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Old 30th Jun 2018, 2:00 am   #1
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Default VOX AC10 clone

I'm at the pointy end of a scratch build Vox AC15 clone build for my adult son and while it has been a lot of work I have enjoyed doing it. Primary reason for the build (son did not ask for it) was that I had a 60's vintage Woden transformer set - mains, output and choke for a push pull el84 amp along with a Goodmans Audiom 60 speaker. As it was I ended up not using the Woden transformer set as the filament winding would have been running right on the limit or slightly over its rated current and I was concerned about temperature rise.
Which brings me to my question - I still have the Woden transformer/choke set and also an old pair of Celestion vintage 10 speakers so why not build a Vox AC10 twin clone which has 2 less valves and so requires less filament current , I'd like to keep it as close as possible to the original (sans the Vox logo of course) and need to know the 2 piece chassis dimensions does any one have this info it would be much appreciated ?
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Old 30th Jun 2018, 3:54 pm   #2
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Default Re: VOX AC10 chassis dimensions

Looking at this pic - http://www.voxshowroom.com/uk/amp/ac10_hood.html you should be able to guesstimate the chassis dimentions. I laid out similar components and measured 13" wide, ish. I'd say the top was 8" or thereabout deep, depth of bottom chassis 1". Top chassis same width 13", 3" deep. Your bog standard chicken head knob is 1 1/4" across, 3/4" either side = 2 3/4" to 3".

I've seen a measurement of 20", wide, but that's daft, the cab will be built around the speaker, so therefore spkr is 10" x 2 = 24" total max add a bit for clearance.

But as your building a clone, you can make it better and whatever size you want using the original as the starting point.

Andy.
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Old 1st Jul 2018, 1:43 am   #3
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Default Re: VOX AC10 chassis dimensions

Yes, since posting I have been doing some googling and while I haven't found any published chassis dimensions I have done a CAD mockup, using the external dimensions of 24"(60cm) x 18"(45cm) x 9"(23cm) which are often listed in the specs.

I used CAD to draw up the outline of a cabinet add in the thickness of the sides, top etc add internal corner cleats, the 10" speaker and the sliding shelf and that gave near enough to 12cm(5") as the height for the vertical section of the chassis. This link http://www.voxshowroom.com/northcoas...15_TE_blk.html gives AC15 cab dimensions where they quote 5 3/16" (13.2cm) as the height for the vertical portion of the chassis.
This link http://oldamps.weebly.com/vox-ac-10-jmi.html has a picture of a Vox AC10 chassis sitting in an AC15 cab and it looks to be just the right height so I'm assuming my dimension 12cm(5") is a little under size, I should be able to tweak the speaker position and corner cleat size to accommodate a vertical size of 5 3/16" (13.2cm)

Since advising No 1 son that I intend to do an AC10 twin clone he has asked for a master volume to be included and once again using CAD I have come up with a width of 38cm(15") once the master volume is added in, as you say while my build is an AC10 twin clone I can add my own extras.
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Old 1st Jul 2018, 5:32 am   #4
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Default Re: VOX AC10 chassis dimensions

Looks like you've sussed it out. some good detailed pics on 2nd link. Noticed the OPT is a bit diddy, think the original was built down to a price. Can you still get valve bases with retaining clips? It's one thing I don't like about old amps like this, the valves are like bats, it's a wonder they didn't give them little capes and and a pair of fangs : ) My WEM is the same.

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Old 1st Jul 2018, 11:37 am   #5
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Default Re: VOX AC10 chassis dimensions

If that is what you want to use, it is the respected "Contempo" chassis. It is actually in two parts, one steel (cadmium-passivated) and one aluminium. I will quote from "Vox Amplifiers - The JMI Years" by Jim Elyea:

"The basic part of the chassis was the bottom, steel section. Of very simple construction, the bottom portion was a long, rectangular, three-sided box, with the two short ends open. The bottom edges of the long sides were bent out to form a pair of flanges that served as the base for the entire assembly. A pair of cage nuts was inset into each of the flanges to allow the chassis to be bolted to the wooden plinth. The steel chassis held the power section, including transformers and output tubes.
A separate aluminum chassis was bolted at right angles to the back side of the steel chassis. This aluminium chassis was basically an inverted "L" with a flange bent onto each of the long sides. This chassis held the preamp section and the top-facing control panel.
"

I currently have my AC30 chassis out of the cabinet and can give you the dimensions.

