Were the original Vox AC15's wired correctly?

[Amanda]

Hi Ernie,

The output stage of the AC15 Heritage is wired up the same as the
Circuit 3 AC15, I.E. resistors in the screen leads.

As for the sound, they do compare favourably with the original AC15's.

Peter, the resistors mentioned are in the anode leads on the circuit 2 AC15 and in the screen
leads on the circuit 3 AC15, on neither chassis is the output transformer connected to the screen grid!


Regards

Amanda

[ernie1958]

Hi Ernie,

The output stage of the AC15 Heritage is wired up the same as the
Circuit 3 AC15, I.E. resistors in the screen leads.

As for the sound, they do compare favourably with the original AC15's.


Regards

Amanda

Thanks Amanda...so the AC15 Heritage is a good option if you can't find or get the original one..
Btw Amanda...do you think it's possible to add more PLAYBACK heads on a Copicat echo-machine...let's say 6 heads instead of 3..?
I surely would like to find out the "fx" if that's possible...maybe get a kind of Meazzi config out of it..

Cheers,
Ernie

[Amanda]

Hi Ernie,

Anything is possible, a varispeed copicat would be the best
and I reckon there is room to add extra heads on the other side
of the loop, or it may be possible to move the heads around to fit more in.

Make a slightly larger case to put the extra electronics in.

on the Meazzi i'm fixing at the moment the main heads used are 1, 3, 4, 5
(it has erase, record and 6 replay heads!

Amanda

[almano]

Bravo, well done Peter and Amanda, you’ve solved the question of that strange wiring in the old Vox AC15 for me – it does indeed seem like they chose to design the early circuit as a triode connected pentode.

That rogue 100 ohm resistor in the anode is there to suppress RFI (radio frequency interference) – so it has a positive purpose after all.

It appears, from a wee bit of research, that triode connected pentodes were meant to give a relaxed and less bright sound. There is a reduction in output from this configuration, but to offset this, the distortion is kinder as the volume increases – which means it doesn’t sound that unpleasant as it reaches its upper levels.

I must admit that I hadn’t considered this triode option – but it does make a lot of sense, especially after Peter pointed out that present day high end amps now offer switching between pentode and triode modes.

This therefore means that I’ve now got a triode connected ancient / modern design amp in my armoury alongside a standard Fender amp and new (practice) Vox VT30 – plus of course, my old (very wrongly used) Leak set up (which I’ve yet to get working again!).

Cor, it certainly does appear that I’ve somehow managed to acquire a few rather decent amps along the way without actually realising it! So, I wonder, - what price now that old Michealson & Austin TV10 and the Rotel RA1412 amps I’ve still got ferreted away in storage for potential use as good quality guitar rig amps? – they could perhaps (hopefully) prove rather promising!

Thanks to everyone for your input, you’ve made my day!

Cheers,

Alan.

[Amanda]

Hi Alan,

The EL84's in the circuit 2 chassis are not wired in triode mode,
the screens go direct to HT, the anodes are connected to the
output transformer, if the screen grids were connected to the
anodes then that would be triode mode, which is the way it is done on the
AC15 and 30 Heritage amps. Also if the EL84's were wired as triodes the output power would be
lower than 15 watts.

Incidentally The Heritage amps offer the EF86 working in triode mode,
for the last two years of AC15 production '64 - '65 I think the Ef86's
were wired in triode mode!

Amanda

[almano]

Hi Amanda, many thanks for the info – it almost seems to be some sort of odd item I’ve thrown in the ring there with this early AC15.

The circuit certainly seems to be a little unconventional in its anode and G2 arrangements. If it had been ultra linear I would have probably understood it and thought no more about it – but those later circuit variations I had obviously been referring to (without realising there had been so many changes made to this amp over its lifetime) and the, what appeared to be, strange anomalies rather threw me – and hence, therefore, my initial posting on this topic. Thanks for the enlightenment, my curiosity has been more than answered with the superb advice you and everyone have been more than kind enough to put forward on the subject.

