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Peavey Windsor to JCM800 Conversion.
#31
Quote:GV, would 110k be close enough if I parralel 2, 220k for that spot? And, how was that amp even running with 3 of those bias resistors burnt and open? Shouldn't a fuse be blowing?

+ or - 10% should be fine.

Yes, it's a surprise it wasn't blowing out.
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#32
No sound, but nothing is going crazy. I think I may be reading the mod schematic wrong. At first it looked like the jumper when from V2 pin 1, to the bottom of C16. That can't be right. No signal is getting to second half of V2. 

No matter how I look at this mod V2 pin 1, has to hit hit pin 7. I'm thinking it's supposed to be a jumper where C17 was, then go from pin 7 to the bottom C16.

Am I just not seeing something right?
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#33
(05-18-2016, 09:26 PM)karpathion Wrote: No sound, but nothing is going crazy. I think I may be reading the mod schematic wrong. At first it looked like the jumper when from V2 pin 1, to the bottom of C16. That can't be right. No signal is getting to second half of V2. 

No matter how I look at this mod V2 pin 1, has to hit hit pin 7. I'm thinking it's supposed to be a jumper where C17 was, then go from pin 7 to the bottom C16.

Am I just not seeing something right?

Note: V3 not V2. The mod circuit says from Pin 1 to C16. Removing C17 seems to take out V3b  from the signal path, unless it is meant to be jumpered. Then there's that extra stuff in the corner that connects to A,B,C and D. and disconnect P501
 I can't quite picture how all this works at the moment but valve 3b looks like it's working like 2b[preamp] with that weird high voltage input [ if jumpered at C17 ]
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#34
(05-18-2016, 10:14 PM)GoldenVulture Wrote: Note: V3 not V2. The mod circuit says from Pin 1 to C16. Removing C17 seems to take out V3b  from the signal path, unless it is meant to be jumpered. Then there's that extra stuff in the corner that connects to A,B,C and D. and disconnect P501
 I can't quite picture how all this works at the moment but valve 3b looks like it's working like 2b[preamp] with that weird high voltage input [ if jumpered at C17 ]


You are seeing the same thing I am. It has to jump across C17.
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#35
I appear to be.
Have you done all that ABCD stuff and P501. It's important to do.
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#36
It's gone. I'm 100% positive I have all that ADBC stuff right.

I'm looking at the Marshall 2203 schematic hard. If you look at little grid around V3, it almost looks like splits the signal into 2 paths. One for each half of the PI. If I'm looking that right, it's that ABCD stuff. I's like it sends a signal through each side of V3, then one of those goes to each side of the 2 sets of power tubes.

I could just need some sleep.
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#37
Good idea, give it a break and get back to it refreshed.
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#38
(05-18-2016, 10:18 PM)karpathion Wrote: You are seeing the same thing I am. It has to jump across C17.

On reflection ,NO. I compared it to the 2204. The signal gets to Pin7 via the two 1 meg resistors from  Pin 2 [A].
No connection from 1 to 7.
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#39
I stared at those schematics for 2 more hours last night, and I think I'm starting to get what's going on around that PI.

I'm about to lift the PCB again and retrace my steps. I may just have 1 connection wrong. And it's on that ABCD thing. I did it on Vero board instead of a terminal strip. Radio Shack opens at 10:00, I'll go get a terminal strip then and redo that ABCD thing again the way Doug Hammond has it.
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#40


Fuck yeah! I had 1 wire on the wrong pin. My Vero layout was right, I just one of the B leads on the wrong pin. All it took was a fresh set of eyes.

This motherfucker is badass!! It does what it's supposed to do now. All that hiss and fizz is gone. I knew something wasn't right after the preamp mod. Those burnt bias resistors where fucking with it.

I measured my plate voltage at 475V, diveded that by 25w, and came with -52.3ma bias. It's perfect.

This thing absolutely slays! It is THE sound I've been chasing for years, and I'm pretty sure my new cabinet has a bit to do with that too. 

Blues, blues/rock, early metal and hard rock. Whatever, it does it all just like an 800. I'm tickled fucking pink. I'm shaking right now just knowing what I have.

I have a JCM800 for a total investment of $125. Fuck the haters and corksniffers. I know exactly what is sitting in front of me, and costed a hell of a lot less. Just needed a little bit of elbow grease and patience.

I'll try to get some clips sometime today.

[Image: 14fa87.jpg]
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