The Watering Hole

I don't believe I've seen anything more controversial than the AC30 power amp section.  There are more myths surrounding this thing than there myths in the Greek world of olden times.   I will try to explain some of the myths and talk about the reality of this power amp and how it works.

There are two basic stages in the AC30 power section, as there are in most Class AB, push-pull amps.  WHAT YOU SAY??? Class AB????    Blasphemy...   but it is true, this amp is a push-pull, Class AB amp, biased hotter than normall and, NO, it's NOT Class A.

Tube Amp "guru's" will point out the fact that VOX used a "cathode bias resistor" to bias the output tubes and THIS unequivocally proves that the power amp is running in Class A.   Not true, cathode bias is just one way to bias a tube and a cathode bias resistor can be used to bias the tube either Class A, A1, AB, AB1 or B.   The value of this resistor just determines where along the load-line the tube gets biased.

This power amp, as noted above, is biased "hotter" than normal, closer to a Class AB1.  So, let's talk about Class A, A1, AB, AB1 and B, for a moment.  In pure Class A topologies, the power tube(s) amplify the signal for the ENTIRE signal swing.   The bias point is set, for the tubes, either via a "fixed bias" (voltage applied to the grid with a grounded cathode) or via cathode bias (a resistor connected between the cathode and ground).  The bias voltage is chosen to set tube current flow and plate voltage at the center between the "extremes" (cutoff and saturation) for the chosen plate impedance/resistance.  

In Class A1 topologies two, or more, tubes are run together with each tube amplifying MOST of the signal, but not all.   One tube, or the other, "finishes" the final "peak" of the signal swing.    

In Class AB and AB1 topologies, less and less of the output signal is "handled" by one tube, or the other.   The bias voltage, on the grid, again, determines just how much of the signal swing is handled.   The "hotter" the bias, the more contribution each tube has toward the final output.  The "cooler" the bias the less contribution each tube has toward the resultant output and the more "independant" of each other they work.   AB1 is closer to Class A, with each tube running "hotter" and amplifying a significant portion of the other tubes signal.  Class AB is closer to Class B, with much less "overlap" between the tubes.

The AC30 clearly has a "push-pull" topology in the power amp.  The OT is center-tapped (one dead giveaway of push-pull) with the output tube power being fed into the center-tap of the OT.   The top tubes feed the "top" of the transformer and the bottom tubes feed the bottom of the transformer.  

Thus, even though the tubes are biased hot, this power stage would never run in pure Class A.   Another issue, that tells us this can't run in Class A is the high plate voltages (around 300V, or so).   To run in Class A means high current.  High Voltage, coupled with high current = High Power.   In fact, at the plate voltages that VOX has chosen, for this stage, running in pure Class A would burn up the power tubes, as the plate voltage * plate current would literally melt these tubes right out of their sockets.  To get close to running in Class A, the plate voltages would have to be dropped about 50-75 volts to stop this from occuring.

So, sorry to say, the VOX AC30 is just another push-pull amplifier running in Class AB1, not Class A.   Just because it has a Cathode resistor doesn't mean it's Class A.   >:(

So, there must be something that differentiates the AC30 power stage, from the other ones in the industry... what is that thing?   If you look closely, at an AC30 schematic, you'll find that one thing, here, is missing and that's a feedback loop.    The AC30 power amp has no negative feedback which means that it is, essentially, "free-running".

Absence of negative feedback increases the output impedance, of the power amp, signficantly and reduces the power amps ability to "control" the speakers (low damping factor).   Because of this, the speaker/cabinet resonances (low frequency, around 100Hz) are emphasized and the voice coil resonances are emphasized (high frequency rise to a "peak" around 2-4kHz - depending on speaker design/construction).   This high frequency emphasis further contributes to the "chime" and the low frequency emphasis contributes to the "mid-cut".  Together, they create a "dynamic" and "responsive" amp, with a relatively "chimy" top-end, and a "floppy" or "loose" low-end.   However, because of the "slope" introduced by the tone stack and the 675Hz high pass filter in the interstage coupling, the low-end on the amp remains relatively "tight" as there isn't much there, to begin with.  

To summarize, the VOX AC30 power amp is, really, a Class AB1 amp with both power tubes contributing to a significant portion of the others "swing", but they still do "cut off" and don't work for the entire swing of the AC signal.    

The LARGEST contributor to the VOX AC30 power amp tone is the fact that there is no negative feedback in the power stage - unlike the other two topologies we've looked at.    This further enhances the "chime" and the "grind" produced in the preamp as it reduces the damping factor of the power amp and allows the speaker and speaker/cabinet resonances to have much more precedence over the final sound.  In both Marshall, and Fender amps we've looked at, in the past several lessons.

Another thing to think about, and we've covered this, prior... whenever negative feedback occurs around the power stage the power tubes, and all tubes involved in the feedback path DO NOT transition nicely from non-distorted (clean).  When there is no negative feedback around the power amp, the transition of the power amp, from clean to distorted, is much more "graceful" and "smooth" and thus the power amp distortion, on this amp, will be much more pleasant.  

Enjoy... ask questions if you have them.

Dar

Dar

Too ignorant to ask quetions.  ;)

I'm following you, but I'm having to trust what you're saying, in which I do.  :D

Very interesting info though and great reads.
I'm printing them out and putting them in a little folder for later enjoyment.

