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In Reply to: RE: Heaters in series? posted by andy evans on August 06, 2017 at 08:15:04
What would be your objection to using one filament supply? Both tubes are in the same channel. I could understand objections in terms of channel to channel intermodulation of some sort, but that isn't the case here.
I have had a lot of trouble getting this to work as configured and feel like I am close to getting it to work like I want it to. Perhaps I will try your suggestion regarding plate choke once everything is dialed in and I am happy with the sound.
As regards connecting primary and secondary goes the primary leads are actually brown and blue. The connections as I have it are reversed from what you describe. I don't believe there is any absolute phase so as long as each winding is in phase relative to the other it probably won't make any difference. At least I have not detected any treble rolloff as I have it hooked up.
Thanks for all your help and suggestions.
Don't you want the second tube to have higher bias voltage?
Well to be strictly correct wouldn't that be lower voltage?. All kidding aside the second stage is biased with a separate negative voltage supply. I believe the term used is "fixed bias" but I have never understood this since most such supplies have provisions to allow adjustment of the voltage.
The difference in voltage from grid to cathode will be greater in magnitude for the second stage than the first stage.
Fixed bias is called fixed bias because the bias voltage doesn't wander during operation (transitioning from class A to B for example).
If your filament supply has considerable capacitance from each end of the filament to ground, then you may be able to bias up the first stage to several volts, then put fixed bias on the second stage and still run it on the same filament supply without the two stages interfering with each other too much.
Something like this?
So that brings even more complications, lol. The output tube has 3.75V of bias on the filament and some additional negative grid bias while the driver stage is biased at 11.25V with 0V of additional grid bias.
In this schematic the 22K resistor loading the 0D3 could be a 10W part and work just fine (5W is marginal). I don't know why it's drawn as an adjustable, that's strange. You do need one 0D3, loading resistor, and bypass cap per channel though.
The schematic is also missing the bias supply and the 20V filament supply. Again there's a weird 6 Ohm 25W adjustable resistor, not sure where you're going to buy that.
Not everything is immediately evident in this schematic. Filament power and the negative bias supply come from switch mode supplies. Also note the use of mercury vapor rectifier in the power supply which has a large switchable dropping resistor, not sure how this can prevent arcing before the tube has warmed up.
Well I have built my own version and to my ears it sounds excellent.
Filament supply uses an LED supply rated 20 volts, 1.25 amps with a fixed 4 ohm resistor. I have two 25 watt 6 ohm pots if you are interested. The input tube biases up to 170 volts @10 ma. I omitted the VR tube. The output stage uses my own conventional negative supply and with -25 volts I get 20 ma current @ 355volts. Its dead quiet, no discernable noise, and sounds great. Of course the fact that it makes any kind of noise at the end of the day is a constant source of amazement and pleasure for me :)
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