Fisher Tube Tuner Restoration Walkthrough

[LinuksGuru]

Hi, Tube folks,

For anyone who is interested - I wrote Fisher 400 tube tuner/amplifier restoration walkthrough. Your comments and suggestions are very welcome. Upon request I can add/edit stuff which may be valuable for the next creative tube junkies and vintage lovers.

http://www.starlett.com/~web5_macgur...ration_en.html

PS. Next restoration walkthrough is coming soon - tube integrated amplifier Pioneer SM-83.

[Wavebourn]

Originally posted by LinuksGuru
Hi, Tube folks,

For anyone who is interested - I wrote Fisher 400 tube tuner/amplifier restoration walkthrough. Your comments and suggestions are very welcome. Upon request I can add/edit stuff which may be valuable for the next creative tube junkies and vintage lovers.

http://www.starlett.com/~web5_macgur...ration_en.html

PS. Next restoration walkthrough is coming soon - tube integrated amplifier Pioneer SM-83.

Fine, except grid leak resistors called grid stoppers, and your explanation why their values need to be reduced is wrong.

Also, "on schematic diagram it was 15V, main was 10v.", did you mean mine instead of main? And "lethal charge" should be called "lethal voltage".

[SpreadSpectrum]

Nice walkthrough. I have a Fisher X-101-B that I refreshed, then the power transformer went up in smoke. According to the Photofact, the power transformer was rated for less than the quiescent current draw. Since I want a bigger power transformer (for obvious reasons) and there is no room on the chassis, I am considering a separate power supply chassis.

The grid stoppers are the 1k resistors that connect directly to the grids of the power tubes.

[LinuksGuru]

Originally posted by Wavebourn
Fine, except grid leak resistors called grid stoppers, and your explanation why their values need to be reduced is wrong.

Ok, thanks, was is correct explanation?

[Wavebourn]

Originally posted by LinuksGuru


Ok, thanks, was is correct explanation?


There are 2 x 330k grid resistors (so called grid stoppers) and 2 x 0.047 ukF coupling capacitors which need to be replaced with 180k - 220k and 0.1 - 0.22 ukF respectively. Grid stoppers act as high-frequency low-pass filter, cancel possibility of parasitic oscillation on the tube itself, suppress radio frequencies on the input stage plus prevent blocking distortion. New tubes have sligthly different plate/plate-grid curves, and therefore, sometimes may not work stable with 330k grid stoppers

There are 2 x 330k grid resistors (so called grid leak resistors) and 2 x 0.047 ukF coupling capacitors which need to be replaced with 180k - 220k and 0.1 - 0.22 ukF respectively. NOS tubes can have more gases inside so voltage drops on this resistors may cause positive shift of bias causing higher idle current that causes more gases that causes higher bias shift and so on, until anodes are red and melted down. When values of resistors are lower the same currents cause lower voltage drops, so run-away is less possible.
Since values of resistors are smaller to preserve the same time constant values of coupling capacitors must be increased accordingly, otherwise roll-off will start on higher frequencies affecting bass reproduction.

However, my English is not perfect, but my explanation may help.

[LinuksGuru]

Originally posted by Wavebourn



There are 2 x 330k grid resistors (so called grid leak resistors) and 2 x 0.047 ukF coupling capacitors which need to be replaced with 180k - 220k and 0.1 - 0.22 ukF respectively. NOS tubes can have more gases inside so voltage drops on this resistors may cause positive shift of bias causing higher idle current that causes more gases that causes higher bias shift and so on, until anodes are red and melted down. When values of resistors are lower the same currents cause lower voltage drops, so run-away is less possible.
Since values of resistors are smaller to preserve the same time constant values of coupling capacitors must be increased accordingly, otherwise roll-off will start on higher frequencies affecting bass reproduction.

However, my English is not perfect, but my explanation may help.

Thanks a LOT! My English is not perfect either - it is not my native tongue.

[kshallen]

Originally Posted by Wavebourn

There are 2 x 330k grid resistors (so called grid leak resistors) and 2 x 0.047 ukF coupling capacitors which need to be replaced with 180k - 220k and 0.1 - 0.22 ukF respectively. NOS tubes can have more gases inside so voltage drops on this resistors may cause positive shift of bias causing higher idle current that causes more gases that causes higher bias shift and so on, until anodes are red and melted down. When values of resistors are lower the same currents cause lower voltage drops, so run-away is less possible.
Since values of resistors are smaller to preserve the same time constant values of coupling capacitors must be increased accordingly, otherwise roll-off will start on higher frequencies affecting bass reproduction.

However, my English is not perfect, but my explanation may help.

Hi, Fisher X101-b resistor mod, are you saying the both 330K on each tube are to be replaced, 8 resistors total. If yes, then maybe I am thinking wrong, but would not .68 uf be closer to preserving the 3dB point for the filter formed
by 2 grid resistor in series and coupling cap?

[LinuksGuru]

Restoration walkthrough have been moved to:
Fisher 400 Vacuum Tube/Valve Tuner/Amplifier Restoration | MacGuru HQ