50W Plexi, Dual Rectifier, PPIMV Budget Build

[Guitarnut]

Ideally it would be zero, but if it's only a mV or two it is all good.

Connected as you instructed it shows 1mV. I removed line voltage and restored a few times just to make sure the meter was actually reading that low and it went from .000 to .001 every time.

So, I'm good to go as is.

Peace,
Mark

[Guitarnut]

On the inputs, yes keep the 1M at the jacks from tip to ground, tie the switching contact to ground (so the inputs are shorted to ground when no plug is inserted), and then take each tip through a single 68k on the way to the grids.

For my own understanding, I did a little reading on this part of the circuit. Something that stood out is that the two 68K resistors are normally in parallel when the Hi input is used. That would be 34K at the junction of the two. Should I keep both 68K and put them in parallel across the same two eyelets? This would give me the normal 34K vs 68K we've been discussing.

What do you think?

Peace,
Mark

[martin manning]

You are correct, 34k series when plugged in to the high input. You can parallel them or just use a 33k. The value there controls the extreme HF roll-off of the input stage. I have see others use 68k there, but 34k is correct.

[Guitarnut]

You are correct, 34k series when plugged in to the high input. You can parallel them or just use a 33k. The value there controls the extreme HF roll-off of the input stage. I have see others use 68k there, but 34k is correct.

Cool! Thanks!

[Guitarnut]

Some of this has been covered so this is a summary of where the project is currently.

More progress on the chassis. First, I've decided not to mount the board upside down. It might work if all goes well first time around but if I had to replace a part or re-flow a joint it could be a nightmare. So, I decided to keep the board's mounting position and solder components to the back side. This will be convenient but it means building the layout backwards. Which in itself is another challenge.

I spent some time in Photoshop reversing things so I would have a printed road map to follow. Because of the placement of the board, the pot order is reversed so all the leads had to be redrawn. Not my best work but it gives me something to follow. I also reversed all the component values so I can read them clearly.

[IMG898]http://www.crenshawweb.com/plexi50/layout_rev.jpg[/img]

I screwed the pooch on my OT orientation. The cores should be perpendicular to each other mine are parallel. So before re-drilling, as Martin kindly instructed, I tested to see if there would be an issue with magnetic coupling.

Applying line voltage to the PT I measured voltage at the OT secondary and luckily it showed only 1mV. So it's G2G.

[IMG:800:600]http://www.crenshawweb.com/plexi50/build36.jpg[/img]

[IMG:800:600]http://www.crenshawweb.com/plexi50/build37.jpg[/img]

Next was a mock up of the face plate. The center line for pots was 13/16 from the bottom to allow more room for labels and 7/8 for jacks and switches. I used one of the original plates for a test run.

[IMG:800:600]http://www.crenshawweb.com/plexi50/build38.jpg[/img]

[IMG:800:600]http://www.crenshawweb.com/plexi50/build39.jpg[/img]

[IMG:800:600]http://www.crenshawweb.com/plexi50/build40.jpg[/img]

[IMG:800:600]http://www.crenshawweb.com/plexi50/build41.jpg[/img]

I'm concerned about the clearance between pot lugs and the chassis.

[IMG:800:600]http://www.crenshawweb.com/plexi50/build42.jpg[/img]

Currently, the spacing between pots is 1.25". I re-worked the layout to allow 1.50" so I can turn the pots to the side, like the dual pot in this pic, and have plenty of room. This adjustment to the layout also moved the indicator light down away from the end of the board.

[IMG:800:600]http://www.crenshawweb.com/plexi50/build43.jpg[/img]

[Guitarnut]

This is where I landed on the inputs. With Martin's help I decide to parallel the two 68K resistors and go with switching jacks. The switches will be connected to ground so the unused jack will be shorted to ground.

[img:384:319]http://www.crenshawweb.com/plexi50/inputs_rev.jpg[/img]

Peace,
Mark

[Guitarnut]

I started on the front panel this morning. Cut ti from a 36"x2"x.25" aluminum panel from HD. I also cut a panel to fit inside the rear of the chassis to fill the holes from the previous equipment.

[IMG:800:600]http://www.crenshawweb.com/plexi50/build44.jpg[/img]

[IMG:800:600]http://www.crenshawweb.com/plexi50/build45.jpg[/img]

I set up a fence on the drill press and got them in a nice straight line.

[IMG:800:600]http://www.crenshawweb.com/plexi50/build46.jpg[/img]

[IMG:800:600]http://www.crenshawweb.com/plexi50/build47.jpg[/img]

I didn't have a bit large enough for the jewel so I chucked up a round file and brought the table close to the chuck. I just worked it in a circle around the file while the drill was running on a slow speed setting. It worked pretty well.

[IMG:800:600]http://www.crenshawweb.com/plexi50/build48.jpg[/img]

[IMG:800:600]http://www.crenshawweb.com/plexi50/build49.jpg[/img]

[Guitarnut]

Some more progress shots.

