Which cathode voltages to choose?

[shocki]

I am still reading Merlin Blencowes book and I am studying a lot of schematics. But there is one basic thing I just don't understand.
In the various blackface amps the voltages on the plates are different.

The smaller amps have less plate voltages on the preamp and output tubes.

Of course a tube has a certain maximum voltage setting that should not be exceeded. So a 6V6 needs lees plate voltage. This is what I understand.

The Plate voltage has nothing to do with the gain as far as I understand. So why are different plate voltages in used in the varies Fender models when everything else in the schematic is pretty much the same? Especially in the preamp sections?

[soundguruman]

The highest voltage determines how much headroom the amp has, not the gain of the amp.
You can set it for any amount of gain, but at some point you reach maximum, and that maximum is determined by the available voltage.
So, if you have 300 volts, the gain must stop at that point. It can't go higher. If you have 400 volts, then it can go that far only.
The tube has a maximum voltage, yes, but we exceed that all the time. We torture the tube, because that makes a cool sound as it is screaming for mercy.

Then if you make an amp that never exceeds the ratings of the tubes, you have a very dull sounding amp. OR a hi fi amp...

[shocki]

So, if you have 300 volts, the gain must stop at that point. It can't go higher. If you have 400 volts, then it can go that far only.

But don't you set the bias in a middle position anyway? So you set it the way it does not reach the 400 volts. As far as I understood the gain has nothing to do with the plate voltage in the first place?
So if you have an amp with a certain plate voltage you adjust the bias accordingly?

On the other hands in the Fender amps the leave the bias (Rk) the same in all amps and change the plate voltages. In this case you are right. This will mess up the bias and you will loose headroom.
But why don't they leave the preamp voltages the same for every amp OR adjust the bias point if they work with different voltages (for what ever reason) ?

Why would you like to have less headroom in a Deluxe Reberb than in a Super Reverb in the preamp tubes?

[soundguruman]

But don't you set the bias in a middle position anyway? So you set it the way it does not reach the 400 volts. As far as I understood the gain has nothing to do with the plate voltage in the first place?
So if you have an amp with a certain plate voltage you adjust the bias accordingly?

On the other hands in the Fender amps the leave the bias and change the plate voltages. In this case you are right.
But why don't they leave the preamp voltages the same for every amp OR adjust the bias point if they work with different voltages (for what ever reason) ?

The voltage determines the "maximum limit" of gain, not the gain itself.
Try using this to get an idea:
Adam's Amplifiers: Triode Calculator

[shocki]

In the calculator I cannot enter B+. So I don't see the effect.

But let me explain (in simple words) how I understand this.
Lets say we have plate voltage of 150V. The input signal is 1V. The Plate voltage rises to 200V. So we got a gain of 50.
Now lets take 200V and 1V input. The Plate voltage will maybe go also go to 250V. (which is wrong. but anyway)
So in both cases we have the same Gain. But in higher voltage regions.

So as we alread said the gain will be the same but you said headroom will be bigger.

I think I have some fundamental problem understanding what this means sonically. Cannot really mean "louder" since this would have to do with the Gain?

Maybe I used the wrong term when talking about plate voltage. I mean the B+ that goes to the plate.

[exclamationmark]

I don't really understand what you are trying to ask... Are you asking about the effect of the B+ voltage or the effect of the bias point? I'm going to assume it's the latter from your thread title. Anyway, the bias point can affect the linearity of the signal, as well as the maximum signal swing before clipping. For example if you pick a bias point deep into cutoff on a 12ax7 (say -3v with a B+ of 300v), and feed it a low level signal, the signal will end up very assymetrical, as well as having less gain than a hotter bias point. This is because the grid curves are somewhat 'bunched up' towards cutoff and somewhat irregular. A hotter bias point has the grid curves spaced further apart, as well as having the curves spaced more evenly. You can confirm this yourself by drawing a load line and seeing where the grid curves intersect it.

In practice I find hotter biased stages to be more gainy sounding, as well as having a fuzzier sort of sound. Cold biased stages tend to have less gain and a more mid centric crunchier sound. I don't really know what causes the apparent honkiness in cold biased stages, but I've seen it (heard it?) in both 12AU7s and 12AX7's, in both cathode biased and fixed bias configurations.

