First rate Mullard 5-10 stereo home build

[Lavane]

I don't agree with this at all.In fact,many manufacturers (especially in England) produced consumer-grade equipment that could easily meet MIL Spec.And any of these units could have been found in the average home. Well,maybe not the Hiwatt

Compare these photos with the OP's and you can't miss the influence.

Leak:

Quad:

Pye:

And even musical instrument manufacturers;

Hiwatt:

My son is bringing his Red Bear MK 120 over next weekend to have the sockets retentioned. It's supposed to be based on the Hiwatt with a little Marshall thrown in. It sports a quad of Russian 6n3's
Damn that thing gets loud with his four driver cab!

 

[arts]

Those decals are of the old ''water slide'' variety,and were in very common use back in that era,especially (again) in England. They are an absolute pain in the a** to get installed properly and lined up just right,and they have a nasty habit of moving around with the passage of time.They are usually varnished over for protection,but I'm sure the humidity gets to them nevertheless,and then they go for a walk.

Seeing that you are UK based,you might remember these types of decals being used on BSA,Matchless,AJS and many other British motorcycles.I restore old Brit bikes,and these decals (most of which are actually still available!) have certainly strengthened my grasp of the term ''fussy'. Helped improve my use of profanity as well....

 

[petemcfc]

Re., the build quality,I will hazard a guess that it was an apprentice project,as you will know all major engineering companies in the UK ran their own training schools and encouraged development.

 

[arts]

The metalwork is also impeccable. Look closely at the cutouts for the transformers; crisp and clean. Also,the corner brackets for the feet appear to be stamped.Are they spotwelded in place or soldered/brazed? Not the style of construction most basement builders are capable of

 

[primosounds]

I don't dispute that milspec assembly techniques or component choices were not being used by commercial amps but that the amp shown has assembly techniques or touches that are not seen in a commercial designs.

 

[arts]

I don't dispute that milspec assembly techniques or component choices were not being used by commercial amps but that the amp shown has assembly techniques or touches that are not seen in a commercial designs.

Such as?

 

[primosounds]

The metalwork is also impeccable. Look closely at the cutouts for the transformers; crisp and clean. Also,the corner brackets for the feet appear to be stamped.Are they spotwelded in place or soldered/brazed? Not the style of construction most basement builders are capable of

Metal work like that was far more easily available back then. People were more used to having things custom made. In fact the feet that are on the bent pieces of sheet metal are on an amp that was made for me locally. They are spot welded on to the corners and help to brace the chassis, since the corners are not lapped. Actually, this is an "easier" design since it is only one bend on the metal brake and a single cut to make the corner.

 

[Dandy]

The metalwork is pretty well done. I will post a pic of the bare but newly painted chassis before I reinstall components. One detail: the cutouts for the output transformers are rounded inwards at the corners so that the mounting bolts are secured to the alu chassis all the way round. I only noticed this when I removed the OPTs.

 

[primosounds]

Such as?

Such as the feet you mentioned. Obviously in mass production that would be a more time consuming method of making the box strong than having lap joint corners with spot welds, and another short lip for mounting the footers. The end of the box being shaped on a press in mass production

 

[Dandy]

Here are a couple of closeups of the metalwork, taken from the inside. The first one shows the OPT cutout with the inward rounded corners. The second attempts to show that the socket holes seem not to have been punched, but carefully worked by hand. The circles are carefully scribed out, but the holes are not exactly round as you get with a punch.

 

[soundmotor]

I don't agree with this at all.In fact,many manufacturers (especially in England) produced consumer-grade equipment that could easily meet MIL Spec.And any of these units could have been found in the average home. Well,maybe not the Hiwatt

Compare these photos with the OP's and you can't miss the influence.

I agree on the potential influence and like British tube gear a lot. Owned quite a bit over the years too; Pye, Beam Echo, QUAD, and still have a set of Leak TL25+. I am not familiar with British mil requirements but a bit skeptical any of mine could meet US or Brit mil spec. They are neatly constructed to be sure but they were built no better than they needed to be. Built to actual military specification would have added cost where it wasn't needed and reduced performance where it was. Not exactly a winning formula. My Pye Mozart looked great but had the structural integrity of a kleenex box with a front panel made of copper circuit board. Sitting still in a cabinet though it worked just fine and utilized a clever design choice that kept costs down. The Leaks have better chassis but suffer from every cap needing replacement. I've had many pieces of mil gear though with caps still in tolerance. Not surprising when they are hermetically sealed and rare to find in consumer gear.

