I recently purchased this model, I now find that the speakers voice coil is seized in that when you gently push down near the middle of speaker it does not move up and down. Rather than paying 100 bucks to have it re-coned (I'm a newbie with speaker matters!) I'd like to replace the speaker with a PM speaker that I have which is similar size. The PM speaker measures 6.3 ohms across the black and red leads.
Now from what I have read for info on the speaker that was original is that it is an H-4 has a field coil dc resistance of 3200 ohms output transformer dc resistance of 800 ohms, voice coil impedance of .7 ohms.
So to get this right all i need to do is connect a 3.2K 10 watt resistor to where the field coil was connected ( female connections at plug)? Then connect my pm speaker to the output of the output transformer?
what about the field choke? would i have to do something with that? Would I need to double the size of electro cap #68 to 12 uf vs 6 uf to cut hum? Please find attached drawn up schematic regarding the above. any help would be appreciated!
You have 10 times the VC impedance. This is not good. You have to use a new output transformer.
thanks for your reply Morzh! as per this attached chart for Philco speakers My original speaker was an H-4 , the chart shows that the output transformer has a primary dc resistance of 800 ohms, so what output transformer would i be looking for then to properly drive my 6.3 ohm voice coil in my PM speaker? sorry I'm new to this and appreciate your help!
Looking through my parts here I found a Jensen universal audio transformer, atttached is the directions looks like if I want to drive a 6 ohm pm voice coil I need to know the tube resistance. I have push pull 47's. Therefore if I look up the 47 tube in a tube manual am I looking for "load resistance"? if so it is 7000 ohms. So do I double that ( because i have 2 tubes) so that would put me at 14000 . if looking at this chart so on the secondary of the audio transformer would I use lugs 2 & 4 and go with what is close which would be 16000 under the 6 ohm column to connect my PM speaker to?
Hi Dirk,
>I now find that the speaker's voice coil is seized
On a lot of the early Philco spkrs the cone with the voice coil can be removed pretty easily. Unsolder the vc wires at the terminal strip. Remove the screws around the perimeter of the spkr frame and the one in the center. Remove the cardboard ring and gently lift the edge of the paper cone, it isn't glued in just held in by the screws and ring. You may find that wiggling or giving the cone a gently twist to get the vc freed up from the pole as yours is stuck (center piece). Once you get the cone out you'll want to clean out around the pole. It maybe just be dirty or it may have some rust which can be removed w/ some 400G sandpaper. Stick it down into the pole and frame gap and give it a light sand. Vacuum out the pole and reassemble.
Here's a couple of pics of a spkr from a model 80. Cone is badly torn.
GL
Thanks for the reply, I may try that but I would also like to learn on how to replace an electrodynamic speaker with a PM speaker in case I need to in future restorations.
If the output tubes connected as pentode (tetrode) the impedance doesn't necessarily double. I cannot find the 47 pushpull but it might be 10k rather than 14k.
http://philcoradio.com/phorum/showthread.php?tid=215
Go to the link above, scroll down a bit, and download the file
speakers.zip.
Unzip the file and open read-only in Excel.
This spreadsheet gives data on all Philco speakers from 1929 through 1938.
If you have the late version 112 with two 47 oputput tubes, the original speaker was H-4.
The primary impedance of the original output transformer is 13,000 ohms. You will want an output transformer with a primary impedance of, or close to, 13,000 ohms and with a secondary that will match the 6 ohm speaker you want to use.
Mike and Terry have both given you excellent advice. You would do well to heed their advice. I can only add that any vintage radio will perform best when using the original speaker designed for it. There are exceptions, but given how the early Philco sets aren't comparable with 1950s high fideltiy, they usually do best with their original speaker or a replacement that is identical to the original.
H series speakers of this type often pop up on the auction site if the original cannot be repaired.
General rule is that pentode output tubes 2A5, 6F6, 6K6, 6V6, 41, 42, 46, and 47 all have a reasonably close plate load impedance and can use the same output transformer. The exceptions are if they are in a triode connection (plate and screen grid connected together) this lowers the impedance. Philco higher end sets use 3 #42 tubes in which two are in a triode configuration, they are abt 5.9K vs 12K in the standard pentode configuration.
