New build : 18w El84 plexi with EVH spec'd 68 board

[Pat_rocks92]

Hello i'm beginning a new project after my jtm50 micro build. I'm stepping up in wattage slowly.

this time it will be an el84 plexi. I got 2 el84 sylvania black plates specially for this build.

the power transformer is a GDS 18w transformer which is a copy of the original transformers on the 2061x plexi

The outpout transformer is a GDS transformer and is an updated copy of the old radiospares transformers found in the 2061x plexi too.

I noticed that many small boutique amp builders used them for small plexis : those transformers are made by heyboer and sold here : https://gdsamps.com/parts/

As johan segeborn demonstrated it the 2061x can sound very close to a 100w plexi that's the reason i'm using its transformers.

instead of going for a 2061x plexi, i'm going for a tubetown PX which is a 20w plexi designed by tubetown i will use its power section

https://www.tube-town.net/cms/userfiles/downloads/px18-schem.pdf

https://www.tube-town.net/cms/userfiles/downloads/px18-layout.pdf

then i will add a 68 specd preamp section :

for the van halen specs here is what dave friedman told us :

Post by RACKSYSTEMS » Wed Feb 02, 2011 11:40 pm

Split cathode V1a 250uF/820,V1b .68/820
All coupling caps are 0.022uF,Bright channel coupling cap is 0.0022uF

470k mixer resistors
500pf [red out of 69 amp] mixer bypass cap
Bypass cap on V2a is .68uF and a 220uf to 470uf
33k/560pF lemco tone stack combo
100k NFB resistor at 4 ohm tap
220k bias splitter resistors
0.1 uF cap on presence control
.022uf output couplers

filtering is
2] 100uf's f&t in series mains
2] 32uf's f&t in series screens
1]100uf lcr phase inverter
1] dual 33 x 33 f&t preamp


finally i'll add a pre phase inverter master volume with diode clipping.

and probably an extra tube for another gain stage.

the layout for the turrets has already been made on inkscape :

this layout has been made for mallory caps, 330uf rifa 64vdc, and wima 250uf 6vdc. the dual electrolytic is a dual 32 uf from erie 275v.

here is the layout for anyone interested in the board

next step is printing with an A4 the layout cuting it and gluing it to a board. Then i'll use a center punch where the lines intersect and drill the holes for the turret.

 

[Lowerleftcoast]

Recommendations off the top of my head...

Change the presence circuit to match the '59 Bassman. This would take DC off of the presence pot (no scratchy pot).

Take the 68k resistors (R1, R2, R4, R5) off the board. Preferably hang them from the tube socket pins for better RFI rejection. Otherwise, hang them on the input jacks. (You don't want to have them on the board when you decide to retrofit with shielded wire.)

Place a snubber cap from Phase Inverter pin1 to pin6. This is done to keep ultrasonic high frequency oscillation at bay. This was included on the 5F6A Bassman and several Marshall amplifiers. Fender used a 47pF but I find 100pF provides a little more protection and the high frequency cut is still above what the ear can hear.

 

[Dacious]

I think the filter caps are way overspecced for that transformer - they'd be for a 100 watter.

 

[Pat_rocks92]

I think the filter caps are way overspecced for that transformer - they'd be for a 100 watter.

I guess that's a lot but people say good things about this amp so i'll try with this amount of filtering. Usually higher filtering makes an amp easier to play.

 

[Pat_rocks92]

Recommendations off the top of my head...

Change the presence circuit to match the '59 Bassman. This would take DC off of the presence pot (no scratchy pot).

Take the 68k resistors (R1, R2, R4, R5) off the board. Preferably hang them from the tube socket pins for better RFI rejection. Otherwise, hang them on the input jacks. (You don't want to have them on the board when you decide to retrofit with shielded wire.)

Place a snubber cap from Phase Inverter pin1 to pin6. This is done to keep ultrasonic high frequency oscillation at bay. This was included on the 5F6A Bassman and several Marshall amplifiers. Fender used a 47pF but I find 100pF provides a little more protection and the high frequency cut is still above what the ear can hear.

i'll try to put the 68k resistors close to the tube socket, i saw that john suhr does this too on his builds. For the rest it might be worth trying !

 

[Pat_rocks92]

Update :

for those interested in building this amp : i'm building in a tight small size the chassis is a hammond Chassis AL 1444-1372 : 330 x 178 x 51 mm

here is the pdf for the plates print and to order them just upload the pdf (the spacing for the fx loop is for the zero loss fx loop from metropoulos) : https://www.tube-town.net/ttstore/custom-shop-faceplate-pro-reverse-glossy.html

There are the layouts for hole driling too :
you'll have to print them with an a3 sheet.

I use a center punch on the crosses then use a small drill bit to make a tiny hole and continue with a step drill bit the results are not perfect but it does the job.
I think it would be way better to use a drill press with forstner bits. But as i'm a student and still at home well...

as you can see it can align well (the fuse hole has been purposedely made smaller because i'll use a smaller fuse holder.

The big issue is when you have to drill polygone shapes... I have a small dremel and use this to make the hole for the power outlet.

here is the chassis with the top layout. I made a mistake and began to drill the outside transformer line haha. Hopefully i only cut about 3cm. I'll fill it with some epoxy and cover it with a sticker on both sides.

 

[Pat_rocks92]

Update : Board almost finished

it just lacks the trimpot and 220k resistors which i forgot to order x).

i couldn't decide between a 250uf or 330uf on v1 but this wima reads 290uf it's in between and i finally decided to try with it.