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Author Topic: Problem with 2203 JCm  (Read 33038 times)
kevilay
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« Reply #30 on: January 13, 2010, 09:51:51 PM »
Yeah, thats what Nik told me to do. I swapped the leads warmed it up and biased it. Then plugged a guitar in and it played for about 30seconds and popped the 1A fuse. I touched up the 2 ends i swapped to make sure they were a bit cleaner and tried again and about 6 seconds after i turned it on it popped the 1A fuse again.

Kevin
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cmoore
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« Reply #31 on: January 13, 2010, 10:27:17 PM »
You must still be pulling to ground somewhere. What were the plate voltage and current when you set the bias?
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kevilay
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« Reply #32 on: January 13, 2010, 11:00:33 PM »
I didnt measure the plate voltage, Nik told me to shoot for 35mV-40mV for the bias So I set it to 35

Kevin
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cmoore
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« Reply #33 on: January 14, 2010, 12:17:34 AM »
The HT fuse is usually power tubes, but in a new build it could be a number of things.  I would check all of your wiring against a schematic, not the layout.  Check every connection on the schematic with a bright marker as you verify it.  It is also a good idea to run through the chassis with your meter and check continuity. Make sure nothing is shorted to ground, if your meter talks, all the better.
I would do those two things; check continuity, and verify your connections to the schematic.
If you have or can borrow a set of power tubes that is another quick way to see if your tube(s) are bad.
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kevilay
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« Reply #34 on: January 14, 2010, 02:09:06 AM »
I dont have access to any other tubes. Does Nik post the schematics anywhere?

Kevin
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cmoore
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« Reply #35 on: January 14, 2010, 02:53:07 AM »
Nik does not normally supply a schematic. I imagine his reasoning would be that his amps are either a matter of public record, or they are proprietary; and not available at all.  Your amp circuit is pretty much communal at this point.
Good Luck

http://www.ampwares.com/schematics/marshall/jcm800_lead_mstvol_100w_2203.pdf
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kevilay
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« Reply #36 on: January 14, 2010, 03:27:07 AM »
Whats a good way to go about learning about how a guitar amp actually works? Are there any good sites or books that breaks down why everything is used how it effects the circuit and what not. As for the amp I'm still in the process of checking for shorts, I found someone nearby that said they would give me a hand. Hopefully we can get it working.

Kevin
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kevilay
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« Reply #37 on: January 16, 2010, 02:37:16 AM »
I got it going now but its all crunchy sounding and the volume is really really low. I brought it to a local amp builder and he said he thinks it might be a bad resistor. He referred me to another guy with proper test equipment. Any of you guys have any ideas?

Kevin
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kevilay
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« Reply #38 on: January 16, 2010, 11:41:14 PM »
Got it working, turns out it was 2 bad tubes.

Kevin
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cmoore
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« Reply #39 on: January 17, 2010, 12:37:08 AM »
Congratulations!!!!
You really "hung in there" with this amp.  Tubes are one of the first things to suspect when you have a problem. They can, and do, go bad at any time.  They can be fine at the dealer, and go bad during shipping.  They can be perfect at rehearsal, and go gad at the gig.  They can work fine while you trouble shoot three other problems, and then die themselves just when you think you have it all fixed.  If you have a tube amp, you must have spare tubes. It is that simple.
Good Luck
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kevilay
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« Reply #40 on: January 17, 2010, 05:16:35 PM »
This guy that told me what the problem is is an amp designer. Hes an awesome guy he showed me how I can change some minor things to get more distortion

Kevin
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kevilay
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« Reply #41 on: January 17, 2010, 06:17:43 PM »
So all I need now is a set of power tubes. Any recommendations I noticed theres a tone of different types. I think i got the JJ EL34s in it now. I play stuff from Clean chili peppers to metallica, Im looking for mostly that heavy sound. I noticed they have selection of early and late distortion n stuff. Just looking for some advice on tubes I know little.

Kevin
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cmoore
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« Reply #42 on: January 17, 2010, 08:08:16 PM »
Canada has a very good tube dealer (maybe more than I know)  http://thetubestore.com/

We use mostly SED and TAD.  The fact is you will just have to try a few to see what you hear.  I do not think you need to spend the large amount of money that is NOS tubes, unless your income is not a factor.
For guitar amps there are basically only three tube manufactures:
1.  JJ
2.  New Sensor
3.  Shuguang
Virtually every (modern day) tube you buy will be made by one of these three companies, including the tubes I recommended. Do not pay extra for Gold anything (pins etc.)  do not pay extra for balanced preamp tubes of any kind.  Do not hesitate to pay a few extra dollars to a good dealer for the same tube you might be able to buy from an unproven source.  A good dealer will burn in, and subsequently, cull a large number of the tubes they purchase.  The cull rate might be as high as 20%. It is worth a few extra dollars to know the tubes you buy have gone through at least this cursory 24-48 burn test.  Do not pay more for "quiet tubes".  There is no way to guarantee micro phonics, or rather the lack of it. 
Matched power tubes are not "necessary" but it is a big convenience, and has always been worth the extra money to us to have matched power tubes. That's about it.
Good Luck
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kevilay
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« Reply #43 on: January 18, 2010, 12:10:23 AM »
Nice, after playing for a bit I noticed just by setting my front pickup volume i can go from a cleaner sound to a nice heavy distortion with a pickup switch.  How could I go from the really clean tone to a heavy distortion during a song with this amp?

Kevin
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