Pro Junior Series # vs. schematic Rev. and 'any other docs exist?' questions

[Murrayatuptown]

Hi:

I am talking to a friend about his Pro Junior he is sure is a II, but after downloading the 10-page docs PDF from The Tube Store (I think), and poring over Revs thru F and what his looks like, I cannot be certain which Revs align with what Series #...but since Fender and Gibson released all kinds of hybrid transitional creatures labeled one way, with different circuitry inside, could be anyone's guess.

One of the mid-to-later rev. board layouts shows a 2002 copyright, as does his, and he has the 0.01 uF capacitors across the FWB rectifiers, so I'm thinking it's gotta be a pretty late model. I didn't see a II, III or IV on it. That's not a terribly important concern...just thought I'd ask.

I may end up doing a Fromel mod kit installation for him, but in discussing what that does, and what else he wishes were different, I have been looking at other circuitry to explain Pro Jr. 'stock' bias, the Fromel mod's bias change, and what cathode bias means vs. 'fixed' grid bias. He stays awake & keeps bringing up add'l questions, but I don't know if I'm helping or baffling. He'll tell me one way or the other.

I never seem to do anything the simple way, so I started plotting how I could mount a perfboard temporarily above the PJ PCB to allow some experimentation without butchery.

Then I found SLUCKEY''s November.PDF file and there are a lot of answers and ideas in that!

Before I extract the amp chassis, I am scrutinizing the layout for a minimally surgical approach to options.

Extracting the board will let me see which end of each component corresponds with what schematic 'nodes, but the component-side view of the board and the layout drawing aren't quite enough - can't see the traces.

Does anyone know if there was ever a PCB solder-side view, like in a service manual? I doubt it, but gotta ask.

Thank you

Murray

[Murrayatuptown]

2002 (Series III?) PCB

I will eventually figure it our when I extract the circuit boards, but I was curious if anyone already did figure out what the W1 and W2 jumper wires are in the Fender Pro Jr.

W2 is located right next to R7, the volume control but not shown on the schematic.

W1 is on the main board, between the 'ribbons' near the V3 & V4 sockets. It is only a few ohms 'above' chassis ground. This is the most interesting to me as I wonder if that is how the EL84 cathodes reach ground...might be a simple place to insert Sluckey-style options like a current sampling resistor, or switchable (I'm thinking relay) grid/cathode bias.

I do a lot of staring, studying & thinking before surgery...

I also want to map out the 'ribbon' cables for two reasons: people don't like breaking them on their repetitive disassemblies/reassemblies...might as well determine what each wire does...may also present opportunities for mod 'injection points' rather than on-board changes involving cutting traces.


Thanks

Murray

[sluckey]

W1 is just a wire jumper. It's used to connect two pcb traces together. This simplifies trace layout by jumping over traces, especially useful on a single sided board such as this. One end of W1 is connected to the main board ground. The other end of W1 connects to the two small ribbon cables that connect to the output tube board and finally to pin 3 (cathodes of each EL84.

W2 is used to connect two pcb traces together also in order to simplify trace layout. It connects the volume and tone pots back to V1B plate via C1, C2, and R5.

Both jumpers are needed.

[sluckey]

BTW, this may interest you...

https://el34world.com/Hoffman/Pro_Junior_Build.htm

[Murrayatuptown]

Thank you very much.

I was going to do the same 'photo shoot'...might not need to now...or I can do more mapping before 'going in'...less time it's apart, the less there is to forget.

[Murrayatuptown]

Yes. All helpful. Thanks.


Ed.: Corrected misstatement about trace cuts in proximity to W1. They were in series with W1 but on the socket board.

I cut traces between V3 & V4 ribbons and the pin 3 (V3, V4) cathode PCB traces and installed 1 ohm WW cathode current sampling resistors, then ran a short shielded 3-conductor cable from the top of each resistor and the common (to W1 & ground) to tip jacks mounted on the chassis between the output jack and the end of the socket pcb.

I added a pot (25k) and parallel resistor (10k) in series with the 15k bias resistor to make bias adjustable from the too-hot setting voltage to points cooler. I foolishly let a simulation (different results than measured) convince me to lower the 15k to 13k...so I now have about
-(9.3-13.7) range. If I left the 15k, it would have (duh, of course) given me the as-built bias voltage as the max. (least negative). But now I can measure cathode current on each tube.

The parallel resistor was to a) provide open-wiper protection and b) limit the sim range. This shunt-based bias adjustment method behaves differently than the 'classic' all-series dropping method. In the latter (as commonly built), open wiper loses bias so a safety resistor biases the tubes off.

With a pot (wired with wiper & one end connected) in series with 15k, open wiper removes the shunt branch that lowers the bias...raising it to (sim) somewhere between -70 and -90 V. Might not bother the EL84's but would anger the 35V bias electrolytics. So the 10k fixed value across the 2-wire pot limits it to something much lower than -35 (don't recall. Maybe around -20).