JCM800/1987

[nmonaghen]

Hi, I am building an amplifier and I was wondering if these modification I made would work well. I did something similar to the one wire mod on a super lead. My goal is to make the high gain channel as JCM800 like as possible, and make the clean channel as much like a super lead as possible. I am not sure the best way to accomplish this, though.

I have two possible ideas. The first one is to run the high gain input like a JCM800, and feed that directly into the low gain input of the 1987. I know the low gain input would work correctly, but i have no idea if the high gain input would sound any good, or if it would work at all. My second idea is to make the high and low gain inputs both work like a JCM800. That way I know the high gain and the low gain inputs would both work correctly, but I do not think I would get the sound I want from the low gain input. It would be a JCM800 sound and I would like to be able to get that also.

The low gain channel uses the bright channel and the high gain uses the normal. Right now my plan is to leave V1 biased like a super lead normally is. I don't know if there is any reason to change it (unless it is helpful to change it on the lead channel, I would prefer to keep the low gain as stock as possible). I used a pre phase inverter master volume. I have been reading about ppimv mods, and I don't think I would like it as much.

Idea number one:
http://imageshack.us/photo/my-images/32/1987nathan.gif

Uploaded with ImageShack.us

Idea number two
http://imageshack.us/photo/my-images/81 ... athan2.gif

Uploaded with ImageShack.us

If anyone could help me it would be much appreciated.

Me and my friend have build a 1987 and a slo100 clone before, so I am not worried about the actual construction of the amp. I am trying to take it a step forward and do some design/modifications this time.

[klauswilliams]

If you are looking for stock 800 AND stock Super Lead in the same amp do a search for "SDM's 2 in 1". Of course you can tweak from there for whatever, but that setup will give you both amps stock in one box via switch. Lots have done it, including me, with great results.

[nmonaghen]

Thank You! That is exactly what I was looking for. Now it is time to start ordering parts.

[Reeltarded]

Build a 1987 circuit then cross the mixer for the normal channel back to the junction of the 68k grid resistors for the bright channel input. You probably won't even have to use a single piece of shielded wire.

The normal channel isn't a low gain channel. It's actually higher gain than the bright channel.

This sounds great without any other changes. Bright input is the same, normal input is cascaded. You might add another 470k at the mixer to ground.

[nmonaghen]

Ok, I finally got around to building this amp. I ended up simply building a 2203 and will do the 2 in 1 mod at a later date. I am having two problems with it at the moment. First problem is I cannot get the high gain input to work. I have tried replacing both jacks, checking contacts, and resoldering joints. The low gain channel sounds wonderful, but when I plug into the high gain I get nothing. Sometimes I can her an extremely faint sound if I crank it all the way up. When I pin 7 on v1 I get no sound, but when I tap the lead coming off of pin 6 I can hear the tapping. I have tried switching tubes and that didn't help.

My second problem is redplating. I have some pretty serious redplating going on. It is usually worse on V5 than V4, but then it is occasionally worse on V4 than V5. I considered that the tubes may be bad so I tried a set I know work and I have the same problem. Another part I thought was weird was it is worst at idle. I See the tubes begin to redplate (20 or 30 seconds after startup) and I can hit a chord and the redplating goes away almost instantly as the guitar starts playing. As soon as you mute the note it comes back just as quickly. It makes no difference where I set the bias. The power transformer also gets very hot.

I measured each half of the output transformer primary and I got 65 ohms and 70 ohms. Then I powered it up and measured the voltage on each half and it kept climbing. It started at around 20v on each side, with the side with lower resistance having a slightly higher current at all times. Then it climbed to around 25v. After a minute or so it was redplating very badly and I was reading 28.3v and it was still climbing. I calculated that as .44ma of current, my reading is probably on the low side as well, and the voltage was still climbing. I tried ajusting the bias potentiometer and it didn't seem to make any difference. I would turn it all the way one way and the voltage was higher than before. Then I would turn it all the way the the other way and the voltage would be higher. I guess one question with this part I have is if you want to lower current draw, do you want to increase or decrease the bias resistance? Does anyone have any suggestions as to what I should look to do to try to remedy these problems?

Thank you very much in advance,
Nathan

ps. This is the schematic I used

http://sphotos-a.xx.fbcdn.net/hphotos-a ... 7912_n.jpg" onclick="window.open(this.href);return false;

[julkke]

A picture tells more than thousand words...

[nmonaghen]

Ok, I will post some pictures when I come home from work.

[nmonaghen]

http://sphotos-b.xx.fbcdn.net/hphotos-a ... 6028_n.jpg" onclick="window.open(this.href);return false;
http://sphotos-b.xx.fbcdn.net/hphotos-a ... 0239_n.jpg" onclick="window.open(this.href);return false;

I don't know how useful these pictures will be considering the mess the wiring is. The wires and the switch to the furthest side are not connected to anything so you can ignore those. if there is anything specific you would like a photo of, please let me know.

Also the jacks are made opposite from each other if that throws anyone off. the side that disconnects/shorts is on the bottom of the top jack and the top of the bottom jack.

[julkke]

Hmm, I don't wanna be harsh but you need to clean that amp up. Your redplating problem could be caused by bad lead dress around the power tubes leading to RF oscillation. It is hard to tell from the pics but your soldering also seems a bit sloppy. Test connections for continuity with a multimeter. Where are your screen resistors, I can't see them? Take a look on some of the fine builds here and copy the lead dress.

[nmonaghen]

Don't worry about sounding harsh, I can take it I will try cleaning up the wiring next. The chassis wiring is pretty neat overall, but I was hoping that I could leave the board wires long until I get it working to get some room to move around. The screen resistors are there (on the tube sockets), you just can't see them in the picture.

Thank you very much! Hopefully that fixes the redplating problem.

[julkke]

Don't worry about sounding harsh, I can take it I will try cleaning up the wiring next. The chassis wiring is pretty neat overall, but I was hoping that I could leave the board wires long until I get it working to get some room to move around. The screen resistors are there (on the tube sockets), you just can't see them in the picture.

Thank you very much! Hopefully that fixes the redplating problem.

A little of space to move the wires is cool but that is way overkill. Don't worry, you'll get it fixed!

[demonufo]

Um, wow. Even that chassis layout is all wrong really for this type of build. That is going to take some very neat work to make it work properly. And that is far from neat work.

What is that chassis, exactly?

[nmonaghen]

It is a weber 8cm100 chasis.

[Coot Boy]

Err, which part of the wiring did you say was "pretty neat" Everything's wrong with that thing.

[nmonaghen]

Well I am going to redo the wiring and come back after I am done. Hopefully my wiring was the problem. While I knew my wiring was a mess for the most part, I had figured that it would be a bit noisier that if i had done it neatly and would be easier to fix if I had made a mistake; I could then rewire it once I knew it was working. I did not realize that messy wiring could cause problems like that.

I did find the problem with the Hi input, so if the wiring fixes the red plating then I will be good!

I was also wondering, would there be any advantage to using shielded wire for the leads to the power tubes or the negative feedback? If my amplifier is having oscillation problems I would think that that may be helpful. I do not know for sure though, if it would be helpful, or just make more of a mess.

Thank you all for the help!