[morzh]

Yes 5W is plenty for a single 47.

[MrFixr55]

Hi Bob,
Any output transformer from any radio of the period that uses a 2A5, 42 or 6F6 should do, as they all have the approximate load resistance, plate voltage, current, etc. as the 47. The 2A5 was the logical next step on evolution of the power pentode from the 47. Other than filament (heater) voltage and, in the case of the 6F6, pinouts, the specs are the same.

For RCA Radios for which I have Ryder Schematics that list output wattage of a single 47, 2A5, 42 or 6F6, the maximum out is 5 Watts. Typical "undistorted" Output is 2.5 watts. RCA's definition of "undistorted" at that time was 5% or less. The RCA Receiving Tube Manual lists power output for a 47 to be 2.7 Watts at 6% distortion, so a 5 Watt transformer should do.

[Greg]

Hi Bob,

If you are powering the radio through an isolation transformer (think you are), how do the line caps provide an RF path to ground?

I'll have to try popping the capacitors out of the Bakelite blocks from the "top". It looked pretty slick.

Like the videos.

Greg

[Bob Andersen]

Bad news. Field coil is open on the used speaker I've been inquiring about. We struck a deal for a lower amount and I'll see if I can do something with it.

Yes, I am powering it through a Sencore PR57 isolation / variac. That is an excellent question and I wish I had an answer. Perhaps there are some line filtering caps on the isolated output inside the Sencore ?

[morzh]

Greg


Perhaps through parasitic capacitance of the isolation xfmr?

[Bob Andersen]

I've moved on to resistors now and was surprised to find this 70K open. It's part of a voltage divider that feeds all the 24 screen voltages.

Also, a 490K resistor going to the RF tube AVC cap had doubled in value.

I have not fired it back up yet and am curious to see if the is any performance improvment.
[Image: https://live.staticflickr.com/65535/5156...5433_c.jpg]

Speaking of 24 tubes. I noticed the tube chart and schematic all call for 24s while I'm using 24As. Am I correct that 24As are superior and completely compatible for this radio?

The Philco 90 speaker arrived today. The cone, spider and frame are in good condition, but alas, the field coil is indeed open.
[Image: https://live.staticflickr.com/65535/5156...a8a7_c.jpg]

[Image: https://live.staticflickr.com/65535/5156...2cbe_c.jpg]

I removed some of protective wrappings in the hope of finding a break but no luck.
I also experience sticker shock when I priced a one pound spool of 36 gauge enameled magnet wire at $50. When did wire become so expensive ?

I'll start slowly unwinding and hope I find the break near the outside. If not, I'll try to get the pole out and rewind it.
[Image: https://live.staticflickr.com/65535/5156...9d43_c.jpg]

[morzh]

The coil wire does not look burnt so maybe, just maybe if you are lucky, it has rotted or broke right at the top.
If not.....you seem to have a very good intact cone and the spider. Try to remove it without damaging, if you intend to press out the centerpole in order to remove the coil and rewind it.

And....copper has been expensive for quite a while. So is the magnet wire, being practically copper by weight. Pure copper on Amazon is $25 a pound.
As a commodity, it is about $5 a pound.
And you need to draw the wire, coat it with lacquer......

The resistor on 20/70/90 almost always double in value. I have seen maybe 2 or 3 open ones, but if not, teh value is 70 to 100% up. With a notable exception of large divider resistors (10K/16K, a 2W and 1W ones): the large one almost always (in my experience) goes down.

[Arran]

I found these: https://www.ebay.com/itm/371652497041,  https://www.ebay.com/itm/371183860250,  https://www.ebay.com/itm/363090262105 I don't know whether they are suitable or not?
Regards
Arran

[morzh]

Arran

As long as the gauge is correct, I do not see a reason why not.

[Bob Andersen]

A new arch support and feet just arrived courtesy of Steve Davis. Thanks!
[Image: https://live.staticflickr.com/65535/5160...175e_c.jpg]

[Phlogiston]

You can probably remove the debris from the field coil and carefully wrap with masking tape. I know that the coil is close to the pole but there is usually enough room to do this. Then you can paint it black.

[Arran]

It looks like the second generation pie pan style speakers have the rear bracket attached with those metal pins that you can drive out, but then you would have to press out the pole piece to remove the coil. Fortunately it looks like it's easy to remove the cones in speakers like this as it has a series of screws holding the cone, and cardboard gasket, in place without glue, so you can access the pins. Still a little acetone around the rim will help should it stick. I was just wondering whether the wire I found on fleabay was being correctly represented, is it actually #36 gauge, not whether #36 gauge will work? From what I read it's not hard to rewind a field coil, just time consuming, finding the break is always the first course of action.
Regards
Arran

[morzh]

Arran,


My experience with K pancake speakers is that EVEN if the cone looks great, it cannot be handled. Too brittle. Some kind of different paper from the one that was used later. It is dense and glossy vs later matte and filter paper-like.
So if one attempts to remove it, which will probably involve separation of the outer lip from the metal frame, I am not sure it will be removed as one piece.

[Bob Andersen]

I plan on digging into the field coil tonight to look for a break. If I can't repair it, I'll go with another speaker for now. Getting that pole piece out is a little beyond me at the moment.

Assuming I can mend the field coil, perhaps I should go over the cone with some diluted Aileens Tacky Glue to reinforce it ?

[Bob Andersen]

Well this project is getting very frustrating. I finished with the recap, replaced all out of spec resistors and put in good tubes.

After a few minutes of playing, the volume drops about 80%. Nothing will get it back other than letting it cool off a while. I've replaced the detector mica cap, tried swapping tubes, wiggling, tapping, cold spray. Nothing helps. Rechecked all the solder joints.

B+ stays about the same when the volume drops. Have not checked all the grid voltages yet.

My next thought is to replace the few remaining dog bone resistors.

Any other ideas ?