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Re: Ampage's Vox AC30 week...

7/18/2000 1:52 PM
Randall Aiken
Re: Ampage's Vox AC30 week...
Has anybody tried wiring up an AC30 clone with a fixed-bias switch? Seems like you could get a more dynamic sound at high volumes using fixed-bias... (While everybody makes a big deal of the Vox amps being "Class A" perhaps it is also the absence of negative feedback that contributes to their sound...)  
Funny you should mention that, Steve!  
I recently took one of my amp prototypes, which has a fixed/cathode bias switch (the basic circuit is a 2-EL84 design using my standard DC-coupled cathode-follower output drive, no negative feedback, and a 4-holer plexi-based preamp with reverb) and did a blind A/B comparison with three excellent guitarists as the test subjects. The fixed bias current was set to the same value as the cathode bias current. The surprising outcome was that all three of them picked fixed bias *every* time as sounding better. I think you may be correct - the majority of the AC-30 "mojo" comes from the lack of negative feedback and the way the preamp section of the AC-30 sounds. I'll have to wire up an AC-30 style preamp section into the prototype and try it out in an A/B test when I get a chance.  
It looks like it may yet again be time to revise my entire philosophy on amp design! Now, can we put this "class A" thing to bed? :)  
Randall Aiken
7/18/2000 2:34 PM
Dave H.

As the bias currents were the same perhaps fixed bias sounded better because it had less crossover distortion when overdriven than cathode bias. With your DC CF drive there would be no bias shift in fixed bias but you would still have bias shift because of the cathode RC time constant in cathode bias.  
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7/18/2000 4:04 PM
Randall Aiken

That's a good possibility, Dave. I'm definitely going to have to get to the bottom of this as soon as I get a bit of time. I have always used DC-coupled cathode drive with my fixed bias amps, but have never tried it in a cathode-biased amp before this. The difference in tone didn't sound like crossover distortion to me. Most of the guys in the test described it as having "better clarity", "my sound", "smoother", "better feel", or "I dunno, it just sounds better." Could it be that they are just used to the sound of Marshalls/Fenders/etc., and that is the sound they hear in their head as the "right" tone?  
Randall Aiken
7/18/2000 10:19 PM
Matthew Springer

Could it be that they are just used to the sound of Marshalls/Fenders/etc., and that is the sound they hear in their head as the "right" tone?  
As for me, I have this Jerry McPherson lead tone I've been hunting for forever, and after looking at a recently discovered website look slike a fender into VOX thing cranked to 11.  
As for me, I would definately shell out some clams for the right amp that has the right sustain/clean tone combination. Kinda like the Top Hat Club AC30/ tweed Deluxe hybrids.
7/19/2000 3:55 AM
Steve A.

    Even with the cathode bias set a bit hotter than the fixed bias, I think that you get better dynamics and response with no NFB and the fixed bias. Without the NFB loop, the sound can get pretty wild so cathode bias will take the edge off for a smoother sound, but once you start cranking the amp up it tends to get a bit flabby (not necessarily bad as lots of people like the Matchless DC30).  
    You mentioned your test subjects being excellent guitarists; the combination of no NFB and fixed-bias can be very unforgiving and if you don't have the chops you might prefer cathode bias... (Hmmm, maybe that explains why I keep adding cathode bias switches to all of my amps... ;)    )  
    At lower playing volumes, I do like the way cathode bias makes it sound like your amp is cranked up much louder...  
Steve Ahola
7/19/2000 8:46 AM
Steve A.
Vox section added to Blue Guitar site...
    I'd been talking about this for 8 months but since this is "Vox Week" I figured that I'd better do something about it right away. ;)    I've included the sources for the drawings when available. (BTW I posted Joe Piazza's AC50 drawing because he doesn't seem to currently have a site). If anybody has something to add, please e-mail it to me. Thanks!  
Steve Ahola  
P.S. A few files worth mentioning: the ac30elek.gif and ac30elek.pdf drawings (from Elektor Electronics April 1989, p.29) are very sharp looking. And ac30volt.jpg was drawn up by a tech who had measured the voltages on several AC30's so that should be very useful. Enjoy!
7/19/2000 4:36 PM

Steve, that's some serious Vox info!
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