12-07-2022, 09:27 AM
Hi Bob,
First off, it seems that Electrohome was very generous with the capacity of the filters to begin with, so I would not increase the capacity. Per the RCA Receiving Tube Manual, for a "Capacitor Input Filter" (of which this), the maximum value of the input filter cap is 40 uF (same as MFD). Per the schematic you provided, the filter "caps" are 40.0 and 50.0 uF. This as as much as or more than what was used in later model radios with a PM Dynamic speaker with no field coil or choke.
As for the resistor, start with about 1.2KOhm. the goal is to get about 250V at the center tap of the output transformer.
CAUTION!! Do not run this radio without the output transformer. The screen grids of the '42s will take all the current, glow white hot and burn out, causing shorts.
The Hammond 125B transformer should work. However, if you search "the auction site", you may find a used output transformer for less. I think that a plate to plate primary impedance of about 8 KOhm will do for 250V. The manual I am looking at says 10 KOhm but that is at 315V.
If using the Hammond 125B, use secondary taps 2 and 5 if using an 8 Ohm speaker. If using a 4 Ohm speaker, use taps 3 and 5.
Hope this helps.
A Blessed Christmas, Chanukah and Holiday season to you and all in the Phorum!
First off, it seems that Electrohome was very generous with the capacity of the filters to begin with, so I would not increase the capacity. Per the RCA Receiving Tube Manual, for a "Capacitor Input Filter" (of which this), the maximum value of the input filter cap is 40 uF (same as MFD). Per the schematic you provided, the filter "caps" are 40.0 and 50.0 uF. This as as much as or more than what was used in later model radios with a PM Dynamic speaker with no field coil or choke.
As for the resistor, start with about 1.2KOhm. the goal is to get about 250V at the center tap of the output transformer.
CAUTION!! Do not run this radio without the output transformer. The screen grids of the '42s will take all the current, glow white hot and burn out, causing shorts.
The Hammond 125B transformer should work. However, if you search "the auction site", you may find a used output transformer for less. I think that a plate to plate primary impedance of about 8 KOhm will do for 250V. The manual I am looking at says 10 KOhm but that is at 315V.
If using the Hammond 125B, use secondary taps 2 and 5 if using an 8 Ohm speaker. If using a 4 Ohm speaker, use taps 3 and 5.
Hope this helps.
A Blessed Christmas, Chanukah and Holiday season to you and all in the Phorum!
"Do Justly, love Mercy and walk humbly with your God"- Micah 6:8
"Let us begin to do good"- St. Francis
Best Regards,
MrFixr55
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