First rate Mullard 5-10 stereo home build

[Hi-Q]

My brother built a pair back in the day. Then the two main choices of output transformer was either parmeko or partridge. Partridge were deemed the more superior and that is what my brother used. Congratulations in finding them in that amp, if they are in good condition you will enjoy

 

[BinaryMike]

Reminds me of the terminal board I built for CatSkin. I started with a fiberglass 'vectorboard' predrilled on 0.100" centers, then just enlarged the holes that I wanted to use for terminals. It doesn't look as clean because I installed almost all of the wiring topside, but it's still eminently serviceable.

 

[nj pheonix]

I don't think I ever saw that article before.
Thanks Mike.
I'm going to have to read (reread) that a few times and chew on it a bit.
It's very different than anything I remember seeing before.
Dandy sorry about the sidetrack. Congrats on a cool (and unusual) piece.

 

[Dandy]

Have begun checking the carbon comp resistors carrying high voltage. Many have drifted high and need replacement. I'm using new carbon film ones that I have to hand, and it sounds even quieter. The turrets make replacement work so easy. These seem to have a slot into which the lead can be 'clicked' before soldering. Amazing.

 

[gadget73]

CC resistors are terrible for noise and holding value. Any flavor of film is an upgrade in both departments.

 

[Audiovet]

http://www.r-type.org/articles/art-003e.htm

I am going to be hated for this ......

Referring to the above schematic: The value for R12 and R13 is given as 820K-ohm. Thereby hangs danger ....

To avoid the trap H.J. Leak fell into with his Stereo 20 amplifier: He used 1Meg resistors in the originals; the specs declared that to be the maximum allowed values. So many amplifiers came back with EL84 failures that his reputation was almost ruined - the EL84s went into grid rectification current and failed. He had to reduce those values to some 680K, later using 470K grid resistors with EL84s to be on the safe side.

Data sheets still indicate a maximum of 1 meg, but the publication dates may have preceded that of Leak's experience. Possibly the batch of EL84s used was at fault - no further datil available.

I would not use 1 meg as grid return resistors. It is hardly ever necessary to use more than 680K. In the prsent amplifier going to 680K will make no difference at all. Nothing else needs be changed.

 

[Dandy]

Thanks for looking in audiovet. My next chance to look at this will be over the weekend. I'll try moving the 820k resistors down 680k.

 

[Dandy]

I replaced the resistors for 680k ones without a problem.

A separate problem arose when I replaced the RCA input sockets with better quality ones--one side started oscillating. It stopped when I shorted the input.

I double checked my soldering on the socket and that the ground of the RCA was grounded. And checked my soldering on all the other replacement components for that channel. I even replaced the 8uf 450V decoupling caps. Nothing changed. Then I changed back to the cheaper RCA, and the oscillating went away again! Not sure why the high quality replacement RCA caused the oscillation.

 

[gadget73]

ground change maybe? Are the originals isolated from chassis where your new ones are not, or vise-versa?

 

[Dandy]

Yes I think it was a ground issue. Both old and new sockets were/are chassis grounded, so maybe just a bad ground connection. The ground buss is just galvanized wire. More updating needed.

 

[Dandy]

Well I've been running this amp for a while now and thought I'd post an update. I got the amplifier unit itself running well after replacing most of the caps and resistors, but the more I worked on it, the more the age of the components concerned me. There was a particular problem with the connector between the power supply and amp. I temporarily bypassed this by connecting heater, ground and B+ connections with separate wires.

Then, recently, I located a supplier of the obsolete Belling Lee parts and I knew I could restore the connectors. When the parts arrived I decided to go for a complete teardown, respray and rebuild. The respray gave me some problems as the metal primer didn't bond well with the steel transformer covers, but I went ahead anyway. The result is shown in the pictures. Despite careful handling, two tiny flakes have come off the transformer covers, so they'll need attention sometime.

Despite problems with the paint job, I was glad I stripped and cleaned it completely as there was some age-related corrosion everywhere, including under the transformer covers. (See pic.) At one point the power supply and amp must have been stored in damp conditions. As I only have external Belling Lee connectors for one cable, I took two of the four power connections out of commission, and that allowed me to reduce the thickness of the wires for the power to the filament heaters.

It was very satisfying getting it all cleaned and rebuilt with new wiring. I couldn't restore the neon power indicator and one of the fuse holders broke, revealing and earlier glued repair, so there are a few new components. The ground buss turned out to be copper, just very corroded from the decades.

My wiring is also not as neat as the excellent original, but I still intend to stick the cables in place using silicone, as I've seen Dave G do with caps. I was amazed at the care taken with the original build, except that the builder located the two mains inlet fuses right over the rectifier! Not wanting to mess with the chassis, I have left them there.

With the power supply restored, I'll get on with the amp itself. Aiming for a better paint job.

