Re: Mullard 5-10 amplifier
 

I am not an expert and I am sure others will chip in but in principle you want to achieve a star earth to a single point on the chassis.
In your amp I would start with the following.
Separate the HT power supply components from ground initially (only temporary), make sure that the 0v of the transformer secondary, centre tap or bottom of the winding, goes to the smoothing capacitor negative terminals.
Connect the smoothing capacitor negative to the star point. This ensures that the charging currents for the capacitors only circulate in the power supply.
Separate the heater centre tap from the HT secondary 0V on the transformer and take this point separately fo the star point.
The strip of Tin Copper on the terminal strip can probably remain but again separate it from the "daisy chain" and take it to the star point.
The centre spigots on the valve bases can probably go to chassis by the valve base mounting bolts, keep these short and local.
The input socket grounds likewise should go to starpoint.
I assume from the picture that the octal socket is for power to the preamp, if so the 0v from this should also go to star point.
If you have more than one 0V to the preamp avoid joining them together at the preamp end, this will cause a hum loop.
Hopefully this will give a good starting point but the most important part is separating the power supply and only connecting once to the star point.

Peter

Re: Mullard 5-10 amplifier
 

I'm talking about the actual resistance of any wiring in the ripple current loop (otherwise referred to as the charging pulse loop). The ripple current/charging pulses will develop a voltage across this resistance which will be added to the inevitable ripple voltage from the capacitors' ESR and reactance if this wiring is common to both the ripple current loop and the dc output circuits.

The circuits I drew illustrate this- the wiring resistances in the BAD circuit are common, in the GOOD circuit, they're not. You can't eliminate all common resistance but you can minimise it by careful layout.

It IS more of a problem in low voltage high current power supplies than in valve amp ones, but it's still worth keeping an eye on.

Re: Mullard 5-10 amplifier
 

NO Put the switch IN SERIES with the heaters so you can just switch them out for a few seconds.

Re: Mullard 5-10 amplifier
 

Lot's of possible fixes reccomended there but for a start find out where this 100hz (from your oscillogram) is getting in, BTW what is the amplitude, couldn't see what your volts/div was set to.

Get your scope and check the input to V1, then V1's anode, but check your scope can handle how many volts are on V1 anode, then go through the rest of the amp systematically. Tack a 630v 0.1 or similar cap onto anode pins if anode voltage exceeds scope Vmax in. Your right a PP OP stage should reject common signals, but only to a point.

You could also pull out the ECC83 and power up briefly to check which bit of the amp the 100hz is sneaking in. for more details on the 5-10 see here - http://www.r-type.org/articles/art-003e.htm

Andy.

Re: Mullard 5-10 amplifier
 

You may want to disable the feedback loop, otherwise once hum has got in anywhere, you will see it all the way round the loop which is confusing. Also the loop should be reducing it, so running the amp open-loop should show exaggerated levels to make things easier.

David

Re: Mullard 5-10 amplifier
 

Hello,

Just a quick thought, but could there be charging pulses and power supply filter noise etc. from the axial reservoir and filter capacitors being fed into an unfavourable point on the ground?

Can’t make out where the reservoir and filter capacitors are actually grounded, but on the 5-10 I have in the workshop they are returned more or less to the CT and OV mains transformer end of the ground buss so as to keep the pulses and noise close to the transformer CT and OV point?

Again just a thought.

See attached pic.

Regards
Terry

Re: Mullard 5-10 amplifier
 

^^ +1 for that.
The pictures posted earlier show a distinct 'blob' of solder right where the cap negative would expected to be connected.

Re: Mullard 5-10 amplifier
 

….and I just noticed that the original smoothing can looks like it was isolated from chassis and just earthed to the bus bar. I vaguely remember doing this to mine when I built them. The smoothing can had a clear plastic cover and I made the hole in the chassis just slightly larger than the diameter of the can so it could 'drop through' by about 1/4 inch. I then earthed the negative tag to the bus bar.

This means you may have to rewire your replacement smoothing caps and earth them to the bus bar as originally done rather than taking the negative connection to the nearest chassis point. You'll have charging pulses all over the chassis otherwise.

Re: Mullard 5-10 amplifier
 

I totally agree that the problem with this amplifier is being caused by 'modification' of the unit from the original well proven design in that the original smoothing cans have been removed and the replacements that are mounted below deck don't have their negative/ground connections returned to chassis correctly and as of the original design.

There is absolutely no hum problem with this amplifier as built in its original form - I should know, I've had one for decades.

The first question that I would ask is how did it perform originally before all the re-capping? I'm going to guess that you don't actually know because you didn't try it before doing a blanket replacement of capacitors, but I really hope you're going to tell me I'm wrong on that guess.

The paper capacitors that you've replaced so far certainly do need replacing in this amplifier if they're the original type. The electrolytic capacitors in the cans may well have been ok to leave, although in my particular amplifier I did have the smoothing section of the large can develop internal leakage causing burning of the associated resistor and warming of the can, so that had to be replaced.

