The Fine Things are Always Hand Made
I posted about this a few weeks back, looking for a replacement.
I’ve been unsuccessful in finding a replacement, and nor can see how to fix the original. I’ve dismantled it and can't see any damage or lack of contact.
So I have a question: the 20000 ohm tap, is I understand, to boost bass response at low volume. Am I right in thinking that this effect can be exactly replicated by increasing the bass via the bass control, or is something else happening here? Advice appreciated as always.
I see Mark Oppat came thru for you like he did for me <sarc>....He didn't even have the courtesy to respond to a couple of very patient PM's on Antique Radio Forum....I don't need him anyway.
500K audio taper pots are readily available. The tap can be created by using conductive epoxy from the resistive pot material out to a wire which becomes the tap. It has been done before and works. I purchased some conductive epoxy and a few 500K pots to make a few. See here: https://www.youtube.com/watch?v=NzafFqC1C6A
On 2 original Philharmonic pots I measure the tap at 100k. I have a hard time believing that the original tap could have been 20K on a 500K pot, so I think the 100K I measure is more likely correct. I purchased a couple of 100k pots and was thinking about adding a resistor in the 400K range to the high end of it and putting the bass response circuit to the high end of the pot. So, in effect, the wiper will always be below the "tap". Sure, I will loose volume range, but 1/4 of a turn on the factory 500k is LOUD. All the originals wear out in the first quarter. Nobody need the power of quad 6L6's for a radio in their home. I think about 1/8 of a turn is really loud on these and having the volume go up at a slower rate would be nicer. This is just an idea I had and I need to try it out.
Thanks Scott, that’s very helpful, esp. the link to the video.
Interesting video. Worth trying.
Seems the 20K tap on a 500K pot was consistent on the diagrams for the 1937 Philharmonic of 5/1937, Sixteen and Super 12. The 1939 Philly diagram (Riders 14,pg 43-44) shows a 50K tap on the diagram.
I see the Philharmonic service notes has a list of 1939 parts revisions, and on page 16 says a 40K tap.
NOS tapped pots are reported scarce now. So maybe take what you can get between 20K and 50K.
In more modern 1950's Hi-Fi terminology, it was a "loudness" control cutting in at low volume control setting, somewhat variable depending on exactly where the control was set in the 1st 1/4 of advance.
Also appears the volume control could not silence the radio with a good antenna and strong local station. On my 1940 BOL Philharmonic remote control, I finally added a resistor to ground just ahead of the volume control to quiet down the radio at low volume control setting acceptable to me.
I had to replace a broken tapped volume control, on a model Sixteen as I recall. The resistance strip had broken where the tap was riveted. Appears the method in the video might work to bridge such a break, saving the original volume control.
It would be nice if someone else had a set with an original vol pot that they could measure too. Most have been replaced and it is hard to come across an original one.
We know that the EH Scott schematic and documentation has numerous errors, and I am not saying the volume tap is one of them, but having a tap at about 25% is most common in old radios. This is about 50% rotation due to the audio taper. A 20K tap is 4% which is unusually low. A 50K tap is 10% which is still low, but closer to where these taps most commonly are in radios. If I had the time I would test the frequency response of a set with a low value tap, but that is not going to happen soon.