series. For extra geekpoints and a little more output voltage, use Schottky rectifiers.

With the 2x 100R virtual center tap, the output current would be very low, since it effectively all has to flow through the 100 ohm resistors. The resulting voltage across the resistors may spoil the hum balance, too. I leave adapting the circuit to a heater winding without CT as an exercise for the reader

[martin manning]

With the 2x 100R virtual center tap, the output current would be very low, since it effectively all has to flow through the 100 ohm resistors. The resulting voltage across the resistors may spoil the hum balance, too. I leave adapting the circuit to a heater winding without CT as an exercise for the reader

I see your points... Answer: with no CT on the heater supply, use a separate winding or a separate transformer for the relay supply.

MPM

[hamfist]

For extra geekpoints and a little more output voltage, use Schottky rectifiers.

Thanks for all the info Steve.

However, as for the Schottky rectifiers (whatever they are, I'm sure they are superb !), I don't think I'm quite experienced enough to consider geek-points just yet ! Maybe in a couple of years !

[Old Tele man]

...FYI: schottky diodes have very low forward voltage-drop, even less the Ge diodes, so are more efficient and thus pass-along more voltage to the tube filaments instead of "wasting" it as voltage "drop."