To keep this valuable and desirable amplifier authentic, you really need to replace the smoothing cans with proper replacements, mounted as original on top of the chassis. Leaving holes in the top where they've been and fitting single replacements below deck will ruin this amplifier and destroy its value. Having said all that, I will admit that on my particular amplifier, when the smoothing section of the can failed, I did a temporary repair by isolating the bad capacitor section and mounting an axial type beneath the chassis - and that was perhaps nearly a decade ago, but it's my amplifier and I've no intention of selling it or passing it on to anyone else in my lifetime. If you're repairing your amplifier to sell on or even repairing it for someone else, then you need to do the job properly, unless of course, the person wants a cheap job done and is happy with having holes left in the top of the chassis.

To stop the hum problem I think you either need to return the negatives of the replacement electrolytics to as near as possible to the original design and reinstate that missing bus bar connection, or fit proper direct replacement capacitors, also reinstating that bus bar line.

Below are the original drawings of the amplifier layout, and the first one says it all as regards the connections - no extra chokes needed. This was a proven design, ok, designers do get it wrong sometimes, but my advice would be don't try to modify their design unless you have the technical knowledge to do so;)

Re: Mullard 5-10 amplifier
 

Hi guys,

Thanks for all your help.

I did unfortunately replace all the caps before powering this up. I don't intend to part with this amp anytime soon and I don't really care if it looks a bit different to how it did so long as it works well. I admit that I've messed this one up but you learn from your mistakes and I won't do that again!

Anyway I didn't get a chance to look at it today and I won't be able to till Tuesday and will keep you all posted as to progress. I really want to understand what I've done wrong electronically and why I'm getting hum where I shouldn't and will have to read through all your posts again.

On Tuesday I will get the caps as close to how they all were as possible. I did notice that the hum completely disappears when I remove the EF86.

Thanks again,
Tim.

Re: Mullard 5-10 amplifier
 

Hi Terry,

I think you're right, the caps are grounded on the nearest valve base, which is the EF86.

So that means the high gain pentode stage has pulses across it? When I pull the EF86 out it kills the hum.

Re: Mullard 5-10 amplifier
 

Techman - by original design, do you mean the Mullard design, or the design of this kit version (I'm pretty sure it is a Sterne-Clyne kit - 1960s - , which may use the same circuit, but differs considerably in physical design and layout)?

Mike

Re: Mullard 5-10 amplifier
 

That's a brave thing to do with the Mullard 5-10 (actually pulling valves out is a pretty brave thing to do with any piece of kit). The EF86 anode is DC-coupled to the grids of the following triodes. The grid potentials will go positive until the grid current through (what used to be) the EF86 anode load and HT dropper stops them. I'm not sure how much grid dissipation an ECC83 is rated for, but it may not be very much.

Cheers,

GJ

Re: Mullard 5-10 amplifier
 

It was a joy to build these amps back in the 70's. Those diagrams brought back the memories of bending the 16SWG bus bar to fit between the tags of the tagboards.

Incidentally you can still buy smoothing cans that should fit in the original mountings....if it was me doing the job, I would. Google 'Ask Jan First'. It will mean ordering from Germany but you will be supplied with first-class brand new capacitors (not NOS) and they are available in single, double and even triple cans. OK they are not cheap...expect to spend around £30 each for suitable multi-caps but you will never need to replace them again and the amp will retain its looks and value.

Re: Mullard 5-10 amplifier
 

The reservoir capacitor really needs its own separate path back from the negative terminal to the transformer. This capacitor gets charged in a series of hefty current pulses; and those current pulses will create a voltage across the return connection. If its charging circuit is effectively in series with anything, then that voltage will appear downstream of that connection.

Re: Mullard 5-10 amplifier
 

I can confirm that vampyretim's 5-10 is indeed the Stern-Clyne version. Yes, it does differ considerably in layout from the original Mullard design, but I think we can regard it as a well-proved version of the design.

I wish vampyretim all success with curing his problem with it.

Les.

Re: Mullard 5-10 amplifier
 

Regarding Dual Can capacitors, Cricklewood Electronics stock 2 x 16uf 500v., 2x 32uf 500v, and other values. The 2 x16uf is currently £7.80 inc. VAT (P&P Extra) The makes are usually F&T, the same as those stocked by 'Ask Jan First'

Re: Mullard 5-10 amplifier
 

....and dont forget to check the centre tap of the heater supply is earthed well.

Re: Mullard 5-10 amplifier
 

Well yes, primarily this particular kit on this occasion.

I'm gathering that you're not so enamoured with it yourself from what you've said in a previous post. I think what we have to remember is that it had to be made down to a price to be affordable. If it had been made like a Dynatron LF59 with an industrial grade steel plate chassis, seam welded on all the corners that you could park an British army tank on, then it just wouldn't have sold to the target market. It is what it is and I think it's very good for what it is and for the price it was at the time.

Re: Mullard 5-10 amplifier
 

I would look at the earth bus in the first photo in post#30 and as the earth for the main power capacitors are at the moment effectively connected to the audio screen next to the preamp valve move them to the actual earth of the HT secondary between the two 300V terminals. It would give a tiny bit more isolation from the small signals and it will be no more effort than putting it back where they were in the original design.
As there are now two cans I would be tempted to use two negative wires and join them at the transformer.