Vox Experts: adding a bit more "fatness" to an AC15HW

[JIMMY JAZZMAN]

Before alot of modding, the tone cut on the AC15 is very useful for what you desire. Most amps, a tone cut creates a
somewhat "muddy" tone the more you turn, but with the Vox AC15, it may get you where you want to be. I've had many amps, and the VOX is the only one where I can actually use the tone cut for better tones.

 

[Dirty Dave]

Mad question, but have you tried playing through the TB channel, with the normal channel dimed?
Even if you haven't jumped the channels, turning up the the normal channel on my AC15HW adds so much huge low-end to the TB side.
I've never seen it written down anywhere, but I saw a guy do it on Youtube, and it works.

edit: Sorry, just seen that you've dimed both channels.
Forget it.

 

[gusfinley]

Looking at the AC15HW1 schematic, you could get a little more gain out of the Normal channel if you jumper across R17. With R17 in the circuit, it is cutting 3/8th of the output. It also would get a little darker because the C13 bright cap would be out of the circuit when the normal channel is at maximum.

Other than that the AC15HW isn't skimping much with lots of 100nF coupling caps on the normal channel and in the phase inverter.

 

[Mark1406]

If you are going to the trouble of putting a Mercury Magnetics output transformer into the amp. The Marshall meets Vox sounding one is the Albion transformer. The Haddon is brighter than the Woden. They are cool sounding transformers.

Regards

Mark

 

[JamesAM]

Mad question, but have you tried playing through the TB channel, with the normal channel dimed?
Even if you haven't jumped the channels, turning up the the normal channel on my AC15HW adds so much huge low-end to the TB side.
I've never seen it written down anywhere, but I saw a guy do it on Youtube, and it works.

edit: Sorry, just seen that you've dimed both channels.
Forget it.

No worries, this is great advice! I discovered this trick by accident a while back when I didnt realize what channel i was plugged into. I didn't realize that the channels were that interactive, almost like a 5e3, but way more subtle. Crank the normal channel while you're plugged into the top boost and you get a slightly fatter sound.

 

[ashtone54]

Ya cain’t git 4-12 tone from a combo amp.

 

[GearGeek01]

I've owned 2 x Vox AC15's... both threw so much treble the only word I could think of is "ice pick"... nasty, trebly, yucky amps... definitely not for me... sell that trebly ice pick and buy something that sounds the way you want it to sound

I never understand people pouring money into stuff they hate, trying to re-engineer a turd...

 

[Arfage]

Hi all,

I'm looking to see if it's possible to modify the AC15 circuit to get a bit more of a thick matchless or even plexi sound out of it at higher volumes. The top boost channel has the right amount of gain, but doesn't have enough thump in the bass. The normal channel has the right amount of bass, but lacks the gain of the TB channel. Jumping the channels muddies up the low end too much with my ES-335. This amp is SO CLOSE to the sound I'm chasing in my head, but falls just a bit short.

What are some ways to squeeze a bit more bass or gain out of this circuit? I know coupling cap value changes are one way, but has anyone tried anything else? I'd prefer easier changes as opposed to something like adding a pentode in the preamp, but I'm open to any ideas you might have. Or, is this a fools errand, and I should just build an 18 watt or a 6v6 plexi?

Forget the Marshall, not gonna happen. But of course Matchless did use the AC30 as a springboard for some of their models. You just need to study the schematics of both amps, no other way. And either you'll find it, or a reason why it won't.

 

[Arfage]

Hi all,

I'm looking to see if it's possible to modify the AC15 circuit to get a bit more of a thick matchless or even plexi sound out of it at higher volumes. The top boost channel has the right amount of gain, but doesn't have enough thump in the bass. The normal channel has the right amount of bass, but lacks the gain of the TB channel. Jumping the channels muddies up the low end too much with my ES-335. This amp is SO CLOSE to the sound I'm chasing in my head, but falls just a bit short.