 

[Pio1980]

I have a single Leak TL-12 Plus buried in storage somewhere. Exemplary build quality.
Probably the preeminent American example of the Mullard 5/20 is the Marantz 8/8B, the previous model 2 and 5 are also Mullard 5/20. The model 9 is related, but not strictly speaking, Mullard. The Marantz amps are exceptionally well crafted and assembled with the best parts then commercially available (outside of milspec). Dunno that Allen Bradley 'J' pots and Sprague Vitamin 'Q' caps would have improved the performance.

 

[Dandy]

Not all UK made amps from that period are the same quality. In the 50s, I believe, there was still a lot of military surplus transformers and components around that DIY builders could get cheaply, but manufacturers were cost conscious and some components have not lasted well. Also, some design layout was really awkward. I had a pair Beam Echo mono integrateds had a slide out chassis (a bit like the Heathkit AA50), which was really annoying to access, though it looked elegant (again like the Heathkit AA50).

 

[petemcfc]

The larger holes may have been cut with what would be called a tank cutter,they can run out a bit,though going off the square cut out they and the square could have been chain drilled then filed,have a look for file marks within the holes.
It has been done by hand on a pedestal drill,the clue is in the de-burring of the smaller holes,see the somewhat hexagonal shape that has been made with a twist drill,the person who drilled them did not have access to a rose bit to correctly de burr it.
It seem odd that the correct tooling was not used,so again I suggest that it is an Electrical apprentice project,or a skilled man with Electrical bias made it at home or in a shed.
I am a Mechanical Fitter and I recognise how the man has gone about his task.
Also,in the industry at the time factories employed hundreds or thousands of skilled men and they all met up in the pub, any little "jobs" could be done on a Saturday morning, such as the sheet metal fabrication.
I should know,I've had stainless steel fish tank hoods knocked up for a few pints.

Addition.
I take it there is not a stamped rating plate riveted to the casing or evidence that one had been fitted.
If it had been professionally made the plate is mandatory,if made as a one off skill test or hobbyist and it was not made for sale no plate would be fitted as there would be no order book/serial number tie in.

 

[Dandy]

Thanks for the interesting fabrication information Pete. There is no plate,nor evidence of one having been fitted.

I often wish we still lived in a time when there were lots of skilled factory workers around, doing technical hobbies like ours.

 

[petemcfc]

I used to work on high voltage switchgear anything up to 11Kv,sometimes above,the lay out of the wiring reminds me of how wiremen presented their work,your in the same area as me and there was factories as you know such as Whipp & Bourne,GEC,Mather & Platt as well as RR and British Aerospace,though it would have been Avro back then,all with Electrical knowhow,its a bit of an engineering hotbed even now,as for the decals,think of Airfix kits,I believe its a school or shed job,if only you could find out.

 

[Pio1980]

The decal could be the builders initials.

 

[Dandy]

I'm now rewiring the amp at night as time permits, and under orders to get it done as I'm doing this in I've added more wiring to that area since this pic was taken. doors! The build has been done in layers, with the filament heater wiring (pic 1) covered by a screen (pic 2). Then the top layer with components attached to turrets (pic 3) has the wiring connections underneath it (pic 4), almost like a forerunner of a PCB.

Pic 1.

Pic 2

Pic 3

Pic 4


For assembly, the wiring to the tube sockets will have to be drawn up through the socket holes, then all the layers screwed down. After that the connections to the tube sockets will be cut to length and soldered to the tube sockets from above. It looks amazing and it's easy to change components, but if there is a problem with the connections underneath, I will have to desolder all the tube socket wiring to get underneath and then start again! Wish me luck.

 

[Dandy]

All buttoned up, but as feared I have one problem, with the plate and cathode voltages of the 12AX7 in one channel.

One channel is fine, but the problem channel has no sound, though there are crackles when I prod certain resistors, or measure voltage. The voltages of the 6BQ5 tubes are okay, and so, allowing for a problem with the 12AX7, are those in the EF86.

But what is the cause of the high plate voltages (265v instead of 185v) in the 12AX7 and 0v instead of 71v on its cathode? I've checked the cathode resistor and continuities from it to the cathode and to ground. Then I checked and replaced the resisters from the B+ To the plates. I then thought I'd found the problem with a broken socket pin for one of the cathodes, but fixing this hasn't fixed the voltages either. I should add that the grid voltages are right.

Anything obvious that I've missed checking?

 

[gadget73]

0v at the cathode and high plate voltage means its not conducting or the grid voltage is wrong relative to the cathode.

The trick with a 5-10 is that the grid is direct coupled off the EF86, so if something is really wrong there your ECC83 voltages are going to be all wrong too.