Philco 41-280 question

[DavGoodlin]

I am working on a pretty standard set here. After a few easy restorations like this one, its a stumper. I had an open audio output transformer in a 40-195 once, which was obviously easy to pinpoint but that set used a larger trans and the more powerful push pull 42s . This set uses 41s and has very low audio output, hardly normal for these.
A check of DC voltages on both 41 output tubes is normal and seem to correspond with the values in the tube manual. The bias is correct and 7C6 1st audio scopes out fine for signal voltage gain.

What I dont understand is that, while I have 203 volts on both the screen grid G2s and 190 volts on the plates, the
original 32-8120 output transformer primary measures 2250 and 1280 ohms DC resistance when the schematic indicates 190 and 170 ohms! The speaker field coil is right on at 1090 ohms, so what is up here?
Wouldn't a transformer fail mean its shorted or open? Can it be partially open?

[Reece]

All I could think of would be that there are bad connections from the leads to the windings or high resistance "almost opens" in the windings, just tenuous stuff. If you clip on your ohmmeter and wiggle the wires going to the primary, do the readings change? Are any windings shorted to the transformer frame?

As a test I'd sub in any push-pull transformer I had around just to see how the output stage worked with it.

[old_coot88]

Originally Posted by Reece

...As a test I'd sub in any push-pull transformer I had around just to see how the output stage worked with it.

Lacking a p-p output xfmr, just for checking, you can use a known good power transformer with a center tapped HV secondary. Use that as the primary, with either the 6.3V or 5V winding driving the voice coil. (been there, dun that)

[DavGoodlin]

Thanks ! I wouldn't ask if I dint think something odd was going on.

I just ordered two P-P 8 watt transformers from Antique Radio Supply, the only place that seems to have them. The data sheet lists impedances, not DC resistance, so Im taking a chance that the 20 ohm difference spec'd for the two halves of the primary are not critical. The 41 tubes do not employ a phase splitter stage, but have an odd circuit where one G2 is to B+ and the other G2 via a 3.9K resistor with feedback capacitor to the other tubes' G1.

Reece, I think that partial open is correct. I pulled an output transformer off a speaker I bought for a Philco 42-1008, which had the original one under the chassis. Testing that one and the 42-1008s, both were open alsodue to the tiny wires, dried up wrapping and loose leads allowed to move.

Old Coot, I must try that. I have a 240/120-6.3v filament transformer.

[old_coot88]

Originally Posted by DavGoodlin

Old Coot, I must try that. I have a 240/120-6.3v filament transformer.

Um.. you're referring to single-ended application, right? Not sure that would work too good, because the line-voltage primary won't present high enough impedance to the AF output tube.
Whereas the center-tapped HV winding in a power xfmr is a usable match for p-p output tubes (and the split winding supplies flux cancellation).

Power trannys will pass decent broadcast-quality audio, with sharp roll-off above 5Khz or so. As a kid scrounging make-do parts for projects, i used them strapped back-to-back for Heising modulation in transmitters.

[wa2ise]

Originally Posted by DavGoodlin

The 41 tubes do not employ a phase splitter stage, but have an odd circuit where one G2 is to B+ and the other G2 via a 3.9K resistor with feedback capacitor to the other tubes' G1.

The cost cutting went too far there. That arrangement won't sound that good. I'd install a real phase inverter tube somewhere, probably under the chassis.

[old_coot88]

Originally Posted by wa2ise

The cost cutting went too far there. That arrangement won't sound that good. I'd install a real phase inverter tube somewhere, probably under the chassis.

Short of adding a real phase inverter, another way to get p-p output on the cheap is to use a grounded-grid, common cathode setup.
That's where both tubes' cathodes are strapped together and unbypassed.
Tube 1 is driven in the totally conventional manner (ie., thru a coupling cap from the first AF stage).
Tube 2 has its control grid (G1) grounded. Since Tube 2 shares common cathode with Tube 1, it is driven 180 degrees out of phase via its grounded grid. Ergo, push-pull.

The cathode resistor's value is chosen to bias the tubes' current at (roughly) the most linear spot on the curve.

This method has a few downsides, but offers real push-pull with minimal parts.

