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Homeskillet's House
of Audio Electric Oddities

The circuits shown here are most certainly not appropriate for general use. I build them for my own enjoyment and curiosity. They usually are dismantled in a few days and the parts are used for something else.

The schematics and designs here are all bench designed using a basic circuit idea and then trial and error to arrive at specific values. They may have problems.

They are connected to a variac with a fuse and current meter. They are turned on and off by slowly turning the variac up and down. They are not subject to any of the rigors of consumer use and they are not intended to be.

I always wear eye protection and turn the power off by turning down the variac. Nothing is subjected to hard switching or a quick turn on. I also work with one hand behind my back if the circuit is to be approached while powered on.

I also continually check to see if the power is truly off as I work on a circuit by verifying the variac is off and the voltmeter and current meter reads zero. It is easy to be interrupted and resume working on a circuit to find that you haven't powered it down so checking continually as one works helps prevent this.

I also power the unit down and let it drain before applying test leads or making any change whatsoever. This part is really a big deal.

If I do post anything that is a working design suitable for general use I'll make a note of it and probably make a big deal of it.
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Posted 3rd August 2013 at 04:55 AM by homeskillet
Updated 5th August 2013 at 05:34 AM by homeskillet

Hi. Here continues the tinkering continued from the forum post:

I took out that EF184 and the plate to plate feedback loop since I read that devices with a more stable performance without feedback sounded better once it was applied than devices that rely on feedback for their very operation. This seemed about right and I was tiring of the sound a bit.

The plate to plate feedback was the only way I could find to stabilize the EF184 and 6550 in this basic configuration and give any reasonable results and wiring it satisfied my curiosity so apart it came.

I replaced V1 with a 12AT7 and wired it in parallel since I figured it could use all the help it could get to drive the 6550.

The global feedback is my first attempt at current feedback. It's basically part of my first attempt at feedback at all.

I got the idea from JC Morrison's blog but I was unable to use the cathode to cathode feedback he describes since I have only two stages. I understood I needed a low impedance source of feedback so I figured it could be had from the secondary of the output transformer somehow.

I ended up using the 4 ohm tap as a center tap with the hope that distortion products that make it through are fed as negative feedback to the cathode of V1.

The effect is pretty extreme. With the cathode of V1 unbypassed the response climbs as the frequency lowers as well as decreases as the frequency climbs. The sound is like turning a tone knob all the way down.

This sounds concurrent with what I read except that I was anticipating a rise with the high frequencies as well as the low instead of the roll off in the highs that I saw.

I remembered then that bypassing the cathode with a low value capacitor gives an uneven boost to the highs.

I had a .22 cap on hand. It's an old vitamin Q by the way but non-polarized anyway. This picks the high frequency response up and it turned out pretty even on a sweep from zero to 40kHz. A larger cap really messes things up. The .22 uf cap is working so in it stays.

Please feel free to comment. I believe I set the comments for members only but I chose to not moderate them myself.

Thank you for looking. -Fred

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  1. Old Comment
    Michael Bean's Avatar
    There's no feedback happening there, you need to ground one side of the output TX secondary.

    Posted 20th August 2013 at 11:55 AM by Michael Bean Michael Bean is offline
  2. Old Comment
    homeskillet's Avatar
    Hi. Sorry I didn't see your comment here. It certainly is odd and was just an experiment that I have not pursued any further.

    The signal is changed quite a bit by the connection in a way that resembles feedback (output is lowered) though it is quirky as was pointed out in the forum that I posted it in.

    I wouldn't call "no feedback". I would just call it weird. If there were no feedback then nothing would be changed whether it is connected or not.

    I appreciate your comment and when I employ a feedback loop that includes the output transformer I ground one side as you mention.
    Posted 20th December 2013 at 04:42 AM by homeskillet homeskillet is offline
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