Gain stage question?

[Rod]

Hi!

In a gain stage with a cathode resistor of 1.5K, what adds more gain?, increasing the plate resistor from 100K to 220K, for increasing the cathode bypass capacitor from 1uF to 22 uF?

with kind of frequencies will i get by increasing the plate resistor from 100K to 200K? and what are the side effects?
also what will be the side effects and frequencie results from increasing the cathode bypass capacitor from 1uf to 22 uF?

[nickb]

Here, you can find out for yourself: Cathode Bypass Capacitor Calculator

[loudthud]

The "Amplification Factor" that you will find on tube data sheets is basically the gain you would get with and infinite plate resistor and a fully bypassed cathode resistor. You can find the gain for a specific plate resistor by plotting a load line on the plate curves of the tube. The frequencies below those where the bypass capacitor on the cathode ceases to be effective will have reduced gain. The easiest way to take everything into account is to use a modeling program.

[Jazz P Bass]

I don't see the plate resistor change as frequency dependent.
Now the cathode cap.
That is a frequency mod.
A typical Fender first tube circuit will have a 25uf bypass cap.
This will 'pass' most all frequencies.
A smaller value cap will 'pass' progressively less bass.

[Chuck H]

The best explaination can be had using the calculator above. The cathode bypass cap will not change the gain potential, but will adjust which frequencies realize the gain potential. The plate resistor will effect the overall gain potential, which won't be realized by all frequencies without a cap value that allows full bypass.

Probably the easiest explaination would be to say:

With a 100k plate load the stage has a gain of about 30 with an unbypassed cathode resistor. Any frequencies passed by a cathode bypass cap will have a gain of about 60. If fully bypassed the gain is 60 at all frequencies.

With a 220k plate load the stage has a gain of about 46 with an unbypassed cathode resistor. Any frequencies passed by a cathode bypass cap will have a gain of about 75. If fully bypassed the gain is 75 at all frequencies.

[soundguruman]

Lots of people will tell you that adding the cathode bypass cap does not increase the gain...
but it does, overall about 30 db. (between cap and no cap)

It causes a HUGE increase in the sensitivity, and low frequency extension.

Compare 1 uF to 22, the boost comes at about 500 cps and below, and makes a really big difference.
Maybe not to a tech, but to a guitar player it does. The difference between "thin and fat."
and there is such a thing as too fat, where the sound comes muddy

Changing 100K to 200K will give you some gain boost BUT,
the problem being is that the tube can only have so much gain before it breaks into oscillation.
as you increase the gain, the resistor drops more voltage across, after that, the system breaks down at some point.
the tube will squeal like a little piggy, when it reaches some threshold.

There is all kind of squelch controls, that work by feedback or limiting the frequency response, etc...and mechanical damping, to accomplish more gain without oscillation.
you can crank it higher without the squealing...until it's over-driving like a chainsaw...etc...
but the art in all this is to do that without making the amp sound muddy or noisy.

and so there are a number of fixes to suppress the hiss, noise, oscillation, microphonic tendencies of a high gain circuit.
you cannot boost the gain without compensating for the increased noise, etc...

The first thing to understand is that at the input there is some amount of noise.
This noise will be amplified thousands of times, and it will be delivered to the speakers, just like the guitar is delivered.
and so, how are you going to keep as much noise as you can, out of the audio?
the noise, hiss, mud, squealing, oscillation?
That is the 'art' part of the equation.
that is why it's not as easy as it appears - to boost the gain of a guitar amp.

It's also going to spill over into your guitar.
The pickups, the wood, the other hardware and the wiring.
your guitar is a microphone, at some threshold of gain.
And it will also squeal at some point in the audio chain.
In fact I really think that the guitar and preamp squeal in concert.

[Jazz P Bass]

An unbypassed cathode resistor actually develops negative feedback (between the cathode & grid).
So a 'bypassed' cathode resistor must have more gain.

[eschertron]

I know this post is about preamp stages, however, my one experiment with bypass caps is on the 6V6 of my little Fender Champion 600. The theory is pretty much the same. I took out the 22uF cap and initially replaced it with a 1.8uF cap (very similar to the OPs values). The smaller cap [filtered out / reduced compensation for / allowed NFB for] bass frequencies well into the lower midrange, reducing the apparent overall volume (gain) of the amp and making it a whimpy, thin-sounding amp. The second cap I tried, 8uF IIRC, did much better at eliminating the farty sounds of the stage while keeping the overall sound and gain intact. So for the OP, replacing a 1uF cap with a 22uF will make a noticable difference in loudness and bass response, just like Jazz P Bass said.
The technical info I've read on bypass caps says the difference between unbypassed and fully bypassed is 6dB at all freqs, or a perceived volume difference of about twice as loud.

