Mullard 10W High Quality Stereo Amplifier

[Pamphonica]

Here are a few notes on this project, finally finished after more than two years sitting half-finished while other higher-priority repairs took over the bench.

It started as a supposedly working amp bought at Audiojumble for £100. The seller claimed it was Pamphonic but I'm afraid Pamphonic never made one of these! The detective work was discussed in this thread), with "as-bought" photos. It turns out to be a TRS "10+10 Stereo" from the early 60s.

I took a look at the circuit and worked out it was a version of the Mullard Ten Watt High Quality Stereophonic Amplifier. (Details here). It was not a lovely amp, badly built and hacked about, so I decided on a metal-up rebuild, following the original Mullard circuit and layout recommendations.

This meant a new transformer from Ed Dinning to give a proper centre-tapped HT and 5V heater winding for the new GZ34. 1W Piher resistors were donated by Roy (VCM163), Colin (HG MICKE) donated the sufflex caps for the tone control section and Phil (A223bfg) donated the 3-section smoothing cap). The rest came from my parts drawers or Ebay.

Mullard's design note is really helpful and the earthing arrangements were very carefully followed. This includes two tiny screens across the ECL86 valve holders between the triode and pentode sections. My version has a little hiss but no audible hum which is fine.

A couple of design note errata from the layout drawings if anyone else wants to build this design:

1. For each channel there are two wires that come down to supply HT to the main amp boards, and decoupled HT to the EF86 board. As shown they come from a small tagstrip holding R11/C5 and R111/C105. The wires that leave this tagstrip must be swapped. The HT wire from the first (LH) tag must go to the main amp board, and the next tag to the EF86 board etc.
2. The balance control wiring as shown is reversed. The front section of the control carries the right channel. If you wire is at shown you will get balance Left-Right reversed.

My version has original output transformers so can match to 3.5 or 15 ohm speakers with a switch to select which. That also involves switching in extra feedback components as required.

I decided on solid wire for the interconnects, to allow neat cable runs. The heater wiring was carefully dressed into the chassis corners. I also made custom front panels and labels to give it a professional look (using the Mega Electronics laser print plus adhesive aluminium panel system)

Getting the correct pots (2M twin gang) was made a lot easier by ordering them as specials from BloreEd, who were very helpful and efficient, and reasonably priced. About £5 each for four pots to my exact spec, including a 1M log/antilog pot for balance.

The only major problem I had was instability on both channels when first powered up. After some good advice on fault finding ("break the feedback loop and test each section") from GrimJosef, I located the problem.

My original tidy wiring scheme allowed the feedback signal from the speakers to run parallel to an input signal wire for a short distance. This was enough to set up feedback. With feedback wires (blue in photo below) rerouted to run above and at right angles to the signal (green) wires, all instability vanished.

I also had an ECL86 develop a heater cathode leak/short after a few hours running. I am still not happy with the balance control. I think I may need to revert to a linear pot, as it is not a smooth pan between channels.
And the hiss could be better. I suspect that the (variable) biasing of the EF86 grid via the volume pot is causing some of it.

But I have just had some lovely JJ Cale blues playing on the amp while I write this and it sounds very smooth. I am definitely not an audio buff. I just like electronics and get a kick out of making and mending stuff. This is probably the first audio amp I have built from scratch in about 45 years!

Regards

Jeremy

[Pamphonica]

Here are the photos of the amp as I received it. Not nearly such a pretty sight!

Apart from anything else, the capacitors had been mounted on the back side of the tag-strips, making repairs or replacement near impossible.

Jeremy

Tidy job, I like it.

[ukcol]

A very nice bit of work Jeremy, enjoy it.

[Radio Wrangler]

Looks so pretty under the chassis you'll have to mount it upside down!

David

[Pamphonica]

Yes I know - when I was building it, upside down, it did seem a shame to have to put it right way up at the end! (Although it does have a nice matching baseplate on it)

Jeremy

[joebog1]

A craftsman at work!!

I like it!

Joe

[Hybrid tellies]

A very nice interesting project.

[Diabolical Artificer]

I agree, beautiful wiring Jeremy, I could look at it all day. Well done.

Andy.

[Pamphonica]

To be honest, the solid-core wiring actually made my life a bit easier, as it stays where it is put! I decided not to lace the wiring. A good decision as it turned out, as re-wiring laced-up sections would have been a real chore. But I did put small twists of scrap solid-core wire round certain sets of wires, to keep them constrained.

I have diagnosed the problem with the balance control. It was my error. I failed to specify the pot fully. The pot as delivered from Blore-Ed had the antilog section at the rear. But the anti-log section was meant to be for the Right-hand channel. As laid out by Mullard, the front pot sections control the Right-Hand channel. I will have to re-wire the balance pot so the rear section controls the right channel.


Jeremy

[Peter.N.]

Beautifully laid out, some early TVs were like that, pleasure to work on.

Peter

[sp10mk11]

That is really some thing to be proud of, it looks stunning underneath.
I hope it gives you lots of pleasure

Gary