Width of steel chassis - 5.25"
Height of steel chassis - 1.25"
Width of flanges on steel chassis - 0.5"
Thickness of steel - 1/16"
Height of aluminium chassis - 5"
Width of aluminium control panel - 2.75"
Flange on aluminium control panel - 0.625"
Thickness of aluminium - 1/16"

The AC30 chassis is actually 21" long, but the AC15 must be shorter. Stephen Grosvenor, in his excellent book "A Service Engineer's Guide to the Vox AC30 Valve Amplifier", quotes the size of the cabinet of the AC15 single as 20" wide, 21" high and 10" deep. The AC15 Twin has the same height and depth, but the width is 27". Knowing that the control panel on the AC30 sits ~3.75" in from the outer edge of the cabinet, I would make an educated guess that the length of the AC15 Contempo chassis would be ~14" (bear in mind that the same chassis would have been put into both the Twin and the Single). There is also some addition to the covers of the transformers, such that bolts go through lugs on them and into the aluminium chassis to strengthen the joint between the two chassis. I should also add that on my AC30, the aluminium chassis only has a flange on the short leg of the "L", not both.

I have used Imperial units here, because I'm certain that Jennings Musical Industries would have done, but I had to convert to decimal fractions generally, as I don't know how to get Imperial fractions on this forum. I have used the American spelling of "aluminum" when quoting Jim Elyea, but the English spelling when they are my words. I apologise if any of this is confusing.

If I can give you any more info, just ask. My AC30 is out of the cabinet for a while, as I am giving it a bit of a restoration, putting period-correct carbon composition resistors and such like back in. Incidentally, don't use metal-film resistors in a classic amp like the AC30, even if it is a clone. It won't sound right. If you can't find carbon composition resistors, use carbon film - please!

Good Luck, Colin.
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Old 1st Jul 2018, 1:25 pm   #6
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Default Re: VOX AC10 chassis dimensions

Thank you for the detailed reply.
For my AC15 build I copied info from another diyer (I've lost the link) and saved all of the info on the chassis and cabinet size without actually doing a great deal of research, it turned out that the dimensions given were on the large size and it was not until I came to build the cab that I realised my chassis would not fit into a standard sized AC15 cab - it was too late to change the chassis as this was completed, the end result was around 1inch bigger all round- however it was not a disaster as the aim was to create a look alike with the same sound. The vibrato was a bit confusing as not ever having heard guitar vibrato I wasn't sure if it was working correctly and the only way I can describe it is that it is like a warble.
I built the chassis from 2mm aluminium as this is what I had on hand, most of my AC15 build dimensions were close to the AC30 dimensions you've given with the exception of the height of the aluminium chassis (overall width excluded of course), mine is 6.3 inches, I'm now guessing that the diyer I copied had to increase the height to accommodate turret boards available to him, I ended up making my own turret boards. I'm not fussed about dimensions in metric or imperial, I'm now retired and grew up with feet and inches but now tend to use metric mm for all but lathe and mill work (still stuck with thou's here) so I can visualise both.

Thank you for the heads up on the resistors but I did use metal film resistors for my AC15 build as I heard that carbon resistors can be noisy - I was bit shocked when I saw them they look so puny. I don't think carbon composition resistors are that easy to come by but I do have a supply of carbon film 1watt resistors in just about all values from 22R to 1meg popular sizes. I'll be sure to use these for the next build.

Having been a bit off with my AC15 build It would be nice to be a bit closer to the mark with the AC10 twin I'm planning on next. I've included some pics of the AC15 build it is more or less finished just waiting for the tolex and piping to arrive.
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Old 1st Jul 2018, 5:42 pm   #7
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Default Re: VOX AC10 chassis dimensions