What are some ways to squeeze a bit more bass or gain out of this circuit? I know coupling cap value changes are one way, but has anyone tried anything else? I'd prefer easier changes as opposed to something like adding a pentode in the preamp, but I'm open to any ideas you might have. Or, is this a fools errand, and I should just build an 18 watt or a 6v6 plexi?

Also you could just increase some tone stack or preamp cathode cap values and see what happens..

 

[enorbet2]

Just FTR "Start Bright, end Dark" is the formulka that Paul Kossof used to get his killer tone. It's really difficult to get more bottom from 15 watts but it's trivial to roll off high end which sounds similar. You can do a non-destrctive test by shunting the driver tube plate resistors with caps. Ideally use a capacitor substitution box but if you don't have one, start big with a 0.001 cap and reduce value as needed if too much highs are rolled off.

 

[gusfinley]

I mentioned changing R17 in a previous post. Hopefully you have stumbled across this site:

http://www.nikplayer.com/2012/01/ac15hw1-modifications.html

http://www.nikplayer.com/2012/02/ac15hw1-modifications-part2.html

Do a search for R17 on that page and you can see how the author and other commenters felt it changed their amps.

 

[Mark1406]

You can increase the treble cap in the tone stack from 50pF to 500pF. This fattens up the sound. You can even rewire the tone stack so it is like a Marshall one. There are alternatives. I would start with replacing the treble control (if you can) with a pull pot with the treble cap (50pF) across the switch contacts and the 470pF to 500pF where the 50pF was. When you pull the pot out engaging the switch, the 500pF is working on its own and the amp will sound fatter. You get to keep the original amp tone too.

Regards

Mark

 

[fiveightandten]

Thanks, I’ve been eying the axiom output trans by MM (the same one Morgan uses I think), but they sure are expensive and they don’t list any voltage specs.

I was looking at lowering the filter caps to JMI spec as an alternative, but I rather like the tightness the higher capacitance gives me now. I’m not sure the looser low end that a smaller power supply would give me will equate to more bass. I’ve seen several threads by folks who changed the coupling cap values, but all of them said it made a negligible difference compared to altering the power supply.

might be worth looking at another speaker or a closed back cab as you and other folks here have suggested. The hw cab itself is actually birch ply and sounds pretty good, but a closed back cab probably would get me where I need to be. I remember the ac10 I had was really bassy because of its sealed cabinet, and it was a tiny mdf job. I’ll give it a look.

A late reply from me on this, but yeah, if you don't want to do any circuit mods. I think a speaker change would get you the low end them you seek. The question though, is, if it will change other stuff that you don't want to change.

I don't have an AC15HW, but I do have a Morgan AC20 (the normal channel Vox clone), Morgan Abbey (the Top Boost channel Vox clone), an AC15C1X, an AC-30TBX. Here's my take on a few speakers with those amps.

Celestion AlNiCo blue - This is the sound. I'm not sure if you're running this or a Greenback, but IMO this is the calibration point from where you start if you want the Vox sound. Woody midrange with an upper mid quack, natural compression that gives the attack some spring to it, and some gritty attitude when driven. From here you decide where to go if it's not working for you.

Celestion AlNiCo Gold - This has a similar dynamic response to the blue, but it has more low end and more heft to it. It sounds more meaty in the low end, but it doesn't have the upper mid thing that the Blue has. It's more of a lower vowel-like quality to the mids. As Voxes don't really put much of that out, it sounds more contoured than the Blue. This is a viable option for a bit more low end, but it's not "A blue with more low end and more power". There is a different voicing to it.

Celestion G12H-75 Creamback - Morgan loads his Vox clones with this speaker. I scoffed at it. A 75W H magnet ceramic in a 15W Vox? Ha! But seriously, it's a nice pairing. Now, H magnet speakers generally put out more lows and highs than M magnet or AlNiCo drivers, and they can sound scooped in comparison. The G12H30 is an example of that. But the H75 Creamback still has some midrange to it. The highs are smooth and not grating like some H mags. It's pretty balanced, and in a Vox it'll add a notable amount of heft and tightness to the low end. Highs are clearer than an AlNiCo, but not in a harsh way. This is the speaker to get if you want a 15W amp to punch above its weight class.

