The 14A7 should work fine. Its actually a better choice if you don't mind that it is not factory original.
The extra 6 volt drop across the 14A7 filament will be compensated for by today's higher AC line voltages. 115 V was the norm back then, so with 120 V AC you will be just about right on spec.
Good news. I received a 7B7 replacement from my area local Amtique Radio Club. (http://vrps.org/) Radio is up and running. One thing to resolve. Not sure which direction. All local stations come in well but a slight hum. I was wondering if the RR electrolytics are the cause. The original radio had a PM speaker retrofittrd installed but they installed a choke coil in lieu of subtituting with resistors and larger Caps. The original schematic calls for 20UF x2 with the original speaker, but the radio had 2 30Ufs I Replaced closesest 32UFs x2. Are these electrolytics too large?
Just about all of the ac/dc set w/pm spkers use 30/50mfd caps or larger. Just for convenience I use 2X 47mfd.
When my pals were reading comic books
I was down in the basement in my dad's
workshop. Perusing his Sam's Photofoacts
Vol 1-50 admiring the old set and trying to
figure out what all those squiggly meant.
Circa 1966
Now I think I've got!
Radiorolyn: Would increasing the size UF of those two electrolytics reduce the baseline HUM? I have replaced all caps in this radio and as noted the previous 30 UFs were subed with 32Uf,, ? Its a baseline hum in the background. Radio receives lots of stations.
AC/DC sets for the most part are a half wave rect. Before going to high on the UF of the Ecaps check spect on the rect tube and see what it wii handle. Try reversing the plug into the wall outlet. If using LED or CF light bulbs can be a problem. If you have a touch lamp can be a big problem mine turned off still is a problem have to unplug. Some sets will always have a very low hum between stations with my hearing not a problem. Stay Safe David
As David mentioned there is a recommended maximum value for the input capacitor which is usually about 40ufd. The reason for this is that when the set is first turned on that capacitor being devoid of any voltage looks like a dead short to the cathode of the rectifier tube. This can put a lot of stress on the cathode for the first few seconds of warm up time. The higher the capacitance the more stress. Once the cap starts charging then that dead short starts to look like a resistor going higher and higher in resistance till the only thing that is drawing current are the circuits in the radio and not just the cap in that's p/s.
Hum can also be from leakage between the filament/cathode or cathode/control grid. Any AC voltage getting where it shouldn't be will cause hum.
You could have the set near a strong magnetic field. Move it to different rooms in the house and see if that helps.
This modern age that we live in has so many devises that can wreak havoc on these old sets it's like a cancer. Take the signals produce by the average switching p/s used in a battery charger. They have a lot of amplitude and harmonics that can creep into the rf and IF stages. To the receiver it looks like just another AM signal. If we try to filter it out, then we filter out what we want to listen to also.
It's not unusually to have a little bit of hum. The older sets w/field coil spkrs were always more hum free than the resistance coupled filters that were used in the post war sets. Back in the tube days you have ac (filament voltage) running around near high impedance grid circuits that can be influenced by this ac and cause hum. Now days in semiconductor audio and rf circuits there is no ac running around near the high impedance dc circuits. Thousands of microfarads of filtering and the dc coming out is clean as a whistle.
Another common occurrence with ac/dc sets is that with the set turned all the way down you can still hear stations coming thru. I've been told that it's from leakage in the diode to triode sections the det/1st audio tube.
Enough of this boring stuff...
When my pals were reading comic books
I was down in the basement in my dad's
workshop. Perusing his Sam's Photofoacts
Vol 1-50 admiring the old set and trying to
figure out what all those squiggly meant.
Circa 1966
Now I think I've got!
A few updates on my HUM issue and asking for input. After changing out all of the tubes after testing and no improvement, I added an additional 10UF to the PS filter caps no help. My question,, Since the Radio was converted to a PM and the the mod still has a coil in place for the filter caps vs changing to resistors , I decided to check that "choke?" transformner. The schematic indicates 500 ohms resistance . Isolated Mine is reading 2 Ohms. /// Note this is not the output coil... its a transformer inserted when the EM was removed. vs doing the resistor mod.
>Since the Radio was converted to a PM and the the mod still has a coil in place for the filter caps vs changing to resistors , I decided to check that "choke?" transformer.
The "coil" you mean the field coil? So you have the back half of the original spkr and the pm spkr together? True be told there is 3 coils on the electrodynamic spkr. Voice, hum bucking, and field.
The transformer doesn't really have much to do with the power supply or filtering. It's there to match the impedance of the plate of the output tube to the voice coil of the spkr. In this case about 2000 ohms to 3 ohms.
If you are measuring the resistance of the field coil and it's 2 ohms that's a problem. It's darn near a dead short. That will make it hum. Replace w/a 500 or so ohm 2 or 3w resistor.
When my pals were reading comic books
I was down in the basement in my dad's
workshop. Perusing his Sam's Photofoacts
Vol 1-50 admiring the old set and trying to
figure out what all those squiggly meant.
Circa 1966
Now I think I've got!
I'm thinking your hum is from that transformer doing a poor job of filtering. Looks like a shorted winding. You should be able to disconnect the winding and temporarily connect a resistor of 800-1200 ohms, 5 watts. Leave the added filter capacitor in the circuit.
When my pals were reading comic books
I was down in the basement in my dad's
workshop. Perusing his Sam's Photofoacts
Vol 1-50 admiring the old set and trying to
figure out what all those squiggly meant.
Circa 1966
Now I think I've got!
RodB you may have described it better than me. I am sending the original photo and you will see 2 transformers solderd to the chasis. 1) goes to the speaker and the other is in the circuit between the 2 electrolytics.
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