Rectifying heater current

[dave g]

I don't really see any amps with the heater current rectified - something that in theory could reduce a lot of noise issues. I realize that it's not possible with most standard PTs due to the voltage drop across the diodes, but what about a custom made PT with a higher voltage tap? Is there a good reason why nobody does this?

[yeahyeah]

also interested! Doesn't fuchs do DC fils?

[ayan]

I don't really see any amps with the heater current rectified - something that in theory could reduce a lot of noise issues. I realize that it's not possible with most standard PTs due to the voltage drop across the diodes, but what about a custom made PT with a higher voltage tap? Is there a good reason why nobody does this?

I suspect the reason for not doing it is the current rating of the filament windings, actually, and cost considerations (perhaps). If you rectify 6.3V you end up with 8.9V minus diode drops, so that's still good enough to feed into a 6 V voltage regulator, but you end up using more current.

Personally, I have found the D-style amps I built to be consistently quiet, so I don't believe one would buy much by using DC on the heaters... Twist the wires tightly, observe the polarity of them and you should be fine with AC.

Some amp manufacturers do use DC on the preamp tubes; there is no sense in doing so for the power section in a push pull amp since the noise introduced by the power tube heaters will cancel out there.

Cheers,

Gil

[dave g]

You're right about D-style amps being quiet, I think a lot of that has to do with the large amount of negative feedback present...so in the case of a D-style, then it's not so much an issue. I was just pondering this because I've been trying to think of some ways to reduce noise in cathode biased amps with hotter preamp sections.

As far as getting 8.9 volts, however, that's just the AC peak voltage. You can't get your DC voltage to be higher than the AC RMS voltage, so after the diode drops you actually end up with significantly less voltage.

[jelle]

Gil,

What do you mean with: " observe the polarity of them"

Is there a difference?? Please explain,

Jelle

[novosibir]

What do you mean with: " observe the polarity of them"

Gil meant, insure, that i.e. on all pin 9's is the same phase of the AC swing of the heater current. And yes, there's a difference - even a huge difference, hum-wise.

Larry

[novosibir]

I don't really see any amps with the heater current rectified - something that in theory could reduce a lot of noise issues.

I categorical do have regulated heater DC in my both amp models and they're really dead quite.

But it isn't absolute necessary to do it, to quiten an amp. You i.e. might set the virtual heater's center tap (the conjunction of the both 100 ohms) to an elevated potential off of ground.

Simply make the 'FET simulation resistor', usually a 220K...330K from two smaller resistors in series, so that you'd end up on their conjunction with about 65V...75V. Now hook the heater's virtual ground point to this 'new created' voltage and bypass the lower resistor w/ a 10µ...22µ filter cap to ground.

If there was a slightly hum before, after this mod the amp will be dead quite!

Larry

[jelle]

Thanks Larry!
I already did this like you have explained....

Never knew it made such a difference...

Thanks again,
Jelle

[ayan]

What do you mean with: " observe the polarity of them"

Gil meant, insure, that i.e. on all pin 9's is the same phase of the AC swing of the heater current. And yes, there's a difference - even a huge difference, hum-wise.

Larry

Exactly what I was referring to. Thanks for the clarification, Larry.

Gil

[odourboy]

FWIW, my Rivera R30 uses regulated DC on the preamp tube heaters. It's an unusual setup in that they use two 18v regulators to give 36v and drop it across three 12AX7s in a daisy chain. If the heater fails on one or one tube is pulled, all three tubes go out.