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Originally Posted by Blooze
Jeffhs - which schematic is the one for the K731 on techschematic.org? Is it the KM07, as I couldn't find one specifically for the K731?
And thanks so much for the info!
I glanced at the tubes in mine yesterday real quick and noticed that the 35C5 has been replaced with a 50C5. Gonna have to change that. I didn't pull the ones with the shielded sockets yet.
Shane
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Shane,
You are more than welcome.
I had the same problem when I was trying to find a schematic for my K731. Try looking under K725, which is practically identical to the K731 as far as the chassis goes; the only difference I can see on my schematic for this set is the chassis number (7K06 for the K725, 7K07 for chassis K731, L, M, R, W, Y) and a different style of cabinet used with the K725 (plastic as opposed to the '731's walnut cabinet). My 731 has chassis 7M07. It could be the later versions of the K731 (the ones that came out in the '60s) used chassis 7K06 and 7K07.
As to that 50C5 in your set rather than a 35C5 I don't know what to tell you, except by all means replace the 50C5 with its 35-volt equivalent. I did some figuring as to total voltage drop across the filament string with a 35C5 and a 50C5 in the output stage; there is a difference of over ten volts between the two (114 with a 35C5, 129 with a 50C5). Look at the tube layout diagram on the bottom of your '731. Is there a 35C5 or a 50C5 shown? If it is a 35C5, and you have a 50C5 in the output tube socket, the set's former owner may have switched tubes, either inadvertently or not knowing any better.
It is always best to use the exact tubes specified in the tube layout chart, especially with a series-string filament circuit; using a 35-volt tube in place of a 50-volt one, or vice-versa, could (likely will) overload other tubes in the string, leading to their premature failure. Series-filament tubes with heater-cathode shorts can also upset voltage drops along the string, causing one or more tubes to fail as well. The total voltage drop across a series filament string must very closely approximate the line voltage. If there is a difference, it is compensated for by a filament dropping resistor.