[letterbeacon]

I'd like to use LEDs in my tube builds, so I was thinking of using a 7805 to get 5V from the 6.3V filament windings that supply the tubes.

Is there anything wrong in doing this (assuming I don't go over the power capacity of the winding)?

[ruffrecords]

Nothing wrong with it in principle. Just be careful not the ground the dc supply. The only other thing to watch out for is the rectifier can add noise to the heater circuit which might find its way into the audio path.

Cheers

Ian

[abbey road d enfer]

I'd like to use LEDs in my tube builds, so I was thinking of using a 7805 to get 5V from the 6.3V filament windings that supply the tubes.

Is there anything wrong in doing this (assuming I don't go over the power capacity of the winding)?

Why do you need 5V for LED's? LED's are current devices, most of them work with almost any voltage higher than 3V and a resistor in series.

[mjrippe]

+1 to what Abbey said.  Also, a 7805 will not regulate from 6.3v.  It needs at least 7.5v

[ruffrecords]

+1 to what Abbey said.  Also, a 7805 will not regulate from 6.3v.  It needs at least 7.5v

Yes but 6.3VAC rectified is 8.9V odf raw dc.

Also, to add to what Abbey said, you can work the LEDs directly off the 6.3VAC with a series resistor as long as you also add a reverse diode across the LED to limit the reverse voltage.

Cheers

Ian

[squarewave]

Is it also used for tubes?

You don't need a regulator. Just a resistor in series with the LED is fine.

But if it's also used for tube heaters, I would be careful about drawing too much current in one direction. Tube heater wires are balanced to cancel out magnetic fields. Too much of an imbalance might cause the tubes to pickup and amplify heater noise in the form of mains hum.

[letterbeacon]

Great thanks for the advice all.

[abbey road d enfer]

But if it's also used for tube heaters, I would be careful about drawing too much current in one direction. Tube heater wires are balanced to cancel out magnetic fields. Too much of an imbalance might cause the tubes to pickup and amplify heater noise in the form of mains hum.

I don't understand this. I agree that introducing harmonics on the heaters is just a bad idea, but I don't see what it has to do with balance and magnetic field. If the waveform is distorted beacuse of asymetrical current draw, the disturbance will be shared between both wires. If the heaters are electrically balanced, this should cancel out. Different story if they are unbalanced.

[mjrippe]

Yes but 6.3VAC rectified is 8.9V odf raw dc.

I see, my friend.  While I assumed 6.3VDC you went with 6.3VAC.  The OP only stated 6.3V, but AC is more likely. 

[ruffrecords]

I see, my friend.  While I assumed 6.3VDC you went with 6.3VAC.  The OP only stated 6.3V, but AC is more likely. 

Fortunately the OP said 'from 6.3V filament windings which supply the tubes'  which it is reasonable to assume are AC. However, I confess I saw 6.3V and ASSumed he meant AC.

Cheers

Ian

[squarewave]

I don't understand this. I agree that introducing harmonics on the heaters is just a bad idea, but I don't see what it has to do with balance and magnetic field. If the waveform is distorted beacuse of asymetrical current draw, the disturbance will be shared between both wires. If the heaters are electrically balanced, this should cancel out. Different story if they are unbalanced.

Mmmm, yeah true. But an LED would only be drawing current for half a cycle and that's current not going to the heaters for that half of a cycle. So the magnetic field will be stronger on one half of the cycle vs the other. So that's sort of an imbalance.

[Tubetec]

Heater might draw 300ma on a small signal tube , what would a led draw ?
I have a Russian tube mic I like a lot , it used have a blue led with resistor across the heater supply in the mic body to show its working  , I got rid of the led , not because it made any difference to the noise levels , I didnt like the cold blue vibe coming off it

[Tubetec]

We dont really know much about the circuits involved here but you could rectify and smooth 6.3v ac and end up with a hundred or so millivolts ac noise on a 6.3 volts dc heater supply  .Push pull  power tubes wont see much or any benefit from this , small single ended ones might , lower level stages might well benefit signifigantly.

Hum balance(ie 2x100 ohms to ground) in ac heater supplies can make upto 30db noise difference with  certain low noise valves .RC Smoothed dc(not regulated) with a balanced approach to wiring may well improve matters further. Some nice guitar amps like Boogie provide an unregulated 6.3v DC derived off the main 6.3v ac to early stage preamp tubes to good effect in terms of noise .

[Rusan]

Some nice guitar amps like Boogie provide an unregulated 6.3v DC derived off the main 6.3v ac to early stage preamp tubes...

Just be careful not to ground the dc supply.

I have an old Carvin with a DC heater supply for the preamp section derived from the 6.3V secondary, that's grounded on both ends.  Not exactly good engineering practice.... Mesa Boogie made the same mistake on the old Subway Rocket, so evidently Carvin wasn't the only one who missed it.

By the way, I was just thinking that Ian's recommendation of an oppositely biased diode (maybe a 1N914 or 1N4148?) in parallel with the LED to limit the reverse voltage across it would also create symmetrical current draw. 

Rusan

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