This thread identifies the function of the 6GF7(A) tube in John K's and Rwood's copies of this amp. Assuming there are diodes present as indicated below, then the 6GF7 is a voltage regulator controlling voltage to the 'screens' of the output tubes and, except for the 'driver', the other three pre-amp tubes are 12AX7s. I'd guess that for the remaining working copies of this amp, this gadget contributes significantly to how come people think these amps sound so good.
Listed as footnotes here are links to existing LTG threads on the several models of Guild bass amps. While DKL has several schematics, to my knowledge none includes a 5th apparently 'pre-amp' tube, the "... one in the middle..." per Rwood's earlier post. There are 2 versions w/ 4 tubes in the preamp; one has 6L6 outs, the other 8417s; these tubes have the same pinouts, circuits very similar but apparently no schematic for this particular model. Finally, there is yet another model, also without a schematic, w/ 6550 outputs - like matsickma's and possibly megadan's - and probably with 5-12AX7s, 1-12AU7A,and 1-7591A; more on the 7591 below.
These are pics of the '5th' tube amp; 1st not Rwood's, the others are and regenerated from the footnoted posts:
Tube deck view with pesky little pc boards; must be removed to check voltages or other maintenance - 6GF7 in the approximate center:
Under chassis view:
6GF7(A) - The 'X' tube:
This tube was widely used in older TVs to control vertical deflection and is a twin-triode; similar to the common 12AX7 family. Unlike a 12AX7, the two sections have different voltage-handling characteristics but it's function isn't audio amplification.
The blue square box looks like the diodes/rectifier; if they are small caps to filter RF, then the diodes could be mounted on the strip in the rectangular box. The red square is the '5th tube' / 6GF7; the screen voltage regulator.
The 7591 tube, commonly used for output including the Thunderbird combo amp and found in the in 6550 output model is probably also a voltage regulator for the 'screens'. The 'screens' are metal structures inside the tubes that, well, without overcooking this post, control the tube's operation. Regulators are common in hi-fi audio however most modern gear uses transistors for the application.
And Rwood, if you're out there, the 7247 'driver' is likely the one in the yellow circle - look for the wiring leading from the center/'wiper' leg of the master vol control to the 4.7K input resistor.
Existing LTG Posts:
http://letstalkguild.com/phpBB2/viewtopic.php?f=7&t=80
http://letstalkguild.com/phpBB2/viewtop ... 839&hilit=
http://letstalkguild.com/phpBB2/viewtop ... highlight=
Other bits and pieces:
http://www.diyaudio.com/forums/showthread/t-73176.html
http://www.audioasylum.com/scripts/t.pl ... s&m=185665
cj
Listed as footnotes here are links to existing LTG threads on the several models of Guild bass amps. While DKL has several schematics, to my knowledge none includes a 5th apparently 'pre-amp' tube, the "... one in the middle..." per Rwood's earlier post. There are 2 versions w/ 4 tubes in the preamp; one has 6L6 outs, the other 8417s; these tubes have the same pinouts, circuits very similar but apparently no schematic for this particular model. Finally, there is yet another model, also without a schematic, w/ 6550 outputs - like matsickma's and possibly megadan's - and probably with 5-12AX7s, 1-12AU7A,and 1-7591A; more on the 7591 below.
These are pics of the '5th' tube amp; 1st not Rwood's, the others are and regenerated from the footnoted posts:
Tube deck view with pesky little pc boards; must be removed to check voltages or other maintenance - 6GF7 in the approximate center:
Under chassis view:
6GF7(A) - The 'X' tube:
This tube was widely used in older TVs to control vertical deflection and is a twin-triode; similar to the common 12AX7 family. Unlike a 12AX7, the two sections have different voltage-handling characteristics but it's function isn't audio amplification.
The blue square box looks like the diodes/rectifier; if they are small caps to filter RF, then the diodes could be mounted on the strip in the rectangular box. The red square is the '5th tube' / 6GF7; the screen voltage regulator.
The 7591 tube, commonly used for output including the Thunderbird combo amp and found in the in 6550 output model is probably also a voltage regulator for the 'screens'. The 'screens' are metal structures inside the tubes that, well, without overcooking this post, control the tube's operation. Regulators are common in hi-fi audio however most modern gear uses transistors for the application.
And Rwood, if you're out there, the 7247 'driver' is likely the one in the yellow circle - look for the wiring leading from the center/'wiper' leg of the master vol control to the 4.7K input resistor.
Existing LTG Posts:
http://letstalkguild.com/phpBB2/viewtopic.php?f=7&t=80
http://letstalkguild.com/phpBB2/viewtop ... 839&hilit=
http://letstalkguild.com/phpBB2/viewtop ... highlight=
Other bits and pieces:
http://www.diyaudio.com/forums/showthread/t-73176.html
http://www.audioasylum.com/scripts/t.pl ... s&m=185665
cj