Retrofitting speaker for Philco 84 - Printable Version
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Retrofitting speaker for Philco 84 - Ctkid - 09-25-2022
Hello,
I previously posted about finding a replacement speaker for my Philco 84. Some of the responses mentioned using a speaker from a Philco 60 if I could not find an 84 model. I have been unsuccessful in finding an 84 speaker and the available Philco 60 sets are either to expensive or the speakers are bad. Is there a way I can retrofit this model with something else modern? I want to get the set working and am not that interested in keeping it original. The cabinet is in great shape and I would like to get it working and am new to the hobby.
Regards,
Bob
RE: Retrofitting speaker for Philco 84 - GarySP - 09-25-2022
Hello Bob. Is your speaker missing completely, or just in very poor shape. Reason I ask is that there are outfits that will rebuild your speaker. Take care and BE HEALTHY! Gary
RE: Retrofitting speaker for Philco 84 - falcon123 - 09-26-2022
ive got two ill never use. you can have them for free but the mail will cost a fortune. that said, if the field coil ohms out.they can be reconed. if the correct impedance voice coil is unavailable a different audio transformer can be used. it sort of revolves around the field coil to repair economically and the field coil doubles as a choke for the power supply
RE: Retrofitting speaker for Philco 84 - RetroRev - 09-26-2022
Nor sure if it's the same setup, but I just replaced the missing field coil speaker with a permanent magnet speaker in my 46-480 by putting in a 50w resistor of the same ohm value as the field coil.
RE: Retrofitting speaker for Philco 84 - Ctkid - 09-26-2022
Thanks everyone for your input. It looks like I will try and replace the field coil speaker with a permanent magnet speaker. Wish me luck!
Regards,
Bob
RE: Retrofitting speaker for Philco 84 - MrFixr55 - 09-27-2022
Hi CtKid,
It is rather easy to replace this speaker with a modern one. Use a 1100 Ohm 10 W resistor (20 W would be better. a 1000 Ohm or 1200 Ohm resistor will be OK also. Please note that this resistor will get real hot, so place appropriately. I would not worry too much about "impedance matching" if using an 8 Ohm resistor. If the output transformer is bad, any transformer with a 7 KOhm to 8 KOhm primary and 8 Ohm Secondary will be good.
This radio is a surprisingly good performer considering the novel 4 tube "Superhet" circuit with no IF, an "Autodyne" 1st Detector (Oscillator Mixer) circuit and a "regenerative" 2nd Detector.
If the set does not work after replacing the speaker, "ohm" the Oscilloator coil. It is common that the "tickler" (the coil in the cathode circuit) is open.
This set is not as historically significant as the Model 20, 71, etc., but is a historically important set IMHO.
This set was originally produced, in the throes of the Great Depression, as a "Loss Leader" to get people into the showroom so the salesman could "upsell" the Customer to a more expensive, higher performance set. This failed, as people bought these sets big time and over 100,000 were produced, and the set was made in various forms and upgraded models until 1937 (The 37-84 was the same circuit but used Octal tubes). The original price was $19.95, and dropped to $14.95. This forced RCA to drop the price of their "loss leader" R28 set. Many companies and dealers were angered by Philco producing this set.
RE: Retrofitting speaker for Philco 84 - Radioroslyn - 09-28-2022
John,
I have a question as I'm a hacker at this. If the vc impedance is 1ohm and it's connected to a secondary that is designed for a 8 ohm load. The primary is design for a 7000 ohm load. By connecting a load to the secondary that is 1/8 what it's designed for how does that effect the impedance (reflected) on the primary?
462ron - 462ron - 09-29-2022
I’m no engineer so with that said, I’m thinking if an 8 ohm spkr reflects back to the primary as 7K with its current transformer turns ratio, then a 1 ohm spkr would reflect back as 875 ohms or 1/8 the 7K value unless I’m missing something!
Ron
RE: Retrofitting speaker for Philco 84 - MrFixr55 - 09-29-2022
I think you are correct Ron and Terry. It is not as bad the other way (8 Ohm speaker on a transformer with a 1 Ohm secondary), at least for fidelity. However, empirically, they say it is better to run a tube style amp with a shorted output transformer secondary than with an open one. It is very important not to run a Pentode tube with a transformer with an open primary, the screen grid will melt and short. For solid state amps, the exact opposite is the case; one should never short the output of a solid state amp. As a kid, I replaced a slew or transistors for friends who hooked up 3 or 4 8 Ohm speakers in parallel.
Remember, resistance and impedance are 2 different things. I am not a radio engineer (Ask me questions about clinical chemistry, I'm your guy) but do understand a lot of it. I think that the members who are General or Extra Class HAMs could explain impedance better. It has to do with reactance, inductance and capacitance as well as resistance. There is an "Impedance ratio", however, if you try to run a transformer from a 6L6 (impedance of 2500 Ohm, 6W output) output transformer on a 50L6 Output stage in an AA5 (2500 Ohm impedance, 1 W output), the 50L6 will be drawing more current then with the correct speaker due to the resistance of the 6L6 transformer being lower than the one designed for an AA5 using a 50L6 even though the impedances are the same.
I think that if the schematic lists a value of 400 Ohm for a primary and 0.89 Ohm as a secondary, they are describing the DC resistance and not impedance. After all, they give the Field coil resistance in Ohms, not the inductance in Henries. (Is that the plural of Henry?)
By the mid 1930s, speaker voice coils were either 3.2 Ohm, 4 Ohm or 8 Ohm impedance (at 400 Hz or 1KHz). I don't know what the original Philco "Pie Plate" or G E "Kellogg Rice" speakers as used in the Victor early RCA, Majestic, etc. units used.
HAMs, jump in here (and save me).
RE: Retrofitting speaker for Philco 84 - Simon Wa3uay - 10-02-2022
I learned enough about impedance (and lots of other stuff) to get my ticket way back in the day, but not being in the trade, or using the math promptly forgot it. but is does has to do with matching circuits so they can talk with each other...
I bet Mike knows a lot......
RE: Retrofitting speaker for Philco 84 - radiorich - 10-02-2022
Hello Guys,
Anyway I know when I was building a little tube amp that member from ARF Peter came up with to get the right impedance the circuit design called for a 8 ohm speaker .
Simon I bet Mike would be your Man !
Sincerely Richard