SchweinHaus
Well-Known Member
Hi gang,
I am hoping to get some on my first tube amp rebuild. After having some great success with this thread finding the right beginner's amp project, I settled on a Magnavox 9302. After something like 12 years of posting, I became a member and got a 9302 on Bartertown. What I really want to do is create a thread translating the famed "Gillespie mods" for beginners. I've come to have a really deep and abiding appreciation for this community and want to contribute to it. But I'm hoping I can lean on it one more time to get me there.
As part of my quest, I bought a pdf of EJ Jurich's Vaccuum Tube Amplifier Basics, and I've identified this as a key source of my problem: Translating what I'm seeing in a schematic to what I've got on the chassis. I find this harder without the benefit of a PCB. In the book, he gives the following as an example of something that can be read off of a circuit.
View attachment 3519252
I don't want to get bogged down in the specifics of the example circuit, I just brought it up as an example of the skill I've identified that I don't really have.
My first concrete request is for good schematics for the 9302. I don't really know how to find that in the database, and all the ones I google are fairly low quality. Here's what I'm working with so far.
It seems clear that some parts of the schematic are missing, but this is a purely academic exercise at this point, so let's skip that for now.
One thing I have trouble with is figuring out where the schematic starts and end. So I started from the power transformer, and I instantly see that the brown and yellow and brown are clipped and do not lead to Y1 or Y2 (I assume Y1 and Y2 are other schematics I'll need.)
Okay, so that's a dead end. So I went back to another part of the schematic and looked at the input and preamp section. I see channel one going to ground, and then the other lead coming off the RCA joint going to a 470k resistor that is grounded, and then I see a 47k resistor that feeds into a 100 mmf capcitor (non polarized? but I'm not sure what the asterisk means?), but this is where my amp diverges. Here's my amp.
So the RCA input is grounded directly to the chassis. That checks out. then I follow the white wire to the connection block where I see the 470k R107 going to a ground terminal one slot over. Check. And then I get the 47K R118 going directly to pin 8 of the preamp tube without any connection to C107. And then on pin 9, I see R109 (Red red silver is, what, .022 ohms?) which should be connected to the other leg of C107, but instead goes to the balance pot (R117).
I'm starting to see this come together. I can mostly read the balance pot going to the output transformers through R112 and R212, but the x's in the diagram are a mystery to me.
It seems like the fix is as simple as soldering a 100 mmf capacitor with one leg on pin 8 and the other leg on pin 9 of the preamp tube, but that seems too easy? I don't know enough about how the circuit actually works to know if that would change the values of the resistors or anything else, and I don't have all the resistor values in the schematic.
Oh, and the pilot light is completely bypassed. See the two empty solder joints on the bottom left.
It looks like the the green line out of the power transformer on the schematic (far right of the pitcture) skips the pilot light and goes to pin 4 of the first power tube and then is daisy chained over to pin 4 of every other tube. The green and yellow line looks like it skips the pilot light and goes to pin 5 of the first power tube and then gets daisy chained over to pin n5 of the other three tubes, and then those lines get tied into an on light added by the someone along the way. I'm confused by what V1-6 mean section in this part of the schematic because I see the 4 power tubes listed above, and V1 and V4 are the preamp tubes, which don't appear to be connected to the green or green yellow lines off the transformer.
So this amp has had some cowboys mucking about in it, which I'm actually quite excited about. I would guess someone did some mods to this, and I'm wondering outloud if they got rid of the coupling caps, just based on having heard that phrase so many times and the location of the missing cap.
My plan is to get this back to dead stock, then do the Gillespie mods. From there, I want to add a phono section, preferably tube, and an input switch (which I can already figure out how to do). Then it gets my other trade skills in some kind of a custom base. I might go whole hog and build a new chasis and add some carpentry flourishes and I might design a bit of interface for it. All aesthetic stuff, but fun and part of the joy in it. It miiiight even get VU's. And I do plan on documenting everything in a thread or two for the next person who comes along.