Power triodes and ac/dc style output tubes (like 25L6, 35L6,43, 50b5,50C5, and 50L6 have a some what lower load generally around 1.5-2K per tube.
Ron, I went to the thread you linked and dlownloaded all of the zip files you listed there. They all worked fine after unzipping except speakers.zip. I could open the other Excell file, phildata.xls fine using Lotus 123, but when I tried to open Speakers.xls it said it couldn't open a password protected file. You may want to check that it is not so protected and re-zip and upload an unprotected version of it again if it is
Thanks for the replies folks, I'm going to tackle removing the cone as per Radioroslyn suggestion when I have time here. But getting back to my original question and the first post with my attached drawn up schematic, IF i had to replace the speaker, say with a PM speaker, Would I have to A) Change the output transformer to one that will be able to drive a 6 ohm PM? B) Replace the field coil with a 3.2 K resistor? If so what wattage?
C) Increase the 2nd Electro cap from 6uf to 12 uf? D) Do I have to do anything with the filter choke? am I on the right track here ?
(03-24-2018, 03:37 AM)mikethedruid Wrote: [ -> ]...when I tried to open Speakers.xls it said it couldn't open a password protected file. You may want to check that it is not so protected and re-zip and upload an unprotected version of it again if it is
Nope, I plainly stated in post #8 above...
Quote:Unzip the file and open read-only in Excel.
(emphasis added)
The file was made with and intended for Excel. It works as intended with Excel. It also works with LibreOffice Calc and OpenOffice Calc (both of these free solutions automatically open the file as read-only without asking for a password).
The file is password-protected for a reason (unwanted changes). In Excel, when you open it, just click the Read Only button and you're good to go.
(03-24-2018, 06:49 AM)dirkbuddy Wrote: [ -> ]Would I have to A) Change the output transformer to one that will be able to drive a 6 ohm PM?
Yes.
Quote:B) Replace the field coil with a 3.2 K resistor? If so what wattage?
Yes. The formula P = I squared * R will find your total wattage. In this case, we know from the
speakers.zip file that the original speaker was rated at 40 mA (0.04 A) of current and has a DC resistance of 3200 ohms. So,
(0.04 * 0.04) * 3200 = 5.12
You'll want to double that for safety, so a minimum 10 watt resistor is indicated to replace the field coil. Personally, I would go with 25 watts as that resistor is going to run very hot, but that's your call.
Quote:C) Increase the 2nd Electro cap from 6uf to 12 uf?
10 uF is probably enough, but you can go with 12 if you wish - and if you can find one. It's easy to find 8 and 10 uF capacitors, but not so easy to find 12 uF.
For both input and output electrolytics, I would replace with film for longevity.
Quote:D) Do I have to do anything with the filter choke? am I on the right track here ?
No, you leave the filter choke alone and
leave it in the circuit.
A) Change the output transformer to one that will be able to drive a 6 ohm PM?
If the primary is ok (not open) it will work ok. Is it a perfect match no but the audible difference is negligible. Typically the Philco spkr is lower abt 2 ohms.
B) Replace the field coil with a 3.2 K resistor? If so what wattage?
Yes it does has to have either a choke or a resistor there. Wattage would be determine be the current flow thru it. Doing some quick calculations the current flow thru the field coil and #57 (they are in parallel w/the hv) it comes to 5.4 watts. You'll want to double that so the resistor doesn't get flaming hot.
C) Increase the 2nd Electro cap from 6uf to 12 uf?
I would leave it unless hum is an issue. The field coil isn't doing much if any filtering.
D) Do I have to do anything with the filter choke?
No.
E) am I on the right track here ?
Depends some of us like to keep things original as possible so repairing the spkr would been top on my list rather than replacing it w/a pm job.
GL
ps started to post and made breakfast but didn't look to see if anyone answered your Q's before finishing the post.
As Ron suggested, I'd go with 20-25W resistor. Field coil is huge and gets warm, but resistor is smaller. Also a 25W resistor is not as big as one might think.