The PHILCO Phorum
38-15 Chairside Antenna - Printable Version

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38-15 Chairside Antenna - robsradioactive - 09-05-2017

Hi all,
I recently picked up a nice 38-15 Chairside and since the cabinet was nice, I just went through the chassis restoration.
(Recap, clean all contacts, replace out of tolerance resistors, new tubes as needed, complete alignment, etc.)

It works very well except that it apparently needs an outside antenna to properly receive weaker stations.

This is problematic for where I chose to put it. Are there any good internal antennas for these types of radios available? I could put in an RF amp, I suppose, but don't want to. It is certainly not as sensitive as many other 5 tube radios...and I also wonder if this is normal for the prewar tube types that it uses.
Anyone have a similar experience with similar models?

Thanks!
Rob

Schematic is here: http://www.nostalgiaair.org/Resources/download.asp?FN=\M0013239.pdf


[url=https://drive.google.com/open?id=0B7nZyVR-b56DNG1GMnpVTVNjelE][/url]

RE: 38-15 Chairside Antenna - Radioroslyn - 09-05-2017

Hi Rob,
I've had a few of those style sets and have found them to have good sensitivity. Could have a weak tube or two. If you can't put up an outside ant a loop or active loop would be a good alternative. The type of building construction your in can diminish reception if it's something like steel and concrete.

RE: 38-15 Chairside Antenna - morzh - 09-05-2017

My 38-15 (the tabletop one), that has it's antenna pigtail and nothing else connected, even receives shortwave. Even in the basement. I'm with Terry, look for weak tubes. Especially the 6A7 pentagrid, it is known to have weak amplifying section (the oscillator is usually good) to the point of it being 1/4 to 1/5 value listed in the table. It will still work but the sensitivity will be diminished.

RE: 38-15 Chairside Antenna - robsradioactive - 09-05-2017

Thanks guys!
Only the floor is concrete and steel. The main floor is frame. So with your info, I think there might really be an issue with it.
I remember testing the tubes and replacing a few, but now I don't remember which ones, or if I just did an emissions test or a Gm test. (Not that any tester is 100% reliable.)

I think I'll do some more tube testing and switching out. I'll post back with what happens after that.
Rob

RE: 38-15 Chairside Antenna - NostalgiaRadioTime - 09-05-2017

Hi Rob,

If swapping tubes doesn't make a difference you might check out this thread. Some of us have had very good luck with these antennas:

http://philcoradio.com/phorum/showthread.php?tid=16904

RE: 38-15 Chairside Antenna - morzh - 09-05-2017

I doubt a loop antenna will do much if the signal is blocked in the first place. Cases like these call for external aerial.

RE: 38-15 Chairside Antenna - robsradioactive - 09-06-2017

Thanks again you guys! I swapped the 6A7 for a tested and, apparently, N.O.S. tube. That made a huge improvement.
I then tested the original 6A7 on my (calibrated) Hickok 600A and it showed [?] on the pentode section.

Turned the english to 73 and that made the meter read Gm of about 600/1000 (average new tube).
The oscillator section was fine at nearly 500/300(" ").

Emission tests good, so I must have originally just done an emission test (or maybe enhanced emission test depending on who you believe) on my Jackson 648. I don't know how else I could have missed that marginal tube.

Also, I may try that tunable loop antenna!

Rob

RE: 38-15 Chairside Antenna - morzh - 09-06-2017

(09-06-2017, 08:56 PM)robsradioactive Wrote:  Thanks again you guys! I swapped the 6A7 for a tested and, apparently, N.O.S. tube. That made a huge improvement.
I then tested the original 6A7 on my (calibrated) Hickok 600A and it showed [?] on the pentode section.

Turned the english to 73 and that made the meter read Gm of about 600/1000 (average new tube).
The oscillator section was fine at nearly 500/300(" ").

Emission tests good, so I must have originally just done an emission test (or maybe enhanced emission test depending on who you believe) on my Jackson 648. I don't know how else I could have missed that marginal tube.


Rob

Yep. Very typical.

RE: 38-15 Chairside Antenna - Chris h - 09-07-2017

The tunable loop from Eton, that is mentioned above, does work very well for BC use. They are pretty narrow banded, though, which makes DXing a little difficult. They are somewhat directional, as most loops are, that can be nice for nulling out interference. I frequently use one in my basement shop, where it is quite effective.

An out door wire is hard to beat, but these are a lot more portable and you don't have to worry about lightning protection, on the antenna, anyway.

CC Crain makes (or made) an amplified twin coil ferrite loop that works well for BC use too. It is very directional and sensitive. It does need to be powered, though.

RE: 38-15 Chairside Antenna - robsradioactive - 09-09-2017

I agree. An outdoor long-wire is hard to beat. But mine comes in through a jack in another room and attached to an S-20R. Also, last week we had some tree guys out to cut back some branches and they knocked it down with their "cherry picker"...so that's a ladder job before winter.

I am going to try a small tuned loop, just for kicks. I don't used the chairside for SW listening and pretty much always just listen to 640 AM around here when I use it at all. I started building a square loop yesterday. I'll post here how it works--if it works!   Icon_crazy 

Rob

RE: 38-15 Chairside Antenna - robsradioactive - 09-11-2017

Well, so I built this for it and it actually works very well. I used 13 turns on a 10" x 10" frame and put a padder capacitor across it. The antenna impedance at 640 kHz turned out to be about 35 k ohms, and the 38-15 chairside measured about 22 k ohms at the same frequency. I called it good and didn't try any further Z match.
Meanwhile, I also discovered abandoned phone cable running horizontally around the outside of the house and right beneath the window behind the chairside. I tapped into that, and now it seems I don't need the tuned loop at all.
Go figure...

Rob

[Image: https://s26.postimg.org/w1rq2x3w9/Tuned_Loop.jpg]