Bias Supply Source

[mwelch55]

I have seen never seen anyone get the bias from the negative side of a bridge.  Is there a problem with doing it that way?

[galaxiex]

Wouldn't that be ground?

[DummyLoad]

you would have then a +385V rail & -385V rail. so, yes you can, but why? then you need expensive filters (450V or better) to build a very low current supply and power waste of a voltage divider to make it useable.


--pete 

[mwelch55]

The way I have seen most people do it is to tap off of the high voltage and use a diode and a dropping resistor before the bias circuit.  What I am talking about is connecting a dropping resistor on the negative terminal of the bridge rectifier and using that for the negative supply for bias.  There would be no need for a rectifier because it is already negative.  It just seems like a convenient place to acquire the negative voltage without the need of using another diode.  Is there a reason I don't hear about this on forums?  Is it bad in some way?  I don't see any problems with this approach.

[Willabe]

Is there a reason I don't hear about this on forums?  Is it bad in some way? 

Yes, what Pete (DummyLoad) explained. 

If you take it from the FWB's negative, then you are dealing with a much higher dcv that will need to be knocked down to be usable. That will waste B+ current from the PT's B+ secondary, and you'll need higher voltage caps to filter that dcv. What you save in not using a single diode, that you can get for under 10 cents, or less if you buy in quantity, you loss way more in the large filter caps. Plus those filter caps will take up more space in the chassis.

If it were a good thing, companies would have used it a lot. I've never seen it used.
 

[mwelch55]

Thanks for the replies.  I can see how it would be a little less efficient.

[2deaf]

It's not a FWB.  It's the same thing as tapping off of one leg and dropping through a 100K or a 220K for your bias except this time it is full wave instead of half wave.

Just put a larger dropping resister at the point you noted to compensate for the larger full wave voltage and you're on your way.  Less ripple, also.

[PRR]

> connecting a dropping resistor on the negative terminal of the bridge rectifier and using that for the negative supply for bias.

Many-many radios, and several organs, biased this way.

Note that a simple resistor will give a pulsating voltage, you must add filtering.

In a guitar amp, the power tube is 90% of the total drain so this is pretty nearly the same as a simple cathode resistor. It will not give "fixed" bias because the voltage drop varies with current; you can't do this on say a 50 Watt.

In a large radio the many RF/IF tubes add a semi-steady load and stabilize the bias somewhat. However mostly it was about dodging the use of a large electrolytic on the power tube cathode.

In some organs the Field Coils used as much current as the power tube, giving excellent stability even as power tube operation shifted to class B.

So for most cases: it has no advantage, and confuses future technicians.

[2deaf]

> connecting a dropping resistor on the negative terminal of the bridge rectifier and using that for the negative supply for bias.

Many-many radios, and several organs, biased this way.

Note that a simple resistor will give a pulsating voltage, you must add filtering.

In a guitar amp, the power tube is 90% of the total drain so this is pretty nearly the same as a simple cathode resistor. It will not give "fixed" bias because the voltage drop varies with current; you can't do this on say a 50 Watt.

In a large radio the many RF/IF tubes add a semi-steady load and stabilize the bias somewhat. However mostly it was about dodging the use of a large electrolytic on the power tube cathode.

In some organs the Field Coils used as much current as the power tube, giving excellent stability even as power tube operation shifted to class B.

So for most cases: it has no advantage, and confuses future technicians.

Sounds like back biasing.  Attached is what I thought he meant.

[mwelch55]

2deaf,

That is exactly what I am talking about (the drawing on the right).

[sluckey]

2deaf,

That is exactly what I am talking about (the drawing on the right).

Both circuits are identical, just drawn differently.

Keep in mind that this will not work if your PT does not have a center tap.

[2deaf]

That is exactly what I am talking about (the drawing on the right).

They're both the same circuit, just drawn differently.  I've used the one on the left before and I have seen it used on other amps.  I never thought of using a bridge like that and I think it's a pretty good idea if you're going full wave..

[jjasilli]

"The bias supply demands hardly any current so it was sufficient to use a half-wave supply which required only one extra (relatively small) diode, rather than going to the expense of adding a fourth diode for a full-wave supply. When silicon rectifiers finally appeared they were simply substituted for the old valve rectifiers in many circuits, which is why the old fashioned half-wave bias supply is commonly used even in new amps."  http://www.valvewizard.co.uk/bias.html

... which addresses this topic

[Willabe]

It's not a FWB.  It's the same thing as tapping off of one leg and dropping through a 100K or a 220K for your bias except this time it is full wave instead of half wave.

Thanks 2deaf for correcting me.

I should have caught that because I saw the PT's B+ secondary CT. But the diamond 4 diode/FWB schemo drawing got me. 

[mwelch55]

I have several amps that I have used a bridge rectifier, but only used half of it.  I have never had any issue.  I have never done the bias like that, but I thought it was a good idea.

This is the bridge I have been using.

[jjasilli]

You need to match the PT secondary: 


* If the secondary has a CT AND you use it, then you cannot ground the tail end of the Bridge; that would be a short circuit.  Hence you don't need a Bridge as the tail end is superfluous.  (Though you could use it for a (-)supply.)


* If the secondary has a no CT, then you must use a Bridge, and ground its tail end (if you want full wave rectification).


See the annexed Hammond PT Guide.