Hi All:
I am new to the forum but have been collecting for many years. I am restoring a model 20 catherdral and have a couple questions.
Does anyone know what the DC resistance of the filter choke should be?
On the voltage chart it states the grid voltage on the 71A tubes shoul dbe 50 volts, is this correct? Mine measures 7 volts and the radio works fine but I think it should be more.
Last one, the speaker is in perfect condition but sounds very poor and I would say that is due to the fact the cone has almost no movement, the spider seems to be hard as a rock. Would anyone know if this was a flexable piece 80 or so years ago?
Thanks
How are you measuring the 71A bias voltage? These tubes are biased with a combination of fixed and cathode bias, so you can't measure the grid bias voltage with respect to the chassis. It must be measured from the filament to the grid instead.
Actually, all of the model 20's voltages are measured with the filament as reference, as a tube socket test adapter was used for the measurements.
Hi Mondial
I was measuring to ground and all other voltages are within a few volts of spec. I did a voltage drop mesurement on the cathode resistor and applied some ohms law and total current is 30ma so a little less than the spec says but I think that will be just fine.
Thanks
Hi Mondial:
I did the voltage measurement with reference to the filament and the grid voltage on the 71A tubes is exactly 50 volts as per the spec sheet.
Thanks for the heads up as I did no know that.
Yes it is a bit confusing since the measurements are not referenced to chassis ground. With heater-cathode tubes there is not much difference, but with the directly heated filament of the 71A, the self bias resistor throws off the reading.
As to the speaker problem, are you sure the spider is stiff or is the voice coil rubbing on the pole pieces? I have a 20 speaker that wont move at all because of this.
You could try removing the screw in the center of the spider so it is free to move and see if the cone movement is still restricted.
Now that I have figured out how to attach pictures here are the chassis shots of the 20 I am working on at the moment. All bakelite blocks and metal can caps have been restuffed. All resistors have been checked and replaced with old dog bone style if needed except the one which I didn't have so had to use a new style. All the tubes are Philco engraved base globes except one of the 171A tubes is an RCA. It seems to work really well but still have a speaker issue to work out.
The best way to see what you can hear is to unsolder the output transformer's output from the voice coil, take a regular PM speaker and solder it to the transformer's output. The lowest possible voice coile impedance speaker is the best choice, as the impedance of Philco speaker is 0.7 Ohm and most speakers today are 4 or 8 Ohm which is an order of magnitude. It will still sound OK. So you could see if the speaker could be improved.
(This post was last modified: 01-09-2014, 12:11 AM by morzh.)
Thanks Morzh, I was going to give that a try but the wires are all inside the speaker frame and not easy to get at. That is the plan though and will keep you posted.
I have two speakers lying in front of me - one is unmolested 20 and one is unmolested 70 (they are largely the same).
The wires coming from the output winding to the voice coil are outside the speaker frame, and then they solder to two solder posts to which the voice coil solders from the opposite side (and yes it comes from inside but you do not need to touch it). Is this not what you have?
Again, you ONLY have to unsolder the output transformer's wires leading to the voice coil, (and as you probably understand you have to keep the speaker plugged in in the chassis anyway as it bears the transformer and the field coil.)
Hi Morzh:
Yes on mine the cable comes from the radio to the speaker through a grommet and all of the wiring is inside the steel frame of the speaker. I did manage to get it all apart last night and try a different speaker and it sounds WAY better and you dont have to crank the volume near as much so I think I will send this one out to be re-coned.
One other thing I found when restoring the chassis was the center tap on the driver transformer was connected to ground and not the bias resistor like the schematic shows. On very close inspection it looked like a factory connection but I moved it anyway as I was puzzled by no grid bias on the 71a tubes and they were running very hot which is not normal. When I took the speaker apart I noticed a lot of the original potting wax was melted out, HUMM makes a person wonder how long it was like that, maybe right from new.
Actually speaking of bias, whether connected to Chassis or to B the centertap will be sufficiently negative towards the cahodes (filaments) as, as you cn see, the filaments connect to the C voltage which is biased towards B using the 1-2 section of the big resistor #26 which is 1.4kOhm. The output tubes being the heaviest drawers will likely self bias enough for the grids to be negative. However you are right to reconnect the centertap to B as this is what is required by the sch.
Which brings up this question: is the mentioned resistor 1.4k as required?
(This post was last modified: 01-09-2014, 03:34 PM by morzh.)
The resistor measures at 1.44k, really close per my Fluke meter. The most notable difference is the output tubes run much cooler to the touch. I don't consider the 71a tube a high power tube at all and they were pretty hot before moving the center tap.
They still are warm to the touch but better than before.
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