Biasing heater supply above ground.

[jackinnj]

if the windings of the transformer aren't perfectly matched, won't you induce some millivolts of hum into your B+ supply.

if the insulation of the transformer isn't rated for the B+, might you not develop a short to ground? (let's add a little humidity to the equation.)

[NickC]

i will give this topic a go
first biasing the heater with 30volts above the cathode sounds better why?
first like mention before there tube protection, each tube recommends a difference of voltage between heater and cathode

next is how the tube conducts
we know that the tube conducts by the electrons following from cathode to the anode so the anode has to attrach the electrons
when the difference of voltage between heater and cathode some of the electrons will be move towards the heater due to the lesser potential. this is the same principle of the electrons following from cathode to anode. but in this case some of the electrons are moving towards the heater hence we are creating a tube inside a tube. hence to avoid having the tube in a tube raise the heater then. safe from arcing, better sound and reduce in noise level (AC heated)
hope this helps

[Bas Horneman]

http://www.zwengelamps.com/heater.html

Just like the cathode, when the filament gets hot it also gives off electrons. When the voltage of the heaters is at or near the voltage of the cathode they head straight for the cathode and cause hum. This is especially critical in the early stages of amplification. Any outside noise that gets introduced into the signal path here gets amplified greatly right along with the audio signal. The question now is...how do we alleviate this problem? By raising the heater voltage to a level above that of the cathode voltage, the emitted electrons no longer have a place to go and just remain in a cloud around the heater filament. Now on to the nuts and bolts of how this gets accomplished. It's really quite simple.

The Resistor Network
What we need is to find a DC level for the heater voltage to ride on. We're going to do is to create another filter branch off the rectified secondary voltage parallel to the main power supply. All we need here is a voltage level and very little current. The reason why we're using a 1 meg resistor is so that we don't draw any current and as a result we prevent the power supply from being loaded down.

If you remember your basic electricity, the voltage in a parallel circuit remains constant. This means that if we have 450 volts across the first filter section we'll also have 450 volts across the 1 meg resistor. Clearly this is way too much voltage to put on the heater filaments so we have to reduce this level substantially. Let's say we want to float the heaters at 45 volts above ground. This means we need a 10:1 divider network. With a 1 meg resistor this means that we'll need to use a 100k resistor going to ground to correctly split the voltage. This will give us our 45 volts at the junction between the 1 meg and the 100k resistors.

The Capacitor
What about the capacitor? Since we took the voltage off the first filter stage there is still a substantial ammount of ripple AC present. Because we're using this for the B+ supply several volts of ripple on a 450 volt supply is virtually inconcequential. However on a 45 volt supply this becomes an issue. The way we get rid of the ripple AC is the same way we get rid of it in the main power supply. We'll just use a large electrolytic referenced to ground to smooth it right out! The size of the capacitor really doesn't make a lot of difference. A good thumb rule to use is....when in doubt go BIG. The larger the capacitor you use the more smoothing effect you'll get. Just make sure that the working voltage of the capacitor exceeds the DC level you're going to see across the cap. For example, if you're using a DC level of 45 volts and there is 5 volts of ripple you're going to want to use a capacitor that's rated in excess of 50 volts. I usually use 100uF/100 volt bias filter caps in this circuit with good results. You can use a larger value capacitor if you want but they're going to be pretty good size so bear this in mind. If you're converting a Silver Face Fender to a Black Face bias circuit you'll most likely have a cap left over and this circuit makes a great use for this extra part!

The Potentiometer
If you look at many of the Fender schematics you'll see that the heater is referenced to ground through two 100 ohm resistors. What this does is to guarantee that each half of the heater string gets equal voltage with respect to ground. This is a great idea, and it works well too. But we can do it one better. The Fender arrangement works fine providing the transformer windings to the heater are perfectly balanced. Guess what? This is rarely the case. The way we can fix this is to use a potentiometer to balance the AC. By using a pot in the circuit we can adjust the ratio of the AC voltage seen in each branch of the heater string. This will balance the AC level going to each branch of the string and reduce the hum even further. This pot can be a bit tricky to get hold of and can easily be omitted. If you opt to not use the pot make sure you use the pair of 100 ohm resistors. Either way you'll get great results.

[Bas Horneman]

nt

[G]

Thank you for the diagram and the explaination. I learned a lot reading it. I have one more question for the gallery. The power transformer that I'm using has a 6.3 volt center tapped winding with a current capacity of 10 amps. More than enough for for two 6V6gt's and a couple of 6SL7gt's. My question is that I will have to use this winding for the heaters of the tubes on both channels. Will this lead to some sort of crosstalk? If so is there any way to fix the problem?

G

[Bas Horneman]

Theoretically using a seperate filament transformer for each channel is better, or even each tube.

In practise..it make little enough difference to worry about it.

Hope this helps.

Cheers

[G]

Yes it helps very much. After thinking about it I realized that I am already going to do the best thing you can do in my situation. Eliminate the "tube inside of a tube" by biasing the filaments above the cathode. Thanks again for your explaination.

G