"WAD" Mullard 5-20

[Mark Wood]

Well my first contribution on the forum is to ask the question as to whether anyone else, apart from me, is still using the very first Hi-Fi World Mullard 5-20 amplifier kit?
I bought the kit almost immediately after it was introduced in 1993 and it's given almost entirely trouble-free service ever since. Only one problem springs to mind and that occurred when one member of a set of very cheap EL34s "collapsed" and took out not only the auto cathode bias capacitor but also set fire to the paint on the accompanying 10W carbon resistor! Apart from that they've given superb sound quality (they replaced a Mission 777BU) for 13 years and have sported NOS GE 6CA7s (£12 each from Chelmer in 1994ish!) for the last 6 or 7 years. They're now fed by a Velleman kit valve preamplifier which I reckon was remarkable value for money.
Just to prove that one of these kits still exists, I guess I could post a picture if there's any interest...

[Swithers]

Hi Mark , Would like to see that pic... I have the Maplin 4-20 kit power amps , Which were made around the same time as yours , You can see pic,s by clicking on this link http://smg.photobucket.com/albums/v4...Maplin%20Amps/ . Many Thanks Steve..

[John Caswell]

Hi Mark,
I would be interested to see pictures/cct info/instructions and I am positive Peter Comeau would like any information available to add the the archive information he is compiling.

John aka Dr John

[Allen]

I've got the copy of Hi Fi World where they explain their version of the circuit, a few slight differences from the Mullard book, I'll dig it out and scan it - though I think it dates from before the kit was available.

I'd be interested to see what the kit looked like.

[Mark Wood]

There was actually a mistake in the circuit diagram in the Hi-Fi World DIY supplement that covered the 5-20 kit (April 1993 I think) and the correction was printed in the front of the next issue of the magazine. Being a (sad?) collector of these things, I actually have every issue of Hi-Fi World and all of the DIY supplements going back to some time in 1992 - I've even got the first issue which I really bought for the accompanying CD! So if anyone's looking for an article, I can probably find it. I also have all of the documentation that came with the kit.
The only significant difference from the original Mullard circuit is the change from a valve rectifier to a diode bridge, so like the Maplin kit, I suppose it should really be called a "4-20."
I won't be able to do it until after the weekend but I'll photograph the finished kits resplendent with their mahogany side panels (it was the only iron-on veneer that Wilmslow Audio had at the time!) and post a picture. Seeing Steve's nicely finished Maplin amps in the picture, reminds me that I must do something about "re-panelling" them.

[John Caswell]

Hi Mark/Allen,
All info most welcome, all costs defrayed as usual. In fact supplement info up to issue 5 would be most welcome.
PM me accordingly please.

John aka Dr John

[NealG]

Being a (sad?) collector of these things, I actually have every issue of Hi-Fi World and all of the DIY supplements going back to some time in 1992

You and me too! My wife complains bitterly about it at times and about all the amps. laying around the house

[Family_Dog]

I built two of these from the original Mullard booklet that was issued in the late fifties or early sixties. I recently revamped those same two amplifiers and built another three of them, using them in my HT setup.

More recently, Claus Byrith published an article on these amplifiers. This may be found here:
http://www.lundahl.se/pdfs/claus_byr...fier_30wpp.pdf
This article is a definite must-read!

In this paper he discusses various modifications for the original 5-20, such as converting the first EF86 pentode stage to triode in order to reduce sensiivity (a wise move for today's input sources) and convert the original cathode Bias to a fixed Bias system. Although I do use the triode input conversion, I have stuck with the cathode biasing for the EL34 output tubes, as I do not have provision for a bias voltage source on my mains transformer.

I don't know if one can paste pictures here, but I do have pictures posted under my name in the Vintage Tube group at the url in my signature. The pictures show various stages of construction as well as before and after views.

[Family_Dog]

Found out I can paste images here...



The old (right) and the new (left)...





Top view... The output Transformer has since been resprayed




Rear View...




Under Chassis view




Apologies for the pics, a photographer I am not.

The electrolytic caps here are the original ones, I reformed them and they seem to be still doing their job. This is a photo of one of the original two 5-20s that I had, I used a 50+50uF/500 JJ Electrolytic cap in each of the remaining three amplifiers.

[Mark Wood]

Well, as promised (threatened?), here are a couple of attached photographs to prove that there's at least one example of Hi-Fi World's first ever amplifier kit still up and runing after 13 years. Apologies for the woodwork (and the carpet!), my quickly put together effort. The monoblocks are largely still using their original passive components with only a few changes necessitated by the self-destruction of a cheap EL34 and my opinion is that they sound superb. The kit was only on sale for a very short time (it was replaced by the longer-lived 5881) and was based on just a primitive aluminium chassis. The instructions were somewhat brief (I still have them if anyone's interested) and the photographs in the HFW DIY supplement were essential for working out some component placement and wiring. The supplier of the kits (I believe still a manufacturer of valve amplifiers) did offer side panels and transformer covers but mine never arrived and it soon became clear that the contract with HFW had ended.