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| |-+  JCM 800 2203, 2204, 2550, 2555
| | |-+  Problem with 2203 JCm
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Author Topic: Problem with 2203 JCm  (Read 28967 times)
kevilay
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« on: January 11, 2010, 10:43:24 PM »

Hey guys I just built a 2203 jcm and ran into an issue. I turened it on and the indicator light is on.  I went to bias it but i am reading 0v at the end of the 1ohm resistor with respect to ground. The tubes are glowing dimly. Im not really sure how bright their supposed to be with full power on ive never used a tube amp before. I checked my wiring a few times and dont see any issues. I tried hookin up a guitar and playing it at a very low volume but i get no sound, no hum, no static nothing. Where should i check? I have a volt meter. This is my first time building an amp.

Kevin
« Last Edit: January 11, 2010, 11:36:43 PM by kevilay » Logged
cmoore
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« Reply #1 on: January 12, 2010, 01:03:03 AM »


I would pull the power tubes and make sure you have:
1. Negative DC bias voltage at Pin 5
2. AC Heater voltage at Pin 2 and 7

If either of those are missing you can just work back wards through the circuit until you find where the loss originates.  Let us know what you find.
Good Luck
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kevilay
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« Reply #2 on: January 12, 2010, 04:26:03 AM »

I read 0 on pins 2 and 7, and -850v on pin 5 all with respect to ground. What should my voltages be. There was some confusion on the layout for me as for the heater let me explain what I did.

I daisy chained all the pin 2's with red and pin 7's with black. Then i tied in the 2 brown cables from the transformer (3.15v each according to layout) which also runs to the indicator.

Kevin
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cmoore
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« Reply #3 on: January 12, 2010, 06:29:15 AM »

OK -
Just to make sure, check both fuses with you meter to make sure they are good.
Pin 2 and 7 are AC and you read that with one lead on Pin 2 and one lead on Pin 7, not to ground... and you get Zero volts? That means you have no filament supply and the tubes will not function.  Disconnect (unsolder) the heater wires from the first power tube and measure those two wires just like you would a tube, one lead to each wire.  You are checking for 6.3 VAC.  Make sure the wires do not touch.  If there is no voltage there, do the same to the pilot light.  I am thinking you have a short in one of your tubes, and not a bad power transformer.  If you get 6.3 Volts from the transformer then you must have a bad tube that is shorting the heater supply to ground.
I can see Pin 5 having -85 VDC, but not 10 times that much. I think that tap of the PT only supplies 100 volts. You might want to check that again, meter on VDC, black lead at ground, red lead at Pin 5.
Good Luck
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kevilay
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« Reply #4 on: January 12, 2010, 03:33:48 PM »

ok, i checked pins 2 to 7 incorrectly the first time. With my meter across pins 2 to 7 on AC i measured 11.2 v. As for measuring from pin 5 to ground I have my meter on 200v dc and it says like 872 or something around there. Must have a decimal in there otherwise it wouldnt read that high so 87.2 makes sense. So you think it might be a bad power tube? Is there a way to check them, This is the first time i've ever even seen a tube Tongue

Kevin
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cmoore
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« Reply #5 on: January 12, 2010, 04:09:19 PM »

Usually a bad power tube will pull the AC to ground, so the reading would be Zero. Your number of 11.2 is suspiciously high.  The transformer does not normally supply that much voltage. You need to desolder the heater supply leads from the first power tube so you cam measure the supply from the transformer. If that supply is 11 volts, your PT is wound wrong and needs to be replaced.  It is possible, but bad trannies do not happen very often.  So you need to measure the 3.15 tranny wires and see what they give you.  Just make sure they are disconnected from the power tube and the pilot light.  Like I said, if that says 11.2 then your PT is bad.
Regards the DC, yes, I assume there is a decimal you are not seeing. 87.2 VDC sounds reasonable at this point.
Good Luck
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kevilay
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« Reply #6 on: January 12, 2010, 07:19:56 PM »

Is there a way of testing the tubes? For the PT test I desolder the 2, 3.15v brown wires then touch one lead to each end and measure ac volts correct? Im at school so I cant test it for another 2 hours.

Kevin
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kevilay
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« Reply #7 on: January 12, 2010, 08:27:38 PM »

I remeasured the voltage from the 2 brown heater wires with a different analog meter I have and its reading around 7.5 volts. I think my digital meter is reading AC incorrectly cause its reading 170v out of the wall outlet, this meter reads correctly. Is that closer to what I should have?

Also that -85v on pin 5 is now -50v with this other meter.

Kevin
« Last Edit: January 12, 2010, 09:12:18 PM by kevilay » Logged
cmoore
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« Reply #8 on: January 12, 2010, 09:30:26 PM »

That is close, but it is still pretty high. You may want to change the meter battery for a new one and try it again.  It would not hurt to measure your wall voltage again also.  You might have a lot of voltage from the wall, it would be good for you to know, although that can change throughout the day.
You can hook it all back up and see what you get with the tubes all drawing current.
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kevilay
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« Reply #9 on: January 12, 2010, 09:36:04 PM »

I checked my wall outlet and its exactly 120V

Kevin
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kevilay
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« Reply #10 on: January 12, 2010, 10:05:37 PM »

That negative dc bias voltage reads -48v with the trim pot all the one way and up to 68v with the trim pot the other way. This doesn't change weather it is on standby or not on standby.

Kevin
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kevilay
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« Reply #11 on: January 12, 2010, 10:28:52 PM »

I was checking voltages and noticed the voltage selector is wrong. It reads 100v, 120v and 220v, However the layout says 120, 220 and 240 so I flicked it down to 100v which should be 120v. Now my heater voltage is 6.3, my reverse bias voltage is 42.5 volts but my voltage across the 1ohm resistor is still 0.

Kevin
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cmoore
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« Reply #12 on: January 13, 2010, 12:17:42 AM »

When you say 42.5, you do mean negative don't you? Is your bias cap positive ground?  I think on those JCM's it was a 10uf/150 volt or somewhere in that range?
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kevilay
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« Reply #13 on: January 13, 2010, 12:23:43 AM »

yeah its negative sorry
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cmoore
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« Reply #14 on: January 13, 2010, 01:37:17 AM »

Just to double check.... you are reading the negative VDC from Pin 5 to ground, and your 1 ohm resistor is attached from Pin 8 to ground.
Is your meter auto ranging?  If not, you need to select the lowest possible range for VDC.  This will allow you to read milli volts DC across the 1 ohm resistor.
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