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Old 20th July 2007, 04:31 PM   #1
Rodango is offline Rodango  United States
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Default Output Autotransformers

Recently I bought a Chinese TM300 6DJ8/KT88 based tube amp off eBay. It's a nice amp with good vintage tubes in it but it uses output autotransformers. Has anyone seen an amp use output autotransformers? I now know why the nuts on the output binding posts are insulated....

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Old 20th July 2007, 04:53 PM   #2
Gluca is offline Gluca  Italy
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I think a cap must be somewhere between the hi voltage/anode and the autoformer/binding pos and probably a loading choke too: I do no see any problem with an autoformer at the output. Binding posts are not to be exposed to hi volt. Insulation is there for other reasons.

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Old 20th July 2007, 05:04 PM   #3
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In a parafeed setup the autotransformer should sound slightly better than the conventional transformer.

Shoog
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Old 20th July 2007, 05:12 PM   #4
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http://www.nutshellhifi.com/gary/index.html

(and my version: http://www.ecp.cc/less-pressivo.html )

The only real problem if you are building your own is finding an autoformer that has a tap with a low enough output impedance.
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Old 21st July 2007, 05:26 AM   #5
Rodango is offline Rodango  United States
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Quote:
Originally posted by Shoog
In a parafeed setup the autotransformer should sound slightly better than the conventional transformer.

Shoog
And it does! It sounds very good. The schematic on the transformer shows no coupling cap or any other device. Just one continuous winding with plates, screens and B+ taps toward the top and the 8-ohm and 4-ohm taps near the bottom.
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Old 21st July 2007, 12:58 PM   #6
jduffy is offline jduffy  United States
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I think this does:

http://www.bottlehead.com/et/adobespc/S.E.X./SEX.htm
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Old 21st July 2007, 03:37 PM   #7
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Is there any cost advantage to an autotransformer?

mike
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Old 21st July 2007, 03:57 PM   #8
kevinkr is offline kevinkr  United States
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I hope this amplifier is not built as described, if so it is extremely hazardous! Unconscionable in fact.. While I think this is very unlikely you can check quite easily with a dvm.. Just measure the output taps to chassis and see if there is a large dc voltage present. (Be careful though as this if true makes it quite hazardous - a resistive load with insulated leads is a good idea across the normal output taps.) Obviously you could just take the bottom off and measure for continuity from the plate circuit to the output. Probably just a badly drawn transformer schematic.

The old sex amps used vertical sweep output trannies IIRC - these are not usually autoformers.

I doubt there is any significant economic advantage to an autoformer at these step down ratios - it is very attractive in applications where the ratios are low... While there are real performance advantages in low impedance, low ratio applications, at 20:1 or higher which is typical in an opt from a tube to a speaker I doubt there is any advantage at all.
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Old 21st July 2007, 11:32 PM   #9
Rodango is offline Rodango  United States
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Well, it must be a bad schematic on the xfmr, no continuity between pin 3 of one of the KT88s and the output jacks. Also no DC on the output jacks..
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