I am hoping to get some on my first tube amp rebuild. After having some great success with this thread finding the right beginner's amp project, I settled on a Magnavox 9302. After something like 12 years of posting, I became a member and got a 9302 on Bartertown. What I really want to do is create a thread translating the famed "Gillespie mods" for beginners. I've come to have a really deep and abiding appreciation for this community and want to contribute to it. But I'm hoping I can lean on it one more time to get me there.
As part of my quest, I bought a pdf of EJ Jurich's Vaccuum Tube Amplifier Basics, and I've identified this as a key source of my problem: Translating what I'm seeing in a schematic to what I've got on the chassis. I find this harder without the benefit of a PCB. In the book, he gives the following as an example of something that can be read off of a circuit.
View attachment 3519252
I don't want to get bogged down in the specifics of the example circuit, I just brought it up as an example of the skill I've identified that I don't really have.
My first concrete request is for good schematics for the 9302. I don't really know how to find that in the database, and all the ones I google are fairly low quality. Here's what I'm working with so far.
It seems clear that some parts of the schematic are missing, but this is a purely academic exercise at this point, so let's skip that for now.
One thing I have trouble with is figuring out where the schematic starts and end. So I started from the power transformer, and I instantly see that the brown and yellow and brown are clipped and do not lead to Y1 or Y2 (I assume Y1 and Y2 are other schematics I'll need.)
Okay, so that's a dead end. So I went back to another part of the schematic and looked at the input and preamp section. I see channel one going to ground, and then the other lead coming off the RCA joint going to a 470k resistor that is grounded, and then I see a 47k resistor that feeds into a 100 mmf capcitor (non polarized? but I'm not sure what the asterisk means?), but this is where my amp diverges. Here's my amp.
So the RCA input is grounded directly to the chassis. That checks out. then I follow the white wire to the connection block where I see the 470k R107 going to a ground terminal one slot over. Check. And then I get the 47K R118 going directly to pin 8 of the preamp tube without any connection to C107. And then on pin 9, I see R109 (Red red silver is, what, .022 ohms?) which should be connected to the other leg of C107, but instead goes to the balance pot (R117).
I'm starting to see this come together. I can mostly read the balance pot going to the output transformers through R112 and R212, but the x's in the diagram are a mystery to me.
It seems like the fix is as simple as soldering a 100 mmf capacitor with one leg on pin 8 and the other leg on pin 9 of the preamp tube, but that seems too easy? I don't know enough about how the circuit actually works to know if that would change the values of the resistors or anything else, and I don't have all the resistor values in the schematic.
Oh, and the pilot light is completely bypassed. See the two empty solder joints on the bottom left.
It looks like the the green line out of the power transformer on the schematic (far right of the pitcture) skips the pilot light and goes to pin 4 of the first power tube and then is daisy chained over to pin 4 of every other tube. The green and yellow line looks like it skips the pilot light and goes to pin 5 of the first power tube and then gets daisy chained over to pin n5 of the other three tubes, and then those lines get tied into an on light added by the someone along the way. I'm confused by what V1-6 mean section in this part of the schematic because I see the 4 power tubes listed above, and V1 and V4 are the preamp tubes, which don't appear to be connected to the green or green yellow lines off the transformer.
So this amp has had some cowboys mucking about in it, which I'm actually quite excited about. I would guess someone did some mods to this, and I'm wondering outloud if they got rid of the coupling caps, just based on having heard that phrase so many times and the location of the missing cap.
My plan is to get this back to dead stock, then do the Gillespie mods. From there, I want to add a phono section, preferably tube, and an input switch (which I can already figure out how to do). Then it gets my other trade skills in some kind of a custom base. I might go whole hog and build a new chasis and add some carpentry flourishes and I might design a bit of interface for it. All aesthetic stuff, but fun and part of the joy in it. It miiiight even get VU's. And I do plan on documenting everything in a thread or two for the next person who comes along.
