[dconant]

Sorry Mike no commission. I'll likely be buying the 115 as soon as I can convince myself.

[dconant]

Can someone tell me if my 42 tubes are connected triode or pentode in fixed or cathode bias? How could I tell by looking at the schematic.

My somewhat educated guess is push pull cathode biased in pentode mode.

Any idea what the output watts would be?

[RodB]

Yes, you can look at the schematic. When the output tube screen grid is connected directly to the plate then it's wired as a triode. Cathode bias is developed by a cathode resistor of around 150-220 ohms usually bypassed with an electrolytic capacitor. Fixed bias is developed in the power supply usually with a voltage divider resistor connected to the center tap of the power transformer. Output power is found either by ohms law or looking at the tube manual for the output tube. The specs will list 2 or 3 configurations and give the output power for a specific configuration. In push-pull you double the value. For instance if it gives a value of 4.2 watts that would be 8.4 into the opt and 8 to the speaker. Transformers are about 85% efficient.

[dconant]

Thanks Rod. Since the screen grid is not tied to the plate then my tubes are pentode right? I see the candohm ties to the cathodes and the resistance will be 300 ohms from point 5 to 3 but I don't find the cap. Are the tubes cathode biased? Can you tell me what the Effective Load Resistance and power output is for my 42 tubes from the specs here https://frank.pocnet.net/sheets/093/4/42.pdf

I am trying to determine what output transformer to replace my bad one. I have a Hammond 125C but I am afraid the wattage is too small. It is an 8 watt transformer. I have a speaker that measures 5 ohms with a vtvm. Is that an 8 ohm speaker or 6 ohm speaker and can I use that transformer? I'm thinking I can use pins 4 & 6? I have tried it and it seems ok to me but I don't hear that well so it may not sound the best. Is it hurting the tubes?

[MrFixr55]

Hi DConnat, 

Yes, the 42s are in Pentode Mode, cathode bias.  I am using the schematic from the Philco Library, Link below:

Early Philco Schematics – Philco Library

A few things:

Switching or varying the B voltage to the screen grids was a relatively common method for controlling the sensitivity of a stage.  One use of a "local-distance" switch was to do just that, vary the screen voltage, thereby varying the gain.

If that cap pictured is actually #47, definitely get rid of it and replace it with a modern 0.001 uF cap.  The value of 0.001 uF is not unreasonable, but like Morzh says, I don't know how they developed it with that open metal frame job.  In old RCA sets like the Radiolas, open caps were good for about 0.00025 uF at the most.  Regardless, that one may have shorted, causing catastrophic failures.

A few warnings here:

When a circuit with power pentodes is fired up with the output transformer missing or open, the screen grids draw much current, often more than they can handle.  They will glow to an almost incandescent white, then burn up, ruining the tube.  In addition, the fried screen grid can short to the control grid and /or cathode, wreaking more havoc.  I hope this did not happen to you, especially if you have the globe style 42s (waaay cool tube, first "modern" pentode with indirect heated cathode, yet in a globe envelope).

Please note that on this radio, like many other older Philcos with a 3200 Ohm field coil, not all of the B+ current passes through the field coil.  the output stage, which comprises the bulk of needed B+ current comes right off the power supply, before the field coil.

Looking at the Candohm:  Note that Pin 3 is grounded, Pin 4 connects to the center tap of the HV Supply, Pin 5 connects to the cathodes of the 42s.  The 205 Ohms between the cathodes of the 42s and the HV return provide cathode bias.  The 95 Ohms between the HV return and chassis ground provides grid bias for the 42s through the interstage transformer center tap and provides grid bias for any other tube whose cathode is at chassis ground potential.  Any open here will greatly limit output power or cause total non-operation, but no collateral damage.

The other connections, B+ to the Field Coil to pin 1 of the Candohm (3200 Ohm),1 and 2 (900 Ohm, ad 2 of the Candohm to 3 of the Candohm at chassis ground (2700 Ohm) provide a "voltage divider" to provide a relatively predictable level of lower B+ voltage to the plates of the 1st Audio stage, the detector Amp (through another voltage divider) and to the plates of the IF, Osc / Mixer and RF tubes, from the junction between the field and Pin 1 of the Candohm; and to the screens of the IF, Osc /  Mixer and RF from pin 2 of the Candohm.  The majority of current drawn through the Candohm is from the speaker field. 

Please note that the dotted resistors and speaker are if this is a "221" version, employing 2 speakers.  The drawing can be most easily misinterpreted.

I don't think that the dotted line between pin 3 of the dotted Candohm, #46A connects to the line that it crosses.  I also do not think that it connects to resistor 41A as drawn.  I believe that Pin 3 of Resistor 46A is grounded.  The 2 field coils are in series from B+ to ground, forming the main voltage divider, therefore Resistor 41A is merely a dropping resistor.

Any opening of the string from Pin 1 to Pin 3 of the Candohm will break the voltage divider, causing insufficient current to the field coil and much higher voltages throughout the set, causing distortion.

I would have to think that most of these failures occurred from the radio being 93 years old.  It appears that open output transformers and open field coils are common.  

If you have the single speaker model or are replacing both speakers of a 2 speaker model with PM speakers, definitely use the "121" option on the schematic for the Candohm.   In addition, it may be necessary to increase the capacity of some of the filter caps to reduce the hum introduced by the loss of the choke capacity of the field coil and loss of the hum bucking coil.

Keep us posted on your progress, please feel free to ask more questions, and good luck with your restoration!