Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: ElPedro on August 08, 2016, 02:21:35 pm
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Well, another week, another problem.
Working on a JCM 800 2204 based off the Ceriatone schematic. Only changes - removed the 1/2 power switch, and changed the bias resistor value to get in the right bias range.
Problem - the only way I can get sound out of it is to play a chord very loudly. The amp opens up and sounds the chord and then fades away to static, and then nothing. To me it seems like a bias problem. The readings I get are as follows:
Bias 36mv
Pin 5 of output tubes -34v
Pin 3 of output tubes 437v
These numbers seem OK to me. I have tried different power tubes and preamp tubes.
Anyone able to set me on the right path?
Thanks!
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I don't know that I'd call that a bias issue, the bias current seems about perfect for an EL34. The datasheet I looked up says 55 mA is the max so 70% would be 38.5mA (of course that 36mV you list should have been through a 1 ohm I'm sure, but just checking). Otherwise, if it's not bias, then I'm throwing a wild guess that something is pulling a lot of the signal to ground, maybe one of the pulldown resistors is not at the right level, or something, but that's a stab at little data. You may want to have the DMM connected when playing to see that the pin 5 voltage doesn't do something odd and go near 0 or something similar. Another possible option would be something like negative feedback is off in a way that is making it really strong and diminishing volume. I had an amp I built that had two inputs but they were out of phase, so if I jumpered them it would really mute down the volume at certain volume levels, and I had to push it to get it up there.
Something along those lines 'may' be the culprit...
Edit: Diminishing volume, not tone...
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pompei I think you're confusing a pull down resistor with a grid leak resistor? A pulldown resistor helps eliminate popping from an effects pedal being turned on & off and is placed on the input and/or output jacks.
But this (grid leak/bias issue) is something I was thinking along the lines of also. Usually it's when someone makes a mod of an additional stage and forgets to remember to be sure to have one. The JCM800 employs an extra gain stage where something like this could occur but he's following a ceriatone layout - layouts are sort of hard to mess up but it can happen certainly?
Well, another week, another problem.
Anyone able to set me on the right path?
El Pedro - post PICTURES & schematic or layout you are working from. You cannot get any kind of accurate help without helping us to help you. If you cannot, then you may as well as delete your post as it wastes everyone's time. It's very inconsiderate, ignorant, and lazy asking time of others to help or assist you without doing everything you can to assist in this process as much as you can yourself.
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Sorry for the lack of pics and schematics. I was looking for input from others as to things they have experienced that may have caused this issue, in general.
[url][http://www.ceriatone.com/images/layoutPic/marshallLayout/JCM800_2204Ceriatone.jpg/url]
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pompei I think you're confusing a pull down resistor with a grid leak resistor? A pulldown resistor helps eliminate popping from an effects pedal being turned on & off and is placed on the input and/or output jacks.
Oops you're right, that's exactly what I meant :) Sometimes all these terms become a big muddy mess. At least someone else agreed with my theory, despite using the wrong terms :)
~Phil
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Sorry for the lack of pics and schematics. I was looking for input from others as to things they have experienced that may have caused this issue, in general.
Fixed URl :
http://www.ceriatone.com/images/layoutPic/marshallLayout/JCM800_2204Ceriatone.jpg (http://www.ceriatone.com/images/layoutPic/marshallLayout/JCM800_2204Ceriatone.jpg)
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Sorry for the lack of pics and schematics. I was looking for input from others as to things they have experienced that may have caused this issue, in general.
[url][http://www.ceriatone.com/images/layoutPic/marshallLayout/JCM800_2204Ceriatone.jpg/url]
Are you looking to fix a problem or just whimsical discussion? A layout and/or schematic is worthless for your possible issue. No way to know what you did or didn't do w/out close up macro photos.
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-The thing that strikes me is that the original JCM800 2204 and the JMP 2204 both have much higher negative bias voltages. Are you sure you're not roasting your EL34's? Have you checked for red plates?
http://el34world.com/charts/Schematics/files/marshall/Marshall_2204%20JMP%20Master%20Volume%20Lead%2050watt.gif (http://el34world.com/charts/Schematics/files/marshall/Marshall_2204%20JMP%20Master%20Volume%20Lead%2050watt.gif)
http://el34world.com/charts/Schematics/files/marshall/Marshall_JCM800_2204_Lead_Series.pdf (http://el34world.com/charts/Schematics/files/marshall/Marshall_JCM800_2204_Lead_Series.pdf)