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Old 6th July 2012, 07:29 AM   #1
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Default Why does the Miniwatt N3 have 4 connections to its output transformers?

Ok so I'm still doing a lot of research to understand ultralinear vs single-ended, and I came across an article talking about the output transformers of Single-Ended vs Ultralinear and discovered that a Ultralinear will have four connections to the output transformer and the Single-ended with have two.

So this makes me ask the question why does the miniwatt N3 have four? I took it apart, it has four, I know this for a fact. So whats the deal what am I missing? and I've been reading about converting an ultra linear into a super triode here http://www.mennovanderveen.nl/eng/index.html, and many others also talked about an easier way, which makes me wonder what the person was talking about when they told me that it's not possible to turn a ultralinear into a Triode amp? (maybe that wasn't what I asked? what's the difference from a triode and a single-ended then?)

These are my thoughts after wandering the internet, clarity would be much apreciated Click the image to open in full size. Click the image to open in full size.
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Old 6th July 2012, 09:26 AM   #2
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A transformer need to have a minimum of 4 connections, 2 inputs and 2 outputs. It can have more taps, both on the input and the output, but the number of taps isn't indicative of its intended usage. Try to find if there are any markings.
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Old 6th July 2012, 11:40 AM   #3
DF96 is offline DF96  England
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Quote:
Originally Posted by Thinker1920
which makes me wonder what the person was talking about when they told me that it's not possible to turn a ultralinear into a Triode amp?
Of course you can turn a UL into a triode amp. First replace the OPT, or accept that you will be running with the wrong load impedance if you keep the old OPT and just move g2 to the anode connection. Then, if necessary, modify the driver stage to give the higher drive voltage a triode output is likely to need without compromising diistortion too much. Then, if appropriate and necessary, modify the negative feedback to take account of the change in open loop gain and frequency response.

That's all you have to do.
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Old 19th July 2012, 07:54 PM   #4
frapa02 is offline frapa02  United Kingdom
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I recently acquired a Miniwatt N3 which needed repair. Whilst it was apart, I saw the 4 connections from the output transformer to the PCB which were referred to in the original post above.

So, to answer the question about why there are 4 connections, the other 2 connections are to return the output winding to the negative feedback loop.

I tested the DC resistance of the primary winding which was high at around 1k ohms. This shouldn't hurt but creates inefficiency.

There are some things we could consider doing to the N3. Firstly, remove the unnecessary resistors across the input. I have not yet checked what value they are but the Chinese are notorious for adding these and shunting the signal. Just to be certain of a grid leak resistor, all you need is a 1M ohm across the input which would take over if the pot went bad. I have seen 100k or less across the input on many far eastern amplifiers.

Then, we could look at the coupling capacitors.

There are no grid stoppers so watch out for instability. It may get away with it due to the tight design of the PCB.

The output stage has a current source in the cathode, using a MOSFET and what looks like a thyristor - very unusual.

The power supply is nice an beefy, the fault with mine was one of the 100uF 350V Nichicon HT capacitors was blowing steam and electrolyte which caused multiple big arcs. This was easily replaced but a bit fiddly.

It sounds good but I am missing a bit of depth in the lower mid range, using Mullard EL84s and EI ECC83. What hit me with the greatest impression was the dynamics and resolution of cymbals and drums and of course the mid range purity.

Cheers, Paul.
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Old 19th July 2012, 09:40 PM   #5
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I had the same feeling about the missing depth in the lower mid range, I recently picked up a single ended motorola amp SH12N and I love it so much I sold the miniwatt:P
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Old 20th July 2012, 01:18 AM   #6
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The resistors across the output, they are not useless. They are there so that you don't fry something when you connect the load after powering on.
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Old 20th July 2012, 10:28 AM   #7
DF96 is offline DF96  England
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Quote:
Originally Posted by frapa02
I tested the DC resistance of the primary winding which was high at around 1k ohms. This shouldn't hurt but creates inefficiency.
That is high! Even a cheap 1950s radio set OPT would have a lower resistance than that. Maybe that is the price for getting inductance up enough to get a 30Hz LF rolloff, without making the OPT bigger.
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