Scumback H75 - Scumback makes high quality clones of vintage Celestions. As far as H magnet speakers go, this is a very balanced one. It doesn't quite have the low end of a modern Celestion H magnet like the G12H75 Creamback. What it *does* have is more lows and a fuller sound than an AlNiCo or M mag speaker. It does this without bright highs and does it without sacrificing midrange. It does NOT sound scooped. If an H mag Celestion is too beefy or too countered sounding, the Scumback H75 is a healthy medium. Scumbacks also have the added benefit of having absolutely no offensive frequencies. Celestions can be brash, especially when pushed. Scumbacks have none of that. They're a bit more polite, but still have plenty of character and are never harsh.


You could look into a closed back cabinet. But that would mean carrying an extension cabinet around, as I wouldn't recommend sealing up your existing enclosure, due to ventilation concerns. I think a speaker swap to an H magnet would yield very similar results in your existing cabinet.

The Mercury Magnetics iron is indeed expensive, and not the most cost effective way to change the sound of an amp.

All that being said, if you're handy with a soldering iron, I would try raising the value of the TB coupling cap. You said the Normal Channel had enough bass. Swap the 470pf Top Boost coupling cap for a .022u or a .01u. That may wind up doing exactly what you want. The Normal channel has a huge 0.1u cap, but I wouldn't go that far on the TB channel. A substitution box is a VERY handy tool for figuring out things like this.

https://www.amazon.com/dp/B00R6SOZ00/?tag=tdpri-20

Be warned, the caps that come with this one are 50V and way underpowered for a coupling cap. You'd want to get some 500V caps to load in there so it's useful for any position in a tube amp.

Hope that helps.

 

[Les H]

The best way to seemingly add fatness to a Vox is to go play a blackface fender reissue for about 30 minutes and then plug back into the Vox. Viola....

 

[Maguchi]

I'd buy an inexpensive seven band eq pedal and give that a try.

A Behringer EQ700 pedal.

https://www.sweetwater.com/store/detail/EQ700--behringer-eq700-graphic-equalizer-pedal

Or a Boss GE-7 pedal

 

[JamesAM]

A late reply from me on this, but yeah, if you don't want to do any circuit mods. I think a speaker change would get you the low end them you seek. The question though, is, if it will change other stuff that you don't want to change.

I don't have an AC15HW, but I do have a Morgan AC20 (the normal channel Vox clone), Morgan Abbey (the Top Boost channel Vox clone), an AC15C1X, an AC-30TBX. Here's my take on a few speakers with those amps.

Celestion AlNiCo blue - This is the sound. I'm not sure if you're running this or a Greenback, but IMO this is the calibration point from where you start if you want the Vox sound. Woody midrange with an upper mid quack, natural compression that gives the attack some spring to it, and some gritty attitude when driven. From here you decide where to go if it's not working for you.

Celestion AlNiCo Gold - This has a similar dynamic response to the blue, but it has more low end and more heft to it. It sounds more meaty in the low end, but it doesn't have the upper mid thing that the Blue has. It's more of a lower vowel-like quality to the mids. As Voxes don't really put much of that out, it sounds more contoured than the Blue. This is a viable option for a bit more low end, but it's not "A blue with more low end and more power". There is a different voicing to it.

Celestion G12H-75 Creamback - Morgan loads his Vox clones with this speaker. I scoffed at it. A 75W H magnet ceramic in a 15W Vox? Ha! But seriously, it's a nice pairing. Now, H magnet speakers generally put out more lows and highs than M magnet or AlNiCo drivers, and they can sound scooped in comparison. The G12H30 is an example of that. But the H75 Creamback still has some midrange to it. The highs are smooth and not grating like some H mags. It's pretty balanced, and in a Vox it'll add a notable amount of heft and tightness to the low end. Highs are clearer than an AlNiCo, but not in a harsh way. This is the speaker to get if you want a 15W amp to punch above its weight class.

