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Old 13th Jun 2019, 2:15 pm   #41
turretslug
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Default Re: Mullard 5-10 amplifier

I agree with Refugee, and with julie m's post (#36)- keeping ripple return current out of low-level stage chassis return paths is good practice and very inexpensive, involving only forethought and small lengths of wire. Ripple current has a typically peaky and sharp-edged form, resulting in unpleasant buzzy harmonics that effectively appear to be part of the signal as far as subsequent stages are concerned- push-pull output won't cancel this sort of hum.
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Old 13th Jun 2019, 2:38 pm   #42
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Default Re: Mullard 5-10 amplifier

Another thing I spotted is that the input socket is bolted directly to the chassis right next to the preamp valve so with the capacitor negative going onto the chassis there will cause hum every time.
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Old 13th Jun 2019, 2:51 pm   #43
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Originally Posted by Techman View Post
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.
Well, I agree with you that it was designed down to a price and for a particular market, and I suppose that's fair enough. To expand a bit on my experience - I bought the Stern-Clyne kit to partner the Mullard Type C tape preamplifier that I was building (excellent circuit, pretty well up with the semi-pros), so performance mattered, especially with a tape pre-amp where so much design effort had gone in to eliminating hum. I went to Tottenham Ct Rd to buy the components for both circuits and, being more interested in the tape preamp, decided to get the kit version 5-10 to save me some time. Stern-Clyne's shop there had a built demo model which was exhibiting a low but intrusive hum, and I tackled the salesman on this. He assured me that the hum was simply the result of the connections between the 5-10 and the other source equipment, and wasn't "native". With hindsight, this was obviously a well-rehearsed sales patter to get around an awkward, basic flaw. I was in a hurry at the time and bought one. When I built it, it had exactly the same hum I had heard in the shop - even with the input grounded. That's what convinces me that there was a basic design flaw with these kits. I tried and checked everything, and eventually discovered that the hum could be reduced by connecting leads between various parts of the chassis (I didn't know enough about grounding good-practice back then as a teenager to analyse the earth-paths, and was in a hurry to complete the system). I ended up strapping thick cable across the chassis (and I mean thick - the stuff used for equipotential bonding in bathrooms/kitchens). This reduced the hum to something like what I'd expect from a 5-10. I often wished afterwards that I had bought the parts and built it exactly to the Mullard design. So, I was just concerned that the OP might have the same designed-in problem as I had, and might spend time and money looking for non-existent faulty components when the fault lay elsewhere.
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Old 15th Jun 2019, 7:57 pm   #44
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Default Re: Mullard 5-10 amplifier

Hello,

I use the F&T 50+50uF dual can for reservoir and filter, which are the same capacitors as sold by the BVWS.

Quite some time ago I fitted a regulated supply to a pair of Sterns 5-10’s, this used a 6080, EF91, 85A2 with silicon rectification. There are enough spare 6.3V windings on what is beefy mains transformer for the filaments of the 6080 and EF91. Silicon rectification gave enough raw pre regulation voltage to enable me to set the regulated voltage at 300V or thereabouts.

From memory it improved the noise level and THD, but by how much I can’t fully remember, but for all the effort it didn’t give a vast improvement in the amplifier in practice.

Also, from memory I fitted a 0.1uF between directly to the chassis and the negative speaker terminal to try and help with RFI pick up via the earth of the speaker leads.

Anyway, these are nice amplifiers and give a good performance given their shortcomings.

Regards
Terry

Last edited by Valvepower; 15th Jun 2019 at 7:59 pm. Reason: Triping Errot
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Old 17th Jun 2019, 2:55 pm   #45
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Default Re: Mullard 5-10 amplifier

Thanks guys,

Hum has gone!! I placed the cap earth connections back to the 0v of the transformer.

Am getting some hiss but virtually zero hum.

Thanks for everyone's help.
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Old 17th Jun 2019, 4:22 pm   #46
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Default Re: Mullard 5-10 amplifier

Excellent news- it's always gratifying to get a definite result from suggestions that might at first appear a bit nebulous, and remote diagnosis is always a bit of a lucky dip!

An object lesson in how some earths are more equal than others....
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Old 17th Jun 2019, 6:49 pm   #47
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An object lesson in how some earths are more equal than others...
Lol, I won't be making that mistake again. I would really love some books on how to design valve amps. Does anyone recommend any?
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Old 17th Jun 2019, 6:55 pm   #48
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Default Re: Mullard 5-10 amplifier

You could do worse that start with the introductory general chapters of Mullard's own "Circuits for Audio Amplifiers" (all of them valved). You may be able to find a pdf copy online somewhere. There are more specialised and detailed treatments, but it's a good start.

Mike
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Old 17th Jun 2019, 7:08 pm   #49
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Hi Mike,

I already have that book. I was wondering if there might be something like the art of electronics but for valve circuitry.
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Old 17th Jun 2019, 7:13 pm   #50
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Default Re: Mullard 5-10 amplifier

I don't really "do" valves any more, but I'm sure others will have suggestions.

Mike
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Old 17th Jun 2019, 7:13 pm   #51
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Default Re: Mullard 5-10 amplifier

Ah, bedside reading!

