Mullard 5-10 amplifier
 

Hi all,

I'm restoring a Mullard 5-10 amp one with a lovely Partridge output transformer.
I've re-capped it and all voltages are what they should be. She sounds amazing the D.C coupling is producing lovely rich bass but she's humming quite a bit.
I was wondering if a filter choke might improve things? If I could get the hum down I would go as far to say that this is the best amplifier that I've ever heard.
I have a couple of old radios with a filter choke, do you guys think that I could repurpose one perhaps?

If not does anyone recommend anywhere that I could get one?

I've really fallen in love with this gorgeous little amp!

Thanks.

Re: Mullard 5-10 amplifier
 

Check the ECC83 - leaky heater cathode can cause hum as the cathode voltage is a fair way up from ground.

Re: Mullard 5-10 amplifier
 

Quite simply put THEY DON'T HUM if everything is working correctly. In practice there is very slight residual hum if you put your ear to the speaker with the volume at zero. If you have recapped it, does that include the smoothing cap? Apart from the ECC83 you should also check the EF86. If this has been changed, I've heard it said that some of them currently supplied are not up to the standard of the original Mullards.

Agree that these are fine sounding amplifiers. Mullard knew what they were doing when they designed it. You shouldn't need to make any changes at all.

Re: Mullard 5-10 amplifier
 

Hi guys,

I've swapped out quite a few of the valves to see if that was causing hum but I've only got a couple of ecc83's at the moment and I needed two for the control unit. I did try a few nos Mullard EF86's though so it's not that. When I took one side of the EL34's out the hum reduced significantly so I replaced that valve but the hum was still there.

I replaced all electrolytics and waxies. I will try a different ecc83 tomorrow.

Thanks guys

Re: Mullard 5-10 amplifier
 

One trick you could try to determine if it's coming from the heater supply is to disconnect the heaters while the amp is operating....arrange a switch or something in the heater supply and operate it briefly...the amp will operate for some seconds before the heaters die away. More than enough time to establish if the hum stops of not. If it does then investigate the actual heater wiring...I think it was originally a centre-tapped supply. You could try running the ECC83 and EF86 heaters from DC but as I said, it shouldn't be necessary.

I built two of these back in the 70's....no problems at all with hum.

Re: Mullard 5-10 amplifier
 

Just out of curiosity, how would I go about running the heaters on D.C? Rectifying heater supply?

I'll put a switch across the heaters tomorrow and see what happens. Thanks for the advice.

Re: Mullard 5-10 amplifier
 

If you just stick a bridge rectifier in the heater supply, you A) lose some voltage in the diodes and B) still have a waveform with a strong varying component... 100Hz ripple, so you likely still have hum but double the frequency.

You need to smooth and filter the rectified output of a higher voltage secondary. So it's common to add regulation.

It quickly becomes a big project.

BUT other people have 5-10s working fine on AC heating. So something is wrong. You need to find it and fix it. It's likely less work than going DC heating.

Switching off the heaters is a very neat test and saves what might be an awful lot of work which could be for nothing.

David

Re: Mullard 5-10 amplifier
 

Surely those o/p valves are EL84s. It was the 5/20 that used EL34s.

I wonder if it was built from a kit. I had a massive hum problem with a 5/10 that I built from a Sterne Clyne kit back in the late 1960s. It turned out to be down to the awful chassis supplied - just thin folded alumnium with no proper corner strengthening - the sides,front and back were just loose flapping aluminium. Also, they had "rationalised" the underchassis component layout to a single, vertical tagboard carrying all the smaller components (Mullard's original design was very exacting and careful). A picture of the underneath might be useful. I ended up having to strap large connections across parts of the chassis to eliminate/reduce hum loops from currents in the chassis.

But, as others say, an excellent amp when properly executed, and plenty enough volume when feeding into the much more sensitive speakers of its time.

Mike

Re: Mullard 5-10 amplifier
 

" Just out of curiosity, how would I go about running the heaters on D.C? " Bit here - http://www.valvewizard.co.uk/heater.html

Another thing to check is layout as well a htr wiring and grounding, a pic of the layout would help. Do you have a scope?

Andy.

Re: Mullard 5-10 amplifier
 

I had hum problems with a Rock Ola amplifiers ,in the end it turned out to be bad earthing where the tag strips were riveted to the chassis and in some cases where the valve bases were also riveted to the chassis as an earth point, I am now soldering these points to the chassis, Mick.

Re: Mullard 5-10 amplifier
 

Whether this is a fault or not depends on how big a bit, and what fraction of it 'quite' is.

People who are used to solid-state amps expect no hum. None whatsoever. So if they can hear any at all when they press their ear up against the woofer cone they say "Oooh, that's humming !". In fact what they're hearing might be perfectly normal for a valve amp.

The hum-and-noise spec for the Mullard 5-10 was 75dB below 10W. That corresponds to 1.6mV RMS into 8ohms. With a modern speaker of, say, 87dB/Wm sensitivity I'd expect that hum, especially if it's at 100Hz, to be audible by most people if they're within 1m of the speaker, but not really audible if they're 2.5-3m away (typical listening position in a good-sized room).

