Vox AC-4

[PieterGuitar]

hi everyone
I havn't got much knowledge of electronics
but would love to build my own amp and i
allready choose wich one
the vox AC-4
schematic:
http://audiohi-fi.narod.ru/cxems/Ac4.gif
but need some help with it
you know, i know how to solder and all that stuff
but i just can't set the schematic into reality
now i saw that some other schematics also have
a layout with it like this one
http://www.diycustomamps.com/images/...ton_wiring.pdf (this ones from a princeton)
now my question is
if there is someone how can make a layout drawing of the ac-4 schematic
thx
greetz Pieter

[Marc]

Pieter,
Here's a non-tremolo layout I did for a conversion of a Crate VC-508. Be sure to shield the input grids.
Turned out great, once I found an EF86 that wasn't microphonic.

Marc

[tubeswell]

Hi Pieter

I believe you will find a pic of the guts here

http://www.ampaholics.co.uk/vox_ac4_blonde.htm

(Nice choice of amp BTW)

You will need:

An aluminium (probably easier to work with and more authentically voxish) or steel chassis (and cab is nice) and something (assorted drills/chassis-punches) to make holes (esp socket holes) with.

3 x nine-pin sockets

1 x EL84/6BQ5, 1 x EF86, and if you can't find any EZ80s out there, you can sub an EZ91 (JJ makes new ones) - the EZ81 puts out more current (150mA compared to 90mA for the EZ80). EF86s are still made, as are EL84s

1 x power transformer with an HT secondary winding of 250-0-250VAC 150mA, and a heater winding of 6.3VAC (3A should cover it) with a Center Tap is best

1 x small (say 10-15W) Single-ended output transformer with a 4k5 - 5k2 load resistance - best if you can get one with multiple secondary taps (4R, 8R, 16R), so you can chuck in most/whatever type(s) of speaker you can find.

Use high-voltage (400V-630V) film caps throughout (They are cheap as chips). (Do you understand about cap values?)

Similarly you need 2 x 16uF-20uF and 1 x 8uF-10uF hi voltage (450V-500V) electrolytic filter caps

The 3 x small 25uF electrolytic cathode bypass caps only need to be rated about 25V

With all electrolytic caps you need to put them in following the correct polarity (see + signs), otherwise they will explode

A bunch of resistors of assorted values (do you know about resistor values?) If you can get carbon comp resistors - they are worth it. Make sure you get resistors with the minimum power ratings where this is specified (e.g. 5W for the 1st filter resistor and 2W for the EL84 cathode resistor), otherwise the rest can be 1/2W resistors

2 x 1M audio/log pots for the vol and tone controls and a 1M linear pot for the trem speed control (I guess a 1M reverse-audio/antilog pot would probably work better here)

2 x 1/4 inch (6.3mm) grounding tip-switch sockets for the input, and some ordinary 6.3mm sockets for the speaker output(s)

1 x 1A fuse and socket and DPDT mains AC switch and Mains cord/plug/socket setup

If you want a power lamp (suggest you do as a safety feature), you can get a lamp holder and 6.3VAC lamp and insert it in the heater circuit.

Assorted wire and wire clamps and solder tag boards (or turret/eyelet board if you want to have a go at the tradiaional fender/marshall-style way of laying things out), and assorted nuts, bolts and screws

I think that pretty much covers it

[PieterGuitar]

thx verry much you guys
i think i know enough now
and i can't start my first amp
greetz Pieter

[tubeswell]

Pieter,
Here's a non-tremolo layout I did for a conversion of a Crate VC-508. Be sure to shield the input grids.
Turned out great, once I found an EF86 that wasn't microphonic.

Marc

And I bet (with that 2 x diode rectifier setup) you had a center tap on the HT secondary winding? :-)

[PieterGuitar]

now actually the only thing i need
is a list to go to the store
so if you guys could help me out with
short but detailed list that would be awesome :d
greetz Pieter

[tubeswell]

See the list posted in my previous post

[HTH]

a much cheaper way of building an AC4 would be to buy an Epi VJ head and mod that - it's a decent enough platform to mod.

see the picture attached for my 'work in progress' on a gutted Epi VJ.

[PieterGuitar]

so this is what i'll be needing??

3 x nine-pin sockets
1 x EL84
1 x EF86
???EZ91 (but i dont see em on the layout so not sure)???
1 x Hammond125ESE ( can i use this one?)
1 x output transformer ( not sure wich one actually)
400V film capacitors (but not sure wich values and how much of them)
2 x 20uF 400v electrolytic filter caps --->..??see + signs??
1 x 10uF 400v electrolytic filter caps --->..??see + signs??
3 x 26uF 25V electrolytic cathode bypass caps
??What values do i need for the carbon comp resistors and how much of 'em??
2 x 1M audio/log pots
1 x 1M lineair log pot
2 x 1/4 inch grounding tip-switch sockets
1 x 1/4 sockets (for speaker)
1 x 1A fuse and socket
1 x Dpdt Selector switch
1 x mains Cord/Plus/Socket
1 x bulb (into 6,3V heater circuit??)
1 x small eyelet board
and wires, nuts, bolts, screws etc.

plz let me know?

greetz Pieter

[imaradiostar]

125ese is excessive for an EL84 amp- might wanna try a smaller OT unless you plan to use an EL34 output tube.

jamie

[tubeswell]

"???EZ91 (but i dont see em on the layout so not sure)??? - "

The EZ81 is a rectifier tube that is still in current production (as opposed to the EZ80 in the original schematic - the EZ81 offers more current supply)

"1 x Hammond125ESE ( can i use this one?)"

