Using a Combo Amp Speaker with an External Amp

[ozboomer]

I have a couple of problems, amp-wise, that I would like to address:

• The volume control range is too large on my current amp (a Fender Frontman 15R, bought in 2006). I start by inserting a cable between one of my Strats and the input jack of the amp; then I set the volume to "0" (lowest value) and then I turn the amp ON. As I turn the volume up, there's a point very early in the rotation, where I can almost hear the output "turn on". If I do any more than barely touch the volume knob, the output level is too great and things sound too loud; if I had to describe the volume setting, I'd probably describe it as being on "0.5" or less.
  
  Now, I don't believe there's anything wrong with the amp; it's just that it's over-powered for what I need at home, which is something with a low output level.
  
  
• I need a small, portable (battery-powered) amplifier that I can carry easily and is lightweight but still has enough grunt that I can hear some sort of amplification of the guitar's signal.
  

Now, to address these issues, I was going to have a go at building a Ruby or Noisy Cricket amp (which would certainly end up being less expensive than a Roland Micro Cube or VOX DA5, which run ~A$199). Re the 2nd problem above, the Ruby/Cricket would give me something small and yet I understand I'd still *hear* something; I would hope/expect the rotation on the volume for the Ruby would get me up into the middle of the rotation or more, which would be better than the "barely touching the volume is too loud" -style of thing I deal with presently. The only issues I see (for the initial build) is to decide on a speaker. From what I've read, I should probably see if I can find an old 6"-8" TV speaker to use with the Ruby... and this leads to a question about the 1st problem above...

Until I can source a good(?) speaker for the Ruby to plug into, I wonder how it would go driving the speaker in the Frontman? It's an 8", 8-Ohm speaker, so it should be usable... and given that this amp is old enough now that it would be out of warranty, I wonder if I could do a simple mod on it to use the speaker from an external source, namely the Ruby amp?

Here's an initial cut at a design for the mod:

...or, for the sake of possible "mistakes", should I use totally unusual connectors and use a conventional ON/OFF switch (toggle, bezel, etc) to switch between my "external" input and the "internal" amp...?

I have to admit I've never done anything with mains-supply amps and I'm sort-of nervous about dealing with the innards of a solid-state amp... but near as I can see, I'm only fiddling with speaker wires, as such... so it shouldn't be a big deal?

Still, I'd appreciate any thoughts... Thanks.


John

[JohnH]

Your diagram looks OK for what you intend. I suggest the amp should be unplugged from mains if using it as a passive speaker - just to be sure.

Ive heard that some Fender amps try to fool you into thinking they are big and loud by having a very sudden early increase as you turn up the volume. You could maybe address that, by using a log volume pot on the amp if it ciurrently has a linear pot, or adding a switcheable 'pad' comprising a resistor or two. A 15W SS amp should be capable of being used for quiet practice.

I have a small Marshall Mg10 for latenight noodling, that Ive made a coiuple of tweaks to to help it sound better, it can get too loud but turns down too if needed.

John

[Teleblooz]

I wired up an old Fender solid-state combo the same way. When the amp stared to go south I used it as an extension speaker. Worked fine, no problems. I agree with JohnH though - unplug the amp from power.

I've had the same experience with the volume thing and never did find a workable solution (not that I looked all that hard). I'll leave suggestions to those more tech-savvy than I am.

[ozboomer]

Since I started this thread, I've ended-up grabbing an Orange Micro Crush, which works Ok, playing along with the radio, and using anywhere I need a slight, 'open-air' bit of amplification (although, it does seem to lose-out on the bass response at times... but that's another discussion)... but I still want to pursue dealing with this amp/volume control thing.

Jun 28, 2010 0:36:52 GMT -5 JohnH said:

Ive heard that some Fender amps try to fool you into thinking they are big and loud by having a very sudden early increase as you turn up the volume.

[...]

...adding a switcheable 'pad' comprising a resistor or two. A 15W SS amp should be capable of being used for quiet practice.

I was thinking about this a bit...and it would seem the Electro-Harmonix Signal Pad Passive Attenuator would fit the bill... or, more likely, a home-made version, along the lines described in this (Spanish) posting...

...but wouldn't there be some loading issue with using purely passive components in that way? ...or is using a JohnH Buffer with a pot overkill for this?

...just me over-thinking things... as always...


John

[thetragichero]

in terms of bass (and treble for that matter) response being low on a low-powered amp turned down, you are correct
it's just how the human ear works
you would have to overcompensate by turning the bass up

[newey]

Oz-

I'm wondering if there isn't a problem with your Frontman 15R. I have a Frontman 25R of roughly the same vintage, and its volume control turns down nice and evenly all the way to zero. I can easily get practice levels out of it at 1-2 on the volume control. And I have played through Frontman 15s and didn't experience any unusual volume action.

Go into a music store and try another one, see if it behaves the same as yours. If not, it might be a problem specific to yours.

[JohnH]

Sensible advice from newey.

But in case a desire remains to hack things about, here is a schematic:

support.fender.com/schematics/guitar_amplifiers/Frontman_15G_15R_schematic.pdf

Although it is billed as covering the 15R, it seems to be based on the 15G, but hopefully its near enough.

Here's part of it, where the signals from the drive and normal volume controls get combined, and sent onward to the common tone stack:



An odd observation is that the drive seems to be permanently connected. I wonder if thats a drawing error?

But, as a suggestion for reducing volume, if you increase the feedback in the Op-amp stage at lower right, you could cut its gain from 1:1 down to say 1: 1/2 (which would be a reduction of 6db) or less, by splicing a resistor in parallel across R15. The good thing about that approach is that you can try it out without cutting anything, and it maintains the tone of the amp and the balance between channels. Particularly, by doing this after the drive channel clipping stage, you maintain the drive distortion, but at lower volume.

cheers

John

[elderik]

I luv my Roland MicroCube for low volume applications. They right at $100 new, and even less used

[Yew]

Jun 28, 2010 0:36:52 GMT -5 JohnH said:

I have a small Marshall Mg10 for latenight noodling, that Ive made a coiuple of tweaks to to help it sound better.

John

What are those tweaks? I have an MG10 and it always suprised me how good it sounds with a reasonable guitar