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Old 7th April 2008, 06:08 PM   #1
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Default SE 6550 class project (continued)

Here are some progress pics from an earlier thread that I posted about a month ago. I was just doing an impromptu "fit-up"- and decided to post these. The base still needs finishing- and the ins/outs need labeling.

Comments, criticisms, etc??
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Old 7th April 2008, 06:14 PM   #2
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Default Another pic.......................

N/T
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Old 7th April 2008, 08:06 PM   #3
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Got a schematic? I LOVE the volume knob you're using, pure 50's style class. Let us know how she sounds.
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Old 7th April 2008, 08:20 PM   #4
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Default The schematic...................

Thanks for your kind coments.

The schema is located on this thread...............

My first "from scratch" design, SE KT88..........

There are some minor errors on the schematic because this was the first draft, so please forgive me.

It uses a ccs-loaded 6sn7 that yields a gain of about 14. I am using a heavily modded Adcom GCD-750 that puts out about 3 volts peak. The simulation shows that within it's practical power range, the circuit yields roughly 3% THD. I am considering adding a pentode switch as well.

The volume knob came from a 50's automobile radio. I machined a disc to fit the numbered dial inside because it looked a bit stark without it.
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Old 7th April 2008, 10:35 PM   #5
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Default Re: SE 6550 class project (continued)

Quote:
Originally posted by cadaverdog
Here are some progress pics from an earlier thread that I posted about a month ago. I was just doing an impromptu "fit-up"- and decided to post these. The base still needs finishing- and the ins/outs need labeling.

Comments, criticisms, etc??

I would not use a toroidal power transformer. Any hash or noise from your power line will make it right through. Just an opinion.
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Old 7th April 2008, 10:54 PM   #6
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Default About the toroid....................

I was worried about EM radiation being that the chassis was so small. The EMI self-cancellation properties of the toroid appealed to me in this particular design.

I wish that I had time to do a couple of build-ups both with an EI lamination transformer and with the toroid. Unfortunately, I have to present a working prototype before the class in 2 weeks.
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Old 8th April 2008, 12:45 AM   #7
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Quote:
I would not use a toroidal power transformer.
I have the same Antek toroid on a SimpleSE. No problems with noise even with an electric drill plugged into the same outlet as the amp (one of my usual amp torture tests). I am using a choke in my power supply though. You can hide a Triad choke under the chassis at a later date if needed.

The problem that I see is the B+ voltage. I get about 500 volts using solid state rectifiers with this transformer. I am using cathode bias so the tube sees less voltage. With about 485 volts across the tube (assuming 15 volts lost in the DC resistance in the OPT) you will need about -50 volts of bias. (based on my Electro Harmonix 6550's)
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Old 8th April 2008, 01:19 AM   #8
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Default Woah!!

One of the things not present on the schematic is the fact that I bumped-up the supply voltage to 400VDC, but that admittedly doesn't come close to 500VDC!!.................scary.

With that being said, I bench tested this transformer by measuring the secondaries with a true RMS DVM and got about 410V, then I measured the leg to leg dc resistance and saw about 52 ohms. I entered the values into a simulator and "virtually" loaded the supply with 2K ohms. The sim shows about 408 + or - a few volts and about 190ma.

I'm hoping I come reasonably close to that because I don't care to blow up those $25 Panasonic caps!!

BTW, Thanks for the warning.
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Old 8th April 2008, 04:59 PM   #9
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I'd be worried about overstressing your Tungsol 6550 as well. Make sure you don't exceed their dissipation ratings under your chosen operating conditions.

I'd also make up a nice cover for the toroid, nothing worse than the unfinished look of a toroid on top of the chassis to cause people to focus on the appearance of your project rather than on its performance. I've had this unfortunate experience and actually lost the opportunity for a commercial contract because the focus ended up on the appearance of my engineering prototype rather than on its superior sonic performance relative to its competition.
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Old 8th April 2008, 05:50 PM   #10
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Default Actually.................

............The perforated metal you see in the background of the picture is going to be used as a transformer cage. It'll have wood end caps- and be painted the same shade of "chocolate."

On that note, I believe that an audio equipment should look just as beautiful as it sounds. I had this image in my mind of a 50's looking amp, but with a few modern goodies such as the toroid and the soft recovery diodes, etc.

It was hard to get a cool "utilitarian" look, yet still retain some elegance; that's pretty much why the cage is for covering up the butt-ugly toroid- yet leaving the bottles exposed.

Thanks for your suggestions, they are greatly appreciated!!

PS, I ruined several machine tools when I milled the stainless top-plate. My suggestion is to stay away from this material for DIY!!
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