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Old 9th August 2008, 03:50 PM   #11
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I'll drill out the chassis today. I checked the layout one last time before drilling. I scooted the PT over to the left so it's symmetrical with the OTs. Noticed I used the wrong template for the PT footprint, so I need to fix that and reprint (used the Allied instead of the Hammond). I also moved the board over 1/4" to the right to make room for the RCA input jacks at the far left.

In retrospect, I wish I had put all the small resistors on the other side of the board. Originally I wanted every component to be accessible without having to unbolt the board from the chassis, but now I wish I had the space to try to fit in my big coupling caps. Oh well.

How close can I get a KT88 to an OT? This is a small chassis, and I'm not going to have a lot of clearance between the front power tube and the left transformer. Maybe 3/4" at best...

The board is upside down in the photo. It'll get mounted right side up, underneath the chassis top plate after everything is drilled out.

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Old 10th August 2008, 12:54 AM   #12
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Still much to do, but at this point I think it's more done than not. The wood box frame needs to be made, and the chassis top plate needs to be sanded to get out the scratches. I can't decide if I want to powercoat it black. After that, there's the wiring to do.

I think the chassis top plate turned out just fine.

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Old 12th August 2008, 04:31 AM   #13
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Here's tonight's progress...

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Old 12th August 2008, 02:39 PM   #14
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Cool. Thanks for the update.

So when do we get to the listening part?
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Old 16th August 2008, 03:47 AM   #15
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The listening part comes soon. I put the last few components on the board (current regulators, coupling caps, input connectors).

I had some trouble with the current regulators - after bending the three leads as directed, the mounting hole didn't line up. Rather than try to re-bend the leads closer to the TO-220 package, I used the Dremel tool to cut the mounting hole a little longer. Perfect fit.

I wanted to use my K40Y caps but they seemed just too big. I settled for some 715P Orange Drops I had leftover from a previous project.

I got 80% through the wiring tonight. I still have to finish the line side of the power transformer, and there are a few little jumpers to get installed for the RCA input jacks. My wiring looks a little spaghetti. I should probably cut the excess off the transformer leads, but for some reason I can't bring myself to do it.

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Old 16th August 2008, 04:03 AM   #16
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Quote:
I should probably cut the excess off the transformer leads, but for some reason I can't bring myself to do it.
Make it work first. Then the wires will be eaiser to cut!
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Old 16th August 2008, 03:32 PM   #17
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Can anyone who has used the Transcendar output transformers say with any confidence which way the cathode feedback connections should be made? On George's wiring diagrams he suggests that many of the OPTs he has used require the black lead from the OPT to be connected towards the end of the board closest to the power supply caps and rectifier tube. I've guessed that mine should be connected this way, but it would be nice to hear from someone who has already used the Transcendars on a Simple SE.

What's the recommended rating for the fuse? Slow blow, or fast?
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Old 17th August 2008, 12:08 AM   #18
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Well, that was easy. But, I won't get a chance to really listen to it until after the kids are all put to bed tonight...

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Old 17th August 2008, 03:15 AM   #19
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I have a Simple SE with Transcendars in it. I can check it out when I get home. I should be there maybe tomorrow (currently in a cheap hotel on I-95). More cancer surgery on Tuesday and tropical storm Fay should also be there on Tuesday. I might not get time to take the cover off of the amp, for a day or two.
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Old 17th August 2008, 04:02 AM   #20
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Well, I'm finally getting a little time to play with my new toy.

I did some voltage checks. The PT is 395-0-395 with no load (pulled the rectifier tube). The 5 volt and 6.3 volt windings appeared to be spot on. Line voltage is 120 tonight, which is a hair lower than usual.

After installing a pair of new EH "fat bottle" 6CA7 and a Ruby (Chinese) rectifier 5AR4, B+ is about 455V. I'm seeing 433 volts at the anodes and about 442 volts at the screens. Both cathodes are fairly closely matched at 34 volts. Doing some quick math I've figured 61 mA idle current, and 24 watts total dissipation (plate and screen). It seems darn close to the rated maximum for a 6CA7, but I've been told these EH are good tubes. I see no red plate or glowing screen at the moment. I'll watch them carefully for signs of stress.

I verified proper connection of the cathode feedback. I tried it both ways, and watched the output voltages on my 'scope. I have no doubt it is connected correctly. The wiring is as I had guessed it ought to be originally. The secondary's green wire must be landed furthest from the rectifier / power supply cap side of the board. By the way, has anyone noticed that you can very clearly hear the audio material emanating directly from the tubes themselves? I had dummy 8 ohm loads connected during feedback testing, and I was surprised the music was so audible. I don't recall noticing the effect with the smaller 6BQ5 tubes.

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I've got a slight hum issue, which I've temporarily corrected. My RCA input jacks are not isolated from the aluminum top plate of the chassis. On the far other end of the chassis, I've attached the IEC power connector's safety ground. My other amp was built the same exact way, and it didn't hum. Guess I wasn't so lucky this time. I need to work out a permanent fix, which will most likely involve isolating the RCA input jacks from the chassis.

The amp has quite a bit more gain than the Dynaco ST35 clone to which I am accustomed. I tend to listen late at night, I have sleeping kids, and I usually keep the volume low. My Paradigm 7se Mk3 are relatively efficient, at least >90dB. I'm finding the Simple SE requires me to keep the volume knob on my preamp really close to the bottom. I'm not sure what I'm going to do here, or if this is even an issue.

There's a bit more hiss (or rushing noise) which I didn't see in my ST35. It doesn't bother me near half as bad as hum, and I'll easily ignore it. I'm assuming it has something to do with the much higher gain of this amp (or possibly the much lower quantity of feedback used).

I really like the looks of my layout, and I'm really happy with this build. I tell myself someday I'll stain and poly the wood frame, but it's been a year and I still haven't finished my ST35. In my opinion, the rough and natural look is more pleasing visually and encourages the owner to appreciate more time enjoying the sound. Apparently, my cat concurs.

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All in all, a good build and highly recommended.
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