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Old 10th August 2013, 05:24 AM   #91
SandyG is offline SandyG  United States
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Some parts showed up today.

So started working on the Bias circuit a bit. I changed out the 100k R51, R53, R55, R57 to 51k as a starter. Left everything else as it was. Bias seems like it's in a better range. At the maximum bias tubes are drawing about 42ma so range will allow me to bias up the KT88 better then before. The question for the bias circuit is the 100K resistors that go to the grid resistors (R50, R52, R54, R56) should I also try to reduce those to a lower value? I think most KT88 circuits I have seen all run with 100k on that part of the circuit.

I have 22k and 33k 5 Watt resistors for R31 and R39 and the changes for R29 and R35 (5.1k). So will give that a shot.

Also have the 365pf dual section (730pf total) variable cap to give the feedback circuit another try too.

Sandy
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Old 11th August 2013, 02:28 AM   #92
SandyG is offline SandyG  United States
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Some progress...

I tossed the variable cap on C17 and watched what it did to the square wave. And at first all it looked like it was doing was rounding off the edges. I also saw some instability (as Johan said it would) as I started to bring it up towards the end of the range (maybe 600pf).

Then I looked again and slowly put capacitance into the circuit. Ended up cleaning up the square waves LEADING edge a bit with about 470pf of cap for C17. It took out the bit of ripple that was seen right at the beginning of the square wave.

The trailing edge of the wave maintained it's original shape for the most part.

Some progress.

I also added a 220k 2 watt resistor (R10) to the center tap of the 6.3Vac on the 6SN7's with what looks like an 88v potential to the heaters. That should keep the heater to cathode voltage in range for the CF.

More to come.
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Old 3rd September 2013, 01:58 AM   #93
SandyG is offline SandyG  United States
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Ok, finally did all the Johan recommended modifications. I updated the 4.7K resistors R29 and R35 to 5.1K and swapped R60 to 27K.

Then messed with R31 and R39. First trying the recommended 22k 5w and didn't see much changes on the output shape at 10khz. I looked at the square wave and looked smooth but not so square. I also didn't see any noticeable change when I backed off to 33k 3W for R31 R39. I decided to leave in the 33k for the sake of tube life.

Other then that seems like with those changes can go on to build the second one and try them out as a pair. It sounded very good with one channel and other other another home made KT88 (50 Watts or so) amp. Still need the pair to really hear how good they sound.

Need to order another power trans/output from Heybor and ordered a chassis from Landfallsystems.com who makes really nice custom sized cases that are way better then the Hammond crap.

Attached is the 10khz waveform at something like 15V RMS into 8ohms.

Sandy
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File Type: jpg EZ125_10Khz.jpg (153.4 KB, 217 views)
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Old 16th October 2013, 12:11 AM   #94
SandyG is offline SandyG  United States
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Been a while waiting for the power transformers and the chokes. They arrived a week or so ago and I stated working on the chassis. The chassis was from Landfall systems and were size to about 10"x17" in raw finish. I opted to try the paint route vs. anodizing them. Here is their site, and I can very strongly recommend these chassis for your projects!

https://www.landfallsystems.com/
Attached Images
File Type: jpg EZ125_Top_Panel.jpg (129.5 KB, 179 views)
File Type: jpg EZ125_Hole_Cutter.jpg (138.0 KB, 174 views)
File Type: jpg Cutters.jpg (203.6 KB, 165 views)
File Type: jpg EZ125_Front_Back_Panels.jpg (126.6 KB, 163 views)
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Old 16th October 2013, 08:02 AM   #95
multi is offline multi  Australia
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Default 6550 x 4 amp

A different way to make a similar the amp
I too am just making a 4 X 6550 amplifier, using a Chinese metal punch.
I am using 6n6T x 2 and 6bl7 x 2 based on the GEC x 8 Kt88 amp.
The top plate is 4 mill ali; I am having it powdered coated.
will use a Sowter out put transformer and Chinese made c core transformers.
The Chinese made metal punches are very good.
Phil
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File Type: jpg DSC_3382.JPG (261.1 KB, 79 views)
File Type: jpg DSC_3386.JPG (233.2 KB, 75 views)
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Old 16th October 2013, 01:37 PM   #96
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How much did metal-puch-kit cost?
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Old 16th October 2013, 02:47 PM   #97
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Rather then punching / drilling chassis its much better to order laser or water jet abrasive CNC cutting.

Laser cutting is good for sheet steel (not stainless), and water jet abrasive for everything else. Very high precision, will require few polishing with sand paper.

Google for companies available around for this kind of service.
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Old 16th October 2013, 10:46 PM   #98
multi is offline multi  Australia
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In Perth Australia laser cutting costs a lot of money and is only affordable if you are making lots of the same amplifier; not suitable for one offs. i think would be the same in any country
My punch set cost about $A180.00 less than the cost for one die for the japanese sets.
has dies from 20-63.
Phil
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Old 16th October 2013, 11:21 PM   #99
SandyG is offline SandyG  United States
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The punch kit looks cool, not too expensive, but I don't know how they would work with 1/8" (.125) aluminum. That might be a struggle for them to do a clean hole. They are cool, and not too expensive on Ebay, some larger sized rams (10T and 15T) for not much more.

The Rotabroach and the Holesaw kits I use leave a perfectly clean hole, no sharp edges, and are available in a lot of intermediate sizes too. About $140 for both kits as I recall.

I did these on a small drill press, but previously just used a 1/2" Dewalt 18V drill with good results.

Laser/Water/etc are all readily available here but for only a couple of boxes (as mentioned) they are cost prohibitive unless you have a friend with one and will let you use it (Still working on that). Set up fees and time on the machine are still not cheep. Water Jets are getting ousted out of some shops here in California due to EPA waste byproducts as I was told. Who knows...

The hole saws and rotabroach are fast and give excellent results even on the thick chassis material that I have and make short time of the work, just stick on a paper template and center punch then drill.

The harder ones to fabricate are things like the things on the back panel as you need to do some hand filing to make things fit with the strain relief, fuse holders and bias taps.

All part of the fun!

Sandy
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Old 17th October 2013, 03:09 AM   #100
SandyG is offline SandyG  United States
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Here are a few more pics. Primer is self etching and paint is texture black which gives a nice non-glossy finish much like the paint on old dynaco's and other amps from the era. The Landfall box is easily assembled and easy to add parts prior to that. Makes it easy to tighten things up that are hard to get to while in a box that doesn't have panels. Back panel is ready to go, as well as bottom panel that has some large bumpers used as feet. The bottom panel will get some screen mesh over the larger air vents.

Making some progress!

Sandy
Attached Images
File Type: jpg EZ125_All_Side_Panels_Primer.jpg (141.0 KB, 40 views)
File Type: jpg EZ125_Assembling_Chassis.jpg (93.4 KB, 36 views)
File Type: jpg EZ125_Back_Panel.jpg (98.1 KB, 35 views)
File Type: jpg EZ125_Back_Of_Back_Panel.jpg (82.9 KB, 43 views)
File Type: jpg EZ125_Stuffed_Back_Panel.jpg (102.4 KB, 40 views)
File Type: jpg EZ125_Bottom_Panel.jpg (128.7 KB, 40 views)
File Type: jpg EZ125_Mock_Bottom_Assembly.jpg (133.1 KB, 47 views)
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