The Mullard 'Circuits' book was a good run down on their amplifier designs and their valves. It won't tell you much about the principles of design, but it will feed you some good examples. GEC had a similar publication for their valves.

THen there's Williamson's article in Wireless World on the ultra-linear amplifier circuit.

Fritz Langford-Smith's 'Radiotron Designer's Handbook' is a generous tome that goes into how valves work and how to design with them.

Then there's 'Radio Engineering' by Fred Terman which is good at explanation.

These are really the standards of their day. Some can be found as scans on the internet.

David
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Old 17th Jun 2019, 7:42 pm   #52
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Default Re: Mullard 5-10 amplifier

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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.
...and that also emphasises the point I made. When I built two of these back in 1970 they were the type without controls since I built the matching pre-amps to go with them. I made sure that they were built as close as possible to the original Mullard layout and had no problems.

Delighted that you have sorted it out....just sit back and enjoy. Everyone thought I was old-fashioned using valve amps in 1973 but they did admit that they took a lot of beating....especially when a very 'critical' friend brought his copy of 'Dark Side Of The Moon' round for me to check since he was unhappy with the bass response on one particular track. He had a top Marantz amp at the time...the 5-10s took it in their stride with floor-rumbling bass (using Leak Sandwich 600's). The Marantz just couldn't handle it...I think the power supply was the weak link.
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Old 17th Jun 2019, 8:28 pm   #53
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Quote:
Delighted that you have sorted it out....just sit back and enjoy. Everyone thought I was old-fashioned using valve amps in 1973 but they did admit that they took a lot of beating....especially when a very 'critical' friend brought his copy of 'Dark Side Of The Moon' round for me to check since he was unhappy with the bass response on one particular track. He had a top Marantz amp at the time...the 5-10s took it in their stride with floor-rumbling bass (using Leak Sandwich 600's). The Marantz just couldn't handle it...I think the power supply was the weak link.
I think that the bass response is great on the 5-10, I'm very impressed.

I've got quite a bit of hiss though and would like to reduce it if possible. I don't have any idea what would cause hiss.
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Old 17th Jun 2019, 9:17 pm   #54
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Default Re: Mullard 5-10 amplifier

The hiss usually arises in the earliest stage of the amplifier i.e. the EF86 and its circuitry. Some EF86s are noisier than others - all you can do is work your way through a few, listening for the quietest one. Another possible source is the anode and screen grid resistors. Mullard specified that these should be 'high-stability, cracked carbon' types for precisely this reason. These days we would call them 'carbon film'. Modern metal film ones are even quieter, but, as always, it's important to make sure that they are not running too close to their voltage limits.

Cheers,

GJ
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Old 17th Jun 2019, 9:57 pm   #55
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Default Re: Mullard 5-10 amplifier

Hello,

That’s cracking news you’ve got hum sorted, I thought the charging pulses running back down the ground towards the mains transformer and power supply wouldn’t be helping.

As for a book there is Valve Amplifiers by Morgan Jones – its worth a look at.

Back in 1973 I came by (thanks to my late Mum) a valve Quad II system and played Dark side of the Moon through it to stunned friends!

Terry
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Old 18th Jun 2019, 6:09 am   #56
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Default Re: Mullard 5-10 amplifier

Glad you got it sorted, Valve amplifiers by Morgan Jones is available as a free PDF online as are a lot of other books but the best way to learn IMHO is try and design and build a valve amp yourself. I think the Valve Wizard site is the best straight forward hands on build guide there is, there is good article on grounding.

Then there is Crowhurst Basic, Designing Vacuum Tube Amplifiers and related topics and many other free articles and PDF's but still you learn more by tinkering and making mistakes.

Andy.
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Old 18th Jun 2019, 7:51 am   #57
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Default Re: Mullard 5-10 amplifier

Building things without a bit of theory to explain what you are building is like staggering around blind. Doing theory without ever trying to put something into real prsctice is another sterile pursuit.

Things really come together when you do something and know the basics of what you are doing.

Theorists who start making prototypes suddenly discover there are factors they neglected

Builders who delve into theory suddenly have thoughts along the lines of "Oh, that's what was happening when I...."

Have a foot firmly planted in both camps!

David
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Old 18th Jun 2019, 8:55 am   #58
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Default Re: Mullard 5-10 amplifier

My instructor, 48 years ago, encapsulated your comments, David, very succinctly. As he put it 'Theory without Practice is lame, and Practice without theory is blind'. Whether that phrase was his own or a quote from someone else I know not, but it's undoubtedly true.
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Old 18th Jun 2019, 9:44 am   #59
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Default Re: Mullard 5-10 amplifier

In Post#45 Terry speaks of the 5-10s "shortcomings". I dont know what shortcomings there really are - maybe the power supply could have used a Choke? Of course the mains and output transformers need to be of very good quality, but the unit was designed to give the best possible sound - esp. the UL version - from readily avaiable components and that could be built by a capable home constructor. I knew of the team that worked on this at Mullard House and their trials were exhaustive.
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Old 18th Jun 2019, 10:27 am   #60
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Default Re: Mullard 5-10 amplifier

Hi all,

I'm getting roughly 80mV of hiss at the speakers, I've tried over 10 EF86's some of these were nos.

I'm just about to start checking screen and anode resistors.
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