It is truly wonderful to find there is so much of an abundance of high level knowledge around in the membership of this forum on all diverse matters associated with the Shadows, be it the music, the guitars or the equipment, - that it is almost an imperative to become a member of Shadows Music if you have any sort of interest in the subject at all. I only regret not knowing about the Shad’s Music site years ago – this level of enthusiasm is incredibly inspiring.

This AC15 actually has an additional added ‘extra’ consisting of one of those aluminium chassis (the ones we all used to use for home built projects) with a prototype treble tone boost circuit actually built by Dick Denny himself fitted in it. It is hand built and looks very interesting. I’ll be more than happy to post a photo or two of this prototype tone boost circuit next week in this section of the amp’s forum if it will provide any interest to readers of this topic.

Cheers,

Alan.

[Sparkyshadow]

Just a note re the Heritage 15. Last week I traveled to the USA from Canada to play a Fundraiser. I took the Heritage 15 to see what it would sound like it a large (500 capacity) Hall. It was not quite enough welly so the sound engineer put a mike in front of it. The heritage was cranked all the way up. The result was, the audience froze when the first verse notes of Wonderful land were played. I and clearly everyone else were amazed at the tone coming out of that little amp. It is one awesome amp. Other equipment Yamaha stomp box with charlie Hall echo unit and a HBM Jap strat with Lollar pickups....One great amp. Regards Sparks

[almano]

Hi, I did say last week that I would post a photo or two of Dick Denney’s prototype (just one of quite a few prototypes I believe) treble/tone boost circuit here for anyone who’s interested in seeing some of Dick’s own handiwork.

As you can see, the treble boost circuit is mounted on a separate aluminium chassis. Presumably I imagine the production versions would have been incorporated into the main amp assembly as an integrated unit – but this is something I’m sure someone will be able to advise me on.

The second photo is of the AC15 itself with a view of the treble boost circuit located in its place, on the right, next to the main amplifier chassis.

With this rebuild I am trying to keep everything looking cosmetically correct. I’m doing this by leaving the old electrolytic blocks intact and cutting the leads and gluing replacement capacitors underneath and hard wiring them in – I think this is probably the best approach – unless anyone else has a better method of doing such a refurbishment – I am always more than willing to listen to any advice.

I hope you find the photos interesting.

Cheers,

Alan.

[Amanda]

Just looked at the pictures again and noticed,
check the output and rectifier valve positions,
the EZ81 is usually next to the Mains transformer!!

[almano]

Hi Amanda

Yes, I agree – it does look like the EZ81 is in the wrong place. The logical placement would, or should, be next to the mains tranny – if only in order to keep induced hum down (although, being the output bottles, any pick up of mains hum would be minute) – but with this chassis, the EL84’s are actually correctly sitting right up against the mains Tx as far as the wiring goes. It’s certainly not what would be expected as you say, it does indeed actually look wrong.

Presumably this is something they would have changed to a more logical arrangement with later updates of the AC15 – it would make rather more technical sense I think.

It’s all like one of those strange illogical things such as the signal or speaker cables apparently changing the sound of anything. Technically there is no real reason that they should have any particular influence over the sound. Alright you could say the inductance, capacitance, resistance or even microphony of a cable or wire may have a marginal difference somewhere along the line – but, in a technical sense, the difference is fairly marginal. But, something certainly does exist – I’m sure we must all have noticed the difference in sound with changing the guitar to amp cable to a different one. It is only a cable after all, but it actually does, and can, sound different for some odd reason.

Therefore, could this strange valve arrangement you’ve picked up on Amanda have any influence over the ultimate sound from this early version AC15?

It would be really interesting to know if it has. Perhaps if some enterprising manufacturer was to experiment with these odd wiring arrangements, we could actually end up with the definitive 1959 sounding Hank amp – let’s keep our fingers crossed and hope! – or is it all just wishful thinking somehow? - oh well, I can dream at least!

Cheers,

Alan.