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Old 20th Mar 2018, 11:38 pm   #1
Croozer
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Default Four valve stereo amp (EF86s UL84) revisited

Apologies for revisiting this old thread :https://www.vintage-radio.net/forum/...=129209&page=3

Some time ago I received much support from the forum when I posted seeking guidance on building an amplifier. I got to working drawing 7 and too much hum when I left off last.

I came across this write-up when clearing my electronic desktop and realised I hadn't posted it. So here is the final installment.

After persisting with the dinky American 30A5 output valves I still had hum issues. Tying the neutral side of their heaters back to the chassis via a capacitor gave some improvement so I reckoned that isolation between cathode and heater in this little valve might not be all that it could, and decided to abandon the 30A5s as a bad job.

The build itself was inspired by the performance of a UL84 in a record player. I had a heater supply of 90v from a tap on the primary side of the mains transformer. Serendipitously, the UL84 has 45V heaters, so I ordered a pair from Chelmsford, recalculated for the new DC bias conditions and rebuilt accordingly.

The UL84? A great valve! Same circuit, same layout, no trace of hum and far!, more punch. I thought it was a direct equivalent for the more common EL84, but it is designed for lower anode volts and a smaller anode load (I guess to match cheaper-end consumer electronics with puny little output transformers - like mine - and cheap power supply arrangements- like mine).

It might not be hifi, but it does what it is meant to do, and will make the output from a mobile phone into something that is listenable - and more than enough oomph to get an ASBO in a tenement.

So I've ended up with a four valve stereo amplifier, EF86/UL84. It's not exactly the prettiest piece, but I've blinged it up with a couple of LEDs under the valve sockets.

Final circuit diagram and a photo are attached. Here is a link to a video showing its performance with a Chris Rea LP.

https://youtu.be/pjnzfM3oKW4

Many thanks to all who offered advice and assistance.
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Old 22nd Mar 2018, 7:30 am   #2
Diabolical Artificer
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Default Re: Four valve stereo amp (EF86s UL84) revisited

Glad you got there in the end and solved the hum issue, well done. Your right it isn't pretty, but often it's not the destination that's important, but the journey .

Andy.
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Old 22nd Mar 2018, 9:57 am   #3
Ed_Dinning
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Default Re: Four valve stereo amp (EF86s UL84) revisited

Hi, looks pretty good!

As you have discovered, U series are close, but not always direct equivalents to E series. They may be closer to P series where the same numbers exist.

The UL84 is a valve that can develop hum with age, usually because it is driven very hard to get max output and is normally poorly ventilated.

Ed
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