Well, you could not have picked a more knowledgeable group with Dave G. et al. Your post made it seem as though you have no knowledge about electronics, it would seem that is not the case. There are many people on youtube that basically have no electronics background trying to fix dangerous tube electronics, and I have more than once had to redo that work from uninformed tinkerers. I would think though that you are beyond that group from your response so this project should not provide much in the way of difficulties in fixing for you. I would try to learn a bit about the questions you asked rahter than just ask for directions, that information will help to serve you well as you continue to bring these old amps back to life, good luck!
Question for Dave G. regarding 330K resistors...
- Thread starter gkargreen
- Start date
Service manual does not state what wattage these grid resistors are. They look smaller than the 1/3 and 1/2 watt resistors. I was about to install 1/2 watt, 220k ohm, will that work?
Just fine. Most of the manuals state (in the fine print) that "unless otherwise specified, resistors are 1/3 watt".........
Dave
Dave
I realize this is an old thread but figured I would ask anyway. In your KX-200 in addition to changing the coupling caps to .68uf did you change the grid resistors? on the KX-200 schematic they show 270K (R51,R52 R57 and R58) and in the x100B they are 82K. I am assuming you just changed the caps and left the resistors alone. Is my assumption correct?
richiem46; in your case, if you have "bloated" Low frequency, you can just change the caps.
The cap/resistor change is for FIXED BIAS ONLY, due to grid resistance parameters by the tube manufacturer's (See datasheet circled in RED) and Fisher going to a higher rated resistance to get more power (even if it was only a watt or two).
Grid resistance:.................................................Fixed Bias 300KΩ MAX......................................... Cathode Bias (also called Self Bias) is 1MEGΩ.
This is also for the 7868, and 6GM5 (Same family).
The cap/resistor change is for FIXED BIAS ONLY, due to grid resistance parameters by the tube manufacturer's (See datasheet circled in RED) and Fisher going to a higher rated resistance to get more power (even if it was only a watt or two).
Grid resistance:.................................................Fixed Bias 300KΩ MAX......................................... Cathode Bias (also called Self Bias) is 1MEGΩ.
This is also for the 7868, and 6GM5 (Same family).
Larry, thanks for your response! So in my KX-200 I do not need to fiddle with the resistors then. I am not sure what bloated bass means specifically. I know that the KX-200 definitely has a more "solid" sound to it than the KX-100. I suppose it is a matter of personal preference on the part of the listener. I may try the .068 caps just to hear the difference.
the values of those caps don't affect frequency response quite as much as you might think they do inside a feedback loop. Yes a smaller cap has a higher cut-off frequency but the feedback system will bump output of the driver to overcome it. Until the cap gets really really small there won't be much measurable effect on frequency response, though at some point the driver is simply not going to have enough snot to overcome the too-small cap acting as a choke point.
Following up I ended up replacing the .047 coupling capacitors the original owner had installed replacing the stock units with .068 uf Oil-filled Russian caps that I had purchased and then never used. I insulated them with heat shrink tubing as they are large and a tight fit under the chassis. I didn't expect much of a change but was pleasantly surprised. The amp sounds better with the caps, more "presence" with the individual instruments easier to distinguish. I was not a believer in "boutique" caps but there was a definite improvement after installing them. There were no brand markings on the ones that I removed but maybe they were suspect though they looked like newer units and their values metered in spec.
