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Old 31st October 2013, 02:35 PM   #1
stevenrotterdam is offline stevenrotterdam  Netherlands
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Default Quad 50

Hi guys,
I like the Quad 50 because it is a "tube amp with transistors".

Schematic:
Quad 50E circuit diagram - large copy

Anyone who has a clue about the OPT? I have some biggish 5,5 CT :1 toroids and I wonder if they are suitable for something like this.
Steven
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Old 31st October 2013, 04:59 PM   #2
DF96 is offline DF96  England
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In most valve amps the OPT is a necessary evil, responsible for some of the weaknesses of valve amps. It seems bonkers to add an OPT where it is not needed - or was it just a silly fashion statement? The result is an amplifier which suffers from both valve and SS weaknesses.
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Old 31st October 2013, 05:47 PM   #3
skidave is offline skidave  United States
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A 50E is a Quad 303. As I recall, the 50E has transformer output taps to run it with distributed sound systems...i.e. 70 Volt systems for public address.

Get a 303 off of eBay or build a clone. Great little amp!
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Old 31st October 2013, 07:05 PM   #4
planet10 is offline planet10  Canada
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Quad 50
Quote:
Originally Posted by DF96 View Post
In most valve amps the OPT is a necessary evil, responsible for some of the weaknesses of valve amps. It seems bonkers to add an OPT where it is not needed - or was it just a silly fashion statement? The result is an amplifier which suffers from both valve and SS weaknesses.
That last statement is not always true.

The Q50E was made to specification for the broadcast industry. A 303 with one channel removed and an isolation/output transformer installed. I've not heard a Quad 50E but reports from those that have say it is not as good as the 303 of which i have owned many (close to 20?). Last time i had a 303 i compared it to an Audio Sector chipamp. The 303 was embarrased. I have since given up on 303s.

Rene Jaeger brought a couple high voltage (300-350V) MOSFET power amplifiers with DHT driver stages and output transformers to VI diyFEST a couple years ago that exhibited fantastic performance.

dave
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Old 31st October 2013, 07:38 PM   #5
DF96 is offline DF96  England
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Quote:
Originally Posted by skidave
A 50E is a Quad 303. As I recall, the 50E has transformer output taps to run it with distributed sound systems...i.e. 70 Volt systems for public address.
Ah, that explains it. I guess not many audiophiles have 70V speakers.
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Old 31st October 2013, 08:41 PM   #6
SY is offline SY  United States
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Quote:
Originally Posted by DF96 View Post
It seems bonkers to add an OPT where it is not needed - or was it just a silly fashion statement?
Peter Walker design, with redo (after 50% field failures) by Peter Baxandall. Not likely to be a fashion statement.
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Old 31st October 2013, 10:05 PM   #7
stevenrotterdam is offline stevenrotterdam  Netherlands
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I must disagree with the statement that the 50 is similar to the 303. The transformer may be configured for various output impedances, but that is not its main function. It is a push-pull design with 2 identical output devices, like your average EL34 amp.
The 303 has quasi-complementary outputs which makes it a totally different design. Probably they couldn't make good power PNP's back then.
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