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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets. |
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22nd May 2019, 7:47 pm | #1 |
Nonode
Join Date: Nov 2016
Location: Aberaeron, Ceredigion, Wales, UK.
Posts: 2,883
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Output transformer on Blackbox
I’m doing a bit of background reading in preparation for restoring a Pye Backbox.
I keep coming across reference to the output transformer being “fragile “ and problematic. Could you tell me why this should be? Many thanks. Cheers John |
22nd May 2019, 8:11 pm | #2 |
Dekatron
Join Date: Oct 2003
Location: Croydon, Surrey, UK.
Posts: 7,567
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Re: Output transformer on Blackbox
Maybe they were borderline rating. As the amplifier ages and the coupling capacitors start to leak, the output valves get stressed and draw more current. After a while the transformer will just fail on the primary winding.
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22nd May 2019, 8:31 pm | #3 |
Dekatron
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 13,996
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Re: Output transformer on Blackbox
I'd be more minded to think of classic "green spot" corrosion rotting through the windings.
If the OPT does fail, don't worry: you can easily/cheaply get a replacement transformer [with much-better insulation] from the likes of Hammond. Alternatively, a 12-0-12v mains-transformer rated at 10W or so will work. So it's not "HiFi" rated but unless you've got traceably certified, historically-burned-in Golden Ears™ you won't notice the difference. |
22nd May 2019, 8:56 pm | #4 |
Dekatron
Join Date: Jan 2004
Location: Newcastle upon Tyne, Tyne & Wear, UK.
Posts: 8,194
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Re: Output transformer on Blackbox
Hi Gents, there were several different transformers used on the BB.
The early units using the EL42 were simply PP, later units were UL PP. The later units were UL PP and used the PCL83. They were all pitch covered to keep out moisture and I have not seen many with "green spot". The majority of them have died from excessive current caused by leaky caps; but, the feedback system (3 different loops) on the PCL83 types has been known to oscillate ultrasonically and cause the valves to draw high currents. The anode winding on all these units were a bit marginal and subject to getting hot Note that one of the units had a mains transformer with an unused 60v winding. Ed |
23rd May 2019, 10:56 am | #5 |
Dekatron
Join Date: Oct 2003
Location: Croydon, Surrey, UK.
Posts: 7,567
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Re: Output transformer on Blackbox
Yes that's a good point about HF oscillation....I've had that on the PCL83 version. Fortunately the transformer survived.
__________________
There are lots of brilliant keyboard players and then there is Rick Wakeman..... |
23rd May 2019, 2:03 pm | #6 |
Nonode
Join Date: Nov 2016
Location: Aberaeron, Ceredigion, Wales, UK.
Posts: 2,883
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Re: Output transformer on Blackbox
Thank you all for your informative replies. I already had you in mind Ed if the transformer turns out to be faulty, but I’m jumping the gun, I won’t have the BB for a week or so, if the transformer is faulty I’ll consider my options.
Cheers John |
30th Jun 2019, 7:56 pm | #7 |
Nonode
Join Date: Nov 2016
Location: Aberaeron, Ceredigion, Wales, UK.
Posts: 2,883
|
Re: Output transformer on Blackbox
Just an update. Fortunately the output transformer was ok, so far, so this thread can be closed.
Cheers John |