[IMG:800:600]http://www.crenshawweb.com/plexi50/build50.jpg[/img]

[IMG:800:600]http://www.crenshawweb.com/plexi50/build51.jpg[/img]

[IMG:800:600]http://www.crenshawweb.com/plexi50/build52.jpg[/img]

[IMG:800:600]http://www.crenshawweb.com/plexi50/build53.jpg[/img]

[IMG::]http://www.crenshawweb.com/plexi50/build54.jpg[/img]

[dcribbs1412]

Really looking good
waiting to see the end result

Darin

[Guitarnut]

Really looking good
waiting to see the end result

Darin

Thanks Darin!

It's coming along but may take another week or so. I've been on vacation this week and able to get a lot done but back in the office next week. Build time will slow down to weekends at that point. But I'm very anxious to get it finished.

Peace,
Mark

[Guitarnut]

I decided to cut down the reverb tank for the BC30 to make a doghouse for the filter caps.

[IMG:800:600]http://www.crenshawweb.com/plexi50/build62.jpg[/img]

[IMG:800:600]http://www.crenshawweb.com/plexi50/build55.jpg[/img]

After some clean up and a scrub with naptha and a scotch-brite pad, I sprayed the upper chassis and doghouse with 3 coats of satin black. I was going to paint the chassis silver metallic but I only a few ounces left and didn't want to buy another quart.

[IMG:800:600]http://www.crenshawweb.com/plexi50/build56.jpg[/img]

Before disassembling the face plate for paint prep, I soldered the ground bus in place to keep the pots in line for easier re-install.

[IMG:800:600]http://www.crenshawweb.com/plexi50/build57.jpg[/img]

I checked all the edges and holes for burrs then sanded the face plate with 120 to get rid of scratches. Then down to 1200 grit to smooth it out. I sprayed it with 3 coats of clear to seal it. Very important to wear latex or nitril gloves during this process to keep oils off the aluminum.

[IMG:800:600]http://www.crenshawweb.com/plexi50/build58.jpg[/img]

Then it was time for the decals. They were made with water slide paper in an ink jet printer. After they dry I'll spray 4-5 more coats of clear.

[IMG:800:600]http://www.crenshawweb.com/plexi50/build59.jpg[/img]

[IMG:800:600]http://www.crenshawweb.com/plexi50/build60.jpg[/img]

[IMG:800:600]http://www.crenshawweb.com/plexi50/build61.jpg[/img]

[Guitarnut]

The xformer has 3 black wires and two white wires. Does the following sound like I have it sorted out?

One set of black wires is an odd 11.5V supply that was rectified for the pre tube heaters in the BC30 donor amp. They showed continuity and I tied them back.

This left the third black wire, HT center tap, which showed continuity between the two red and one of the whites which should be the 60V bias supply.

The two blue wires are the heater supply and they show continuity between the remaining white which should be their center tap.

The two brown are the heater supply for the tube rectifier.

[IMG:800:600]http://www.crenshawweb.com/plexi50/power_trans.jpg[/img]

Also, I could use some advice on how to run the heater wires. I'm guessing this build is so non-conventional that it's a crap shoot at this point but I wanted to get a few suggestions before starting.

The two blue wires going forward off the PT are the heater supply. My options are to take them down the back edge of the chassis to the power tubes, around to the pre tubes and then down the front to the indicator. Or, the opposite starting with the indicator first, pre tubes and then power tubes. Or, Y the leads and run the back route to the tubes and take the Y leads to the indicator. This would eliminate a run down the front of the chassis.

[IMG:800:600]http://www.crenshawweb.com/plexi50/build66.jpg[/img]

Peace,
Mark

[Guitarnut]

More progress. I got my last few supplies from Dave at Boot Hill. Thanks again, Dave. Always great service and always a bonus in the package.

I made the filter cap board. My first time installing eyelets. Nothing to it. I didn't have the setting tool but a wide tip punch works well for flaring them and then flip it over to the blunt end and drive them home. Though I did manage to install one backwards.

[IMG:800:600]http://www.crenshawweb.com/plexi50/build63.jpg[/img]

I ran a total of 4 hots. One each for the two rectifiers, and then one for each of the two additional filter stages. Three grounds. One each for the rectifiers and a third for the other two stages.

Two 47uF/450V for the SS rectifier, two 22uF/450V for the tube rectifier and two 22uF/450V for the other stages. 33K bleeder resistors on each of the rectifier sets.

[IMG:800:600]http://www.crenshawweb.com/plexi50/build64.jpg[/img]

[IMG:800:600]http://www.crenshawweb.com/plexi50/build65.jpg[/img]

It's coming together. The two holes in front of the power tubes will be 8Ω and 16Ω outputs.

[IMG:800:600]http://www.crenshawweb.com/plexi50/build67.jpg[/img]

Peace,
Mark

[dcribbs1412]

Looks great
Nice job on the faceplate and doghouse

Darin

[Guitarnut]

Looks great
Nice job on the faceplate and doghouse

Darin

Thanks Darin!