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Re: Strange pot in the Hybrid-A schem.

9/15/2004 6:08 PM
Re: Strange pot in the Hybrid-A schem.
Each PT secondary winding is an independent floating source of AC voltage, related to the primary input voltage by the winding ratio of that secondary compared to the number of turns on the primary.  
On the hybrid-a schematic of 3-15-00 there are only two PT secondaries shown. The lower one is the 6.3V heater winding, there are wires coming from each end of the coil, but there is no center-tap in the coil, so for this application the pair of 100ohm resistors are used to form a virtual center-tap and that point is connected to ground.  
The upper winding is the high-voltage one, in addition to the wires coming from each end of the coil, there is also a center tap which is connected directly to ground. This is the ground reference for any DC that is developed from any of the other wires or taps on this coil. There is not a separate bias WINDING on this drawing, but there is a bias TAP on the high voltage winding, it is intermediate between the center tap and one of the high voltage AC leads. So you will get an intermediate value of AC voltage coming from this wire compared to the main high-voltage AC you get from the end of the coil. Regardless, once you add a rectifier (tube or solid state) to one of the high voltage leads or taps, that rectifier will only let electrons pass in one direction. So you have to choose which direction and that is accomplished by correct orientation of the diode.  
Looking at the diode symbol, it looks like an arrow and that is the direction which positive current will flow. No positive will flow upstream against the arrow. But negative is just like positive except for polarity. So negative is the one which flows against the arrow. The little line at the end of the arrow is the cathode band and represents the end of the component which is marked with a painted-on band. So when you look at a diode, the positive flows into the plain end then out of the banded end. And negative flows from the banded end toward the plain end.  
Now when a diode is hooked up to one of the AC secondary wires while the center tap is grounded, then beyond that diode you will no longer see the voltage varying both positive & negative in a sine wave like AC is supposed to do. The diode will block either the upper half or the lower half of the sine wave, leaving only one polarity to flow through the diode, either positive or negative depending on the orientation of the diode. So a single diode (or even 3 in series) on a single lead of an AC secondary is a half-wave rectifier relative to a grounded tap on the same winding. The diode lets current of your chosen polarity through half the time, the other half wave it is resting.  
Anyway, we need lots of positive energy (basically about all the HV winding will put out) for the B+ supply, so both of the high voltage wires are drawn from using the 3 series diodes on each end of the coil. There are 3 in series just for redundancy. This gives twice the available power compared to just getting B+ from a single high voltage wire. By using both extremes of the AC winding when the center tap is grounded, then when one of the wires dips negative (which happens 60 times per second) the other one will be rising positive. So there will always be some positive voltage available if the results of the diodes are soldered together. The diodes take turns resting, each PT lead has its own half wave rectifier, when combined like this that is now full-wave. That's what happens right before the standby switch, and since you are not getting power from only the upper or lower half of the AC sine wave, that's why it's called a full-wave rectifier circuit.  
So all that positive DC then goes through the standby switch and gets stored and filtered by the main capacitors and you rock. Hopefully, you made sure the filter capacitors are soldered in with the proper polarity, they must be correct relative to the diode orientation.  
Meanwhile, the intermediate tap you are going to use for a DC bias voltage supply. But it starts out AC just like the HV terminals at the ends of the same coil. And it already has the same ground reference since another tap on the same winding is grounded. Plus you do not need nearly all the milliamps which a whole winding will put out, not even half a winding, this is just a typical bias circuit. But the power is there just waiting for you to use it. So you do not need a full wave since even a half wave will give enough power and be easily filtered for hum when performing this light bias duty. Plus for bias you need a voltage which is negative relative to ground, this is by design and it is the negative voltage applied to the control grid of the power tubes which throttles them to the proper DC idle current without meltdown. Due to internal geometry of the vacuum electrodes, a relatively small negative voltage like -30 to -50volts on the grids will leverage the +300 to +500volts of B+ which appears on the anodes, resulting in the proper balance and control of idle current once you get the bias adjusted correctly for the exact set of tubes you are using.  
So that's why there is only one diode for the bias supply, and its orientation is reversed compared to the B+ diodes. Because for bias you want negative voltage from the raw AC instead of positive voltage. And also why the filter cap(s) in the bias supply need to be soldered in with the positive cap terminal to ground and the negative cap terminal toward your rectified voltage. Because the voltage the bias cap is filtering is negative relative to the same ground that the B+ is positive compared to.  
As for the midrange pot, it may be better if it were more unusual than usual. In that position, a pot with reverse audio taper may give you more useful control of the same audio range than a linear pot. The reverse audio taper is more difficult to come by, but you may think it is worth it.  
Actually once you have assembled a few fully functional circuits, you would probably think in hindsight that your progress would have been by far the most rapid if your very first amp is most absolutely simple design you can, rather than with a Dumble derived circuit or anything else nearly as complex.  
Assembly time is linearly proportional to the number of components. Troubleshooting time is exponentially proportional. Expect with a two-tube amp to spend a half-dozen hours assembling, and for a first attempt at a particular design, estimate almost 5 times that many hours of trouble shooting and tweaking before you can really enjoy playing.  
For a dumble type circuit, a couple dozen hours of assembly (once you know exactly where to solder everything), followed by more than 5 times as many hours troubleshooting, especially for a first project. This is a complex circuit though, so you may not actually reach the point of musical enjoyment at this milestone, but at least the amp would be expected to play. If you do not already have years of troubleshooting experience in high voltage audio circuits, then your performance may not be rapid enough to be satisfying unless you start with the absolute simplest of circuits.  
9/16/2004 6:16 AM
Tommy Dale

Thank you for a VERY clear explanation. I reallyh get it now!  
As for starting with an easy project, I built a Champ as my first project, and just finished a Matchless DC30 clone as my second. But none of these had a adjustable bias, so that's why I asked.  
Now I just have to find a transformer that has a bias tap. I suppose that shouldn't be too difficult, even if I live in Norway.  
9/16/2004 9:15 AM
Carl Gigun

You don't absolutely need a transformer with a bias tap. You can generate the negative bias supply from the high voltage winding without a tap (see marshall schems or many others) or add a very small 30v-50v transformer and make the bias supply with that. There's very little current drawn from the bias supply so the transformer can be very small and inexpensive.  
9/14/2004 10:36 PM

Yes, the .047uf cap goes to the Mid pot. Gary covered the bias voltage nicely.  

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