Audio Innovations 500 'Reborn'

[snowman_al]

Well sort of.
This was my last project valve amp build. I thought you might like to see it?

I acquired a very sad AI 500 some time ago. An early one with the 'famed' 16 ohm tapped output transformers. I had had a rough life, and hard use. 3 of the EL34s were melted, the cathode resistor and wiring were burnt out as were the droppers...

http://i63.tinypic.com/68ep9f.jpg http://i66.tinypic.com/2rr1kj4.jpg
http://i66.tinypic.com/33vntqg.jpg http://i68.tinypic.com/200d6wi.jpg



My original intention was to rebuild it, but it was a, a mare to work on and b, too far gone. The other factor was its size, just too large for domestic harmony...
So as I had to strip it down and did not need the phono stage or all those inputs, just how compact could I make it?

http://i65.tinypic.com/o8f2tj.jpg http://i64.tinypic.com/2dj72xl.jpg


This big, just a touch smaller than a Stereo 20 chassis. Cutting the holes was very hard work with a Q-max cutter, the ally is 3 and 4mm thick!


http://i65.tinypic.com/14x0b5w.jpg http://i68.tinypic.com/a0duh2.jpg

But I was pleased with the result.

http://i63.tinypic.com/vhgjlt.jpg T.B.C

[Firebottle]

Very nicely done :thumbsup:

Are you going to cover the transformers or just have them on show?

[ReggieB]

Nice

[southall-1998]

I actually prefer the AI case.

S.

[snowman_al]

... well I wanted to keep as much of the original hardware (ex. chassis) and components as possible. And keep the original circuit of course.
So I stripped everything down, and tested and cleaned along the way.

Much to my relief the transformers were all fine. So I gave them a coat of black to match the new chassis.
The mains transformer had clearly been a later factory replacement, so lacked end bells, which needed a little improvisation to re-instate.


http://i63.tinypic.com/11kzdjt.jpg http://i68.tinypic.com/2mmwrc3.jpg

http://i64.tinypic.com/2iqk3tj.jpg http://i63.tinypic.com/2gsetf9.jpg



I took a while to decide on the layout, but finally chose a 'Citation' style, just for a change from the symmetrical format of most of my other amps. And yes everything 'on show'.

[sq225917]

Looks good so far. Keep it up.

[snowman_al]

Now for the fun bits...

I was able to salvage a lot of the original hardware, valve holders nuts and bolts etc. and re-use them.
Where ever possible I tried to keep the original components too, but apart from the low power resistors and a few capacitors most were 'cooked' and useless. (I'm sure better components were available at the time of manufacture.)

http://i63.tinypic.com/2j69u6f.jpg

I wanted to keep the boards too, but in the end only one had survived, then only just. So I had to make tag strip replacements.

http://i65.tinypic.com/apfko2.jpg http://i68.tinypic.com/k2biwp.jpg

...and they fit in the front rail of the chassis.

http://i63.tinypic.com/161zzm9.jpg http://i65.tinypic.com/1zp1emp.jpg

The only circuit change I made was to have individual cathode resistors and caps for the EL34s, rather than shared. (I reasoned better practice and easy to strap them back together if I wanted to after auditioning.)

[Firebottle]

Excellent work Alan :yesbruv:

[Primalsea]

Well that looks much better inside than the original, nice attention to detail. The only criticism I have is that you do not have the obligatory coffee stain on your notebook. A tea stain would have been acceptable.

[snowman_al]

Hi Paul, yes I will do better next time. Red wine acceptable?

Moving on, as I said at the beginning there had been a catastrophic melt down of the cathode resistors and 3 of the EL34s.
I suspect these might have been a major factor in the problem, all of the 0.47uF coupling caps were leaking, a couple down to the 10s of killo ohms! So much for poly cap reliability... (In fact the only EROs fit for purpose were the 0.15uF ones. The 0.22 and 0.47s were all leaking to some degree?)

http://i67.tinypic.com/jj7o0x.jpg http://i64.tinypic.com/2z6tvmu.jpg

Once rebuilt and new better matched 2 watt resistors fitted, I re-instated it. And time to try it for the first time. Crossed fingers...

http://i64.tinypic.com/2rr85cp.jpg http://i64.tinypic.com/a173th.jpg

[smithie]

brilliant job and well done...be very proud of that:wow:
there's so much talent on this aos forum:respect:
regards
paul

[walpurgis]

Looks great Alan. I'll be interested in how you find the sound. I like EL34 amps. :thumbsup:

[snowman_al]

How does it sound?
Excellent, it has been in daily use since I finished it back in September. As you would expect a fluid mid range and sweet treble, but above all real depth and clarity to the bass. You can identify individual bass guitar notes. Not the 'all one note' effect many amps give you. (Speakers are Dynaco A25s and EPOS ES11s.)

The other thing is it plays the same at all volume levels, low level through to loud enough. It's one of those amps that you don't listen to, just the music.

I have tried different valves and found the ECC83 first valve make has little effect.
The drivers ECC88s have noticeable effect on the treble, I've tried Mullards, Brimars and Teslas (original fitting I think) and by far the smoothest are the Teslas. The Brimars hurt the mid to treble region. Might try some E88CC/6922s if I can find a pair in the future.

For the output valves I started with 5881s, often recommended for the AI 500, and they do sound really good. However they slightly tilt the whole presentation towards the bass, impressive but maybe a little too much? In the end I've settled on an old set of Winged 'C' EL34s as having the most transparent and even spectrum. (A set of Mullards don't really sound much different, anyway they belong in my Radford and a set of JJ are good, but quite as even as the 'C's.)

I know I built it, but it really is my favourite amp right now...

[Audio Al]

Super Job , Much respect , I wish I was clever and could do work like this , However I would end up brown bread :eek:

[snowman_al]

After I built it I used it for a couple of weeks to soak test it for faults. Fortunately no issues other than the heat it generates.

The 100 ohm dropper in the HT line alone dissipates 9 watts (I see why later models used 47 ohms) that and the other droppers all bolted to the chassis added to quite a rise in chassis temperature after a couple of hours of use. Touchable but not too good for long term reliability. I rather guessed it would run very warm looking at the original steel chassis and its burn marks! So I made provision for a little forced cooling in the original design.

Perforated ally sheet and a super quiet, slim line PC fan.

I opened out the holes for the fan intake and sprayed the whole sheet satin black. Then tried various speeds for minimum noise and most efficient cooling.

http://i66.tinypic.com/bi6j3d.jpg http://i67.tinypic.com/14joyg0.jpg

In the end the slowest speed the fan would run at was just under 6 volts, that provides more than enough cooling and the noise is only just audible if you stand next to the amp. It is even quieter if I raise the amp up another 1/2 inch or so off the shelf, so is mostly flow noise and not unpleasant.

All bolted up and ready for more tunes...

http://i65.tinypic.com/2mnojk7.jpg

I've run it all day (8 hours plus) on a couple of occasions and the chassis is only just warm. Much cooler than either the Leak Stereo 20 or Radford STA 15 after the same period.

[Firebottle]

Although I would prefer natural cooling that is a brilliant solution.

I have a large OTL amp that gets ridiculously hot, that is going to get re-built eventually.

[Bigman80]

They have some really silent CPU fans for pennies now Alan.

Sent from my EVA-L09 using Tapatalk

[Haselsh1]

Really seriously loving the look of that and what a fab project to be involved with. EL34 is my fave valve so I'll bet it sounds fantastic. Pat on the back and raise a glass...!

[Primalsea]

:youtheman:

Fantastic job (no pun intended).