VOX power stage characteristics?

Catthan

Amp gurus and techs, would you mind sharing what's the most notable design feature in the power stage of these amps?

In layman's terms if possible.

Say an AC30.

To me VOX have a midrangy grit and snap to their tone (play an AC30 regularly) but I doubt it's the el84s? 

I think it's the tone-stack but I'm really clueless.. The other thing I know is that many lower wattage designs use el84s so it's not a VOX thing and I'm curious whether people who talk about their "voxyness" really get to crank their power stages, at least to the point that they do smth different than compression.

I know I don't crank the AC30 I play but there is smth unique there which I cannot dial out and with the volumes down it's shouldn't be the power stage..

So is that VOX power tube compression different than other el84 amp tube compression? wiring, bias?  

My mesa LSS can sound a bit like a VOX in some settings as did my el34 Badger but I never pushed the power amp to the point that did anything.. well maybe with the Badger (power scaling) but not with the LSS..

Am I right to thing that el84s are just an ingredient (small one) in the VOX thing.. Am I right in my tendency to doubt people who make the vox tone-"chime" connection to any el84 amp they see? 

Please educate me..

Cirrus

Just as a little anecdote - I once hooked up the effects loop out of my high gain ac50cp head to the effects loop in of my AC30. So that was basically high gain preamp - ac30 power amp.

It sounded like the ac50, except a tiny bit more compressed and a little less bass.

The AC30 power amp is quite straightforward - 4 cathode biased EL84s ran very hot, so that once you get to the top of the clean headroom compression and a bit of overdrive comes on quite quickly. I actually think the combo cab and choice of speakers - relatively bass light and mid-pushed - has the biggest influence on people's perception of the "vox sound".

A also think the preamp circuit has a big influence on the character. The normal channel is pretty much as simple as they come - not much in the way of tone shaping - so the result is very raw and unrefined. The top boost channel has a tiny coupling cap value that shaves off loads of bass, an eq stack that is very bass heavy and scooped to compensate, and a cathode follower preamp tube setup that adds compression and asymmetrical distortion.

edit; I guess the valve rectifier has some effect - though the GZ34 doesn't sag voltage as much as some of its sister valves, it does to my ears soften the pick attack into something that's quite characteristic in vox amps. Instead of being bright and chirpy it becomes a bit more... hard to describe with words, i always think of a vox pick attack as going "pl" instead of "CHING!" 

jpfamps

I think a key feature of the AC30 is a lack of negative feedback applied around the power stage.

The lack of negative feedback increases distortion, even when the amp is "clean", reduces speaker damping and bandwidth.

The AC30 also has the speakers wired in series, further reducing speaker damping.

I think the cathode bias is a bit of a red herring in the AC30 sound; EL84s were designed to be almost as efficient in cathode bias as fixed bias.

I would not consider power supply sag to be that important in the AC30 sound as the valves are biased fairly hot, near class A operation, so even when driven hard, there is not that much of an increase in current draw.

I also think the relationship between the HT voltage and primary impedance is important. The AC30 uses a 4K load and around 320 VDC on the HT.

Most other EL84 based amps seem to use a significantly higher HT, which would imply a higher primary impedance, but in my experience most manufacturers use the "standard" 4K. In my experience the lower HT sounds better (and is better for the valves).

Catthan

Thanks guys!

@Cirrus, I'm not surprised by what you're saying (the non-technical part). 

@jpfamps, your description is a bit more technical than what I can grasp I understand that there is smth different (in some degree at least) to the VOX power amp.

Would you say that this difference makes the power stage (or it's part in tone) noticeable in the AC30?

iow, Would you say that the features you're describing are audible?

More/ less than other in amps?

Thanks,

T

jpfamps

Catthan said:

Thanks guys!

@Cirrus, I'm not surprised by what you're saying (the non-technical part). 

@jpfamps, your description is a bit more technical than what I can grasp I understand that there is smth different (in some degree at least) to the VOX power amp.

Would you say that this difference makes the power stage (or it's part in tone) noticeable in the AC30?

iow, Would you say that the features you're describing are audible?

More/ less than other in amps?

Thanks,

T

Yes, the effects are audible in the power stage.

The more/less than other amps is the wrong question, as you are comparing the sound of the AC30 with other amps.

In my opinion the lack of negative feedback is the most audible feature.

NervousJohn

There's a rather brilliant documentary called something like "vox pops : how dartford made rock and roll" about the AC 30 and gives a really good layman's description of negative feedback.

Catthan

Thanks @NervousJohn, I'll check that out.