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Old 11th November 2012, 08:56 AM   #1
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Default sv572 transformer headache

At the beginning of the year I hatched a plan to get my sv572 amp off itís breadboard and into a box. This was mainly to keep it away from prying hands which were bodily pulling components off the board, no matter where I tried to hide it.

I changed the amp to choke input and dc heating using Rod Colemanís regulators. I bought a Hammond 372FX (300-0-300 170ma) and decided to use an RS 2x9v 100va frame I had in the bit box for the heater supply. Rod didnít have an example transformer for sv572 so I thought as the heaters were 6.3v @ 4 amps then 100va would be ok(thatís 2x9v@5+amps) . The B+ requirement is for 500v @ 100ma and 300v @ 12ma. The rebuild is documented here:

Vitalstates - SV572

Well the build went aheadÖ.and bit me in the ****. The B+ transformer got really hot and the heater transformer buzzed like a pneumatic drill. I learned that a ct transformer is no good for full wave bridge(bit late but never mind)Öso I sourced a 0-600 @ 150ma frame and a 2x9v 120va toroid for the heaters. Re drilled the chassis, moved everything around to accommodate and thought what a good plan.

Well the build went aheadÖ..and bit me in the ****. The B+ transformer got really hot, and the toroid got really hot. Rod Coleman explained that the transformer should have 3 times the capacity of the heater requirement(bit late, but never mind)Ö.so I sourced a 2x9v frame at 200va which is 9v at 11-12Amps. I also sourced a 0-600v at 200ma frame(remember, I only need 500v @ 113ma). Also put the heater rectifier on a heatsink at the rear of the chassis. Re drilled the chassis, moved everything around and sat back thinking what a great plan.

Well the build went aheadÖand bit me in the ****. The B+ transformer gets very hot and the heater transformer also gets very hot. Iíve measured both transformers and according to my dmm with temp measure they are both reaching 70 deg C after about 30-45 mins. Also, upside down on the bench, without the base screwed on, all is quiet. As soon as the base is brought anywhere near the chassis all hell breaks loose. By that I mean the base vibrates and makes the whole chassis shake and there is much noise. Steel chassis and steel base.

Amp sounds very nice but with the heat and vibrating when itís got the base on itís destined for the scrap.

The questions:

What is a realistic max temp for a transformer. Is 70degs reason for alarm. Itís way hotter than my Pass amps because I canít apply the 5 second rule, more like 2 seconds. I didnít expect a 200ma transformer to get hot when asked to deliver just over 100ma. Is that a reasonable expectation.

Is the loud buzzing the result of some interaction between all the transformers and chokes and the steel base. Would an aluminium base cure the problem?. The thing that confuses me is that the last incarnation didnít buzz/vibrate and the only thing thatís changed is one choke that used to be on top is now inside and 1 transformer that used to be inside is now on top.

Ed
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Old 11th November 2012, 09:21 AM   #2
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Hi Ed,
Nice looking amp, what mu factor are you sv572-x ? 3 or 10

Best
Karsten
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Old 11th November 2012, 09:31 AM   #3
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Are you shure your rectifyer is actually a bridge or working as one ?
Could it be that your transformer iron is saturating because of half wave rectification ...
would also explain the buzzing.
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Old 11th November 2012, 12:03 PM   #4
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Quote:
Originally Posted by payloadde View Post
Are you shure your rectifyer is actually a bridge or working as one ?
Could it be that your transformer iron is saturating because of half wave rectification ...
would also explain the buzzing.
good thought, I will double check the rectifier wiring.
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Old 11th November 2012, 12:04 PM   #5
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Quote:
Originally Posted by 211hojbjerg View Post
Hi Ed,
Nice looking amp, what mu factor are you sv572-x ? 3 or 10

Best
Karsten
Hi Karsten

572-3 and 811-3

Ed
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Old 11th November 2012, 02:55 PM   #6
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Wat kind of metal are you using for the chasis? Could there be a magnetic shortcircuit?

Joris
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Old 11th November 2012, 03:23 PM   #7
DF96 is offline DF96  England
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Some DC measurements might help. My guess is that something fundamental is wrong, but instead of fault-tracing and correction you leapt to conclusions and kept changing things.
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Old 11th November 2012, 03:53 PM   #8
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very constructive df96!

I'm currently looking at the transformer bolts which don't have any insulation at present. Others have suggested the possibility of a shorted turn. I will post the findings.

Ed
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Old 11th November 2012, 04:04 PM   #9
DF96 is offline DF96  England
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Why not post some numbers, or at least confirm that all the numbers you have already measured are more or less what you would expect to see.
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Old 11th November 2012, 06:01 PM   #10
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Quote:
Originally Posted by DF96 View Post
Why not post some numbers, or at least confirm that all the numbers you have already measured are more or less what you would expect to see.
The B+ power supply is here:

http://www.vitalstates.org/diy/amplifiers/sv572-ps.jpg

On the last transformer(150ma rating) measuring across a 10R resistor between the rectifier and the first choke showed 1.16v(112ma). The power supply has not been touched other than to swap in the new transformer so I have no reason to believe the current draw has changed.

The heater power supply is here:

http://www.vitalstates.org/diy/ampli...72-hps-cct.jpg

The voltages all look valid:
9 vac on secondarys under load
10.43 vdc between rectifier and first cap
10.36 vdc between last cap and regulator.
Adjusting the current on the regulator easily produces the required 6.3vdc across the filament.

Ed
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