Anything to increase pre-amp gain?

[mykey]

That thing should get way distorted in the drive mode. Something is definitely not working correctly.

you are absolutely correct, it should grind without modification; thereby we should suspect
A. weak preamp tubes- JJ preamp tubes are great for low gain, Chinese preamp tubes are great for high gain. I further suspect that Electro Harmonix 12AX7 is a re-branded Chinese tube. I am always baffled why players refuse to buy new tubes until the ones installed go 100% failure.

B. There is really something wrong with the switching circuit or something has failed in the preamp itself.

How to(?) add more gain was the question, not "whats wrong with this amp?"
so we talked about how to add more gain. Maybe we should talk about whats going wrong instead.
None the less a very good point you have there.

[mykey]

...but this still runs counter to my understanding as outlined in my earlier post. When you say the signal will increase overall, if you mean by virtue of the fact that there will be more bass in it, then I get it... but what I don't get it is how there will be more treble in it. I thought that RC networks had a frequency threshold, and that increasing capacitance in the normal stage-coupling arrangement simply extended that threshold downwards. What is it that as it were turns up the treble?

A .1 coupling cap passes more signal than a .01 cap. if you subtract the RC network- that signal is essentially flat response, not just bass. Depends on the RC AFTER the stage output that determines bass or treble, not the stage itself.
So if I look JUST at the stage, its an overall boost. If I add RC that comes AFTER the stage then it MIGHT boost the bass, depending on what the RC is.
It's the series resistance, the high pass cap (100-500pf) thats going to determine if i get bass or treble boost. If I add more series resistance then a high roll-off occurs.
Then more important, I don't want to overload the NEXT stage with too much signal at the grid, cause mud will result.
The interesting thing here is that the larger the coupling cap, the more DC will appear on the next stage grid. Remarkable that the coupling cap does not block DC entirely. As the audio signal rises, so does the DC at the next grid, and with NO signal, the DC reduces to minimum. (this is a very small millivolt DC which typically can only be measured with a scope probe).
And DC on the next grid will cause SUSTAIN, so I want to give this grid just a DASH of DC and not TOO much. If it gets TOO much, it sounds like a fart in a bath tub. If I go beyond and give the next grid a whole bunch of DC, oscillation motor boating occurs. Hence the .01 coupling cap instead of .1.
This is the fine art of getting sustain without the MUD. (mesa boogie has not learned this)....
As you noticed, there is no formula for sustain- but we all want it. So I have just explained the METHOD. (you can call it a pseudo science)
And even deeper than that- what we really are after is sensitivity, not distortion. 1/2 millivolt to drive amp to full output is much better than 1 millivolt. But what we are trying to do is boost the stages to maximum sensitivity without unwanted oscillation and noise. (on the brink) and what happens as a side effect of sensitivity IS "desirable distortion".
It may take some consideration to realize that maximum sensitivity results in more desirable SUSTAIN and distortion.
That is almost too much and another thread is needed to discuss SUSTAIN, and sensitivity which is really the most important factor in a guitar amp.

[Steve Conner]

WTF is going on here? Everyone knows that the value of the coupling cap mainly affects bass. The coupling caps in vintage amps are mostly big enough to pass everything down to the lowest note on the guitar anyway. So making them bigger won't make the amp more sensitive.

My own personal "Secret of sustain" was undersized coupling caps and undersized cathode bypass caps, or even no cathode bypass caps. I think the last time I messed with this stuff, trying to get the most ultra saturated tones I could, I ended up with 3900pF coupling caps and 4 cascaded gain stages.

It turns the amp into a high-pass filter, which rejects the stuff that Mykey calls "fart in a bath tub" and emphasizes the harmonics to make it really grind and squeal. It also gets rid of the time constants that would cause blocking distortion.

[drewl]

mesa did learn that and uses some pf value coupling caps.
And EH tubes are not made in China, but Russia.

[Alex R]

Oh I like to run with things a bit, you never know what you hear, next thing guitarists will walk in and ask me to perform mods based on some new notion that's floating around the forums.

Can't really figure out what mykey means with his subtracting RC networks (tone controls) from RC networks (which coupling caps are part of) I'm afraid - right over my head it flew & I didn't even feel the breeze of its passing - but evidently there's some chat around about how a bit of DC getting through coupling caps along with the the signal might cut down the bias on that stage temporarily and thus increase gain. I'd have thought that would give a bit of expansion, and cut sustain, if it happened at all, so I don't see where mykey's sustain comes from, but I guess I'm jus' plain ign'rant.

[Enzo]

Or someone likes the way an old beater amp sounds, finds there are leaky caps, and then rationalizes how the leaky caps MUST be the holy grail. I would take Mykey's explanations with a grain of salt.

[Alex R]

I would take Mykey's explanations with a grain of salt.

Improves the flavour considerably.

[mykey]

WTF is going on here? Everyone knows that the value of the coupling cap mainly affects bass. The coupling caps in vintage amps are mostly big enough to pass everything down to the lowest note on the guitar anyway. So making them bigger won't make the amp more sensitive.

My own personal "Secret of sustain" was undersized coupling caps and undersized cathode bypass caps, or even no cathode bypass caps. I think the last time I messed with this stuff, trying to get the most ultra saturated tones I could, I ended up with 3900pF coupling caps and 4 cascaded gain stages.

It turns the amp into a high-pass filter, which rejects the stuff that Mykey calls "fart in a bath tub" and emphasizes the harmonics to make it really grind and squeal. It also gets rid of the time constants that would cause blocking distortion.

