Anything to increase pre-amp gain?

[sexafone123]

Hey y'all,
I am using a fender blues deluxe, which i am in love with for jazz and blues, but sometimes I REALLY wish i could get some more gain on the drive channel, because even with the drive all the way up, It barely gets distorted at all. Should It be getting more distorted than it is? if the answer is no, is there any way to get more of the same gain that the drive knob gives? any answers would be appreciated.
Kemp

[Enzo]

Well, you might always consider a pedal out front. A bazillion of them are available and players use them everywhere.

Looking at the circuit, this is really a one channel amp. Switching "channels" just switches the various volume controls in and out and messes with level a little. I don't really see a lot of opportunities to gainify this thing without affecting the clean as well.

But that is just my opinion.

[Alex R]

You can usually get some crunch out of those amps though, I'm surprised if there's none at all. Might be worth a new set of preamp tubes if you haven't tried that already. The ones with two parallel shiny metal bands inside (whatever is stamped on them) are I think Chinese and often go weak on gain after a while. Same with older Sovteks. The JJ ECC83S is a good and cheap tube to try. To save cash just buy one and substitute it for each the old ones in turn.

[trevorus]

I second the notion of popping in some JJ preamp tubes. Those things are great.

[rf7]

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

[Euthymia]

I'm in agreement with those who say to drop in a set of new preamp tubes, but my choice for people who want more gain/crunch is the Electro-Harmonix 12AX7EH.

The JJ's are indeed great, but the EH's also sound great and tend to have more gain, IME.

[mykey]

Hey y'all,
I am using a fender blues deluxe, which i am in love with for jazz and blues, but sometimes I REALLY wish i could get some more gain on the drive channel, because even with the drive all the way up, It barely gets distorted at all. Should It be getting more distorted than it is? if the answer is no, is there any way to get more of the same gain that the drive knob gives? any answers would be appreciated.
Kemp

yes you can change the values of cathode bias resistors
(on preamp tubes)
install electrolytic cap across those resistors
(most popular being 25 mfd)
install Chinese 12AX7 which have much more crunch
and gain. all of the above can be used to make the amp
grind like a chainsaw.
most people discount the value of using the right preamp
tubes, however it makes a huge difference.
another factor being the output bias, when adjusted too
cold it takes the balls out of the amp.
the hotter the better.
wait -what kind of guitar and pickups? thats a major factor.

[wizard333]

IIRC, those do not have an adjustable bias, and are biased pretty hot at the factory. The wrong rated output tubes could be an issue there, since they design those non adjustable biases for "medium" ratings; tubes at the extreme ends would throw that way off.

Any of what was mentioned above will add gain; the easiest being adding a cathode bypass cap where one currently doesnt exist. 25uf would be the common value for a Fender clean channel, but for a high gain channel it may get muddy with that unless you turn your bass WAY down. The value of the cap wont affect the gain but will affect frequency response; on a high gain channel I'd try a small one first, 5uf ish, then if you think the low end is anemic go bigger; if its muddy go smaller.

High gain affects the low frequencies first, on high gain circuits you have to limit bass response to prevent mud, knowing that the circuit will make up for it. Hence why when you turn an old Plexi (some had 200uf cathode bypass caps on the non-bright channel!) or an old Fender Bassman, etc, up to 8 or 10 to get crunch, the bass control has to come down to 2 or below to keep things tight. Modern high gain circuits limit the bass response a lot on the front side to prevent that.

For fun sometime, stick a 12AX7 in a 50 watt bassman phase inverter slot instead of the 12AT7, turn the bass to about 1 and the volume to 10, treble way up there, and see what an old 'clean' amp can do.

[mykey]

OK, the bypass cap value DOES affect the gain -a smaller bypass cap will give LESS gain, a larger, MORE. And this gain WILL affect the high frequencies as well as low.
And if you are stuck with non adjustable bias you can install a bias adjust pot, no problem. ALL tube amps should have this.
Tweeking the cathode bias resistor ( and installing bypass caps) in the preamp makes a HUGE difference.
Using better preamp tubes also makes a HUGE difference.
There is no universal modification, experiment to find the right values for your playing.
Humbucking pickups make a big difference, single coil pickups have much less output level. Therefore in trying to get grind from your amp, you are better off with humbucking pickups.
I don't give a shit for fender pickups, they are wimpy. Gibson rules.

[Alex R]

Larger bypass caps = more gain is in my view not an especially helpful point to make. Larger bypass caps will let more low frequencies through. Whilst this might be said to be an increase in gain across the frequency spectrum, it comprises a boost in exactly the wrong frequencies for good overdrive sound, as wizard333 explained. Musicians will often register an improvement in bass ('tightening') if smaller bypass caps are fitted - this is because subsequent stages and the speaker are not being overwhelmed by too much bass and producing muddiness instead. I think these are fairly well-established ideas.