[shocki]

[QUOTE]

I don't really understand what you are trying to ask... Are you asking about the effect of the B+ voltage or the effect of the bias point?

Maybe my thread title was a big wrong. I am asking about the B+ on the different sections inside the amp. I understand how the bias thing for a given B+ works. But I dont't understand why Fender is using different B+ in their different models. To put it simple. All Blackface amps pretty much have the same circuit. The only thing that changes is the B+ (also in the preamp section)

[soundguruman]

If you have a higher wattage output, you need more B+ voltage.
If the wattage output is lower, you need less B+ voltage.
That's why the difference in voltages, of the same essential design.
One amp is 100 watts out, the other is 35 watts out.
But you are right the design is almost the same for both amps. Just higher voltage for more watts output.

I love the sound of EL34s screaming for holy mercy in the morning.
Smells like rock and roll.

[shocki]

OK. And why is Fender choosing exactly those b+ Voltages? How are those calculated?
I think with lower voltages you get more of a brown sound. But why did they choose this sound for their Deluxe Reverb and dont use those voltages in their super reverb also?

[soundguruman]

OK. And why is Fender choosing exactly those b+ Voltages? How are those calculated?

They don't choose an exact voltage. They base it on the hardware available, and how much they can afford to spend...
There will be large variations in the measured voltages, according to what tubes are installed, and what the bias is set at.
It is not unusual to find 20% or more variation in voltages. Also because of large variations in the AC power available.

The transformer must provide "X" amount of current, to deliver X amount of wattage, at the output.
The heaters of the tubes have specific voltage and current ratings. the transformer must supply "at least" that.

But as far as "exact" voltages, there isn't any exact. It's in the ballpark, that's all.
It's a guitar amp, not the space shuttle.

[shocki]

Thanks alot! Now it makes sense. Now I just have to know how to calculate the power of the amp for the given powertubes and voltage

[Tonewood]

So why are different plate voltages in used in the varies Fender models when everything else in the schematic is pretty much the same? Especially in the preamp sections?

shocki, I like this question since I don't know the answer & this made me think about it. But my thought is: As mentioned by soundguruman, headroom is increased with higher voltage. More headroom means the valve can take a bigger input signal before distorting. Bigger (louder) amps need more headroom. More headroom in the preamp will help keep things cleaner longer (louder?) while building the signal up for the power amp.

[soundguruman]

shocki, I like this question since I don't know the answer & this made me think about it. But my thought is: As mentioned by soundguruman, headroom is increased with higher voltage. More headroom means the valve can take a bigger input signal before distorting. Bigger (louder) amps need more headroom. More headroom in the preamp will help keep things cleaner longer (louder?) while building the signal up for the power amp.

By gummie, I think he's got it.
Now you are ready to start your own amplifier company.
remember, these amps can shock the crap out of you,
so, make sure you discharge the capacitors before you become a statistic.
(how do you think my hair became so curly?)

Welp, when I was 2 years old, mom told me to never stick a hair pin into a power receptacle.
And the more she told me this, the more anxious I became to find out why(?)
And finally, I did. That's when I discovered what 'electricity' was.
(or, what it would do, more accurately) (nobody actually knows what electricity 'is,' but we know what it 'does.')
(Tesla claimed that electricity is a disturbance in the space-time continuum, and I have to agree)
And so ever since then, I was hooked on it.
Now I'm going to re-invent it, but until then, don't shock yourself like I did.
(it makes a disagreeable burning smell)

[Steve Conner]

By gummie, I think he's got it.
Now you are ready to start your own amplifier company.

When the lawsuits start coming in, just remember who tutored you, the mighty SGM! Maybe start by modifying Tube Screamers with a chisel first.

(Tesla claimed that electricity is a disturbance in the space-time continuum, and I have to agree)

Me too, except actually it was Maxwell who claimed this and Einstein made it look respectable later.

[soundguruman]

When the lawsuits start coming in, just remember who tutored you, the mighty SGM! Maybe start by modifying Tube Screamers with a chisel first.


Me too, except actually it was Maxwell who claimed this and Einstein made it look respectable later.

As long as (douchebag) Edison was not involved, it's OK with me.
and thanks for reminding us of Maxwell and Einstein, they are cool dudes too.
I like Tesla the best because he was a hands-on type of dude.