One other question about B+ resistors

[tpaairman]

I know my other question about the resistors in the power supply got plenty of responses, but there's something else that's not making much sense. In case you missed that one, below is the schematic again that I'm using as an example.

In the power supply we have the 10K resistor and the 22K resistor, that both drop the voltage to the levels we need. My question is this: After the 22K we drop to 280 volts, and then both 12AX7 plates have a 100K resistor, taking us down to 190 volts. I know we need the 22K resistor and the 8 µF for further filtering down the line, but is there any particular reason they would have chosen to drop to 280 volts at that point? Or does it mater as long as it' some voltage value higher than the 190?

[Enzo]

You have power supply, which is not the same thing as plate voltage. They wanted the preamp to run on 280v. The 100k plate loads are "decoupled" from other stages by the filter cap at the 280v point - we call such a point a node. 280v node then. it is the 1ma tube current that drops the voltage across the 100k.

Someone already mentioned, the exact voltage doesn't matter a lot. If your 280v were 300v instead, I doubt you would ever notice a difference. They want a high enough voltage that the stages have plenty of head room. In this case, the 280vDC is steady, the cap holds it there. The 190v moves up and down with the signal - substantially. If my 280v were 200v, then I would only have 10v of room for my signal to move. And that would be when the tube was at cutoff.

[Dave H]

My question is this: After the 22K we drop to 280 volts, and then both 12AX7 plates have a 100K resistor, taking us down to 190 volts. I know we need the 22K resistor and the 8 µF for further filtering down the line, but is there any particular reason they would have chosen to drop to 280 volts at that point? Or does it mater as long as it' some voltage value higher than the 190?

280V is the supply voltage and as Enzo said it's not critical. The cathode resistor is chosen to bias the tube plate at 190V in order to have maximum peak to peak signal output at the plate. When the tube is turned off the plate will be pulled up to 280V by the 100k plate resistor but when the tube is turned on its plate will saturate at about 100v so they bias the plate half way between 100V and 280V i.e. 190V for maximum p-p output.

[Malcolm Irving]

There is a bit of a ‘chicken and egg situation’ because we don’t know the quiescent current drawn by each stage, until we know what the B+ is at each stage, and we don’t know what the B+ will be until we know how much current will be drawn through the power supply dropper resistors (and through the internal resistance of the power transformer).

The usual design approach is to decide how much B+ we want at each stage first. (As others have mentioned, this is not very critical.) Roughly speaking high B+ values give a ‘stiffer, more hi-fi sounding’ preamp, while lower B+ gives a ‘softer, less bright sounding pre-amp’. Warning: other’s opinions may vary!

Having decided on the B+ at each stage, we use plate characteristics, etc. to design each stage and the quiescent current it will draw will come out as a by-product of the design. The total current through each dropper resistance is then known and we can calculate the dropper resistance values to give the intended B+ values.

On more thing to throw into this mix, is that each dropper resistance, in the power supply, forms part of a low pass filter to reduce the power supply ripple progressively as we go back to the most sensitive first gain stage. Larger values of dropper resistance give a better filtering effect and so we don’t want to make them too small.

[Enzo]

On filter caps.

The first cap, called the reservoir cap, will have up to a few volts ripple on it naturally. In a push pull amp this tends to cancel anyway. the second node is usually pretty clean, a few millivolts at most. The following nodes I expect to be totally ripple free, so the filter caps are not filtering ripple. What they are doing is decoupling the stages - that means keeping the influence of signal in any stage from getting into any other stage.

The important part of that is that just finding no ripple in a preamp stage B+ does not mean the cap is working.

[Malcolm Irving]

As an example, a power supply stage with a shunt capacitor of 22uF and a dropper resistance of 10k, reduces 120Hz power supply ripple by a factor of about 166. This is not too far away from the factor by which a gain stage amplifies any ripple at its input (say 60).

The decoupling effect of the power supply shunt capacitors is of primary importance of course, because without them the amplifier turns into an oscillator.

However, I think the ripple filtering effect is also valuable, because we want the ripple at the input stage to be down in the microvolts.