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I've been making a lot of noise lately trying to figure out the 5D8 so I'm trying to do a circuit walk and understand each part... not sure exactly WHAT I'm looking at though.
Going to document this on a wiki later.
A schematic is here:
http://www.schematicheaven.com/fende..._5d8_schem.pdf
From what I can tell, there's 2 input sections. The top one uses a bright bypass parallel its volume, so it's probably a bright input. Each input seems to house the preamp section.
The top input on each seems to connect to a sort of negative feedback, anode to grid. I don't understand the arrows.. are those switches, such that the NFB is activated when using the lower connection, and disabled when using the upper? This would make sense to me, since the NFB gets inverted through one 12AY7 stage and hits the other grid in that case.
The next stage seems to lead volume into a 12AY7 voltage gain stage, then into a 12AY7 current gain stage via cathode follower (warning: tube stress). The cathode follower should allow large amounts of current draw to supply the tone stack (lower current on high signal, rather than high current on high signal? My theory's fuzzy); 100k resistor seems to cool the tube (lest it overdrive like crazy) and allow this to be used as a signal path.
The tone stack following is a psychotic crazy mess, what the hell? That 56k resistor next to ground looks like it could be the mid control if we throw a pot there instead.
Following that, 12AX7 phase inverter. Is that an LTP? Uses a local NFB but I don't understand it. This whole section is a mess to me.
That ground point isloates the PI from the bias circuit for the two 6L6 tubes. They output into a center tapped OT.
The signal off the OT hits a global NFB circuit, returning inverted signal to the phase inverter to increase linearity and reduce overdrive. The 5K back there is a presence knob but effectively it acts like a bias adjustment for stage 1 of the PI.
Power stage uses two 5Y3GT tubes to allow more current draw than one 5U4G. Uses a center tap standby switch to kill the B+ for the tubes. Full wave rectification, tubes heated by 5V localized AC that won't travel through the rest of the amp. Power stages have 32uF immediately available for B+ filtering, then 16uF for the screen grids, 16uF for the PI anodes, and 8uF for all of the preamp tubes and tone stack cathode follower; these capacitors are separated by resistors, it makes some kind of difference to performance.
Because of the power supply layout, the preamp and tone stack cathode follower should sag first under load (they take less power, though, so I'm not sure; 8uF may be enough); the PI second; the power tube screens (distortion!); then the tube plates (more distortion!). I don't know how this plays out in real life, especially since the power tubes will demand more faster and probably really sag first, and cause the rest of the system to collapse with it.
Going to document this on a wiki later.
A schematic is here:
http://www.schematicheaven.com/fende..._5d8_schem.pdf
From what I can tell, there's 2 input sections. The top one uses a bright bypass parallel its volume, so it's probably a bright input. Each input seems to house the preamp section.
The top input on each seems to connect to a sort of negative feedback, anode to grid. I don't understand the arrows.. are those switches, such that the NFB is activated when using the lower connection, and disabled when using the upper? This would make sense to me, since the NFB gets inverted through one 12AY7 stage and hits the other grid in that case.
The next stage seems to lead volume into a 12AY7 voltage gain stage, then into a 12AY7 current gain stage via cathode follower (warning: tube stress). The cathode follower should allow large amounts of current draw to supply the tone stack (lower current on high signal, rather than high current on high signal? My theory's fuzzy); 100k resistor seems to cool the tube (lest it overdrive like crazy) and allow this to be used as a signal path.
The tone stack following is a psychotic crazy mess, what the hell? That 56k resistor next to ground looks like it could be the mid control if we throw a pot there instead.
Following that, 12AX7 phase inverter. Is that an LTP? Uses a local NFB but I don't understand it. This whole section is a mess to me.
That ground point isloates the PI from the bias circuit for the two 6L6 tubes. They output into a center tapped OT.
The signal off the OT hits a global NFB circuit, returning inverted signal to the phase inverter to increase linearity and reduce overdrive. The 5K back there is a presence knob but effectively it acts like a bias adjustment for stage 1 of the PI.
Power stage uses two 5Y3GT tubes to allow more current draw than one 5U4G. Uses a center tap standby switch to kill the B+ for the tubes. Full wave rectification, tubes heated by 5V localized AC that won't travel through the rest of the amp. Power stages have 32uF immediately available for B+ filtering, then 16uF for the screen grids, 16uF for the PI anodes, and 8uF for all of the preamp tubes and tone stack cathode follower; these capacitors are separated by resistors, it makes some kind of difference to performance.
Because of the power supply layout, the preamp and tone stack cathode follower should sag first under load (they take less power, though, so I'm not sure; 8uF may be enough); the PI second; the power tube screens (distortion!); then the tube plates (more distortion!). I don't know how this plays out in real life, especially since the power tubes will demand more faster and probably really sag first, and cause the rest of the system to collapse with it.
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