01-31-2018, 07:26 AM
Jcassity;
The schematic you posted in your thread was the one that I kept referring to. The schematic that I redrew was from a Beitman's schematic for the Code 125 produced in 1948 based on examination by them of a sample of this version. In comparing the values shown for the candohm resistor in the versions that are known to exist plus preceding years or successive year models shows how Philco changed their designs slightly for the values of this resistor. The actual negative voltage applied to the output tubes' signal grid varies depending on those values. I see negative voltages ranging from -11VDC to about -20VDC and values in between. What I may do is take a close look at the current in each output tube. A 10 ohm resistor can be placed in the cathode circuit of each tube and the resulting voltage measured will indicate the current conduction of each output tube. Then a comparison can be made to the specifications for the 6K6GT or 6V6GT tubes to see what the current is. Overall fidelity is impacted by how close to best operating parameters the tubes are performing. The range of values for the negative bias resistor mentioned shows that there is a considerable range permitted by Philco's designs. Some of this may be to optimize power output in some models and make some not as strong by comparison as a way of promoting the better sound of more expensive models. In general, the more negative the signal grid voltage becomes the lower the current in each output tube becomes. This extends tube life, while if the negative voltage is reduced the output tubes' current increases and power output increases up to a point. Output transformers are often changed depending on desired power output and frequency response. That is another area of investigation that needs to be considered when looking at the range of parts values and tube current observed.
I find that the permanent magnet speaker in the version that I have is a surprise as Philco continued to produce versions in later year models that used field core speakers. This particular unit may be somewhat of an experimental version in which Philco was trying some new ideas. I have not seen any indication that the speaker was ever changed from a PM type looking at the connections under the chassis.
Thanks for that extra bit of data about the value of the candohm resistor in your set. The one in this set measures close to 265 ohms.
Joe
The schematic you posted in your thread was the one that I kept referring to. The schematic that I redrew was from a Beitman's schematic for the Code 125 produced in 1948 based on examination by them of a sample of this version. In comparing the values shown for the candohm resistor in the versions that are known to exist plus preceding years or successive year models shows how Philco changed their designs slightly for the values of this resistor. The actual negative voltage applied to the output tubes' signal grid varies depending on those values. I see negative voltages ranging from -11VDC to about -20VDC and values in between. What I may do is take a close look at the current in each output tube. A 10 ohm resistor can be placed in the cathode circuit of each tube and the resulting voltage measured will indicate the current conduction of each output tube. Then a comparison can be made to the specifications for the 6K6GT or 6V6GT tubes to see what the current is. Overall fidelity is impacted by how close to best operating parameters the tubes are performing. The range of values for the negative bias resistor mentioned shows that there is a considerable range permitted by Philco's designs. Some of this may be to optimize power output in some models and make some not as strong by comparison as a way of promoting the better sound of more expensive models. In general, the more negative the signal grid voltage becomes the lower the current in each output tube becomes. This extends tube life, while if the negative voltage is reduced the output tubes' current increases and power output increases up to a point. Output transformers are often changed depending on desired power output and frequency response. That is another area of investigation that needs to be considered when looking at the range of parts values and tube current observed.
I find that the permanent magnet speaker in the version that I have is a surprise as Philco continued to produce versions in later year models that used field core speakers. This particular unit may be somewhat of an experimental version in which Philco was trying some new ideas. I have not seen any indication that the speaker was ever changed from a PM type looking at the connections under the chassis.
Thanks for that extra bit of data about the value of the candohm resistor in your set. The one in this set measures close to 265 ohms.
Joe
