13th Jun 2019, 2:15 pm | #41 |
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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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13th Jun 2019, 2:38 pm | #42 |
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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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13th Jun 2019, 2:51 pm | #43 | |
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Re: Mullard 5-10 amplifier
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15th Jun 2019, 7:57 pm | #44 |
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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 |
17th Jun 2019, 2:55 pm | #45 |
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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. |
17th Jun 2019, 4:22 pm | #46 |
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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.... |
17th Jun 2019, 6:49 pm | #47 | |
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Re: Mullard 5-10 amplifier
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17th Jun 2019, 6:55 pm | #48 |
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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 |
17th Jun 2019, 7:08 pm | #49 |
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Re: Mullard 5-10 amplifier
Hi Mike,
I already have that book. I was wondering if there might be something like the art of electronics but for valve circuitry. |
17th Jun 2019, 7:13 pm | #50 |
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Re: Mullard 5-10 amplifier
I don't really "do" valves any more, but I'm sure others will have suggestions.
Mike |
17th Jun 2019, 7:13 pm | #51 |
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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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17th Jun 2019, 7:42 pm | #52 | |
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Re: Mullard 5-10 amplifier
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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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17th Jun 2019, 8:28 pm | #53 | |
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Re: Mullard 5-10 amplifier
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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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17th Jun 2019, 9:17 pm | #54 |
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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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17th Jun 2019, 9:57 pm | #55 |
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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 |
18th Jun 2019, 6:09 am | #56 |
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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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18th Jun 2019, 7:51 am | #57 |
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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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18th Jun 2019, 8:55 am | #58 |
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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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18th Jun 2019, 9:44 am | #59 |
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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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18th Jun 2019, 10:27 am | #60 |
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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. |