Vox-AC50 MkII (early)

This is an amp head I bought on ebay years back. The AC50 is perhaps most famous as an amp head the Beatles used for a very short time. They had used the 30W Vox AC30 in the clubs, but now were in front of more screaming fans than ever. Vox responded by first creating the AC50 (50 watts) and the quickly thereafter, the AC100 (yup, 100 watts!) to help them be heard through the din. Public address was in its infancy in the early 60s, so you had to get it done on stage. And that meant louder amps.

This example appears to be a salvage job of some sort. The cabinet is new, probably made at Northcoast, and is the small-box type used by the earliest AC50 amps. The lower chassis was chromed, the circuits have been restored and both transformers (power and output) appear to have been re-wound, probably by Mercury Magnetics (based on a visual inspection). All in all, it was very cleanly re-built. The only outstanding "miss" was the lack of a shield by the input jacks, but more on that in a minute.

The small box AC50's are so short that modern EL34s will not fit under the ventilation screening, so the screening is not mounted at the moment.

Based on the chassis' configuration, it is correct for a very early MkII amp. It is using a GZ34 rectifier and still has the neon bulb in the bias supply (a clever tweak). That puts the amp around fall '64 to early '65. Vox would soon replace the tube rectifier and move to a larger cabinet, so this is a bit of a rare configuration.

On the rear panel we have (L to R) 8 ohm out, 16 ohm out and then AC Mains on the far right. Vox took the unique approach of using Canon connectors for power and speakers. These are also (confusingly) the standard cable connection for microphones. Their chief advantage is they lock in place, and perhaps prevent you from using 3rd party cables.

The lower chassis chrome is in pretty good shape; the transformer end bells were re-painted the stock Woden grey color. Whoever re-built this did a very nice job.

My home-made shield to protect the inputs

There are two odd things about this interim AC50 that stick out at me.

First, the layout is a bit risky. Wires create magnetic fields around themselves with the presence of alternating electric current. And nowhere is the current higher than in the speaker leads. Those are the 3 leads you see twisted up and running off the bottom of the picture here. Right above them are the input jacks, where the tiny guitar signal enters. Net net, this causes circuit instability and feedback at high frequencies. Other AC50s I have seen photos of all have a shield by the input jacks to keep this from happening. I have a first attempt in place which is better, but not quite there. I used the transformer mounting bolts to hold a pair of angle brackets, and then cut a sheet of aluminum to mount to them. I'll be replacing it (or possibly augmenting it?) with a steel shield that sits tightly against the chassis.

Which brings me to odd thing #2. The circuit as a whole is really a cousin to that OG of all OGs, the Fender 5F6-A Bassman. Now the 5F6-A took a lot of inspiration from the Gibson GA77, and Vox has confirmed they were influenced by the Gibson as well - even keeping it's rather odd tone control setup. But here is the the thing - Vox used a very low gain tube in the first stage here; a 12AU7. The stock tube in a Bassman is a 12AY7 with a mu of 45 and most folks (including some dude named Jim Marshall) pop a 12AX7 with a mu of 100 in there and rock out. A 12AU7 has a mu of 19; less than half of a 12AY7.

When I was in recording engineering school they drilled into us the importance of getting as much gain in the first stage as you can to minimize noise. The signal will never be less noisy. So why swap in one of the lowest gain tubes you possibly could here? Was it to counter the instability inherent in the layout? Did they just have a lot sitting around? They did use them already in the AC30 as part of the Vibrato circuit. (sorry, "tremulant"). They had already used them in the single channel AC50 Mk1. I don't know.

The final piece of that was, for me at least, the sound of the "Brilliant" channel. It was a bit cold and harsh. The "Normal" channel was better but still lacking the classic Vox chime and clear high end the AC30 has. I wound up using it for recording at home, but I never played out with it because I was not that happy with the sound - or the stability.

Now the AC50 shipped with a 2x12 speaker cabinet that had a Midax horn installed to boost the highs, which buyers seem to have disconnected as pre-requisite to playing rock guitar through these amps. Someone was missing the highs.

So let us review: we have a lack of highs, an odd gain structure, and if I am honest, an uninspiring sound from what is usually the better amp channel - the Brilliant channel. Why? These were smart folks who were building some very popular music gear. And the basic design is a kissing cousin of some of the greatest guitar amps ever built.

This drove me to (finally) pay more attention to the schematic:

A beautifully re-drawn copy of the AC50/2 by Chris Devine whose terrific website is sadly no longer up. I hope it returns!

Things of note:

1. That wickedly cool neon bulb in the bias supply - it clamps the voltage, it's a cheap regulator. I love it. Scrappy engineering at its best.

2. The first stage tube runs each half at very different settings. The plate and cathode resistors for the normal channel look pretty ok for a 12AU7 but the Brilliant channel really looks like it was designed for a a 12AX7. There isn't a mfr spec for the 12AU7 with these kinds of parts. What it is, is a 12AU7 that has been biased very cold. It is strident and grainy sounding. Why? What were they going for?

And that got me thinking. There is a tube that is half 12AU7 and half 12AX7, it is called the 12DW7. And coincidentally (?) the 12AX7 is triode 1 which uses pins 6, 7 & 8. Just right for the AC50. The 12AU7 half uses pins 1,2 & 3. I hear master Oogway quietly telling me, "there are no accidents". What a wise old turtle*.

First I tried a 12AX7 to see what would happen, and what happened was glorious. The AC50 became a snarling guitar amp. It got mean, it backed off and got clean. It did all the best guitar amp things. Just a ball to play! I did some quick googling and others have had good luck with this as well. Good to know. And the higher impedance of the 12AX7 "fixed" the highs - I had to roll them off a bit on the guitar!

Both channels worked well with a 12AX7 substitution, but I went ahead and bought some 12DW7s to test out - could I keep the best of the old and the new?

Here is how that went:

The top pair of tables are one or the other - 12AX7 on the left, 12AU7 on the right. The supply voltage changes a lot on the "Normal" channel due to that 56k dropping resistor in the power supply. The 12AU7 draws 2.9 times as much current as the 12AX7.

The bottom pair are the 12DW7 which as you can easily see, matches up really well with both tube types and on the proper channels (based on how I think about the schematic) I filled in the cels like that to make it easier to compare with the prior run.

I am not saying Vox goofed or made any sort of mistake, I am simply of the opinion that there were some curious choices going on that as a guitar player I was not crazy about. With this one completely reversible tweak I enjoy playing the amp so much more now. I have the higher gain Brilliant channel, and I have the same Normal channel I always did.

I still can't explain putting the speaker outs next to the inputs, but there they are.

If you have one, try this tube swap out and see what you think. You certainly wont hurt anything. "Amaze your friends, confound your enemies", all of that.

References:

AC50 site: https://www.voxac50.org.uk/vox_ac50_mark_2_design.htm

* Go re-watch Kung Fu Panda. You're welcome.

Output Impedance Calculator: https://www.ampbooks.com/mobile/amplifier-calculators/output-impedance/calculator/

Loadline Calculator: https://www.vtadiy.com/loadline-calculators/loadline-calculator/