As a tangent, this discussion has been all about sine wave signals. What if we substitute the word 'clipping' instead of 'gain'. Does that change the way you feel about plate resistor choice? I'm asking because as a novice, I've looked at many preamp stage designs and seen a lot of them that are identical across manufacturers, with a few variations. Is it all about where the load line falls?

[nickb]

I know this post is about preamp stages, however, my one experiment with bypass caps is on the 6V6 of my little Fender Champion 600. The theory is pretty much the same. I took out the 22uF cap and initially replaced it with a 1.8uF cap (very similar to the OPs values). The smaller cap [filtered out / reduced compensation for / allowed NFB for] bass frequencies well into the lower midrange, reducing the apparent overall volume (gain) of the amp and making it a whimpy, thin-sounding amp. The second cap I tried, 8uF IIRC, did much better at eliminating the farty sounds of the stage while keeping the overall sound and gain intact. So for the OP, replacing a 1uF cap with a 22uF will make a noticable difference in loudness and bass response, just like Jazz P Bass said.
The technical info I've read on bypass caps says the difference between unbypassed and fully bypassed is 6dB at all freqs, or a perceived volume difference of about twice as loud.

As a tangent, this discussion has been all about sine wave signals. What if we substitute the word 'clipping' instead of 'gain'. Does that change the way you feel about plate resistor choice? I'm asking because as a novice, I've looked at many preamp stage designs and seen a lot of them that are identical across manufacturers, with a few variations. Is it all about where the load line falls?

Hi eschertron,

The Champ uses 470 ohm / 25uF so the design -3db frequency due to this is 13Hz. 1uF comes out at 340Hz and 8uF is 42Hz. Those numbers tie in with your observations and is something that is worthy of note. For a preamp stage the cathode resistor might be 1.5K ohm so to get the same frequencies we have to reduce the capacitance by the same ratio i.e we get 2.5uF instead of 8uF.

On plate resistance choice, there is more to than just gain. This often chosen to maximize the dynamic range. It sets the quiescent voltage on the plate. If you make it too high you will reduce the range and drive the tube into cut off. On the other hand too small, the range is again reduced as you are are limited by the inability of the preceding stage to supply the necessary grid current. IMHO, this is where some of magic comes from. The choice has a big effect of the sound of the amp in overdrive. Mess too much with the gain of the stage and you might not be able to get satisfactory overdrive at all.

Nick

[eschertron]

Thanks, Nick!
I'm setting up to breadboard a 12AX7 stage soon, so your design observations are greatly appreciated!

[Chuck H]

What happened to Rod???

Incidentally... Since we're starting to complicate the issue anyway. IMHE there will also be a bias shift using the higher value plate load resistor. The bias will shift hotter. This not only changes the dynamic range but also the nature of the clipping. The gain does indeed go up but, since it was left at that, performance also changes.

[soundguruman]

As long a flames spit out of the speakers, you know it sounds hot.

[J M Fahey]

Another truckload of manure.
Oh well, where did I leave the shovel?
The BIG one.

Lots of people will tell you that adding the cathode bypass cap does not increase the gain...
but it does, overall about 30 db. (between cap and no cap)

The op did not mention *adding* a cap but *increasing* its value, from 1uF to 22uF , a very different thing.
30 db? **Thirty*** ?? Not in your wildest dreams !!!!!

And please, link to somebody who said that: "adding a cathode capacitor does not increase gain"
C'mon, you said "lots".

Compare 1 uF to 22, the boost comes at about 500 cps and below, and makes a really big difference.

Humbly suggest you do your Math.

If too dificult, just use:
Cathode Bypass Capacitor Calculator

You'll easily see that with a 1uF cap the -3dB point lies at around 130 Hz and with a 22uF cap, well, 22X *below* that, to the musically important (pun intended) frequency of 6 Hz.

Did 500 Hz come out in any of these calculations? .... no? .... I thought so.

Now to gain "with cap" vs. "gain without cap".

Using 22uF to make the difference stand up more in evidence:
With 22uF: 35.5 dB
Without cap: 29.5 dB
Grand total, meaning MAXIMUM capacitor related gain difference: around 6dB or 2:1

Where's your 30 dB?
Can't find them.

