Pro Jr rebuild up and running but looking for a bias tip?

[toomanyslurpees]

So I got my first not from a kit amp build up and running and so far I'm super happy with it, it's almost the stock schematic just built on a turret board with better components, a bias mod, C2 and the negative feed back loop both on switches. I'm just using the existing transformers and the on/off switch. I'm looking forward to taking it to the band's practice space so I can turn it up a bit. The only thing I'm waiting for is some 1ohm resistors that I ordered with all the stuff to fix up another amp. Right now I have the bias set as low as my adjustment goes (my 'X' voltage is -14 from the non-adjusting stock -10.4VDC) When I get these resistors what current am I looking to achieve through pin 3 of these EL84s? I know I could just actually check the current with my meter especially since that pin on the power tubes is just temporarly hacked in anyway but I'm just enjoying playing it rather than playing with it...

[sluckey]

PaMax for an EL84 is about 12 watts. Many people like to set the bias for 70% static dissipation, in this case, 12 x .7 = 8.4 watts.

So, you measure the cathode current and the plate voltage. Multiply these two numbers. Readjust the bias pot until plate voltage times cathode current equals 8.4 watts. Note, you must recheck plate voltage and cathode current everytime you adjust the bias because both will change.

The above method does not take into consideration the screen current. But since cathode current is really the sum of plate current and screen current, the above method will err on the side of safety, ie, the plate power will actually be slightly lower than the calculated value.

This is the method I use to get to the 70% point. I may slightly readjust bias by ear once I find the 70% point.

If you must know exactly how much power the plate is dissipating, you'll have to know the screen current and subtract that amount from the cathode current. The result will be the actual plate current which you will multiply times the plate voltage. To get screen current, you must measure the voltage drop across the screen resistor (be careful) and divide this voltage by the resistance value of the screen resistor. (I very seldom need to know screen current unless I'm troubleshooting a problem such as burnt screen resistors.)

[toomanyslurpees]

Perfect! Thanks, that's exactly what I needed.

[toomanyslurpees]

So I got my 1ohm resistors in the mail yesterday so I fired them in today and played with the bias pot, the numbers I ended up with is 8.2W on V3 and 8.8W on V4, are these poorly matched tubes or is this pretty normal?

I'll add some pictures of the finished product, I was wondering if my wire routing from the tubes looks decent as far as minimal noise is concerned, it's not too noisy or anything, but if I could make it less noisy I would (I was originally planning on using shielded wire on the signal path but wasn't sure if it would be worth the trouble.)

Well, here it is:




Before:

After:

[simonallaway]

It might benefit from tighter twists on the wiring. My first (only) build got quieter when I re-twisted the filament power. I used solid core 20 AWG which was a breeze to not only twist, but stay twisted.

Nice neat build though :)

[FYL]

8.2W on V3 and 8.8W on V4, are these poorly matched tubes or is this pretty normal?

Should be fine. V3 and V4 are less than ±5% apart: a ±.3W diff relative to a 8.5W median is peanuts.

[conger]

I was wondering if my wire routing from the tubes looks decent as far as minimal noise is concerned, it's not too noisy or anything, but if I could make it less noisy I would (I was originally planning on using shielded wire on the signal path but wasn't sure if it would be worth the trouble.)

It's a neat and unusual build.  I started to modify and improve my PJ and then rebuilt it into many different amps.  It has been a Spitfire clone for the last year or so and I am most pleased with it.  It has no hum or noise.  Yes I would use shielded cable for the signal going to and from the pots.  In the PJ chassis the 2 pots are offset away from the pre amp end.

Also your wiring looks too long and IMO would benefit from being redone to shorten it.  You have a lot of wires running parallel to each other.

I notice your unusual mounting of the power caps which puts the power chain close to the tone pot which may or may not introduce noise.

The best thing that you can do to a PJ is to replace the cheap poor quality OT for a decent one (18Watt OT is perfect) and then you can lose the conjunctive filter and lose the 47pF cap across the PI plates.  This will transform the amp and really open it up.  

Here is an 18watt built into a PJ chassis:

[sluckey]

Very neat conger. I love the green/yellow filament string.

[conger]

Very neat conger. I love the green/yellow filament string.

