Economy Mullard 3-3 stereo amp.

[Murphyv310]

Hi.
I'm in the process of building my second Mullard 3-3 Stereo amp on an economy basis.
I have now made four of these amps in the last couple of years and each of them brings a smile to may face with their performance and quality.
My latest is for my own use and is built on an old chassis retrieved from a Codar CR66 that was missing it's case and coil pack.  
It's actually built back to front and I wasn't sure if I'd land up with hum or feedback. After completing one channel all I can say is I'm impressed
In this version I'm using an EZ81, fed from a 350-0-350v winding for more head room and resistor and choke smoothing. The anodes of the EZ are fed via 100R 5W5 (10watt) gold resistors the cathode for one channel via a 350R (R15) resistor to 100uf, choke and 47uf (C9) smoothing, the cathode resistor will be reduced once the second channel is complete.
   
On the stereo version we don't need R12 & C6 in both amps as they can be fed from one pair, R12 should be dropped in value and I find 2.7K 2 watt is fine, no audio ripple can be detected on C6.
The circuit shown above is the one less the bass and treble controls, mine has the controls fitted.
Now the layout I'm using is totally different to Mullard's but I have no detectable hum and the use of quality modern components also means there is no detectable noise.
Once I've completed the job I'll post up the underside layout and do a video of its performance.

[Murphyv310]

           

Hi.
As can be seen in the pictures this project has been built on an old scrap chassis. It is actually built back to front with the output transformers mounted right at the front behind and above the controls. I thought I may have instability so only one channel was made up first. All is well with no problems whatsoever, hum is undetectable and quality spot on.
Some of the components are secondhand and I have not adhered to Mullards values in a couple of places.
The second channel is built but not tested as yet as I will have to change a couple of resistors to bring up the HT to 300v, also the function switch is not connected as yet. An ECC83 has been fitted for a magnetic cartridge which will come later.
So this is an economy amp with 60% secondhand parts and a layout that Mullard would certainly not approve of, it works and as good as any 3-3 should.
One mod that I will need to do is to move the EZ81 elsewhere as it is too close to the mains transformer and it could cause it to get too hot.

[radioman]

Hi Trevor,

It's good to see parts being 're-cycled'- into something useful  
You'd think having the output transformers so near to the controls would cause problems but clearly not as you've proved !
I look forward to hearing it in action.

Cheers,
Andy

[Aerodyne]

I too have built several 3-3 amps over the years and have never been disappointed by the performance, whether stereo or mono versions. In fact, to my admittedly ageing ears, the 3-3 sounds the equal of most more powerful designs and the frequency-conscious feedback gives an impressive bass range. Nice to see that someone else likes the amp as much as I do (and no, I too tend not to stick to the Mullard layouts nor to the component values, using what comes to hand where possible).
Nice project, Trevor. Thanks for sharing it with us.
Tony

[Murphyv310]

Thanks Tony & Andy.
Here is the completed amp, https://youtu.be/50KVnaHS1DM
The cameras microphone and the speakers dont give it justice but it really is impressive.
Thanks for looking.

[Aerodyne]

I took a look at your video, Trevor, and it is impressive. The bass has that lovely rounded quality that is hard to find on other amplifiers. Truly a great design, the 3-3 and no matter how you build it, whatever component changes you make - within reason, of course - the amp always sounds great.
Thanks for going to the trouble of creating the video. Hopefully, others will see it and become 3-3 converts!
Tony

[Murphyv310]

Hi.
Well Tony I had a Denon mini system to look at today, it had the usual volume control problem with too much grease on the contacts. After stripping it and cleaning off the excess grease I gave it a test. I used the two rather lashed up speakers from yesterday's video. Boy was I disappointed, to get any decent volume the control needed to be cranked up to 30, the bass was dismal even at plus 8 and the loudness on it didn't get near the 3-3. I then got hold of my reluctant assistant..... Anne my other half. Now she hates it when I ask her to do anything like this, but after comparing the Denon to the 3-3 and don't forget she had no idea what amp was on the verdict was clearly in favour of the 3-3.
So much for expensive fairly recent allegedly quality Hi-Fi.

[peter scott]

Sounds very good on YouTube.

Peter

[Murphyv310]

       
Hi.
I've been adding some refinements.
A headphone socket has been fitted along with a suitable attenuating network and some top end frequency lift for my failing hearing, this network is in circuit all the time but a change over switch is added to switch off the speakers and add dummy loading for the output.
A mono stereo switch has also been added.
There is a slight amount of hum detected on headphone but it is leakage from the mains transformer to the outputs more so on the left channel. Perhaps a different orientation of the mains transformer would help or even a drop through type.
Next is to make a template for a facia panel and make one up.

[Diabolical Artificer]

Nice work Trevor. Are they VVT OPT's? I have two of the big EMI elliptical's left if you want them Trevor, you can have them for the price of the courier.

Andy.