Resistors used in Model 20 - Printable Version
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Resistors used in Model 20 - MrFixr55 - 09-26-2023
Hi Pholks,
I am working on a Philco 20. The unit is all original and works, but the Customer is looking for reliability. I have one myself, but I think that it was worked over in the late 1930s or early 40s with replacement caps external to the Bakelite blocks (Whoever repaired this at least did isolate the bakelite caps). The resistors were all the 2" long brown jobs with modern bands using the modern color code, so I don't think that those were original. So for mine, I just replaced all the caps that were replaced lo those years ago. I had to rewind the RF trans between the 2nd RF and Detector due to an open primary. Mine was missing the speaker (and cabinet) so mine plays through a modern 8" speaker in a rather beat up cabinet that I got off ePay. The Customer has a rather nice cabinet of the later style (The 2 columns on the lower front adds a lot to the cabinet IMHO), and he DOES have the pie plate speaker. I am not sure which style spider his has.
Are the glass cartridge style resistors (They seem to have ceramic-like bodies and metal end caps) original equipment in the Philco 20 or should they the ones with wound wire capping the ends? Which of these two styles are referred to as dog bones? (I think it is the type with the wirewound ends, but I defer to you experts.) If replaced with modern resistors, what wattage should be used for the cathode resistor for the detector, the 250 Ohm resistor that is part of Bakelite Block #8, the 1st audio's grid resistor and the dropping resistors for the detector plate voltage? Are half watters sufficient? (The Customer has no interest in the expense of stuffing the blocks or having original resistors but is looking for reliability). The RF Coil Plate windings are good, but original, so I will likely rewind them to prevent the failure that is common to Philco coils of this vintage.
Tubes were all replaced at various times and are of the "ST" variety. The 24As are Philco brand but are marked "For replacement use only".
How reliable or unreliable is the wirewound voltage divider and the main filter cap? I am tempted to replace both, as the customer intends to use this radio.
I would have done John Grady's and Ron R's cap coupled mod to the interstage audio transformer but the primary is blown and someone already "bridged" the interstage transformer.
Thanks for the help!
RE: Resistors used in Model 20 - morzh - 09-26-2023
I think the one having heavy solder caps at the ends are the dogbones, but I have seen this term used towards the painted ones with flat ends also.
The ceramic ones have carbon rod inside. Not sure what inside the other ones.
As I remember, other than the wirewound divider, all of them were 1/2W. I used 1W for all of them, as well as everywhere else, short of the places where I feel 2-3W are warranted.
The cathode resistor for the detector could be any wattage - the voltage there is very small and so the dissipation would be negligeable. 1/4W is plenty.
RE: Resistors used in Model 20 - radiorich - 09-27-2023
Hello Mr Fixr ,
I know on my 38-10T it has one 3 watt Dogbone resistor !
Myself I have a 20deluxe in my collection .
Sincerely Richard
RE: Resistors used in Model 20 - MrFixr55 - 09-27-2023
Thanks Morzh and RadioRich for your responses. Do you have any thoughts on the reliability of the original oil /paper potted filter caps and the wirewound?
I think that the 20 Deluxe is a very handsome radio, second only to the iconic Model 70 among Philcos of this era.
RE: Resistors used in Model 20 - morzh - 09-27-2023
ALL CAPACITORS, with the exception of the micas, HAVE TO GO.
This goes especially to the big can in 20.
Here's my recent experience of potted cap cans: the RCA R-73. When measuring the capacitance, I found the 10uF caps were still 10uF, and the others were fairly close in values to their original one, with the exception of 2 or 3 of them.
So I decided to try to power it and see how they work (RCA can, unlike the Philco 20 or 16 one, is a very hard to take out). Well, I had a huge amount of hum. I did not remove the can, I just cut the wires and replaced them right to the places where the wires from the can would go. After that the radio performed just fine.
And, in Philco 20, it is VERY easy to remove the can, and you do not even have to desolder the panel. The can itself comes out from the metal shallow part around it that also holds the phenolic panel with contacts; the wires to the rivets get cut the same way as you would in a regular backelite block.
Removing the tarred innards may be a bit messy and challenging, but this would be done with the can conveniently removed from the chassis. The replacement caps go right in the panel, and then the freed of the tarred caps metal shell simply goes back.
If it were not for the tar block being a bit stubborn to come out, I'd say this one is no harder than to restuf a regular small backelite cap, and considering the number of caps addressed in one fell swoop, it is even less time-consuming.
As for the looks, and some folks would not even be able to tell 70 and 90 apart, were it not for the feet on 90 instead of the continuous trim base on 70, I prefer the 90 to se as it has a bit different top arch shape, it peaks a little, and to me it is a bit nicer looking than the 70.
The 20 Deluxe is indeed very handsome, I have it. I think, I have most of the 20s: regular, deluxe and the console with the grillwork (no, I do not have the 220 with the phonograph).
RE: Resistors used in Model 20 - Arran - 09-27-2023
Mike;
As a personal preference I always preferred the look of the Model 20s over the later Baby Grand cabinets, though the chassis is only a TRF, they were obviously built to a budget since they use 171As in push pull rather then the more modern #45s, but in a table model that's more then enough. The model 21 is the same set with a Coombs styled cabinet, which was eventually used with the model 70 chassis, the 90 cab is obviously larger, and that may explain why it has a peak, likely to make it look less bulky, but also to offer a perceived value over it's cheaper sibling.