Changing 100K to 200K will give you some gain boost BUT,
the problem being is that the tube can only have so much gain before it breaks into oscillation.
..............................
the tube will squeal like a little piggy, when it reaches some threshold.

Absolutely FALSE.
A tube can have any plate load value, end to end, and it will not oscillate by itself because of that, which is what you are saying.
C'mon, plug a 12AX7 in a socket, get one triode with 1k5//22uF to ground grid to ground with 220K to 1 M, and a 1M trimmer to +B and make it oscillate.
Touching the grid with a finger is considered cheating, by the way.
Twisting a grid connected wire to a plate connected wire or turning a 1KW SSB rig within inches is *also* considered cheating.
[/QUOTE]

Will you PLEASE stop posting mad ideas and wildly inaccurate (that's an understatement) data?
The OP came to this Forum in good faith, trusting us.
You are confusing and driving everybody away.
What's the point?
Do you gain some pleasure out of this?

[soundguruman]

Another truckload of manure.
Oh well, where did I leave the shovel?
The BIG one.


The op did not mention *adding* a cap but *increasing* its value, from 1uF to 22uF , a very different thing.
30 db? **Thirty*** ?? Not in your wildest dreams !!!!!

And please, link to somebody who said that: "adding a cathode capacitor does not increase gain"
C'mon, you said "lots".


Humbly suggest you do your Math.

If too dificult, just use:
Cathode Bypass Capacitor Calculator

You'll easily see that with a 1uF cap the -3dB point lies at around 130 Hz and with a 22uF cap, well, 22X *below* that, to the musically important (pun intended) frequency of 6 Hz.

Did 500 Hz come out in any of these calculations? .... no? .... I thought so.

Now to gain "with cap" vs. "gain without cap".

Using 22uF to make the difference stand up more in evidence:
With 22uF: 35.5 dB
Without cap: 29.5 dB
Grand total, meaning MAXIMUM capacitor related gain difference: around 6dB or 2:1

Where's your 30 dB?
Can't find them.


Absolutely FALSE.
A tube can have any plate load value, end to end, and it will not oscillate by itself because of that, which is what you are saying.
C'mon, plug a 12AX7 in a socket, get one triode with 1k5//22uF to ground grid to ground with 220K to 1 M, and a 1M trimmer to +B and make it oscillate.
Touching the grid with a finger is considered cheating, by the way.
Twisting a grid connected wire to a plate connected wire or turning a 1KW SSB rig within inches is *also* considered cheating.

i see forum bully is in attack mode again.
He does not appreciate what a guitar player would experience,
after changing this cap.
I mean there are techs, and there are techs who play rock and roll.
It's hard for a tech to understand what a guitar player wants.

Yes the data stated is fully correct, for a guitar player.

Many amps rely on switching that cap in and out of the circuit.
Including Fender and Mesa Boogie.
This changes the amp between clean and overdrive...

But a tech really does not fully grasp the concept, until the tech learns how to play rock and roll.

So, I hope he gets counseling.

[Chuck H]

Let's just clear up a couple of things (for the sake of keeping the peace )

I'm sure SGM didn't mean "dB". I'm sure he meant gain factor. I think this because if you replace dB with gain factor in his post then he's reasonably accurate.

Another point in defence of SGM is that with only 6dB difference between cathode bypassed frequencies and unbypassed frequencies the -3dB point becomes a less significant measure. The audible effect may well be closer to 500Hz in his assesment.

I am apt to back Juan on almost any point. And WRT rock or rock players I don't know if anyone here (other than Bruce Collins) has more interaction or has sold more product relative to rock than Juan. And All of his own design (and construction). But Juan. Read my above points. You know where I'm coming from.

Now... SGM... Your post was factually inaccurate and vague enough in areas to elicit correction. No doubt about it. I thought about doing it myself but decided that it would probably do as much harm as good in the scope of things. So...

JM, please recognize the validity where it exsists and make corrections like "I think you mean gain factor and not dB". It's a lot more palatable for everyone.

SGM, please accept that there are some really ef'in smart people here. Wayyy better at this stuff than you and I. If you learn to speak in turn, don't go over your own factual understanding and accept corrections gracefully this forum will be the greatest thing that could happen for you. That is my experience. Don't ever let ego get in the way of personal improvement. It demonstrates limitation. Do you see yourself as limited???

I can understand the urge to become defensive when someone is not only corrective, but also insulting and/or ignoring things that are obviously typos or minor misunderstandings. But if even one party in the dispute is graceful things always seem to work out for the best. And I mean for everyone. Including peripheral readers like me.

JM2C