amaturish q's on preamps and tubes

[mrphotodude]

well...i have gone a long way the past few years...and i can find complex info wherever...but the simple info is hard to find(or at least described very strangely).

Lets take the NYD one bottle for example...
a B+ of 300volts that gets knocked down to 106vdc on plate 2 and 55vdc on plate 1.
I understand reducing ripple in the PSU but what i dont get is why the 300 B+ is needed..
Wouldn't it be sufficient to say have a B+ of 150-200 and then use smaller dropping resistors between the caps in the PI filter?

The whole point of dropping resistors and the psu is to get a voltage needed from the power source, to the load source in a clean matter correct?
Why would it matter if you use a 220k plate resistor or 1k plate resistor so long as you have a well filtered, low ripple, clean dc voltage?

another thing...the zobel network...
what is that all about?
I know the tube needs some impedance on the input from grid to ground, but why the cap? Is that for a frequency stabilization or crossover?

 

[analag]

well...i have gone a long way the past few years...and i can find complex info wherever...but the simple info is hard to find(or at least described very strangely).

Lets take the NYD one bottle for example...
a B+ of 300volts that gets knocked down to 106vdc on plate 2 and 55vdc on plate 1.
I understand reducing ripple in the PSU but what i dont get is why the 300 B+ is needed..
Wouldn't it be sufficient to say have a B+ of 150-200 and then use smaller dropping resistors between the caps in the PI filter?

The whole point of dropping resistors and the psu is to get a voltage needed from the power source, to the load source in a clean matter correct?
Why would it matter if you use a 220k plate resistor or 1k plate resistor so long as you have a well filtered, low ripple, clean dc voltage?

another thing...the zobel network...
what is that all about?
I know the tube needs some impedance on the input from grid to ground, but why the cap? Is that for a frequency stabilization or crossover?

What???

 

[shabtek]

I can't sleep
that v at the plate 55, 106 whatever is gonna go up and down when you waggle the grid right? so if you only had 107v it could only waggle up 1v and then clip and down 106ish v--not really down that low but you can see why you need a 'surplus' voltage over what the plate idles at`

The whole point of dropping resistors and the psu is to get a voltage needed from the power source, to the load source in a clean matter correct?
Why would it matter if you use a 220k plate resistor or 1k plate resistor so long as you have a well filtered, low ripple, clean dc voltage?

huh? power source/load source;clean matter------> <-------
dropping/filter resistor and plate resistor are 2 different things, I think you could use a lower B+ and a lower R in the power supply, but yer plate Rs need to stay the same cause Dave said so.
figure out the idle current , use ohmses law and determine what size dropping R you need for the given B+Voltage
my copy sayeth
R13 (dropping res.) =10kOhm 1Watt 300v in 235 out. 65 v dropped over 10000,
I get 6.5mA for 1 nyd1bottle.
don't know enuff to calculate from the load lines in the 12av7 data but I do trust NYDs indication that you want 235vdc at the top of plate resistors r3,r9
say you only has 250vdc (180vac rectified and filtered????) need to drop 15 quick- and @ .0065A: about 2300 Ohms

another thing...the zobel network...
what is that all about?
I know the tube needs some impedance on the input from grid to ground, but why the cap? Is that for a frequency stabilization or crossover?

what is the zobel network all about? if you have to ask...how do you know that is what it is? as far as I know a zobel network is a way to tailor the freq response of a given model input transfrmr. usually it tames ringing/improves square-wave response I think this is a pretty forgiving circuit even though I have only 1 complete---I used whatever parts I had that were close in value and it sounds really nice.
hope this helps

 

[PRR]

The tube can only pull. For many reasons, it normally pulls-down.

You need something to pull-up. Another tube (oddly wired), a choke, or... a resistor. Ten dollar, ten dollar, ten cents... guess which is most popular.

The tube is a variable resistor, NOT an audio voltage source.

The tube FIGHTS the resistor in a tug-of-war.

The range of resistance possible with a tube is limited. It will not go to zero ohms, only to many-K. It will go very high, but not easily.

The usual tube stage has the tube biased to around twice its minimum resistance, and a roughly equal resistor. Otherwise it isn't a fair fight; rather one side is loafing and we can't have that (electronics are traditionally expensive).

If the tube and resistor are roughly equal resistance, then the plate will sit roughly half-way down the supply voltage.

QED

 

[mrphotodude]

alright..that helps
thanks you

i have known that you can increase the plate resistor in guitar amps to drop voltage and increase distortion...
wasn't sure what was happening though.

i see everything in my mind in a linear fashion.
when i look at a gun i see an xray of the parts moving as i do most things..
but when you dont really know what is happening its hard to visualize.

 

[lernith]

The thing that has most harmed my understanding of electronics was my looking at stomp boxes when I was eight and thinking "Oh, so the opamps do the work and the resistors and stuff are kinda like adapters".  Seeing resistors and capacitors listed under "passive" didn't help, either.  I'm still recovering from this world-view, and that's why I love reading descriptions of how circuits work, even when it's something (to the gurus here) mundane.  It's the way of thinking that one needs to work on.  That's why some people can understand a circuit; even if it's something they've never seen before.
Resistors have this great trick of using current to define voltage, and that alone does all sorts of things.  In your head replace "passive" with "has exactly two connections in the circuit".
At any rate, from this sputtering take the idea that "passives" do a lot of work!

 

[pstamler]

And just to follow up on the Zobel...yes, it is for flattening the frequency response of the input transformer. Note that Dave tells you the Zobel values will differ with different transformers.

Peace,
Paul

 

[sodderboy]

The Zobel network is added to change the frequency response of any audio transformer.  Look at the 1073 for instance.  I always go back to the Neve 80 series for my Zobel learning.  If you could see an 80 series patchbay, there are resistors loading many of the output circuits.  The values are different- different lengths of mainframe wiring have different capacitance and thus require a different resistor value to achieve the desired frequency response.
Mike
PS: the little tube knowledge I have is from old ARRL handbooks and Navpers manuals.  I recommend ARRL pre 1970.

 

[Rob Flinn]

The Zobel network is used to reduce/damp ringing or overshoot in transformers.  It damps the response.

If you apply a square wave to a transformer when it is not damped properly then the response kind of wiggles on the leading edge.  The decay of the wiggle/ringing follows an exponential.  The zobel network damps/reduces this ringing.