Pro Jr. Layout?

[revel]

Hey all. I'm new here. 

I'm looking for a layout for a Pro Jr. There does not seem to be any online anymore.

Short of someone having one, can anyone explain the fundamental differences between a Pro Jr and an 18 watt? Or, put another way, what's an amp that's very near to the Pro Jr. for which there is a layout?

[EL34]

I stashed some drawings I found on the web a while back but have not had time to go over them and check them
You would have to print out the schematic and the 3 layouts below and double check them

Please post your edited drawings back here so everyone can benefit
Now you have your homework assignment :)

Here is the Pro Jr schematic in my library
http://el34world.com/charts/Schematics/files/fender/Fender_Pro_JR.pdf

[JB]

A Marshall 18W (minus the Tremelo channel) is not miles away from the Pro Junior.  Main differences being the 18W has a valve rectifier, is cathode biased and has two inputs, with a triode for each.  Doug's 3rd image is an 18W layout for a Pro Junior chassis - solid state rectified, cathode biased, both input triodes strapped in parallel.

If you go down either route - turret board Pro Junior, or converting to 18W - please report back.  I've considered both of those options for mine, primarily to fix the hum/hiss.  I've taken the modding about as far as I can in an effort to reduce the noise, short of ripping the board out.

I recently played some original 60's Marshall 18W's which sounded fantastic.  If converted it wouldn't quite be one of those - they have a bigger cabinet, different speakers/transformers, valve rectified etc. etc. but some of that flavour would be nice.


 

[EL34]

It's raining and I am stuck inside today
I took the first drawing and compared it to the schematic
I added component values - (I hate those colored coded layouts)
Notes from the original drawing I got someplace on the web
Looks to me like they altered the fixed resistor bias supply and tried to make a bias supply with a bias pot
I did not have time to untangle their bias supply section on the drawing
Maybe someone else can figure it out
The original filter cap values were all 22uf caps
If you use the 40x20x20x20 cap I sell you would instead have 40uf x 20 x 20uf with one 20uf section unused
Or you could just use individual 22uf/500v caps
The 10k resistor coming off the input jacks could be put on the jack itself or on input grid #2 pin
Instead of having it on the board, you could save some board space that way
The drawing did not have the power cord, fuse and power switch, but that is simple to figure out
In the original schematic there is  270 ohm + 2200pf cap in series across the power tube plates
That was not on the drawing.
Note that in my Blues junior design, I left those off without any problems
A couple of the coupling caps on V1 were labelled as .1's
On the schematic they are .01's

Here is my edited drawing

[revel]

Are you in the midwest, too? I don't even remember which day of consecutive rain this is in Minneapolis.

As that first layout was the one that actually aligned with the schematic I was just beginning to do what you did so I'll just use yours.

That bias circuit is...I don't even know. That's easy enough to "untangle." Everything else that's missing is an easy and obvious thing to fill in.

I guess one remaining question is: If building from scratch, what's some decent but still very light iron to use?

Any reason not to insert an MV in place of the 100k resistors?

[EL34]

I should be able to untangle the bias circuit a bit later


Just use the 18 watt iron I sell on this page
http://hoffmanamps.com/MyStore/catalog/Transformers.htm


Someone else may have a MV solution

[JB]

You definitely want variable bias, not the stock circuit - it runs the EL84's way too hot.

Couple of mods/better later versions of the circuit that would be worth doing from the outset:

In the first couple of revisions C1, the first coupling cap is 0.01u and R6, off the volume pot is 56K.  Change these to 0.022u and 22K.  Gives you more bottom end and headroom.  If you change C1 to 0.022u and keep R6 at 56K you may find it gets a bit muddy.

Later revisions have one or both of the first two cathode resistors bypassed for more gain - worth experimenting with.

Reducing the 0.01uF in the tone circuit - the one between the Tone and Volume pots can be useful.  I've got 0.0047uF in mine now which helps with the boxiness of the small cabinet - if you're building from scratch maybe in a bigger/better box you may be ok.

Reducing or making the NFB switchable is a good mod. 

[revel]

Beautiful!

[EL34]

The Pro Jr layout and BOM are still a work in progress
I have never built one or converted a stock one


New builds change as they are being built and I make notes as I do a build
Then the BOM and the layouts change

If you are converting a stock Pro Jr and are using the original PT, the rectifier and bias cicruint will be different than if you are building from scratch and using a 18 watt PT
The layout and schematic in my PDF file will change
It's just a rough draft with notes added at the bottom

Here's links to the work in progress Layout and BOM pdf files

Layout PDF
http://el34world.com/Hoffman/files/Hoffman_Pro_Jr.pdf

BOM PDF
http://el34world.com/Hoffman/files/Hoffman_Pro_Jr_BOM.pdf

[EL34]

My Pro Jr info is here
https://el34world.com/Forum/index.php?topic=24077.0