Forgive me for issuing corrections but I am aware that this forum is archived and often used as a general resource; it's worthwhile trying to get things right. So if I'm wrong, correct me.

[mykey]

Larger bypass caps = more gain is in my view not an especially helpful point to make. Larger bypass caps will let more low frequencies through. Whilst this might be said to be an increase in gain across the frequency spectrum, it comprises a boost in exactly the wrong frequencies for good overdrive sound, as wizard333 explained. Musicians will often register an improvement in bass ('tightening') if smaller bypass caps are fitted - this is because subsequent stages and the speaker are not being overwhelmed by too much bass and producing muddiness instead. I think these are fairly well-established ideas.

Forgive me for issuing corrections but I am aware that this forum is archived and often used as a general resource; it's worthwhile trying to get things right. So if I'm wrong, correct me.

you're wrong about the bass, it's an overall boost, not just bass.

[Alex R]

you're wrong about the bass, it's an overall boost, not just bass.

OK, lets try and get this straight - what we're talking about here is the extent to which a change in coupling cap value will affect (a) overall signal level, (b) EQ. My understanding in layman's terms (I'm a layman and proud of it) is that the slower charge rate of increasing coupling cap values will allow lower and lower frequencies to pass through the stage. As I understand it caps pass no DC, and pass AC according to the rate at which they are able to charge and discharge given their value and the relevant resistance in the circuit. So in my little world the principal effect of an increase in coupling cap values is a concomitant increase in lower frequencies passed by the stage. In the sort of circuits we commonly see in guitar amps with the kinds of resistances therein, with coupling cap values in the ballpark range max 50pf to 1uF those changes in frequencies passed will be audible.

An example: going from .01 to .1 into the PI of a typical Fender 2 x 6L6 amp will produce an audible increase in lower frequencies, I would say (and my experience tells me). Are you saying that the principal effect of such a change is rather an increase in gain at all audible frequencies? And that this is a good way of producing a decent power amp overdrive? If so, I do have a lot to learn from you, and would be pleased to hear your reasoning.

[mykey]

OK, lets try and get this straight - what we're talking about here is the extent to which a change in coupling cap value will affect (a) overall signal level, (b) EQ. My understanding in layman's terms (I'm a layman and proud of it) is that the slower charge rate of increasing coupling cap values will allow lower and lower frequencies to pass through the stage. As I understand it caps pass no DC, and pass AC according to the rate at which they are able to charge and discharge given their value and the relevant resistance in the circuit. So in my little world the principal effect of an increase in coupling cap values is a concomitant increase in lower frequencies passed by the stage. In the sort of circuits we commonly see in guitar amps with the kinds of resistances therein, with coupling cap values in the ballpark range max 50pf to 1uF those changes in frequencies passed will be audible.

An example: going from .01 to .1 into the PI of a typical Fender 2 x 6L6 amp will produce an audible increase in lower frequencies, I would say (and my experience tells me). Are you saying that the principal effect of such a change is rather an increase in gain at all audible frequencies? And that this is a good way of producing a decent power amp overdrive? If so, I do have a lot to learn from you, and would be pleased to hear your reasoning.

no we were talking about BYPASS of the cathode bias resistor.
but yes changing from .01 COUPLING CAP to .1 is going to give you more bass, also more treble at the same time. the signal will INCREASE overall.
But whats confusing here is that you must consider the stage without any tone stack or controls connected to it. JUST the stage alone.
with the controls, tone stack etc connected you are bound to get the effect of more bass and muddyness, because the stage you have just boosted is OVERLOADING the next stage, not because only the bass was boosted.
yes you must thin the signal at the output of the boosted stage to prevent it from overloading the NEXT stage. This thinning is commonly accomplished by using .01-.001 as a coupling cap.
Installing the BYPASS cap on the cathode increases the sensitivity of that stage. with the sensitivity boosted the output needs to be coupled so that it will not overload the NEXT stage. or else as you said- mud. (tell this to mesa boogie).
Sorry for the misunderstanding, my bad.

[Alex R]

You did say bypass caps. Somehow I read it as coupling caps. Now I get it. My bad actually.

But the problem that started the thread - sounds like a bad preamp tube, is the simple answer.

[Alex R]

but yes changing from .01 COUPLING CAP to .1 is going to give you more bass, also more treble at the same time. the signal will INCREASE overall.
But whats confusing here is that you must consider the stage without any tone stack or controls connected to it. JUST the stage alone.

...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?