The vib/trem on these Vox amps is a real weirdo. I am assuming that your AC15-clone circuit is the 1960 era one, with pretty much the same as the AC30. This one has the vib/trem switch and uses an ECC82 and an ECC83. The earlier design uses the pentode section of an ECF82 in the "Vibrovox" unit, which only gives tremolo, or amplitude modulation. Vibrato is frequency modulation and is much trickier to achieve. The circuit that Dick Denney used (pinched from Wurlitzer organs) uses a myriad of valves, as you have found. I have done quite a lot of searching about this and come up with some interesting stuff. I could probably give you a few leads, if you are interested. I notice that you have a normal, panel-mount potentiometer for the preset depth control. You would be better off putting a skeleton-type in there, as a few bumps could cause the pot to come adrift - guitar amps do get banged about a lot! This control set at mid-range gives a reasonable tremolo level, but barely any vibrato. You are right, it is something of a "warbling" effect, but should be relatively slow. It is somewhat akin to the use of a whammy-bar on a guitar. There is a good case for not incorporating any kind of tremolo circuit when building a clone of a classic amp. It is rarely used these days and would tend to be switched off, if existing. There are exceptions to this rule, of course!

I have looked more closely at an AC10 chassis photo and have revised my thoughts on its length. It only has four input jacks, which are quite close (probably as close as on the AC30 with integral top boost), five chicken-head control knobs and the mains controls look to be closer than on the AC30, too. So, I'm guessing at about 16" now. The reason for the bottom part of the chassis in the Contempo being steel is that all the heavy stuff (transformers, choke) are mounted there and aluminium would probably sag under the weight (the earliest Marshall amps were clones of the Fender Bassman, except that they used an aluminium chassis, rather than a steel one. Some people claim that that is a reason why JTM45s sound different to Bassmans).

OK, confession time...Some years ago I stupidly changed all of the small components in my AC30 for modern ones, including replacing carbon-comp resistors with metal-film ones. The sound was a somewhat "cleaner", but I had not only sacrificed the original sound, but I had also reduced the value of the amp significantly. I wasn't so stupid as to throw away all the replaced components, so what I am doing now is to put it all back to what it ought to be, except for replacing out-of-tolerance, faulty or broken parts. OK, so if you are making a clone of a classic, using whatever resistors are to hand probably doesn't matter, so long as you are happy with the sound. I am also making a clone of a Fender Deluxe Reverb (with some mods) and I chose to use carbon-film resistors, rather than searching out carbon comps. Some boutique amp makers favour carbon-film resistors, so I though that was a good argument. Carbon-comps can drift with age, but if the amp is a classic and is working, it may be that the sound you have, while not like when it came out of the factory, is actually what you love. This argument has been put forward by Dan Torres in his book "Inside Tube Amps". All three of the books that I have mentioned are full of excellent information, but I can recommend you get a copy of Stephen Grosvenor's book. You can get it from http://www.thevoxac30guide.com/ and at ŁUK9.99 it is a steal. Jim Elyea's book is rather pricey and shipping to Australia would probably be prohibitive. The copy I have is the Deluxe version with the schematics portfolio. Some of those circuit diagrams are the very devil to locate anywhere else.

Bye-the-bye, the classic "chicken-head" knobs are not all the same. Some have a wider pointer end. The ones used on Vox amps have a relatively narrow pointer. Caveat emptor!

As it happens, when I make a clone amp, I try to make up a logo that is similar to a manufacturer's that I think the amp is similar to. So, I have an "Earp" in ice-cream script because it is "some sort of Marshal(l)", a "Mega/Bluesy" which is inspired by Mesa/Boogie and my Fender Deluxe Reverb was going to have a Fenderesque "Fooldya" logo until I realised that the Deluxe Reverb didn't have a Fender logo.

If you want any more help, just ask.

Good Luck, Colin.
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Old 3rd Jul 2018, 2:05 pm   #8
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Default Re: VOX AC10 clone

I have just had another look at your post on the design of an AC10 Twin clone. I wouldn't bother with a master volume if I were you. Most of the circuit designs for master volume controls are not very good and require a relatively expensive twin-gang potentiometer. For one thing, they pretty-much eliminate the possibility of pushing the output valves and it is output-valve distortion that is the most desirable. Just having pre-amp valve distortion is a bit pointless, IMHO. The ten watts that two EL84s are putting out isn't ear-splittingly loud anyway and since they are capable of getting close to twenty watts, the channel volume controls will have pretty much the same effect as a master volume. As there is no negative feedback in the AC10 circuit, you can't put in a "presence" or "mojo" control either. I would look for a power-brake circuit. This goes between the output transformer and the speaker(s). You could either buy a ready-made one, or find a circuit and build one yourself. Augmenting this with an overdrive pedal will get you the best of both worlds; pre-amp valve overdrive and power-valve overdrive. That way, you can get the output valves singing as much as possible and throttle back the overall volume.
I might have some circuit ideas for power-brakes.
Colin.

