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In the Forum: Melquiades Amplifier
In the Thread: Planning my DSET
Post Subject: I had it wrong all along...Posted by anthony on: 7/26/2020
Work has been so crazy busy all last year and through these strange times right now that I have not been having any time to think about the audio project let alone indulge in advancing it.  One DSET/Macondo has been playing mono and I have really been enjoying it.  Then a few weeks ago we took a family holiday to the beach (no C19 community transmission in my state of Australia...restrictions are reasonably loose) and I resolved to try to take a weekend for myself here or there.

The first weekend was spent cleaning and reorganising the room.  It is amazing how much dust gets through things and how long those Bass Cannons take to wipe down properly.  That was a load off my mind.  It felt so good to do something towards the audio.

Then this weekend, even though I should be working, I decided that the time was right to have another go at biasing the second DSET properly.  That week by the ocean had given me time to think about the problem I was having with the amp, and although I had already spent probably more than a week (or two) over the past 18 months trying to get things to work properly, my electronics beginner status seemed unchallenged.  By having the problem leave my head for six months, then revisiting it, where the problem lay became clearer and over the next couple of weeks I snuck moments to think about the method I would use to verify and ammend it.

It worked straight away.  Plus it has led to me finding a problem with the DSET to which I have been listening since March last year.  The first stage of Channel B has been running hot...270V at the anode where it should have been 200V.  Not sure how I could have missed that...but I did.  The fix for that issue is more challenging.

Romy, perhaps you are able to remember some of the lessons from when you were developing these amps all those years ago.  Basically, the problem is that the first stage of Channel B is biased at -7.15V where the remaining channels are in the vicininty of -4.2V.  I can change either the 12.1kR resistor to a smaller value (about 2.9kR) to bring B back into the fold, or I can double the size of the 402k resistor (between the aforementioned 12.1kR and 10uF cap).  Both will work according to my calculations.  I think you are going to suggest to change the 402kR to circa 800kR rather than 12kR to 2.9kR, but the former is much easier for me to do than the latter.

Happy days though!  I am going to let the amp soak in downstairs on the test bench this week and assuming no problems will grab a couple of blokes for the lift upstairs into position in a week or so.

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