Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: 12AX7 on March 04, 2014, 07:03:03 pm
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In the schematic in the link below for a dual channel JCM800, the PI has some grid values I've never seen and not in balance. Any reason for this that i'm missing? By the way, looks like schematic heaven is closing again, this time maybe for good if no one takes up the cause as this guy did. Might wanna sve some of those schematics !
http://www.webphix.com/schematic%20heaven/www.schematicheaven.com/marshallamps/jcm800_splitch_50w_2205.pdf (http://www.webphix.com/schematic%20heaven/www.schematicheaven.com/marshallamps/jcm800_splitch_50w_2205.pdf)
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looks like schematic heaven is closing again, this time maybe for good if no one takes up the cause as this guy did. Might wanna sve some of those schematics !
It would be kind of cool if someone could assimilate those. :icon_biggrin:
http://www.el34world.com/Forum/index.php?topic=16613.0 (http://www.el34world.com/Forum/index.php?topic=16613.0)
:think1:
With respect, Tubenit
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In the schematic in the link below for a dual channel JCM800, the PI has some grid values I've never seen and not in balance. Any reason for this that i'm missing? ...
The low value for R41 (330kΩ) is not that out of line with some I've seen. Due to bootstrapping its effective value is higher, meaning it can be made smaller than a typical grid reference resistor and still not load down the previous stage. If its value was small and C25's value was also small, then bass would be somewhat rolled off going into the phase inverter. But C25 is relatively huge at 0.022uF (where 0.001uF or so might be typical).
Not sure about a reason for differing values for R41 and R44. There should be no significant current flow through either resistor, so the voltage at the grids of the phase inverter will be equal and the same value as the voltage at the top of R45.
GUESS: The phase inverter is the first stage inside the feedback loop (from phase inverter to speaker output). The interaction of C30 and R44 causes a low frequency roll-off, and making R44 smaller shifts that roll-off higher. If there were too many roll-offs too close to each other inside the loop, then the negative feedback could become positive at the frequency of those roll-offs. Given this spot in the phase inverter is also the feedback input, I'm thinking they were trimming one of the phase shifts in the feedback loop to ensure stability.
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Also because the grid that R44 is connected to is shunted to ground at AC (and therefore there is no signal on that grid which could be otherwise affected by the (bootstrapped) input impedance that R44 provides), then I daresay you can get away with a smaller grid leak for less potential hiss AFAICT.
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I am no where near your levels of understanding, so this may be a stupid thought. But i notice that with some marshall outputs sections, as you go louder it seems as tho NFB is lost to some extent. Could the value of that side be so low as to allow more NFB to balance that issue out as you go louder?
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... But i notice that with some marshall outputs sections, as you go louder it seems as tho NFB is lost to some extent. ...
I don't know what you mean here, so you'd have to point to a schematic for such an amp.
The one we were looking at does not change anything in the feedback loop when volume is changed, so I doubt it makes up for any change of volume knob setting.
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Hard to explain, but in any case I was just curious as to why the unmatched grid resistors, especially being so unmatched and so low. I've never seen that on another amp.