Improving the Fisher X-101C
- Thread starter dcgillespie
- Start date
They are badly mismatched certainly. From the information available, it's very hard to tell if the tubes are actually "bad". That's because you don't know if the tubes drawing less current are doing so because their cathodes are well worn, or because of manufacturing tolerances causing their current draw to be less than other tubes. You simply don't know. The only way to truly know is to subject the tubes to a power output test. If the tubes can output at least 81% of the nominal power that new tubes typically produce, then the tube is good, and just requires less bias due to manufacturing tolerances (the reason there are bias controls to begin with). On the other hand, if the power output is low, then the cathode is well worn, and the story has been told. Without a power output test, the only way you'd know if a tube was bad is if it required sooooo little bias voltage to approach normal quiescent current levels that it would be rather obvious that the tube is bad.
Dave
Dave
On the EFB Control Grid Regulator there is a 51K resistor connected between the Base an Collector of the inverting (input) transistor. If you increase this to 68K (same wattage rating), the available bias voltage at the output of the regulator will increase by about -7 vdc. If that is too much, then use a 62K (increase of about - 4.6 vdc), or 56K (increase of about -2.1 vdc). Use whatever value places the bias control in the upper half of its rotation for the tubes you want to use, once the unit is stabilized.
I hope that helps!
Dave
I hope that helps!
Dave
An IBAM should be used as icing on the cake -- not as a way to wrench way out of balance tubes into submission. Whether a full complement of bias controls exist or not, It is always best to use matched tubes, as that will deliver the best overall performance. Then use the controls to tweak the match.
Dave
Dave
The easiest way to dynamically adjust the control is to adjust it for equal current draw in the output tubes (pin 5 readings) when the amplifier is loaded and driven to about 20 watts output. Output tubes should idle at about 34 mA under quiescent conditions.
Dave
Dave
If one of the transformers was out of phase, then you'd have the problem you had - oscillation as the feedback would then be positive. Not sure where to tell you to look, but I'd look elsewhere. If you short the hot (high side) of the volume control together, does the bass return? If it does, then the problem appears before the volume control. If there is no change, then I'd be looking at the speakers, their wiring, or their placement.
Dave
Dave
CORYK
Active Member
I found it. You were correct Dave, it was at the volume control. These two amps are quite different in layout. My voltage at CR1 is 498v. Should I change R124 to a different value to bring the voltage closer to the schematic? With my new 7591's in I have 34-40ma at pins 5. My bias pot is maxed out to deliver -27v and the a/c pots are at maximum counter clockwise position. I have 347v at pin 1 of V5 and V6.
What method did you use to set the AC Balance controls?
Dave
Dave
For now, I'd set them according to the method I developed that Larry posted as a sticky for this forum. The pin numbers referred to may be different, but I think you'll get the idea. The procedure is performed with the unit unplugged and turned OFF.
Dave
Dave
Adjust the AC line voltage if the AC heaters are operating more than 5% above nominal. Once they are correct, and the output tubes are drawing the correct quiescent current, then the main B+ will be what it will be. The B+ voltage in the unit does not affect the setting of the AC Balance control.
