#1
I'm trying to get more headroom out of my Vox AC15 C1; currently, it starts distorting past 9 o'clock on the normal or 8 o'clock top boost channel (and master). According to this bitmo mod, you can increase the headroom a bit. Does anyone have any experience with this mod on an AC15 (or any amp for this matter)? How much of an increase in headroom can one expect? Are there any other mods that could increase headroom? I've tried switching out the preamp tubes on the phase inverter to increase headroom, but something wasn't jiving right and there was a shill (almost chimey) reverb-like overtone to everything I played. I haven't tried switching out the speaker, and my only speaker consideration would be to upgrade from a Greenback to a Blue, but I've heard the Blue breaks up even faster than the Greenback and so I've hesitated on switching the speaker. I play with a full band and for most of our loud stuff, the break up is not a huge issue, but when we do soft songs, it makes my guitars (both my strat and epi sheraton) sound a bit undefined. Thanks!

http://www.bitmomodkits.com/servlet/the-119/BITMO-VOXMO15-MOD-KIT/Detail

regarding the mod, the price isn't a huge issue ($30), but I'm afraid of voiding the amp's warranty and drilling a hole in the chassis for a mod that isn't going to get me anything worthwhile.
Epi Sheraton II or MIA FSR strat > wah Q535 > fuzzface > Keeley C4 > Qtron > Whammy > Liquid Chorus > Triboost > Hotcake > BigMuff > Small Stone > VolumePlus > SpaceEcho > Cathedral Reverb > LoFi > MemoryMan > BYOC vibrato > DD-7 > Vox AC15 C1
Last edited by cheeekens at Apr 22, 2011,
#2
The vox is an interesting amp. I built an amp that is heavily inspired by the old vox designs not too long ago. The trick that I see to it is to learn how to use the volume knob on your guitar, as well as your playing dynamics.

If you just have your guitar set to full all the time then your not going to be able to get much headroom at all, or if your just slamming your strings the whole time.

What I tend to do is set the volume to about half of its range, set the eq, and then set my amp until it just starts to break up. From there I can increase the guitar volume for more grit, or back off for cleaner stuff.

The only real way I can see of getting more clean headroom from a Vox would be to add negative feedback to it, but that would kill the entire Vox vibe as it is.

An AC30 has a bit more headroom than the 15, but I don't think your wanting to buy a new amp.
#3
hmm... I'll give the volume setting a shot... I do try to clean up with my picking dynamics, but I don't think I've tried messing with the guitar volume. Thanks! Yeah now in hindsight, I regret not saving up for an AC30 for the clean headroom alone, but I get so excited about new gear that I can't save up more than a few hundred dollars at a time anyways lol thanks again!

Just out of curiosity, with whatever knowhow you have regarding Vox electronics, do you know what is it about the Vox amps that gives it that chimey compression with interactive dynamics? I'm not really electronically/technically-inclined, but does it have to do with not needing to bias the tubes? I've heard people say that voxs are true "class A" amps, but I've never truly understood it. Just curious. Thanks!
Epi Sheraton II or MIA FSR strat > wah Q535 > fuzzface > Keeley C4 > Qtron > Whammy > Liquid Chorus > Triboost > Hotcake > BigMuff > Small Stone > VolumePlus > SpaceEcho > Cathedral Reverb > LoFi > MemoryMan > BYOC vibrato > DD-7 > Vox AC15 C1
#4
Vox amps run extremely close to class A, but not entirely class A, while also running the EL84 tubes in them quite hot. Due to this all of the tubes conduct almost 100% of the time. Because of this the screen voltages can drop quite a bit under heavy playing and cause some compression or squash.

Class A basically means all of the amplifying elements in the output section are conducting 100% of the time during a signal. Class B means both conduct 50% of the time, one on at a time. Class AB is somewhere in between, and the Vox is very close to class A.

Another thing that gives the Vox its sound is the fact that the power amp is not a traditional dropping chain. So all of the power supply nodes are indepenent of each other really. This keeps the preamp voltages higher all the times so it retains its clarity.

Another reason that a Vox amp sounds the way it does also has a lot to do with its lack of negative feedback. This is one of the reason why a Vox amp can be made so touch sensitive. While other amps are also touch sensitive, at the right setting you can get just about every tone possible out of the amp just by varying how you pick.
#5
Thanks you definitely know your stuff.... are you going to school for this kind of stuff by any chance or just a hobby? Also, what exactly is negative feedback? is it just the alternate amplitude of the sound wave? I'm trying to remember the physics I took as a college freshman lol
Epi Sheraton II or MIA FSR strat > wah Q535 > fuzzface > Keeley C4 > Qtron > Whammy > Liquid Chorus > Triboost > Hotcake > BigMuff > Small Stone > VolumePlus > SpaceEcho > Cathedral Reverb > LoFi > MemoryMan > BYOC vibrato > DD-7 > Vox AC15 C1
#6
I'm in school for Electrical Engineering, well... I will be for the next three weeks and then I'm free till I start my Masters. But guitar amp work has been a hobby/job of mine for about 5 years now.

Negative Feedback is when you take the signal from one part of the amp and combine it with the signal in an earlier part of the amp. The two signals also have to be completely out of phase, so this reduces the gain but increases the bandwidth. In the sense of a guitar amp it just means that you usually need a larger signal to overdrive the power amp. The guy that desgined Vox realized that people would intentionally overdrive his amps, so he left negative feedback out.
#7
ah ok I get it! Thanks! yeah sometimes I wish I'd had taken a few audio electronics courses back when I was in college, but instead I focused on bio sciences (not that they'd let me take any audio electronics classes anyways, as all the audio electronics my college offered were only for declared electronic sciences/majors), but I try to pick up as much as I can when it pertains to amps and effects =)
Epi Sheraton II or MIA FSR strat > wah Q535 > fuzzface > Keeley C4 > Qtron > Whammy > Liquid Chorus > Triboost > Hotcake > BigMuff > Small Stone > VolumePlus > SpaceEcho > Cathedral Reverb > LoFi > MemoryMan > BYOC vibrato > DD-7 > Vox AC15 C1