PS: A presence control takes the feedback signal to ground progressively via a capacitor and potentiometer, while a "mojo" control reduces the feedback signal by the agency of a fairly large variable resistor. They have similar (but not identical) effects.

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Old 4th Jul 2018, 8:27 am   #9
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Default Re: VOX AC10 clone

The master volume is at the request of No 1 son he seems to think it is needed, I notice that the new Vox hand wired heritage has no master volume, and one of the older hand wired series of AC15 has a master volume but also what looks like a master volume bypass switch, so what you say is probably right I did initially say to him try it first with out a master volume and he did agree but before he picked it up to try it out I went ahead and fitted a master volume even though I wasn't 100% happy about doing this. It was a bit of a hassle as I had to also fit a tremelo speed pot with a push on push off switch to change between vibrato and tremelo. The master volume I fitted was simply a 500k pot wired between the grids of the output valves - turning the volume down shorts them together, this system is used on some of the cheaper printed circuit type Vox amps. It's not the best way to do it but the easiest. I have a small selection of speakers and he is going to fit them one by one to see which one he likes best I'm sure he will also want to try with the master volume removed I would prefer this as I can return the tremelo/vibrato switch back to it's correct location.
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Old 23rd Jul 2018, 2:37 am   #10
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Default Re: VOX AC10 clone

On the subject of vintage carbon comp resistors - I have quite a few populated tag boards with 10% and 5% carbon comp resistors, I went through them and found I have enough to fully complete an AC10 so I started to unsolder the ones I wanted checking them for out of spec as I went, I only got as far as removing 5 before I gave up, every one was out of spec by anything up to 20-25%, they were the fat type around 1/4" diam and just over 1/2" long with a white body so so looks like I'll be dipping into my supply of carbon film resistors.

No. 1 son has agreed to dump the idea of the master volume, he called around to check out the amp and give it a trial as I was keen to have the chassis done and dusted so I could concentrate on the cab and tolex etc. For the trial I had removed the master volume and refitted the vib/trem switch and put in the Goodmans Audiom 60 speaker that I have, he commented that he was stunned at how good it sounded to him, he does have a modern VOX AC30CC (about 5yrs old) and to him the AC15 clone had a far better sound, he has band practice coming up and is keen to try it and see if it is loud enough to be heard over the drummer.

When chicken head knobs were mentioned I decided to have a look through my knob supply and I was lucky to find that I have 8 chicken head types labeled Mcmurdo that look identical to those used by Vox they are shorter than the regular ones available today, didn't even realise I had them must be old age fogging my brain - this means I can shrink the chassis down to a size closer to the original as the size of the knobs will have some bearing on the width of the front panel and chassis. I redrew the front panel layout and have a width of 14" or 355mm and this is probably what I'll settle on. I'm not going to bother with the front panel voltage selector or even include a dummy one, I know this is a departure from the original but I don't think they are relevant today - I have seen these set to a lower voltage in an attempt to get more volume.
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Old 23rd Jul 2018, 7:17 am   #11
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Default Re: VOX AC10 clone

Good to see you making progress, pics?

Andy.
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Old 30th Jul 2018, 12:07 pm   #12
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Default Re: VOX AC10 clone

I've been busy with the tolex which is now only just finished so not that much to do before it is 95% done (still have the front panel graphics/text to do) here are some pics of the cab with finished tolex and partway through one of the gold piping bands that run around the perimeter of the cab, I did misjudge the width of groove that is needed for the piping and it is around .75mm too narrow, only way I can get the piping in is to squeeze a short length of it with pliers (about 1") and then tap it into the groove before it has a chance to spring back, I do admit though that it really is the wrong type of piping, it is upholstery piping and is meant to be stapled around the edge of something, I have cut off the tab that is normally takes the staples, this makes it almost round there are also some close up pics of the internals of the chassis and the finished baffle board.
I'm going to let No. 1 son take it with a temporary paper label on the front panel while I investigate ways to complete the front panel, the quickest and cheapest option is to draw the panel with graphics software, have it printed in colour by someone like Officeworks, glue the print to a strip of aluminium and finish it off with 3 or 4 coats of satin spray varnish. I would however like to see if I could make a masked anodised front panel and will investigate using photosensitive film (same stuff I use for pcb boards) to mask off the areas of text etc before the anodising process, then once the film mask is removed I should be left with an anodised panel and natural AL coloured text, first though I have to perfect the anodising - I've never done this before.

