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Old 26th April 2012, 02:21 PM   #1
soeren is offline soeren  Greenland
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Default SE with PP OPT's?

I have some PP OPT's and many tubes lying around. I was tempted to try an SE configuration, which could be inspired by Tubemasters direct coupled EL84. Using Klipschorns 1.5W is fine for me.

My idea is to put B+ to the center of the OPT, one end to the anode of the EL84 and so on. The (new?) and maybe controversial idea is to connect the other end of the primary to ground via a resistor which in this in this case could be of about 5kohms, adjusted until symmetry is obtained.

Has anybody tried this, and does it work?

Søren
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Old 26th April 2012, 02:27 PM   #2
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There is a proven method for using PP O/P "iron" in a SE amp, the parafeed connection. Choke or CCS load the O/P tube and cap. couple to 1 end of the O/P trafo primary. Ground the other trafo end and tie the CT off.
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Old 26th April 2012, 02:41 PM   #3
soeren is offline soeren  Greenland
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I know that, but then the quality of the choke and capacity begins to be the limiting factor. Another point of view is that with my proposed solution I only have to find a few power resistors, the rest I already have.

I guess that the reason for not use my proposed solution is that it is extremely inefficient, but as long as we are only talking of a watt or two I find it not to be a problem.

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Old 26th April 2012, 02:45 PM   #4
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Then use a CCS. A Class "A" EL84 runs at about 40 mA. That's not a particularly difficult challenge for the available high voltage FETs and BJTs.
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Old 26th April 2012, 02:55 PM   #5
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From this thread:

Spud, Schade, PP, Anti-triode ECL86
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Old 26th April 2012, 03:19 PM   #6
soeren is offline soeren  Greenland
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Thanks for the answers.

So far I have not seen an argument against my proposed solution other than higher efficiency at the cost of a higher complexity.

My intention is not directly to make the 'best' amplifier but to make the simplest possible and quite simply to try something I have not seen realized before.

Søren
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Old 26th April 2012, 04:23 PM   #7
kevinkr is offline kevinkr  United States
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Provided the PP transformer has a large enough core you can use it as is. Note however that you should connect across the entire primary and tape off the center tap. Do not use the 5K resistor on the other tap as this is reflected directly to the plate of the output tube as a load in parallel with your speaker. Note also that using half the transformer gives you just 1/4 of the primary impedance available and in SE this represents a very steep and non-linear load line.

I used a pair of unmodified PP 20W EL84 transformers in a 45 based SE amp almost 20yrs ago, it's still running fine and sounds good according to the guy I loaned it to about 3yrs ago. Distortion performance while not stellar is about what you would expect for a 45 DHT in SE connection into a 4K load regardless of transformer used.
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Old 27th April 2012, 04:22 AM   #8
Fenris is offline Fenris  United States
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I used a single primary toroid in a parafeed configuration to get a few watts of power. It works very well:

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