Still curious if anyone has any thoughts on straightening the dial pointer or reproducing it, but while that stews, I wanted to get some other hypotheticals out of the way.
My wife would like to be able to use Bluetooth with this radio, and in fact, I think I will try to incorporate a switched Aux Input to most of the radios I'm going to work on. I was going to pattern how to do that on the RCA 6K3, but where the 6K3 has the phono input/radio jumper terminals between the 2nd Detector/AVC (6H6) and Audio (6F5), the Silvertone 2014 uses a 12SQ7 as Detector, AVC, and AF Amp. Any thoughts on how to get the signal from the jack into this circuit?
Did about 2/3 of the capacitor and resistor replacements tonight. It appears there were some mis-picks in my order so going to have to wait until the right pieces come in for one resistor and three capacitors. Once that's done, I should be able to do a preliminary power-up test. Much simpler, as has already been stated, than either the RCA 6K3 and the Philco 37-620.
Anyone have thoughts about adapting this design to include an Aux input, or is it not practical?
Okay, with the assistance (as much as I hate to admit it) of Google AI, here are the steps that I have cobbled together. Let me know if anything looks amiss:
Parts for adding aux input:
1x SPDT switch
1x 3.5mm jack
2x 1K ohm resistors ***
1x 1:1 600 ohm audio isolation transformer
So, the plan is to have the audio switchable like I did with the 6K3.
Going to disconnect the wire running between the high end of the volume pot and the 2nd IF transformer at the volume pot terminal, route that via shielded cable out to the switch
Will route the output from the switch back to the volume pot high end also on shielded cable
The secondary of the audio isolation transformer will provide the other input to the switch
The output ground of the transformer will run into the chassis and attach to the B- ***
For the jack, the left and right channels will be combined via 1K *** ohm resistors, the combination of which will go to the isolation transformer primary input
Ground from the jack will go to the transformer primary ground
All of the components will be mounted on some combination of a PCB base and/or plastic project box onto the non-metallic rear cover of the radio
Now, the asterisks above represent clarifying questions:
Resistor value: for the RCA 6K3, this was accomplished with 100 ohm resistors; here Google has suggested 1K ohm; is the difference because this is an AC/DC radio; because of where and how the signal is being introduced into the radio - the 6K3 utilized an existing phono input; or, not actually different at all and I'd be good with 100 ohms here as before?
Ground from transformer to B-: does this need to be shielded cable as well? and it shouldn't matter where it reconnects, as long as it's to a B- point and not the chassis?
I'm also going to add a polarized plug end to the cord and probably will try to relocate the switch to the hot side, though that's going to probably require adding a terminal strip somewhere - there's a lot going on on the back end of that power switch. Will also plan to add a 0.5A fuse before the switch.
Also, I should have asked this probably sooner, but where C11 and C12 (0.05uF each) connect to the line and B- and B- to chassis respectively, should either of both of those be safety capacitors? Same value as original?
Schematic attached.
Best wishes,
Roy
(This post was last modified: 10-13-2026, 10:22 PM by glyphmon.)
Definitely move the switch to the "Hot" side and the wide bade of the power cord directly to the B- / Heater String "Low Side", usually at the 12AV6 / 12SQ7
I would use 100 Ohm resistors to merge the Stereo signals to the isolation transformer. 1K Ohm resistors may attenuate the signal too much
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Thank you for the confirmation. On the topic of safety caps, for C11 and C12, should/could they both be Y2 or should C11 be an X2? I used two Y2s in the 37-620 to go from both sides of the line to ground on the guidance from the forum. This application doesn't seem terribly different.
for 100 ohm on the resistors - I can use what I ordered last time for this purpose.
I don't believe that you need a Y2 cap for C12 but definitely use a Y2 for C11.
The purpose of safety caps is for the cap to be able to handle continuous current (Caps conduct AC) but also to be able to handle voltage spikes to a defined level. The difference between X2 and Y2 caps is that X2 caps fail shorted and Y2 caps fail open. Ironically, X2 caps are placed across the power inputs, so when they short, they will either blow a fuse or smoke a switch, itself or whatever (However, ai believe that they are supposed to be fireproof. This is the reason that any cap that goes between power line and chassis as part of filtering must be a Y2 cap in order to keep the chassis from becoming hot.
On a transformer powered set, caps are usually placed between Line and Chassis and Neutral and Chassis to filter out common mode noise. The reason for safety caps in this case is that these caps are essentially in series across the power line with the chassis at the "halfway point". You will actually feel a "tickle" when touching an ungrounded transformer powered chassis. Therefore, Y2 caps are used for both caps. This should be the case for your RCA 6K3
However, it is different on an AC-DC transformerless "warm chassis" set. In your Silvertone, you can use an X2 cap if you fuse the power line, as this cap is across the power line after the switch. Most folks, however, use a Y2 cap for this cap. AC-DC sets are generally not fused as the heater of the rectifier acts as the fuse in most cases. In your set, a short across the B+ supply would take out the 35Z5 heaters, the pilot and the 150 Ohm resistor, so yeah, definitely put in a 0.5A fuse.
C12 is not meant as a noise filter cap. In an AC-DC set, the purpose of this cap is to provide a path from B- / Common (The return for both the B supply and the heater string back to the power line) and the chassis for high frequencies while providing sufficiently low leakage current to prevent electrocution if a person contacts the chassis while the B- / Common is connected to the "Line" of the outlet. ("Line" is that current carrying conductor which is 120V above earth ground. "Neutral" is that current carrying conductor that is at earth ground potential). Therefore, it is not carrying the current that a cap across the power line is carrying. There is certainly no harm in using a Y2 cap for this cap, and in theory it would be safer, but since there is no "hard ground" connection to the chassis (External grounds are not connected to the chassis on a transformerless set), no current is carried by C12 and spiking should not affect it.
The bigger issue is the power switch in the "Common" as opposed by the hot. This was done by radio manufacturers from the time if first introduction of transformerless sets to the end of the tube era. I believe that the theory was that if the switch was in the neutral, there would be less hum introduced into the audio, as the power switch is usually attached to the back of the volume control. However, this creates a condition where the chassis can become hot (or warm) despite the position of the power cord. For example, if a polarized cord is installed so the "Line" (or "Hot") connects to the rectifier input and "high" side of the heater string, but the switch is in the neutral, the set is safe with the power switch on but with the switch off, the chassis becomes hot (or warm) due to the very low resistance of the tube heater string when cold. Therefore, I think that it is a good idea to do so.
It is important to note that most transformerless chassis are "warm" (cap between Common and chassis, but several are "HOT," including most late 1950 Zenith AM radios, most radios on PC boards and most transformerless AM / FM radios, for example, the RCA 8X71 and many Zenith tabletop models.
Sorry for the length of this, especially if you are already aware of all of this but I write with the idea that folks new to this hobby will read a thread between experts looking for knowledge.
The one instance where changing the location of the power switch from Neutral to Line will not create a safer condition is transformerless sets that use a full wave voltage doubler. There will always be a voltage between chassis and neutral. This is not the case with half wave voltage doublers.
Safety caps arrived today. Soldered one in and had a successful first power-up. No magic smoke escaped and it tunes in with its built in radio (though better with the external).
Now just need to implement the aux input modification when I get my stranded wire back which is currently serving as the antenna for my 6K3 at my workplace where it is fascinating some young scientists.
Thanks for the guidance on this one. Will post a final update and picture when I get the mod done and polish up the cabinet a bit, gently.
for 100 ohm on the resistors - I can use what I ordered last time for this purpose.![[-]](https://philcoradio.com/phorum/images/bootbb/collapse.png "[-]")