Unfortunately it's not my work but it is a good example of how to wire up a PJ.

[Fresh_Start]

Looks really good overall, especially for a first scratch build. 

Having the bias supply in the middle of the board is a bit unusual, but it seems to conserve space here nicely.  My only concern would be having raw AC from the PT secondary in that general vicinity.  Is there any way you can squeeze the "range resistor" and bias diode down at the PT end of the board, then float the connection to the bias filters "above" the board?

This is mostly personal style, but I would tuck the heater wires into the bottom edge of the chassis away from the board to keep them on the chassis and away from everything else.  Also, are the PT heater leads tied down between the positive ends of the first 2 filter caps?  I'm assuming the two green PT leads are for the heaters but obviously could be wrong.  If not, why can you run the green PT leads straight to the first power tube? 

Plate leads should lie flat on the chassis, perpendicular to and away from everything else.  Leads going to control grids are especially sensitive, so I'd have them come straight off the board (up from the board's perspective) and then down to the tube sockets.  Cathodes aren't as sensitive but I tend to keep them floating as well.  Note that only works if you've moved the heaters to the inside lip of the chassis.

It took me a few builds to really start thinking about lead dress in three dimensions but that can help a lot IMHO.

One safety note:  power cord wiring should always be done so that the green safety ground would be the LAST thing to break free.  IOW you need more slack in the green lead of the safety ground than in the hot or neutral wires.  Can't tell from the pics if you've got that there.

Sorry if this seems like nit-picking... you did ask for it! 

Congratulations on the fully functional build!

Cheers,

Chip

[toomanyslurpees]

No worries about being nitpicky, it's all good as far as learning everything I can from building this thing. I didn't realise this is an unussual place for the bias supply, so I take it in the future it's better on the other side of the filter caps? The green is the heater wiring, it's hard to see in that picture but it's going first off to the pilot lamp, that's a plastic one, not the standard Fender kind but I discovered it would take the standard amp jewel lens and I had removed two of them from an airplane so I thought I should put them to use. Does this count for mojo; the plane I took them out of was a twin otter that the Pope flew around central America in back in the 80s? It was actually the Paratrooper jump light in one of only two proof of concept militarized Twin otters ever built (though the rockets, search light, 360 degree search radar, the 50cal were all long gone before the Pope used it, but the wiring was still there so I do get to claim to have removed the rocket wiring from the Pope's plane....)

On thing I didn't do is isolate the output jack which I'm going to do. Like I say, it's not crazy noisy but there is room for improvement. Thanks for the feedback.

[Rev D]

 Shortening the wires as mentioned is a good thing, but I'd be proud as you must be of the first build, mine had crazy long leads and wasn't nearly as nice as that. As mentioned also if there's one (well two really) places to spend money on the PJ it would be a better output transformer (#1 thing IMHO) and a better speaker, I'm not sure what you have in it (a third place would be a bigger cabinet to improve its sound a bit and make it a tad less boxy sounding, although the stock PJ didn't seem as bad as the BJ in my opinion in that regard). But those two things make such an enormous improvement over what Fender used its amazing. They're great amps though and having a proper turret board and hand wired tube sockets was definitely a step in the right direction which will improve the durability of the build dramatically. My blues jrs was needing to have its socket board reflowed once a year or so to keep it happy and not redplating and had I kept it I would have for sure have looked into doing something like you did. Good job!

Regards,

RD

Ps: I'm sure theres some Papal mojo going on, anything his Holyness rode in had to be blessed, so it couldn't hurt having that pilot light in there.   Would love to hear some clips of it some time too!

[toomanyslurpees]

Sweet, I took this thing to band practice and it can wail pretty good and the noise is totally within acceptable limits, I still want to tweak it to see if I can make it more quiet. The one weirdness I do get is if the tone is almost all the way down I start getting some strange noises, I'll probably have to record them and post them sometime to get some thoughts on what's going on (it's only at higher volumes but at low volumes I can get some high pitch squeal if the tone's al the way down, I wonder if I have a value off or something.

Oh yeh, and there is a weber speaker in there, but I am temped to look at a bigger cabinet, or at least try it through a bigger cabinet to compair the sound.

Thanks for all the tips.