Regards
Arran
RE: Resistors used in Model 20 - morzh - 09-27-2023
Arran
>>they were obviously built to a budget since they use 171As in push pull rather then the more modern #45s,
What are you talking about: 71A sells for $100 today wheres a 45 is no more than $20
RE: Resistors used in Model 20 - Ron Ramirez - 09-27-2023
One has to take the Philco part numbers of the original resistors used in model 20, and compare that with known data from vintage Philco paper, such as old parts catalogs.
Here are the resistors used in Model 20, excluding the large multi-section "B-C" resistor
(12) - 50K - Part No. 4237 - 1W
(15) - 250K - Part No. 3768 - 1W
(17), (20) - 500K - Part No. 3769 - 1W
(18) 100K - Part No. 3767 - 1W
So, all those small resistors under a model 20 chassis were originally 1 watt each.
I agree, most if not all of them are probably overkill, wattage-wise. But why take a chance? Just replace them with new 1 watt resistors. Your radio will be happy, and so will you.
RE: Resistors used in Model 20 - radiorich - 09-28-2023
Hello Ron,
Wow 1 watt I guess Philco was thinking not like some other companies with no margine in the wattage !
Sincerely Richard
RE: Resistors used in Model 20 - morzh - 09-28-2023
As I said, I use 1W across the board, as this lets me not having to think about what wattage I need and where, short of, of course, the wire-wound dividers.
Also 1W axials ares also more convenient, having thicker leads. Even though none of them as thick as the original resistors' ones.
This said, anything above 200K does not need to be 1W (even if you have a dropout of 200V across it, the dissipation would be 0.2W), and so if you have the needed value of 1/2W rating, this will do just fine. But if you don't, buy 1W resistors.
In radios that use larger resistors divider (like that 13K-15K-10K-10K one ins a 16) where the 1st resistor, like the 13K, is pretty big (looks like a 2W) I use 3-5W size.
RE: Resistors used in Model 20 - GarySP - 09-28-2023
Mike, are you sure about the price of 71A's? Findstube lists new 71A at $21.00, and 45 at $41.00. I hope your price is wrong ... I've got three radios that use those 71A's! Take care and BE HEALTHY! Gary
RE: Resistors used in Model 20 - Ron Ramirez - 09-28-2023
Yeah, I started buying 1W resistors exclusively a couple years ago as I replenish my resistor stock. They are still much smaller than the original dogbones. For that matter, some modern 1W resistors are even smaller than older 1/2W carbon composition resistors.
> Findatube lists new 71A at $21.00
That reminds me... I need to order a few tubes from Bob D. ...not 71As, though.
RE: Resistors used in Model 20 - morzh - 09-28-2023
Gary
I was kinda tongue in cheek with this one
But I do remember people complaining about 71A getting up in price, in some places being offered to $100 (not that I even would go for that kind of price).
Plus there was a fad (I hope, short-lived) building low power triode Class A stereo amps, using ... 71A. Maybe that affected the price too. No, of course it is not worth $100.
RE: Resistors used in Model 20 - MrFixr55 - 09-28-2023
Thanx for all of the advice, folks. I did, in fact, recap the big can on my own 20 a few years ago; it was exactly as you said. I had to wait for Ms. Fixr to be gone for the day before using the oven to gut the can. She would not be happy with me putting radio parts in the oven, or even the kitchen (or even the house). Since my "customer" is not looking for accurate restoration, I will just cut the wires inside the cans and hang them on the chassis side. I DO hope that the original resistors are good, as the chassis is totally unmolested.
As for $100.00 71As, I will sell to anyone for $50 (only kidding). They actually sell for about $15 - $20 on ePay----Wait!! What the Heck!!!
2 for about $15-$20, but a bunch for between $35 and $70 apiece, and they don't seem to care if they are globes or STs, and more than $200 for a trio of Arcturus Blue 71As. No bids on any though, so maybe the price IS going up, but I don't know why.
I have a schematic for a DIY 71A SE Amp and there are several on the internet. There seems to be a following for "Single Ended" amps, especially those using the Western Electric 300B. There are some on the internet that claim that if you have a very efficient speaker, that 71As, even at 0.7W (700mW) sound "as sweet as a 300B" and nicer than a 45 or 2A3. In fact there is a circuit with switches to select a 300B vs a 71A as the installed output tube. Since I don't have the ears or money to tell the difference, I am not exactly running out to build one of these tomorrow.
RE: Resistors used in Model 20 - morzh - 09-28-2023
>> I DO hope that the original resistors are good
Never saw thtat in a 20 (have restored 3 of them). All were off.
Thing is, 20 used the older style, large metal caps (made of solder) and ceramic body resistors. When I opened one of them, it was a thin carbon (or carbon covered?) rod imbedded into those solder caps. This cannot possibly be very stable, even though over the course of my dealing with those I saw some being righ in the value, especially smaller ones.
I distinctly remember one #15 resistor (250K) being 800K, and another fully open.
Maybe you get lucky, but her's the thing: it is few of then, easy to replace, and they will sooner or later (thermal cycling) go totally bad. I'd replace them.