50W Plexi, Dual Rectifier, PPIMV Budget Build

[Guitarnut]

This is my second build and I'm really excited about it. The Bassman I built first was a great education and this Plexi circuit is familiar but very different in the end result.

I started with the Ceriatone layout and made some changes. I'm going dual rectifier with switching and reservoir caps for each rectifier. I also removed the voltage and impedance selectors. I'll be using 6L6GC and 12AX7 tubes with a 5AR4 rectifier.

[IMG898]http://www.crenshawweb.com/plexi50/plexi50mod2.jpg[/img]

This is a budget build since I had iron from an Epi BC30, tubes, sockets and most of the caps and resistors on hand. The PT is a bit on the hot side but should play nicely with the 6L6GCs. OT is 7.2K primary and I'll be using only the 8Ω tap.

Just placed small parts orders from Mouser and Antique Electronic Supply.

The chassis will be an old 1U rack mount box. I think I'll build the head TW Express style so all the weight sits nicely at the bottom. Might go with a combo...not sure at this point. I'll machine a new face plate and take advantage of the AC connector cutout in back for a removable AC cord.

Looks like I'll have just enough room for the 4 inputs, 7 chicken heads, pilot and 2 switches. (power and tube/standby/solid state). I'm not sure about the layout. I want to get the pre tubes as far as possible from the OT.

[IMG:800:600]http://www.crenshawweb.com/plexi50/build04.jpg[/img]

Peace,
Mark

[Guitarnut]

Both small parts orders shipped same day so I might have parts by the weekend. If not, I have work to do on the chassis first anyway. I picked up some step bits at HF last night to get socket holes cut.

I finally found a bit that would remove the security screws on the front panel and was able to strip the chassis. I'll pick up a small sheet of aluminum and get started on machining the new face plate. I might make one to cover the holes on the back as well. I think I'll paint the entire chassis silver metallic. I have some DupliColor Candy base left over from my Strat builds. I'll use water slide for the input and pot labels.

[IMG768]http://www.crenshawweb.com/plexi50/build05.jpg[/img]

Here's the cool AC socket. It's already grounded to the chassis and appears to be well mounted.

[IMG768]http://www.crenshawweb.com/plexi50/build06.jpg[/img]

I came up with another layout that gets each set of tubes back in a straight line. Still not sure what I'm going to do on the layout.

[IMG768]http://www.crenshawweb.com/plexi50/build07.jpg[/img]

I stripped two of the chassis and saved the power supplies. Will definitely come in handy for projects down the road. I'm not sure if they're AC or DC. There are some healthy sized diodes on the boards so I'm guessing DC. They're only marked V1, V2 and V3. I'll have to power them up and meter them.

[IMG768]http://www.crenshawweb.com/plexi50/build08.jpg[/img]

I'll also save the PCB mount XLR amd BNC connectors as well as several banks of trimmers.

[IMG768]http://www.crenshawweb.com/plexi50/build09.jpg[/img]

Peace,
Mark

[Guitarnut]

Latest layout. Seems workable except for changing the rectifier tube. It gives me about as much distance as I'm going to get between OT and pre tubes.

[IMG:800:600]http://www.crenshawweb.com/plexi50/build10.jpg[/img]

Face plate layout.

[IMG165]http://www.crenshawweb.com/plexi50/face_plate.jpg[/img]

Peace,
Mark

[Guitarnut]

I got the chassis marked and drilled this morning. Couldn't resist a mockup.

[IMG768]http://www.crenshawweb.com/plexi50/build11.jpg[/img]

[IMG768]http://www.crenshawweb.com/plexi50/build12.jpg[/img]

[IMG768]http://www.crenshawweb.com/plexi50/build13.jpg[/img]

Peace,
Mark

[Guitarnut]

In the interest of saving space on the face plate and reducing signal cabling around the pre tubes, I'm thinking about reducing the inputs to 1 normal and 1 bright. Seems easy enough...just eliminate a 68K resistor from each channel. Shouldn't have any effect on the rest of the circuit since in the usual setup the unused jacks would be grounded anyway.

Both inputs would be hi impedance since I don't own any guitars with active pickups. I guess I could add switches to take the 1M out of the circuit.

