Jennings J40 Preamp Schematic

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Jennings J40 Preamp Schematic

Postby Jigawatt on Sun Apr 26, 2015 6:42 am

Thought I'd give this a shot. I made a schematic for the J40 preamp. Feel free to point out any obvious errors or add suggestions. ha

You might want to open this image in another tab. Its cropped off here.

Image
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Re: Jennings J40 Preamp Schematic

Postby dlvoots on Mon Apr 27, 2015 7:31 pm

Very nice job, and quite readable.

In fact, the very idea that I can easily "read" it leads me to ask the following questions about the location and purpose of some of the components (a good thing):

1. The 10K resistor that forms a voltage divider between itself and the 24K resistor (and the junction of the two is connected to the collector of the input transistor)........I'm struggling to understand its purpose. I've stared at it and I'm not sure what its function is. Is it connected properly in the schematic?

2.) The presence control doesn't look right. I'm assuming that the Off / On_1 / On_2 positions map to the three positions on the switch. Based on my understanding of presence controls, off would be no presence, On_1 would be a bit of presence, and On_2 would be maximum presence. I would think that Off would mean only the 2.2uf cap in parallel with the emitter resistor (flat frequency response), On_1 would be 330nf without the 2.2uf (emphasis on high end), and On_3 would be 470nf without the 2.2uf (emphasis on hi end and upper mid range). It may be that I have the switch positions for the 330nf and the 470nf values backwards, but you get the idea.

3.) The third observation is that you need to diagram the input jack wiring. At least one of the three jacks should be a shorting jack, where the third lead of the jack is connected to ground and shorts the input if no instrument is connected. Otherwise, the amp will hum and buzz in an obscene and forlorn manner.

In the average garden variety two input amp / two 68K input resistors per-channel design (Fender, Vox, etc.), the inputs are wired so that using Input 1 only means the 68K resistors are in parallel, resulting in a 34K input resistance. Using Input 2 only means that the 68K resistors transform into a voltage divider, where the input signal is connected to the top and the amp is connected to the middle -- only half of it gets to the amp. Finally, using Input 1 AND Input 2 at the same time means each input is isolated from the other input by its own 68K resistor and also the 68K resistor on the other input (thereby forming a Poor Man's Passive Mixer). Using both inputs at the same time results in a 68K input resistance for either one or both of the two inputs.

Not sure how they've got this critter wired up, but it is important enough that it gets included in the diagram.

Dave
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Re: Jennings J40 Preamp Schematic

Postby Jigawatt on Wed Apr 29, 2015 8:41 am

I just double checked everything.

dlvoots wrote:1. The 10K resistor that forms a voltage divider between itself and the 24K resistor (and the junction of the two is connected to the collector of the input transistor)........I'm struggling to understand its purpose. I've stared at it and I'm not sure what its function is. Is it connected properly in the schematic?


I think its there to limit the current to the first transistor. Since the voltage is the same in parallel, there would only be a drop in current at the 10kΩ resistor.


dlvoots wrote:2.) The presence control doesn't look right. I'm assuming that the Off / On_1 / On_2 positions map to the three positions on the switch. Based on my understanding of presence controls, off would be no presence, On_1 would be a bit of presence, and On_2 would be maximum presence. I would think that Off would mean only the 2.2uf cap in parallel with the emitter resistor (flat frequency response), On_1 would be 330nf without the 2.2uf (emphasis on high end), and On_3 would be 470nf without the 2.2uf (emphasis on hi end and upper mid range). It may be that I have the switch positions for the 330nf and the 470nf values backwards, but you get the idea.


There is definitely an off position where there is nothing flowing through. I however did wire the switch selector wrong, its supposed to go straight to ground instead of in series with the 33kΩ resistor. I'll update that.
I dunno how to correctly calculate the cut off frequencies ( Im used to it being a lot easier to calculate with only a single resistor and not a few extras in parallel). But i think the 330nF in the off position acts like a high pass filter (or emitter bypass cap?) while the other two act like a low pass.


dlvoots wrote:3.) The third observation is that you need to diagram the input jack wiring. At least one of the three jacks should be a shorting jack, where the third lead of the jack is connected to ground and shorts the input if no instrument is connected. Otherwise, the amp will hum and buzz in an obscene and forlorn manner.


Each of the inputs do indeed go through a 68kΩ resistor in parallel with the input of the 220nF capacitor. The grounds from each of the inputs are tied together on a ground post on the board.
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Re: Jennings J40 Preamp Schematic

Postby jyvoipabo on Fri Dec 18, 2015 2:40 pm

Hello,

Thanks for schematic.

I give you some remarks.

There is 2 models of J40 may be more ?

Early models (may be prototype with no serial numbers) have a splited body with amplifier and main transformer is in the bottom.
They differs by value of the controls.
Self =0,300 H Treble = 5K Bass = 20k Volume = 50 K
Models serial number 8xx, Self = 0,600 H Controls are 1K 10k 50k

There is some variations in the wiring of input jacks, depending of the model.

Remarks

Presence switch return to ground to improve High frequency gain of this stage.

100k resistor is added between 16µ capacitor (-) and Gnd.

Signal enter to Hot lug of Treble pot and cursor go to Change tone switch. This lug are inverted.

Cold lug of Bass is Gnd and cursor returns to Gnd by 100Ohm

Signal enters on Hot Lug of Volume and Output is cursor. (Very classic disposition)

So, there is an inversion of lugs of all potentiometer.

I don't verify the power amp schematics because i think it comes directly from RCA application notes.

All the the chemical capacitors need to be checked for dry.

Because tone is very special, wiring of controls need to be checked for dirty modifications.

I have some pics of these amplifier.

Merci encore car sans vos schémas je ne m'en serai pas sorti, et passez de bonnes fêtes. :cheers:

JP
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