I think you may have read the post a little to quickly. You seem to be in a race to criticize rather than consider the fine points of what was written. You should be testing the circuit before making such hasty declarations. There is a lot of truth and wisdom contained in that post I made; disregarding what "everybody knows" in favor of careful independent observation.
The sooner we make our own observations instead of relying on assumptions, the quicker we will forward the art of the guitar amp. Whether you care to make an independent observation or just keep doing what everyone else does is a matter of personal choice.

[siggo]

I have a bluesdeluxe too, with an american fender strat.
I think tone is great but to have a bit of overdrive I should turn Drive pot at maximum and keep a general volume not too low.
I think you can play with preamp valve to find your preferred ones but modifyng circuit (on a PCB) is something that I did not on my amp (especially if you think to sell it some day).
I put a pedal in front of it; there are a lot and if you want to play with electronic you can do it yourself (save money, have fun and voltages are not dangerous).
There are also pedals using tubes.
Ciao.

[Jag]

The sooner we make our own observations instead of relying on assumptions, the quicker we will forward the art of the guitar amp. Whether you care to make an independent observation or just keep doing what everyone else does is a matter of personal choice.

How have you verified your observations? Did you do background research to establish support for your observations or just say "this is what I've observed therefore my assumptions are correct"? If you've got third party data to back up your observations, it might benefit you to present that information as well.

I have no doubt that you have made these observations and have come up with theories. I’m just not sure you’ve done due diligence in ascertaining that your theories are correct. You seem to "get out there" in your explanations.

I'm not saying you are right or wrong but, to me, you seem to take a lot of left turns. Maybe you can show the rest of us where we are going wrong?

[mykey]

How have you verified your observations? Did you do background research to establish support for your observations or just say "this is what I've observed therefore my assumptions are correct"? If you've got third party data to back up your observations, it might benefit you to present that information as well.

I have no doubt that you have made these observations and have come up with theories. I’m just not sure you’ve done due diligence in ascertaining that your theories are correct. You seem to "get out there" in your explanations.

I'm not saying you are right or wrong but, to me, you seem to take a lot of left turns. Maybe you can show the rest of us where we are going wrong?

Ive built amps for 33 years, lets see YOUR "third party data". The reason you would not build the circuit yourself and test it is because you DON"T want to know, afraid that someone else is right.
The only thing that you are doing wrong is not observing for yourself. Never question the status quo, you will be unpopular, and popularity instead of knowledge is a bad thing. Everybody knows that too.

[Alex R]

Well mykey the trouble is, it's hard to get you to respond in detail and with support to back up your assertions so people can understand them and judge their validity. 'Truth and wisdom' are high ideals to bring to amp design, but good luck in your quest. In the meantime your readers are not obliged to accept their presence in your design suggestions without some illustration.

For instance I had not heard before about the business of coupling caps leaking dc onto grids along with rises in the signal. OK, that might be interesting. You said that this would create sustain, which I understand as a product of compression. But it seems to me that the dc leaking along with the signal would do the opposite - bias the grid more positive and increase gain along with the increase in signal, then cut the gain down as the signal decreased, steepening attack and decay curves and creating audio expansion rather than compression. I can't see the sustain in there. But it's hard to get you to apply your wisdom to such details.

[Jag]

What claim have I made that you would like data on? That you take left turns? Read your posts objectively and recognize the fact that many people here have expressed speculation regarding what you have posted. That's my third party data.

In case you can't recongnize the obvious, my post was to express my confusion regarding your posts and to request clarification. We do that around here. I was not trying to be antagonistic.

In other words, maybe I DO want to know. I might actually be afraid someone else is wrong.

I will state one thing FOR SURE. You have no grounds to make ANY statement about what I am or am not observing, I've made my observations CLEARLY.

[wizard333]

First, lets examine the question asked: How can I increase pre-amp gain?

By this question, we can assume that the person asking it is not an expert on the subject, they are looking for a quick, easy way to noticably improve their pre-amp gain.

Then lets look at your assertion Mykey, that larger cathode bypass caps result in larger gain. Now, they certainly result in a frequency response that dips further into the low frequency range. But more gain? If by that you mean more overall signal, then yes. Is it "more gain" in any meaningful way for a quick and noticable way to increase pre-amp gain, which is what this guy wants? NO. Its a fine technical point, more overall signal is by definition "more", but thats not what this guy is looking for.

Adding a cap vs not adding a cap will make a noticeable difference. The overall gain of the tube as set by the cathode and plate resistors and the relationship thereof, can be increased by an appropriate change in those. That will be noticable.

But larger caps? If they add any more gain in the usable frequency range, its certainly nothing close to a linear relationship, or even much noticeable, and the "more gain" benefit can easily be overwhelmed by the muddy bowl of crap your tone will be if you add a bunch of low end to a high gain circuit, not to mention power robbing low end oscillation you could end up with.

If what you say is true, the 'non-bright' channel on an old Plexi, with its HUGE cathode bypass cap, should have WAY more gain than the treble channel. Heck with a cap that big, it should have more gain than god. It doesn't. Modern high gain circuits that dont require bass controls to be set to nearly 0 to sound right should all have huge cathode bypass caps then right? They dont, they are typically small in value.

Don't take my word for it. Get a bunch of 220uf caps and put them in your amp as cathode bypass caps and crank it, see what you get. Compare it with what you had before. Was it an improvement in gain?

Consider your audience and who asked the question when you make claims like that. The guy that asked it isn't interested in a 'how many angels really fit on the head of a pin' sort of picky technical issue, or something that may 'show up on a scope' but not be heard by the user.

If he went and put a bunch of big bypass caps in that amp based on what you said, he wouldn't get what he's looking for. I know it, you know it, so why bring it up.

[siggo]

Interesting link

Here's:
http://www.duncanamps.com/technical/bluesdeluxe.html

I did'nr tried that, but seems reasonable.