Gain stage question?

[soundguruman]

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

Sometimes I fear all the rage, built up in Fahey's skull, will pop out his ears.
Are you referring to dbmw? dbmv? or actual db measured SPL? Those terms are quite oft confused.

[Chuck H]

You shouldn't get semantic in an attempt to save face. It illicits correction and eventually argument, like, "which dB reference would have been accurate in your post above?"... See, I suddenly found myself being sarcastic and shitty. Don't let ego limit personal growth. Smart friends are great unless one has a personality such that they need to be the center of attention. Smart, nice friends are even better. This can be a very friendly place. There's always some poking, jibing and the occasional drink induced smear. Think of it like a bar and we're all just having a beer.

[pdf64]

'Sometimes I fear all the rage, built up in Fahey's skull, will pop out his ears'
Reading post #6, I fear a similar fate.

'Are you referring to dbmw? dbmv? or actual db measured SPL?'
Please see Decibel - Wikipedia, the free encyclopedia

'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)'
As the dB value was stated without suffix, and made in the context of a voltage amplifier, the only reasonable reading of this is dB of voltage gain.
However, I agree that the cathode bypassing arrangement of a gain stage (especially one subject to being overdriven) have a greater impact in terms of user perception than the numbers suggest; this may be due to the change in clipping character according to the degree of bypass.
Pete

[Steve Conner]

Juan may be the hot-headed Latin type, but at least he actually has something inside his skull. %-D

With the usual range of cathode resistor values, the difference between bypassed and unbypassed (hence also the boost provided by a cathode bypass cap) amounts to about a factor of 2 to 3: 6 to 10dB. You could get a 30dB boost by making the cathode resistor extremely large, like in the cold clipping stage of a SLO, but a cathode bypass cap is never used in this stage, that I know of.

A small boost can have a big subjective effect if it covers a large frequency range. The cathode bypass cap boosts everything above its turnover frequency, which is quite a big range.

[Chuck H]

HE MEANT GAIN FACTOR! NOT dB! He just won't admit it because the mistake is in print and he's been called on it. Though I think SGM would like Steve's sig.

[Steve Conner]

Even if it was gain factor, it would still be wrong. \[O_o]/

[Chuck H]

Not really. If an unbypassed 12ax7 triode has a typical in circuit gain of about 30 and the same stage with a bypass has a gain of about 60 that would acount for the number "30" as the increase. But it ain't 30dB.

[Steve Conner]

What?! If the gain was 30 and it goes up to 60, it has gone up by a factor of 2.

New SGMtubes, now with 30 extra gains! But watch out, if you pile on too many gains they squeal like piggies.

[tubeswell]

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?

Fully bypassing the stage (with 25uF) produces more overall gain than increasing the plate resistor to 220k (the later option increases output impedance quite a lot if the stage is not bypassed, and this affects the gain that can be 'passed-on' to the next stage)


And when you bypass the Ra=100k, Rk=1k5 stage with 1uF, you get the 'mid-treble boost' effect:

And just for completeness, when you bypass the Ra=220k Rk=1k5 stage with 1uF (if the 220k stage is fully bypassed it will have the most gain of the lot, but I didn't bother doing that pic)




There you go

[Chuck H]

original:

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)

edit:

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

See? Just remove dB and it makes sense. I automatically removed it as soon as I read it. Assuming it was a mistake. Not 30 times. Not 30 log. It shouldn't be 30dB. Just 30. The gain factor goes from 30 to 60. For a gain increase of 30. Yes that's a factor of 2.

If we're all going to dogpile SGM let's just remember what a dogpile is made of

Anyone reading SGM's posts could guess that he knows at least enough that he can't think that a bypass cap increases gain by 30dB. Did ANYONE read that and actually think that SGM believed that a bypass cap increases gain by 30dB? Or are we just using his error as an excuse to be mean? Don't get me wrong. I absolutely think a correction is in order. But we're past that and on to something else, aren't we? On that note...

Gary just popped back in after a long time away. His post was concise and relative. And R.G. made it a point to key in and tell him to play nice. I hope he does. But it's at least a little antagonistic. This is going to offend, I'm sure, but Gary and SGM both have issues here. And maybe elsewhere. But we could still handle misinformation from either of them in the same way we would anyone else. Until they ask for it. Then the gloves are off. But each incident should be handled on an individual basis. If for no there reason than avoiding flame threads. At the very least you can say they were asking for it. I'm just sayin'. It doesn't impress me when the people I respect here come off as the antagonist. Making a mistake isn't antagonistic. Even being wrong isn't antagonistic. Being snide, short, insulting or otherwise discourteous before it's warranted is antagonistic.

Rant over.

[soundguruman]

So, to get back to what I was doing earlier...

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?

Either one or both.
They both add more gain. But usually that plate resistor is kept at 100K or less, because there is such thing as too much gain in one stage.
If you already have 1 uF, changing it to 22uF is going to give a nice boost in lower mids and bass. This is the boost that many guitar players like.

[leydenjar]

I love this place!