Scumback H75 - Scumback makes high quality clones of vintage Celestions. As far as H magnet speakers go, this is a very balanced one. It doesn't quite have the low end of a modern Celestion H magnet like the G12H75 Creamback. What it *does* have is more lows and a fuller sound than an AlNiCo or M mag speaker. It does this without bright highs and does it without sacrificing midrange. It does NOT sound scooped. If an H mag Celestion is too beefy or too countered sounding, the Scumback H75 is a healthy medium. Scumbacks also have the added benefit of having absolutely no offensive frequencies. Celestions can be brash, especially when pushed. Scumbacks have none of that. They're a bit more polite, but still have plenty of character and are never harsh.


You could look into a closed back cabinet. But that would mean carrying an extension cabinet around, as I wouldn't recommend sealing up your existing enclosure, due to ventilation concerns. I think a speaker swap to an H magnet would yield very similar results in your existing cabinet.

The Mercury Magnetics iron is indeed expensive, and not the most cost effective way to change the sound of an amp.

All that being said, if you're handy with a soldering iron, I would try raising the value of the TB coupling cap. You said the Normal Channel had enough bass. Swap the 470pf Top Boost coupling cap for a .022u or a .01u. That may wind up doing exactly what you want. The Normal channel has a huge 0.1u cap, but I wouldn't go that far on the TB channel. A substitution box is a VERY handy tool for figuring out things like this.

https://www.amazon.com/dp/B00R6SOZ00/?tag=tdpri-20

Be warned, the caps that come with this one are 50V and way underpowered for a coupling cap. You'd want to get some 500V caps to load in there so it's useful for any position in a tube amp.

Hope that helps.

Thanks so much for this awesome reply! I think some combination of an h series speaker and maybe some coupling cap (and maybe normal channel resistor) mods might do the trick. Thanks so much for your insight!

 

[fiveightandten]

Thanks so much for this awesome reply! I think some combination of an h series speaker and maybe some coupling cap (and maybe normal channel resistor) mods might do the trick. Thanks so much for your insight!

No problem. Perhaps buy a few used speakers so you can re-sell what doesn't work and not really loose any money. Solder together a substitution box so you can try out different coupling cap values in real time, and find the right combination.

I can't overstate how helpful a substitution box is for auditioning components when an amp is close, but not quite there. You can also make your own, cheaply and easily, with a rotary switch, an enclosure, and some leads.

 

[SamIV]

Did not read all the replies, but I use a Boss bass eq (GEB-7) on mine. This has helped for me to play at low volumes using an overdrive pedal to increase the bottom end of it while not affecting the high end. I just happened to have one. Did not buy it purposely for that, but it worked out quite well.

 

[Digger73]

I was chasing the same thing in terms of bass punch and retaining the chime. I purchased the amp new and it came stock with a Celestion G12M. Tubes replaced with Tubestore Preferred series EL84 and two Preferred series 7025, Sovtek 12AXLPS Phase Inverter, and Raytheon NOS Rectifier helped. I have since went through a litany of speakers-Scumnico purchased just for this amp, Eminence Red Fang (LOUD), Celestion Cream 90, Celestion V Type, Celestion G12H-75, and maybe a couple others in the last three years. I tried all of these open and closed back. I finally sent the G12M to Austin Speaker Works to have it "retoned" to a ASW Crossroads. This process essentially replaces much of the internals, spider, cone, voice coil, etc. and it becomes a 70W speaker. It is advertised as a proper 60's Greenback and it is. This is what did it. I installed it and put the back on and it is now all there! I have played single coils, humbuckers including Filtertrons with and without the normal jumpered to TB, both 2 to 2 and 2 to 1. I now have another Crossroads in 8 ohm in a cab that I connect to Fender amps at times and that is excellent too.

 

[Digger73]

Uploaded twice somehow...must be a guitar player suffering from brain disconnect