Dropping rectifier filament voltage

[Nightpuma]

Hi.

I am gathering some parts for an experimental tube linestage project. I have searched around for a pwr transformer with 4VCT winding for european direct heated rectifiers, but no luck. However, Hammond have a range of nice trannies with 5VCT tapping. Do any of you know if it would be OK to drop the voltage to 4VCT with resistors considering B+ is to be taken from the CT?

Best

Theo

[Poindexter]

Sure, you can do that.  Be aware that you're going to burn 1V × Xamps (whatever the rectifier heater draws) in the resistance, so size accordingly and leave some space around the resistor(s).  When it gets up in this range, I'll usually split the resistance, just so I don't have blazing hot objects in the chassis.

Aloha,

Poinz

[yagoolar]

Have you tried to order a custom made xformer? I used to do it in Poland and, think, it is cheaper than Hammond's. Write the specs and I will check with locals.

[Eli Duttman]

Theo,

IMO, you will do better by employing 4X Schottky diodes arranged in "yin/yang" pairs at each end of the 5 VAC winding. Instead of resistively dissipating power as waste heat, the forward drop of the diodes achieves your purpose.

Schottky diodes, with their "noiseless" operation, are essential to the tweak. PN junction diodes will introduce switching noise into the B+ rail.

[Poindexter]

Watt's Law never sleeps, Eli.  If we drop Avolts at a draw of Bamps, we shall inevitably burn A×B watts.  Depending on the rectifier draw, there may be problems with a diode of high enough current rating, but otherwise it sounds like a cool idea to me, as long as the heat is taken care of, and if the voltage we need to drop is some multiple of the diode drop.  The voltage will automatically be correct, and we'll simply burn the amount of power needed to do it.  It usually takes me two or three tries to get it just where I want it with resistors.

Aloha,

Poinz
AudioTropic

[Eli Duttman]

Poinz,

The forward drop in Schottkys is "typically" 1/2 V. 2X "yin/yang" pairs achieve the OP's purpose. Do you really think there will be trouble if diodes rated for a continuous 5 A. forward current are employed? We are dealing with a 50% duty cycle. PIV is not much of a concern in this application. FWIW, 10 and 20 A. diodes are available, should that much "beef" be adviseable.

[Poindexter]

I very much doubt that there will be a problem with 5A diodes, Eli. The gentleman has not stated what the heater (or filament) draw of his rectifier is, but I do not know of any power amp rectifiers that draw over 3A heater.  You're right that the 50% duty cycle is germane; a 1A diode can typically pass 30A for a few milliseconds.  It's a heat thing, same as always.  With a little more data about that rectifier, we could advise the gentleman rather better.

Aloha,

Poinz
AudioTropic

[rdf]

Originally posted by Nightpuma
... Hammond have a range of nice trannies with 5VCT tapping....

Hi Theo. You may find the Hammonds run high, it's been my experience 100% of the time. Personally I'ld look at hanging a 5 watt 'white coffin' power resistor off either end of the winding, easily available in a wide range of values under 1 ohm. Bombproof, no danger of added switching noise, a minuscule increase in power supply impedance and a bit of cold filament current limiting; what's not to like for a less than a couple dollars?

[dsavitsk]

Originally posted by Poindexter
but I do not know of any power amp rectifiers that draw over 3A heater.

Push Pull 2A3's draw 5A.

[Nightpuma]

Hi

Thank you all for the replies and advice. First of all, to clearify, the rectifier will be AZ1 that draws 1.1A. I have tried out some different rectifiers, and i really prefer direct heated soundwise. It has of course some other drawbacks, but do they sound good:-)
Resistors is easy and cheap to apply, so that is a safe choice.
Regarding the ying/yang schottkys, that sounds intersting, but what would the benifit be besides less heat? And what about the B+ of 300+V running through them?

Best

Theo