Not sure if anyone has done this before, just before the amp was trialed I swapped the russian EL84's for a pair of new Miniwatts and not looking closely I did not realise that I actually pulled the EZ81 and one of the EL84's, on turning it back on I waited for the valves to warm up and waited and waited and waited and just as I thought something had gone wrong it sprang into life, did not sound out of the ordinary, played a few cords (the sum total of my guitar playing) and turned it off closed the workshop and retired to watch a movie with my wife, next morning it took a while before the penny dropped after seeing an EZ81 lying next to the russian EL84, one of Miniwatt EL84's had done a great job plugged into the rectifier socket.
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Old 30th Jul 2018, 1:02 pm   #13
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Default Re: VOX AC10 clone

Quote:
Not sure if anyone has done this before, just before the amp was trialed I swapped the russian EL84's for a pair of new Miniwatts and not looking closely I did not realise that I actually pulled the EZ81 and one of the EL84's, on turning it back on I waited for the valves to warm up and waited and waited and waited and just as I thought something had gone wrong it sprang into life, did not sound out of the ordinary, played a few cords (the sum total of my guitar playing) and turned it off closed the workshop and retired to watch a movie with my wife, next morning it took a while before the penny dropped after seeing an EZ81 lying next to the russian EL84, one of Miniwatt EL84's had done a great job plugged into the rectifier socket.
That has to be one plucky little EL84! While the heater and cathode pins are common with the EZ81 and the anode_2 is also correct, the other anode pin on the EZ81 is marked as "internal connection" on the EL84. That means that some unspecified internal structure was acting as an anode, unless of course the valve was only acting as a half-wave rectifier, but then the HT would have been half of what it should be.
Colin.
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Old 30th Jul 2018, 7:38 pm   #14
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Default Re: VOX AC10 clone

Less than it should be, but not specifically half. How low would depend on the effective series resistance of the half wave rectifier, the size of the reservoir capacitor and the HT load. At no load, it would be the same.
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Old 31st Jul 2018, 6:18 am   #15
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Default Re: VOX AC10 clone

Nice job.

A.
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Old 5th Aug 2018, 10:16 am   #16
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Default Re: VOX AC10 clone

Most enjoyable and informative read well done.
Cheers
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Old 9th Aug 2018, 12:25 pm   #17
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Default Re: VOX AC10 clone

The amp is now 99% finished tolex, piping and grill cloth done and chassis mounted back into the cab just the front panel to do.

The Goodmans speaker is getting close to 60yrs old, it came out of a chuch hall PA cab and saw weekly use from around 1959-1960 until 2005 when it was scrapped and replaced with a new Mackie powered speaker. Old age, constant use and possibly insect attack has seen the edges of the cone become very thin and quite floppy/compliant, it was so thin that looking into the cab from behind I could see light coming through the edges of the cone in places. In addition the paper voice coil had shrunk just in the area between the cone and actual copper coil, the voice coil gap tolerances are quite tight so the coil rubbed on the pole piece when the cone was fully depressed, it will need a recone in the future.
In an effort to prolong the life of the speaker cone and with nothing to lose I removed the magnet assembly (held on with 4 bolts) and turned up a 1.75" mandrel/dolly with a very slight taper which I inserted into the voice coil this forced the shrunken portion of the voice coil out to it's original size, I then painted some diluted epoxy cement around the expanded section, which held it's shape once the epoxy had set and the mandrel removed.

For the cone I diluted some contact cement with MEK to the consistency of water and painted two coats around the cone edge, the first coat did soak in to some extent and the second just strengthened it. This stiffened it up quite a bit and hopefully has extended its life somewhat. Guitar speaker cones are almost all doped with some sort of pliable glue/rubber or whatever - I'm not sure what it is. There are a lot of different ideas out there on the net, contact cement came up quite a bit so this is what I tried, from experience getting it on your fingers it seems to retain a rubber type consistency once it has set.

For those interested here is a 5min sound clip - I managed to talk No. 1 son into putting it through its paces when he came over to pickup the amp. https://www.youtube.com/watch?v=WUQtsEAAWyM
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