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Old 23rd Jan 2022, 9:08 pm   #1
ToneArm
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Default Mullard 5-20 with Haddon transformers

Hi folks

In 2015 I built a 5-20 around a Partridge UL2 output transformer. It was very good, but when things got tight in 2020 the transformer was one of the things that got sold. For a top price, happily, someone wanted to make a pair.

The remains of the amp languished in a box until Frank McVey offered for sale a suitable Haddon output transformer and matching choke a few months ago.

Unfortunately my chassis needed major surgery to fit them so the amplifier was dismantled, hacked, filled and re-painted in matching hammerite. I took my time with the re-build. I have done a few 5-20s now and happy to find my wiring gets a little bit neater each time. Still a long way to go and lots to learn yet, but I'm happy enough with this one to include an under-the-bonnet picture!

It worked almost straight away. Some noise but as expected, since the feedback was for the 8 ohm tap of a different transformer. The Partridge had been a bit of a fiddle to set feedback, using a trimmer pot and lots of trial and error. For the Haddon I reverted to the standard Mullard values and it was stable and quiet straight off. I was pleased!

One silly error. Among the noise at initial switch on was a weird ticking followed by a short buzz every 5 seconds or so. Suspecting a dry joint I spent an hour re-flowing and inspecting with my magnifying glass. Finally I twigged that the ipad I was using as a source had wifi, bluetooth and mobile data switched on! Once turned off - voila!

I'm so chuffed with this re-build, thanks to Frank and I hope he is pleased to see his lovely transformers put back into use. Its stereo partner has a Sowter UA21 but they sound perfectly well matched to me.

Cheers
Glyn
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Old 23rd Jan 2022, 10:16 pm   #2
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Default Re: Mullard 5-20 with Haddon transformers

Excellent work.Les
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Old 23rd Jan 2022, 10:19 pm   #3
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Default Re: Mullard 5-20 with Haddon transformers

Looking great, well done
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Old 23rd Jan 2022, 10:38 pm   #4
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Default Re: Mullard 5-20 with Haddon transformers

Looks very nice!!! Well done

Joe
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Old 24th Jan 2022, 1:11 am   #5
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Default Re: Mullard 5-20 with Haddon transformers

Very modest claims about your wiring, I must say!

Lose the cable ties on those electrolytics for some nice metal clips and it'll be the bee's knees
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Old 24th Jan 2022, 8:35 am   #6
frankmcvey
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Default Re: Mullard 5-20 with Haddon transformers

Quote:
I'm so chuffed with this re-build, thanks to Frank and I hope he is pleased to see his lovely transformers put back into use.
I'm so pleased to see that beautiful transformer and choke fulfilling their destiny! Very nice, professional-looking job. Congratulations!

Cheers,

Frank
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Old 24th Jan 2022, 9:53 am   #7
vinrads
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Default Re: Mullard 5-20 with Haddon transformers

Wow! Glyn fantastic! better than some so called professional builds. Mick.
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Old 24th Jan 2022, 1:21 pm   #8
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Default Re: Mullard 5-20 with Haddon transformers

That's lovely work. I'd love a picture of the pair together.

Cheers

Aub
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Old 24th Jan 2022, 4:52 pm   #9
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Default Re: Mullard 5-20 with Haddon transformers

Wow, that's a stunner. complete with the name tag.
Must be quite a window rattler. What speakers have you paired it with?
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Old 24th Jan 2022, 5:14 pm   #10
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Default Re: Mullard 5-20 with Haddon transformers

Good heater wiring, and the rest...
 
Old 24th Jan 2022, 7:33 pm   #11
ToneArm
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Default Re: Mullard 5-20 with Haddon transformers

Thanks for all the kind feedback. I used aluminium clad cathode resistors on a copper upstand from the tag strip which seemed like a good idea when I had it but it has made a very effective radiator pointing at C14! Revert to chassis mounting I think.
Cheers
Glyn
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Old 25th Jan 2022, 5:55 pm   #12
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Default Re: Mullard 5-20 with Haddon transformers

I was thinking of building my first valve amplifier this year and plan to buy all new transformers and valves as I don’t have anything suitable. I was going to build the 5-10 but now wondering about the 5-20. What was the primary function for the 5-20. I want a very basic Hifi amplifier and was going to build a pair. I actually wanted one of the quad valve amps but again just too tight. Should I consider the 5-20 ? Disappointed that there doesn’t appear to be any kits around and was hoping to buy a punched chassis.

The amp you built is a thing of beauty very inspirational

Thanks Chris
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Old 25th Jan 2022, 7:16 pm   #13
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Default Re: Mullard 5-20 with Haddon transformers

Glyn,

Really nice build. Nice use of turret boards, which make for nice tidy build.

The Sowter UA21 is nice transformer.

Over the years I’ve collected together a few one-off workshop made 5-10 amplifiers. These have been - like yours - lovingly made in a workshop. It saddens my heart to see them heading to a possible indignant fate of being stripped down (harvested) for the transformers and valves.

Re 5-20…
The 5-20 being a higher power amplifier found favour driving larger speakers and less efficient (modern) current hungry speakers. The 5-20 will clip at 30-35 watts.

The 5-20 circuit found its way into a couple of musical instrument amplifiers like the Linear, Northcourt and RSC amplifiers. I've seen a Barratt Guitar amplifier using the 5-20 without the UL tap to get maximum power from a pair of cathode biased EL34's.

A friend back in the early 1970’s used 5-20 with a 15” folded horn speaker and horn driver in his Sound System and Disco!

Terry.

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Old 25th Jan 2022, 9:36 pm   #14
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Default Re: Mullard 5-20 with Haddon transformers

Thanks Terry. Sadly it's the destiny of a home-made amplifier to be dismantled and its value being only the value of the parts. Would it even legal to sell it as a working amplifier? I'd think not. But there's so much satisfaction in making your own stuff, even amplifiers you don't actually need. I really should increase my repertoire!

Chris, my guess would be a 5-20 would be no more difficult to construct that the 5-10. I have built five 5-20s now, the first pair did use PCBs (redundant stock I believe from an attempted re-launch of the Beam Echo DL7-35 in the 1990s). The PCB helped a bit maybe, although not massively. The beauty of this amplifier is that the Mullard instructions are comprehensive, intended to guide people with limited equipment and experience (like myself) to get to a successful project.

My Sowter pair are in daily use as my "best" system. I also have some quad ii's paired with a croft preamp, a 33/303, 34/405 and a nice Pioneer receiver. I'm no doubt biased but I favour the 5-20s.

Cheers
Glyn
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Old 26th Jan 2022, 7:40 pm   #15
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Default Re: Mullard 5-20 with Haddon transformers

Looks good maybe I will give it a try
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Old 27th Jan 2022, 11:53 pm   #16
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Default Re: Mullard 5-20 with Haddon transformers

Saw these for sale a few years ago for £1700. Homemade. I costed the parts at £700 approx.
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Old 29th Jan 2022, 11:30 am   #17
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Default Re: Mullard 5-20 with Haddon transformers

Hi,

I stand to be corrected, and I maybe wrong, but I think that amplifier could have been made by the guitar amplifier company Matamp.

I had a chat with Jeff Lewis, the owner of Matamp, about the Matamp 5-20 amplifier and it was their foray into the world of valve hi-fi amplifiers using their valve guitar amplifier expertise. From memory they only made 20 or so pairs.

Attached a couple of pictures of the Matamp 5-20.

Terry
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