Amp, well, preamp or tone controls education

[Dazed]

I have a combo. It is a guitar combo. It has a valve pre and power amp. It’s tiny and I like it. 

However it only has a single tone control, voiced for guitar so does little for bass. 

Where do I learn more about modifying this to make it more bass appropriate? Or valve amp circuits in general. 

Perhaps giving it a bass /mid/ treble instead of the single tone. I don’t know if that would even be possible. I suppose an alternative would be to use a preamp pedal but ideally I’d like everything to be in a single unit. 

Or any recommendations for amp techs that could this kind of work could be an option although it’s not an expensive item at the moment and I don’t particularly want to make it one. 

Thanks 🙂

 

[Dan Dare]

There's a limit to what a tiny valve guitar combo can do, no matter what you do to it. Preamps/eq are complex topic. Unless you know what you are doing, I wouldn't DIY mod a valve amp. Apart from anything else, the internal voltages are usually very high, so bodging things can be dangerous. I'd be inclined to get something like an eq pedal - Boss graphic or similar - and run the bass through that into the combo. It still won't be great, but it will be better than nothing and a lot safer.

[Phil Starr]

The biggest problem with a guitar combo will be the speakers which aren't designed for bass, you'll probably blow them unless you cut the power by a lot.

If you just want something at very low volumes then why not get one of these, basically a bass pre amp with all the tone controls you need (but not as many as you might want of course) and a bit of overdrive if you need it. Take the output from this into your guitar combo and keep that volume right down.

[Woodinblack]

22 hours ago, Dazed said:

I have a combo. It is a guitar combo. It has a valve pre and power amp. It’s tiny and I like it. 

However it only has a single tone control, voiced for guitar so does little for bass. 

Where do I learn more about modifying this to make it more bass appropriate? Or valve amp circuits in general. 

The internet is good. Do you know electronics already? If so, there isn't much to it.

If it just has a tone it will be the simple resistor / capacitor type, so you just change your capacitor to get the inductance of the capacitor to match the resistor at the frequency you want (with the normal 2 pi fc) http://www.electronics-tutorials.ws/filter/filter_2.html

there is quite a lot of stuff on valve amps. In general although there is a lot to actually doing valve amps, electronically they are actually easier to understand than transistors, as they are voltage based rather than current. They are also fun to work on as there is a lot more room to work on them and a lot more scope for modifications. There is also because of that a lot more scope for it to go bang (when capacitors go, they go fast and very dramatically), melt your valves, catch fire and electrocute you, so as other people have said, check the voltage if you are doing any work on it, if it has a valve power section there will be high voltages and they really do hurt like a mother (assuming they don't do worse), to the point I can remember vividly every time I have been electrocuted.

You will probably find the circuit is on the internet if it is a common thing, something based on a fender junior or similar, and if it is really common like the harley bentons 5w or fender junior that it is a clone for, there are whole websites for them.

 

22 hours ago, Dazed said:

Perhaps giving it a bass /mid/ treble instead of the single tone. I don’t know if that would even be possible. I suppose an alternative would be to use a preamp pedal but ideally I’d like everything to be in a single unit. 

It is perfectly possible, just involves knowing what the circuit is and a bit of modelling. There are standard ways of doing thing like that, google is your friend, start with baxandall filter stages and all is there.

22 hours ago, Dazed said:

Or any recommendations for amp techs that could this kind of work could be an option although it’s not an expensive item at the moment and I don’t particularly want to make it one.

 

Any amp tech could do these things for you, but you could quickly find it would get more expensive than the original amp was worth.

 

[Bill Fitzmaurice]

Chances are that tone control is the same as the one in your bass, a simple high frequency shelving filter. You can change its knee frequency by changing the value of the capacitor, but nothing you can do will turn an inexpensive guitar combo into a capable bass amp.

[Beer of the Bass]

The problem with adding a more complex tone stack is that passive treble/bass or treble/mid/bass circuits tend to be more lossy than the simple single knob types.  So it may not be a simple matter of adding the tone controls, it may need an additional gain stage too.  At that point you're looking at a rebuild of the whole preamp section, which may not seem worthwhile.  

[Passinwind]

On 12/30/2017 at 07:50, Dazed said:

I have a combo. It is a guitar combo. It has a valve pre and power amp. It’s tiny and I like it. 

However it only has a single tone control, voiced for guitar so does little for bass. 

Where do I learn more about modifying this to make it more bass appropriate? Or valve amp circuits in general. 

Perhaps giving it a bass /mid/ treble instead of the single tone. I don’t know if that would even be possible. I suppose an alternative would be to use a preamp pedal but ideally I’d like everything to be in a single unit. 

Or any recommendations for amp techs that could this kind of work could be an option although it’s not an expensive item at the moment and I don’t particularly want to make it one. 

Thanks 🙂

 

What amp, specifically?