Mullard 5-10

[stuart_morgan_64]

I have a pair of these amplifiers, both seem to work ok now, but on one the mains tx seems to get too warm, they seem well made. Are there parts that should be replaced? Or check through the parts. Any recommendations please.

Just added another picture, litle concerned, made for a smaller amp.

[whiskas]

Hi Stuart,
Besides checking the valves in a tester for any problems, and checking the power supply voltages etc.

I would then take a look at the electrolytics as a probably cause of potential / brewing problems.

First check the power supply smoothing caps (with great care they hold some very high volts!!), then any cathode bypass caps. Look out for symptoms of drying or leaking/shorting capacitors. The types in there are very old and probably well past their best now. To be honest, I would just change them all anyway as a routine service to protect the amps from damage that dud & dying caps can cause.

Don't worry about the Mullard mustard caps, they are virtually never a problem. They're probably the most reliable caps from that period and are seldom faulty.
See how you go from there..
Good luck with it!

[stuart_morgan_64]

Ok thanks, just added another picture of the transformer label, says for the 3-3.

[whiskas]

Hi Staurt,
If the circuit is distressed by bad caps, it may be drawing too much current. This would make the transformer get hotter than usual, and may lead to damage to the transformer.
I suspect the electrolytic caps first - usually they are the most common problem in vintage equipment like this, and a common symptom is excessive current being drawn due to caps with leaks/shorts etc.
It's best to just replace the electrolytics anyway, to protect the amplifiers.
Also, it would probably be best to avoid running the amps until the caps are all changed.

[stuart_morgan_64]

Thanks for the advice, I have checked some and low ESR, so will change elcrolytics, will look for some PSU ones, or posibly seperate ones.

Thanks

[whiskas]

I see what you mean about the transformer being labeled for the 3-3, which is a single EL84 amp, giving about 3 /4 watts.
In that case the transformers might be a bit under-spec'd as the 5-10 I believe uses 2 EL84's per amp, which adds about another 50ma of current draw.
I'd still change the electrolytic caps as they're too old to be reliable now and will cause problems sooner or later.
For good quality and a vintage look, I would reccomend using the BC/Vishay types stocked by Farnell, RS and others.
[Please don't skimp and use cheap Chinese stuff as they'll likely let you down just as the old ones will.]
If you need help finding the BC/Vishay let us know and I, or one of the other chaps here will help you source them.

[GrimJosef]

The 5-10 could be run in a number of different configurations. Yours appears to be set up for so-called 'low-loading'. This puts 437ohm (390ohm+47ohm) resistors in each of the EL84 cathodes which reduces their quiescent current and thus lowers the load on the mains transformer. I can see that the large pink Welwyn resistors in your picture are marked four hundred and something ohms but I can't quite read the text clearly.

Anyway, even in low-loading your mains transformer will be running right at the limit of its specs and I'm not surprised it gets warm. All it would take would be either for the cathode bypass electrolytic capacitor to go a bit leaky or for one of the EL84s to go a bit gassy and the valve's anode current would then rise and the transformer be in trouble. If you're confident that you can safely make measurements inside the amp while it's powered then I'd recommend that you measure the DC voltages on the EL84 control grids relative to signal ground (they should be no more than a few hundred millivolts, and ideally a lot less) and also the resistance from the EL84 cathode to ground (positive meter probe to the cathode) after the amp has been powered down and the valves have cooled significantly. This is a more tricky measurement. Ideally you want to make it while the bypass caps are still warm, since that's when they're most likely to be leaky. But if you dive in too quickly there will still be a bit of residual emission from the valve's warm cathode and that can confuse the reading a good deal, especially if you're using a modern digital meter.

Cheers,

GJ

[stuart_morgan_64]

Thanks for the info, I will check current draw on both LT and HT once caps are fitted, have all caps in stock, apart from the HT so have ordered from RS. Will update, thanks everyone.

[cathoderay57]

It is usually informative to measure the valve electrode voltages (anodes, grid2 (where applic), & cathodes) and tabulate the results for both amps for comparison. Also if you have service data compare the measurements with those in the service data. A very useful guide is the "Green Book" entitled Mullard Circuits for Audio Amplifiers" published in various editions from 1959 to at least 1966. It contains everything you ever wanted to know about the 5-10 circuit but were afraid to ask...…. Copies turn up regularly on eBay. If you are stuck then let me know and I can post the voltages here. Cheers, Jerry

[Biggles]

I find that an in line power consumption meter can be useful in cases like this. You could pull the rectifier valve out and see if the power consumption is the same on both amps. If one is pulling more than the other this would eliminate the main smoothers etc, and point to a fault in the mains transformer itself.

[wd40addict]

The low loading version of the 510 draws 60mA of HT. I built one in 1981 and had to build the low loading version as that was the rating of my mains transformer. The transformers for both the 3-3 and 5-10 are niominally 300-0-300 so also a bit puzzling that yours is a 250-0-250 unit.

What voltage do you get across C15?

I should also say my transformer never gets more than slightly warm.

[terrykc]

Originally Posted by cathoderay57

Also if you have service data compare the measurements with those in the service data. A very useful guide is the "Green Book" entitled Mullard Circuits for Audio Amplifiers" published in various editions from 1959 to at least 1966. It contains everything you ever wanted to know about the 5-10 circuit but were afraid to ask...….

Here's a reprint of the original article: http://www.r-type.org/articles/art-003e.htm

[Techman]

The fact that the transformer is designed for a 3-3 indicates that it's not going to be up to the job for a 5-10, although both transformers appear to look the same. Proper measurement of voltage and current will soon tell you everything you need to know. The unit is obviously somebody's home brew construction from years ago, so you never know what you're going to be up against with folk skimping on using the correct parts such as the mains transformer. It probably never worked properly and hence why it got sold on.

[thermionic]

Looking at the original article as posted by Terry, then the minimum HT current specified is 60mA. The heater requirements are satisfactory. The only mystery is the 250 - 0 - 250 volt HT voltage. I wonder if this is a misprint on the transformer label?

The amps look competently built, and sport a nice output transformer!

SimonT.

[terrykc]

The plot thickens ...

Here is the 3-3: http://www.r-type.org/articles/art-003h.htm and the transformer is also clearly shown with a 300-0-300v HT winding!

[PJL]

The Mullard 5-10 construction notes do include 60mA HT transformers when built for low loading and as the HT voltage is also reduced the actual quiescent current will be less than low loading.

One option might be to use solid state rectifiers in place of the EZ81. This would reduce the mains transformer power demand by 6.3W and increase the HT voltage to nearer the design value.

[stuart_morgan_64]

I have ordered smoothing caps from rs. Will take measurements tomorrow when all fitted. Read all comments thanks.

[stuart_morgan_64]

Caps fitted, all electrolytics replaced. 2 gn LT less than 1A
HT reading from rectifier to caps 57mA
DC from rectifer 300VDC
Current at mains socket 22mA
Mains TX cold, will soak test tomorrow, so far so good, thanks.

[stuart_morgan_64]

How do I know what impedance speakers it has been set up for?

[GrimJosef]

The speaker impedance setting will be determined by the way in which the secondary turrets on the output transformer have been wired. This is the bunch of turrets you can see in your third picture with the twisted purple wires going to them. A good close-up photo of that, ideally showing any labelling alongside the turrets, might allow us to work it out.

Cheers,

GJ