You want an OT that offers a 4k-5K primary load resistance (for a single EL84) - If the Hammond one has multiple secondary taps you might be able to ge the appropriate load resistance out of it

"1 x output transformer ( not sure wich one actually)"

That's what we were discussing above


"400V film capacitors (but not sure wich values and how much of them)"

Looking at the schematic again, you need 1 x .047uF, 1 x .001uF (1000pF), and 1 x .1uF in the main amp, and 1 x .02uF and 2 x .01uF in the trem circuit



2 x 20uF 400v electrolytic filter caps --->..??see + signs??

Yep (for the positions that say "16" on the schematic - except these'll probably be at least 450V rating as commonly available electrolytic caps) The difference in capacitance between 16uF and 20uF is not critical


1 x 10uF 400v electrolytic filter caps --->..??see + signs??

As per above, but for the position marked "8" on the schematic



3 x 26uF 25V electrolytic cathode bypass caps

These can be 22uF or 25uF (not critical)

??What values do i need for the carbon comp resistors and how much of 'em??

All the resistors (except the higher power ones), i.e 3 x 100k, 3 x 1M, 3 x 220k, 1 x 10M, 1 x 5M6, 1 x 3k3, 1 x 1k5, 1 x 6k8

The higher power ones 150R 2W and 1k 5W should probably be ceramic/wire wound resistors, as they will see higher-current.voltages.


2 x 1M audio/log pots
1 x 1M lineair log pot
2 x 1/4 inch grounding tip-switch sockets
1 x 1/4 sockets (for speaker)
1 x 1A fuse and socket
1 x Dpdt Selector switch
1 x mains Cord/Plus/Socket
1 x bulb (into 6,3V heater circuit??)
1 x small eyelet board
and wires, nuts, bolts, screws etc.

[PieterGuitar]

So actually
the layout that marc gave me isn't 100% an AC-4??
hmm
so i need an AC-4 layout that
fits in with the schematic
but i'm tottally not good at settting
a schematic to reality

sorry that i'm asking so much questions
but this is like my first amp so..

[tubeswell]

Ooops did I say 3 x nine-pin sockets??? - I meant to say 4 - sorry (EF86, 12AX7, EL84, EZ81), and it might pay to buy a couple of spare resistors, just in case you screw-up the soldering first time round (Carbon comps don't like too much soldering heat).

FWIW If you are getting the power transformer especially made-up, then get them to put an extra 6.3VAC 1A winding with center tap (for the EZ81). You can run it off the normal 6.3V heater winding if that is center-tapped, but it might be better to have a separate winding for the rectifier.

Re; the layout Marc gave you, it doesn't have a rectifier tube - it is diode rectified instead. There is no particular disadvantage to diode rectification with a single-ended amp, because there is no sag to speak of in SE operation (unless you build in a sag resistor in the B+ line). The only thing is that when you switch a diode rectified amp on, there will be more of an initial power-surge than there would be with a tube rectifier, which some people say will shorten the life of the other tubes. (is the just alot of mojo talk?? I dunno - however you can overcome the tendency for surge-damage by building in a standby switch). With silicon diodes you would save on the extra tube and socket, although you probably should allow for a lower voltage HT secondary winding, because silicon diodes drop less voltage than tube rectifiers - so to get a loaded B+ of around about 270VDC with full-wave 1N4007 diode rectification, you'd go for a 200-0-200VAC winding rated at (say) 100mA current draw, and of course a 6.3VAC heater winding with a center tap rated at about 2A draw minimum.

[PieterGuitar]

okay thx
now almost everything is clear to me
except for WHAT Input- and Output transformers
do i need ??? =P =P =P
and off course a layout image =D
thx
greetz Pieter

[HTH]

okay thx
now almost everything is clear to me
except for WHAT Input- and Output transformers
do i need ??? =P =P =P
and off course a layout image =D
thx
greetz Pieter

The Hammond 125CSE will be a suitable output transformer (has a 2k5, 5k and 10k primary)

For the PT you need to have 2.11A capability on the heater winding if you're using the EZ81 with no seperate 1A winding on the secondary.

EF86 - 200mA
ECC83 - 150mA
EZ81 - 1A
EL84 - 760mA
--------------
total = 2.11A
--------------

personally, I wouldn't even bother with a tube rectifier on this amp, there really is no benefit - just use a couple of diodes (1N4007s are fine).

The Hammond 270AX is perfect for the AC4 with its 240-0-240, 50mA HT winding and 6.3vac, 2.5A heater winding.