If you've got a lot more hum than 1.6mV RMS then you ought to be able to detect it, just about, using the lowest AC voltage range on a decent multimeter (check that the hum pickup in the loop made by the meter leads isn't what you're measuring by touching the far ends of the leads together and confirming that you then measure zero). I'd recommend using an 8ohm resistor as the load to avoid any strange impedance resonances which can be a feature of loudspeakers at low frequencies.

Cheers,

GJ

Re: Mullard 5-10 amplifier
 

If the mains transformer doesn't have an earthed centre tapped winding for the 6.3v ac heater supply, a quick and easy fix is to try fitting a 'humdinger' as follows. First disconnect the chassis earth connection from one side of the 6.3v ac heater supply and fit a couple of 2W 100R resistors in series across the 6.3v winding, and earth the centre connection between the two resistors. This emulates a centre tapped winding and often works well. You could also use a high power wire wound 200 - 500R potentiometer if you can find one - a carbon track device isn't man enough for the job though. The slider of the pot goes to chassis and you can adjust it for minimum hum. Cheers, Jerry

Re: Mullard 5-10 amplifier
 

I am not saying that adding a choke would help; but I have found the people at http://www.bluebellaudio.com/ to be very helpful. Their website is interesting anyway.

Re: Mullard 5-10 amplifier
 

Hi guys,

As requested a picture of the layout.

Yes, I do have a scope. I always thought that push pull amplifiers tended to cancel out most hum, I can see saw tooth ripple on the output valves anodes.

The amp seems well built to me.

I'll get a trace of the hum and find out frequency etc in a bit. I have modded a Dansette A35 with a hum dinger before and it worked very well.

I was very careful when recapping this 5-10 to make sure that there was still only one earthing point on the chassis which is at the BNC input.

Re: Mullard 5-10 amplifier
 

I've got unsurprisingly 100hz at the speaker but it looks weird, see the picture. I'm just about to wire a switch to the heaters. I'll post shortly.

Btw I have switched ECC83's and the hum is just as loud.

Re: Mullard 5-10 amplifier
 

I put a switch across the heater supply and it didn't stop the hum, if anything with the heaters off there was another hum present. The heater winding is centre tapped.

I think it must be a bad earth or an earth loop. I'm going to re-check all solder joins.

I've tried to get a reading of the hum with my dmm but couldn't get any reading, I don't know if it's sensitive enough its lowest A.C range is 200v.

Re: Mullard 5-10 amplifier
 

May also be worth checking that there's no "common resistance" between the inputs to and outputs from the smoothing capacitors. You want any external resistances in the ripple current loop kept out of the DC output circuits.

This is best achieved by routing the inputs and outputs to/from the capacitors on separate wires. Common negative cans should have 0V in from TX centre tap or bridge -ve on a separate wire from the 0V out (or often the chassis connection to the case).

Re: Mullard 5-10 amplifier
 

This is a common misconception. The capacitor providing an AC ground for the second LTP grid has almost exactly the same effect on frequency response as a coupling cap would have.

Do you mean 200mV?

There are two ways to build a 5-10:
1. do it exactly as Mullard said
2. learn enough about stability, hum, grounding etc. to design and debug your own layout

Your 'scope trace suggests to me that PSU charging pulses are going somewhere they should not go. This could be magnetic or ohmic coupling. Magnetic coupling is reduced by distance and by reducing loop area. Ohmic coupling is reduced by careful grounding design, so the charging pulses stay within the charging pulse loop.

Re: Mullard 5-10 amplifier
 

Looking closely at the photo of the layout I personally think that the grounding needs rerouting. It seems that all the 0V connections on the transformer are connected together at the transformer, they then go by a thick piece of tinned copper to a valve centre pin and then to the tag board, along the tag board and then via another piece of TCW to the octal socket, presumably for the pre-amp. Other grounds are connected to the tagboard such as the switch. It is not clear where the capacitor 0V connections go, probably to chassis locally.
As has been suggested by others on this thread the charging current loops for the power supply should be separate and only connected to the chassis / 0V at a single point, likewise the centre point of the heaters should only be connected to ground at this point.
On the plus side I see the heaters wires are twisted and run clear of other wires which is good.

Peter

Re: Mullard 5-10 amplifier
 

Hi all,

I moved onto the Stern preamp as I wasn't getting anything out of it and I came to use the scope and as soon as I connected the earth of the scope probe onto the metal work the thing burst into life albeit with the background hum still present. The hum is only noticeable at low levels and quiet passages and is independent of volume.

I'm a bit confused Chris by what you mean with the wire resistance for filter caps. I've been very careful not to pin the negative cap terminals to any point on the chassis it has all been wired by me to comply with the one earth point on the chassis.

You're correct Pete in how the 0v connections are. How would you recommend rewiring it?

Thanks for all your help guys.