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Old 30th July 2006, 09:25 AM   #1
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Default What kind of output transformer?

I found this schematic in the web and I want to build one of this amplifier, but the problem is what type of output transformer is using in this schematic? I'm living in Malaysia and here is very difficult to find output transformer, even if you can find one, it will be very expensive. I think custom made is the only choice here. Anyone out there can give me some useful guide to make this output transformer?
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Old 30th July 2006, 09:26 AM   #2
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Default Schematic

This is the schematic
Attached Images
File Type: gif 60wel34.gif (36.0 KB, 502 views)
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Old 30th July 2006, 09:53 AM   #3
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Making output transformers is something of an art. Nevertheless, the opriginal Williamson article gave details on how to make the output transformer and that transformer would be entirely suitable for your circuit (which bears more than a passing resemblance to a Williamson). I believe Hi-Fi World is available in Malaysia, and their subsidiary World Designs sell a booklet comprised of all the original Williamson articles.
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Old 30th July 2006, 12:09 PM   #4
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More like a Mullard, I think? This appears to be a circuit out of Audio from 1959 (March). The transformer is an S152A, 4000 ohms plate-to-plate. The amp is good for 50 watts as shown.

If you can find iron out of an old Dynaco Mark III, that will work just fine. There are quite a few commercially available transformers with 50-60W ratings and 4000-5000 ohm primaries that shoudl work just fine.
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Old 30th July 2006, 01:36 PM   #5
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Hey Matthewong,

Nickc, DIY member here, would be able to help you out.

Promitheus Audio


regards,
zul
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Old 30th July 2006, 01:53 PM   #6
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Quote:
Originally posted by SY
More like a Mullard, I think?
A Mullard-type input stage and phase splitter combination, but Williamson output stage biasing.
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Old 30th July 2006, 02:03 PM   #7
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Williamson was cathode bias, this is fixed. Same idle current balancing topology, though.

Interestingly, the original article put all the focus on the use of silicon rectifiers, a novelty at the time. The doubler was used because of the difficulty of getting a rectifier with more than a 400PIV rating.

The voltage divider on the screen of the input tube, referenced to the cathode, is an interesting trick.
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Old 31st July 2006, 09:50 AM   #8
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Quote:
The transformer is an S152A, 4000 ohms plate-to-plate.
That's as may be, but 4000 ohms p-p is NOT an optimal load for EL34s in UL PP. UL offers a fairly broad range of OP impedance but 4000 ohms is too low and will give more distortion than the recommended and optimal 6.6k p-p.
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Old 31st July 2006, 11:44 AM   #9
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That's interesting because Dynaco always used 4300 ohm p-p in their fixed-bias EL34 amps.
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Old 31st July 2006, 02:35 PM   #10
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I'm going by what Claus Byrith stated in his article "Power Amplifiers with Valves," in which he partially redesigned the Mullard 5-20. One of the changes he made was to replace cathode bias with fixed bias. He stated that the ideal load p-p for the EL34s in UL would be 6.6k but he was obliged to use a Lundahl OPT and the closest they had available at that time was 5k p-p.
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