I'm thinking I would use standard 1/4" jacks with a 1M between hot and ground.

[Guitarnut]

Duplicate post removed.

[Guitarnut]

Duplicate Post Removed.

For some reason when I select Edit post and make changes, it keeps posting as a quote. Sorry for the extra noise.

[Guitarnut]

Time to make some sawdust!

Here are my plans for the cab. The width will be 20.5" or 19" inside to inside. 17" tall by 10" deep. The depth will give me room for the 8.5" deep chassis, the rear panels and a little bit of recess and tilt-back for the baffle. All the corners will be rounded over and I'll use the black corners from the Epi BC30 cab.

[IMG768]http://www.crenshawweb.com/plexi50/cab.jpg[/img]

I'll use a small brace across the cab to support the rear of the chassis. Might be tricky with components set almost to the edge.

[IMG768]http://www.crenshawweb.com/plexi50/cab2.jpg[/img]

Peace,
Mark

[Guitarnut]

I got the cab cur and glued up.

I used the box joint jig that I made during my Bassman build. It's really easy to use and it dials in very quickly. So far I've been getting great results with it. Here it is set up on my trusty Shopsmith. I was looking for a dado table insert and found an entire table on eBay for less than what most were paying for the insert...it's all in the timing I guess. Now I just swap out tables instead of inserts.

[IMG:800:600]http://www.crenshawweb.com/plexi50/build14.jpg[/img]

Box joints cut and ready for a dry fit.

[IMG:800:600]http://www.crenshawweb.com/plexi50/build15.jpg[/img]

[IMG:800:600]http://www.crenshawweb.com/plexi50/build17.jpg[/img]

Then I cut the recess for the chassis.

[IMG:800:600]http://www.crenshawweb.com/plexi50/build18.jpg[/img]

All glued up. I'll get the round overs, cleats and baffle done tomorrow.

[IMG:800:600]http://www.crenshawweb.com/plexi50/build19.jpg[/img]

[Guitarnut]

I finished up the pre-vis this morning. I added the front bezel so I could use a piece of grill cloth I have in hand...it's only 12" tall.

Now, I just have to build it.

[IMG768]http://www.crenshawweb.com/plexi50/cab3.jpg[/img]

[Colossal]

Great job so far! Excellent job on all the diligence before building. Will be interesting to see what you think of 6L6s in the circuit. Tube rectification gives a neat sound, I had a rectifier switch on my 6V6 Plexi. I used a dedicated reservoir cap for each rectifier as well as having a VVR. Are you using the 8R tap to halve the primary impedance or actually running your output tubes at 7k6?

[Guitarnut]

Great job so far! Excellent job on all the diligence before building. Will be interesting to see what you think of 6L6s in the circuit. Tube rectification gives a neat sound, I had a rectifier switch on my 6V6 Plexi. I used a dedicated reservoir cap for each rectifier as well as having a VVR. Are you using the 8R tap to halve the primary impedance or actually running your output tubes at 7k6?

Thanks! I was beginning to think I was just talking to myself.

That's one of the questions I have. By halving the impedance do you mean running a lower R drive on a higher tap?

I originally thought I would run an 8Ω Creamback on the 8Ω tap. But at this point I'll be using a 16Ω Eminence Lady Luck 70W on the 16Ω tap for testing and then move to the expensive driver when I know I have an amp that I like.

Peace,
Mark

[Guitarnut]

I leveled out all the box joints and sanded to 120.

[IMG:800:600]http://www.crenshawweb.com/plexi50/build20.jpg[/img]


[IMG:800:600]http://www.crenshawweb.com/plexi50/build21.jpg[/img]

Added the round overs. I have a little bit of filling to do but not much.

[IMG:800:600]http://www.crenshawweb.com/plexi50/build22.jpg[/img]

Peace,
Mark

[martin manning]

Nice re-purposing going on here. I'm confused by the markings on the OT saying that the primaries are 22k? That doesn't make sense. The donor amp had what kind of tubes? The load should be ~4k for a pair of 6L6.

[xtian]

How did you get a nice straight cut on your panel cutout? Did you just use a jigsaw?