First Build :-)

[old tube]

Thought I'd post up the results of my first build as the people on here were kind enough to answer my stupid questions over the past few weeks.

I like the idea of "recycling" old amplification equipment. In my case, I was given a 1964 Philips reel to reel tape recorder by a friend who thought I might be able to salvage some of the valves.

Long story short, inside it had a single ended amplifier which was pretty much an AC4 spec with an EZ80 rectifier, EF86 but using an ECL82 for output. However, the transformers, mustard caps etc. were all there and of the right spec. There was miles of very nice solid core wire in it too. There was no chassis to speak of, it being built "around" the internal machinery of the tape mechanisms.

I'd seen posted elsewhere that the current Vox Pathfinder solid state amp can be converted and it does look like an AC4 and very close in dimensions. Picked up one on eBay for $50. Stripped it out and was ready to go.

First job was to "design" the layout, not too difficult in such a simple amp and the chassis is quite roomy, so I chose to follow the Vox placing of the valves with the earth bar in the "middle" to hang everything off. In practice I needed to use 3 double tag strips as well. Next was to cut, drill etc. the holes required. To fit the transformer I had to make it lay down, only having a drill and tin snips that was a complete pain of a job! The chassis doesn't appear very thick but I reckon you could build nuclear submarines with it.

Once I'd bodged my way through the drilling, and the chassis was ready, it only took a couple of nights to assemble and solder. I comfort myself with the fact that point to point always looks untidy, but i'm about 80% happy with what I did. Only Issue I had on first start was low output which was quickly rectified when I spotted I'd used a 220k instead of a 22k at the EL84. The amp is very quiet electrically, no hiss, even though I used some old CC resistors, no crackle or pop from the mustard caps. Valves are all Mullards except the 12AX7 for the trem.

I did make some mods to the original AC4 circuit. First was the addition of a depth control for the Tremelo. Simply put a 250k pot across the 100k resistor. It works pretty well though is a bit over enthusiastic above half and I'll need to modify a bit to get it to where I want. It's also affected strongly by the overall volume of the amp, which I don't really understand.

I was advised to move the B+ feed to the output transformer from the second can to the first to allow the EL84 to operate a bit more effectively, introduced a little hum but sounded more lively as a result. Like many others before me I dropped the EF86 screen resistor from 5M6 down to 2M to add a bit more gain and grit. I also added a 100pF "bright" cap.

The transformers allow me to use an EZ80 and get perfect "spec" voltages, but I chose to go with an EZ81 as I think the tremolo sounds better with the slightly increased voltages.

Finished the whole thing off with a control plate made by a local engraver. Couldn't get copper so went for red.

All in it's a great living room practice amp. It stays clean most of the way and with the Weber Alnico it is harmonically rich and chimy. Driving it to max does darken it to the point that neck pickups sound muddy .. need to find a solution for that. However, bridge pickups sound very good and the drive is smooth and harmonically rich. It does suffer a little from the inevitable boxiness of a small cab and an 8 " speaker. However, a little reverb really opens it up.

[ampgeek]

Very nice!

Well done,
Dave O.

[old tube]

Thanks!

Forgot to post pics of the "source" material ...

[Structo]

Nice job on that conversion!

My only concern would be the bare terminals on the OT.

Perhaps some heat shrink tubing would help that.

[Blackburn]

Very cool build! I love those retainers. Great way to make that SS Vox awesome.

[HeeBGB]

Very cool build! I love those retainers. Great way to make that SS Vox awesome.

+1 Those retainers are badass!

[M Fowler]

That's a nice looking conversion.

[cbass]

Good job!

[mark enger]

Old tube, your amp looks great, I've been researching a ac4 build myself
and was wondering how you grounded very thing? Did you ground everything over by the Power transformer? I can't see from pix mark

[old tube]

The earth wire (pre-tinned 20swg copper) for all the components runs across the middle of the chassis; simply looped and clamped tightly to the posts you can see which in turn are anchored to the chassis (you can see the screw heads in the photo of the "top" of the chassis). I used star washers both sides to make sure that I had good contact and some vibration proofing. Seems to have worked as I've no earth issues at all. I was originally going to solder the mains earth to that also (as Vox did back in the day) and hence why I ran it down to the mains transformer. However, I was advised that it was better practice to attach the Mains Earth Wire direct to the chassis (it's hidden in the photo).

[mark enger]

Ok thanks OT, I'll give it a try, I was gonna use 2 el84 se or maybe 6gk6 as i have a bunch on NOS on hand with a large OT, and a 10 or 12 inch speaker for a little more volume. mark