Thanks again.

Randy

So, just to make sure I wasn't speaking out of turn and/or starting some new legend about the VOX AC30, I went and reviewed all the information I had about the AC30, both mythical, and non-mythical.  I also grabbed a Mullard, yep the tube company of old, "Tube Circuits for Audio Amplifiers..."

On page 18, under the title Pentode Push-pull Stages Mullard directly addresses the EL84's (the tubes used in the AC30 power stage) with this paragraph:

The conditions for Class AB operation normally recommended and published by valve manufacturers are based on measurements with continuous sine wave drive.  The bias under zero-drive conditions and the anode-to-anode load resistance are so chosen that the optimum performance is achieved when the working points of the valves are displaced under driven conditions.   This displacement is caused by the effect of increased anode to screen-grid currents on the cathode-bias circuit.  For a typical oiutput stage working from a 310V mains supply and using EL84s, the rise in cathode current - and thus in cahtode bias voltage - with a sinusoidal signal voltage is about 40% at full drive...

There are two interesting points, here, to note... first, Class AB operation utilizes a DC voltage, on the plate, of about 310V (320V for the AC30).    So, we're in line with that.   The other issue is, as noted above, that the plate current * plate voltage = plate dissipation (watts).   With the plate current generated by such a high supply voltage, the plate dissipation, in watts, would be too high and the tube would burn up running in class A at this voltage.   The RCA Tube Manuals (RC-14, RC-20) also indicate roughly the same thing.

There is one other thing that is in this, that I'm going to cover in another topic, as well, about "partial driving of the power amp..."  but that's another topic... the AC30 is, truly, a Class AB1 amplifier without negative feedback.  

Also interesting, this book has a preamp design with an EF86 Pentode, two 12AX7's and two EL84's that is almost component-for-component similar to the VOX AC15!   That, too, is described as a "Class AB" power stage!  

Dar  

Great series of posts, Dar, thanks a lot! I was particularly interested in your Vox AC 30 posts, having played one for about ten years back in the day. The three different inputs (Normal, Brilliant and Vibrato) were sounding very diverse, so it would be interesting to know, who of the wellknown AC 30 players used which input.
I can contribute a little story: back in 1971 I went to a Rory Gallagher concert. I was curious about how he got his sound, because I played a Strat (boosted with an ElectroHarmonix Powerbooster) through an AC 30, too.
So after the concert I went backstage and had a little talk with Rory. I first asked him, which input he uses, and he told me, he uses the Normal input. Then I wanted to know, why he doesn't use the Brilliant input, and with a painful expression on his face he said:"Too much treble, man, too much treble".
With my next question it became clear, why he was playing through the Normal input. I asked, what that little box was, that he had on his amp, and he replied: "That's a Rangemaster Treblebooster. It's made by Dallas Arbiter, but they don't make them anymore. If you want one, though, go to Music City in Hamburg, I think they still have 'em". Well, next morning I called Music City, and got one of the last two Rangemasters they had.
And yes, the pretty muddy Normal input, driven by the treblebooster, really sounded great.

I'll reply to this topic, here... the AC30 "Normal" channel differs SIGNIFICANTLY from the "Brilliant" channel and, by significantly, I mean significantly.   We talked about the "Brilliant" channel shelving between stage 1 and stage 2 at 600 something Hertz... and then a further "shift" in frequency response, in favor of the Treble side with the tone stack...

Well the "Normal" channel is a relatively "normal" 12AX7 gain stage with a heavily bypassed cathode resistor allowing the stage, from a signal standpoint, to achieve about as much gain as is physically possible.   This is coupled to the next stage via a .047uF capacitor feeding a 500K volume pot.  This, essentially, is flat response, too.   The output of the volume control is "mixed" through a 220K resistor right into the splitter, which feeds the power amp.

Like the AC15, the "Normal" channel consists of only a single, flat response, gain stage with no voicing.   For most guitar players, especially those into Blues/Rock, this channel would be horribly "woofy" and "dull" compared to the "Brilliant" channel.  Most of the folks that used this channel did use some type of "Treble Booster".  

The rangemaster uses a single transistor (PNP) setup in CE-bias.   The input (guitar) is coupled through a .005uF capacitor which is pushed across a 68K resistor.  This forms a high pass filter with a center frequency of about 470Hz.   The output, then is tapped off the collector of the transistor and fed into a .01uF capacitor and that would "feed" the 1M resistor at the input to the VOX.   The output of this stage would, basically, "run flat" into the VOX input.  It's the "shelving" that occurs at the input of the Treble Booster that does the work.

Interestingly enough, the Rangemaster shelving frequency is about the same as the shelving frequency in the interstage coupling of the Brilliant channel.  Not quite, but close.  That would yield a similar character, in the end, as the Brilliant channel but without the additional "slope" provided by the tone stack in that channel.  A lot of "flat response" preamps do very well with shelving frequencies around/above about 450Hz, or so.   VOX's shelf is 600-something, MESA is 800-something, Marshall is 700-something... it all comes down to those shelves as to the final result of the "voicing".   You can't run "flat" and have good distortion/breakup.

Dar

FIXED... sorry about the mixup.

hahahah,

look at the end of your post,  Dar Dar Dar   :)

Thanks... got it covered.

Dar

no need to apologize.  We can just change your name to Digital Delay Dar.  

Or maybe "canyon man"   :)