I'm looking forward to learning more from studying the work of the late builder. I added a detail of the Belling Lee connector and braided cable as these vintage connectors may be as unfamiliar to others as they were to me.

 

[derekva]

Well I've been running this amp for a while now and thought I'd post an update. I got the amplifier unit itself running well after replacing most of the caps and resistors, but the more I worked on it, the more the age of the components concerned me. There was a particular problem with the connector between the power supply and amp. I temporarily bypassed this by connecting heater, ground and B+ connections with separate wires.

Then, recently, I located a supplier of the obsolete Belling Lee parts and I knew I could restore the connectors. When the parts arrived I decided to go for a complete teardown, respray and rebuild. The respray gave me some problems as the metal primer didn't bond well with the steel transformer covers, but I went ahead anyway. The result is shown in the pictures. Despite careful handling, two tiny flakes have come off the transformer covers, so they'll need attention sometime.

Despite problems with the paint job, I was glad I stripped and cleaned it completely as there was some age-related corrosion everywhere, including under the transformer covers. (See pic.) At one point the power supply and amp must have been stored in damp conditions. As I only have external Belling Lee connectors for one cable, I took two of the four power connections out of commission, and that allowed me to reduce the thickness of the wires for the power to the filament heaters.

It was very satisfying getting it all cleaned and rebuilt with new wiring. I couldn't restore the neon power indicator and one of the fuse holders broke, revealing and earlier glued repair, so there are a few new components. The ground buss turned out to be copper, just very corroded from the decades.

My wiring is also not as neat as the excellent original, but I still intend to stick the cables in place using silicone, as I've seen Dave G do with caps. I was amazed at the care taken with the original build, except that the builder located the two mains inlet fuses right over the rectifier! Not wanting to mess with the chassis, I have left them there.

With the power supply restored, I'll get on with the amp itself. Aiming for a better paint job.

I'm looking forward to learning more from studying the work of the late builder. I added a detail of the Belling Lee connector and braided cable as these vintage connectors may be as unfamiliar to others as they were to me.

View attachment 1046550 View attachment 1046552 View attachment 1046553 View attachment 1046557

Suggestion - try putting the amp in an oven at low heat (e.g. 150-175 degrees) for an afternoon to dry out the transformers (and help cure the paint). Not too hot, of course!

-D

 

[Dandy]

Thanks D. I don't know much about painting beyond using rattle cans.

I enjoy watching the Rick's Restorations show and would love to play in a shop like his!

 

[nj pheonix]

It looks nice.
2 possible suggestions.
For sake of safety and neatness, I think I'd use SJ, or SO cable to that connector (Not criticizing I think it looks good)
If that's a real PITA to redo, they make mesh sleaving you could probably add without too much trouble.

 

[Dandy]

Thanks for the cable suggestions, nj. I didn't know what to look out for. I would prefer something more robust for a permanent arrangement and I would have to desoldering one side anyway to add mesh sleeving.

 

[derekva]

I dig those industrial connectors. They look extremely robust.

-D

 

[primosounds]

That really does not look home made,especially the amplifier portion.The componentry,layout and especially the ''style'' of the build screams Made in England.
I'm thinking maybe Leak,Pye or perhaps even Mullard themselves.
Edit: If it was indeed home made,likely the builder was involved in the manufacturing industry.

I would not say home made but more of a "Custom" professionally made. No manufacturer would go to the time and effort put into this amp. It is definitely way over built. There is that dude "Turner audio" in Oz, whose custom designs are out of this world. Building an amp that sounds and looks better than anything you can buy in a box is only a matter of time and money. If you can use your time the money cost is way less than half of any comparable off the shelf unit. It seems that the vast majority of of members here are mostly into commercially made audio equipment especially looking to acquire or repair a vintage unit. This decision is safe but not necessarily the best sounding buck for your audio dollar.

 

[Dandy]

I dig those industrial connectors. They look extremely robust.-D

I agree that they look super cool, though a broken pin in one of them is (I guess) the reason the amp came out of service, and ultimately came my way.

Primo, I agree that this is a HQ custom build. I'm rebuilding the amplifier module now and, as I will show, the heater wiring was laid out on a separately screened off layer below the turret board layer. Disassembly took me a while, because I hadn't realised the neatness of the wiring direct from the tube sockets to the turret board, incorporating the filament heater layer.

 

[arts]

No manufacturer would go to the time and effort put into this amp. It is definitely way over built. /QUOTE]

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:

 

[Dandy]

Arts, you're right. When this amp first came to light on another forum, there was a bit of head scratching about the logo, RDE, followed by Power 250 or Amplifier 250. Close inspection showed that the letters were just individual transfers, put on by hand. The R might have stood for Rolls Royce as they were made by someone working for them, but DE is anyone's guess.