[DavGoodlin]

I had a single ended transformer that measures 260 ohms on the primary. I disconnected one lead of the present transformer from the 41 plate and connected this transformer to B+ and plate, then speaker. Being that this is single-ended and half the power, the volume is about the same. Replacement transformer is in the mail, some improvement needed.

Thanks for the suggestion on modifiying push-pull sounds interesting and may be the only option to improve over original.
I was also considering an interstage transformer with a center-tapped secondary, allowing the bias voltage, -12 volts to go on the tap. This is what was done using type 42 tubes for 118X and 501X models but needs an extra tube to drive the interstage primary. A tube and a transformer is extra $$$$ that may not be expected on a cheaper set.

Obviously, the radios equipped with PP 41s were economy models compared to the ones using the PP 42s.
This model does receive BC and SW very well with a 50-foot antenna outside, its the audio quality that needs help.

[old_coot88]

It might be an interesting experiment to see if the common-K, grounded-grid method would provide any benefit over the current setup. One possible disadvantage might be volume reduction due to the unbypassed cathodes.

I did build a 'hi fi' amp many decades ago using a 6SJ7 driving 6V6s in common-cathode, grounded grid mode, and it had plenty of volume.

[DavGoodlin]

After getting the output transformer in the mailbox yesterday, I had to try it first.
No improvement in volume, yet it measures 240 + 240 ohms on the primary - a huge change from the 2.8 K on the original.

Then i started examining the components between the 1st audio amp 7C6 and output tube grids and it did not exactly match the Riders 12 schemo....then I saw it - the "alternate audio circuit" on another page (12-64), showing a type 37 triode, a phase splitter for this model only.

When I first pulled the chassis for re-capping, I noticed an empty 5-pin tube socket, under the chassis, there were no filament wires on the socket. The tube chart pasted in the radio showed nothing in this position either. To confuse things further, an octal socket was installed by someone to use a 6X5 instead of the 84 rectifier tube.

Luckily, I had a Philco-branded 37 tube to plug in. Connecting the filament, then checking the resistors of this "found" phase splitter circuit, eased it up on the variac.

The volume is good nowbut 3 of the 6 speaker glue joints I had done without using construction paper, now sounded like a bee was in the speaker. After attending to the remaining speaker punctures like the first three, it should sound pretty good.

[DavGoodlin]

Correction to thread title: This is a model 41-290 under test-repair, which has the phase splitter type 37 triode stage (non-amplifying inverter), the Philco 41-280 and most other similar models don't have the tube stage. This makes the 41-290 a stand-out among these models.

Unfortunately I have another console (42-1008 with record player) that does not have the phase splitter for the two type 41 outputs. It does have a second 7C6 for the cartridge pre-amp. This will be an experiment set and I will try the common grid solution that Old Coot 88 described

[DavGoodlin]

Hi All,
The speaker has not taken the repair (patching-cement) too well, and is coming apart due to paper cone being too brittle.
I should get it re-coned but cannot justify the $$$ since I have a Zenith Speaker (P/N 49H416) from an unknown late-30s model i picked up a while back.

The Zenith's field coil measures 580 ohms compared with the 1100 ohms of the original Philco 36-1523.
While it sounds just as good, the power supply voltage to the audio output transformer that was close to the Riders schematic value of 190v with the 1100 field is now 245 volts with the 580 ohm field. This increases the plate voltages to the AF amp and IF stages as well.

I was considering adding a series resistor to make up for the reduced field resistance.
It appears from Zenith's schematics that the Zenith speaker was intended for 6V6 outputs.
This radio has lower power PP #41 outputs, with ratings similar to a 6K6 per the RCA tube manual.

My question is: Does the lower field resistance correspond to the apparent higher power rating of the speaker? If so, would a dropping resistor be a good idea to bring the B+ voltage down to the specified 180 volts? I had also replaced the 6X5 with 1N4005 rectifiers, so that's also a factor.

Regards, Dave G

BTW - The p-p output transformer mounted on the Zenith speaker works as good as the replacement I used, noted above in post #4. I plan to remove the one under the chassis.

[Electronic M]

A dropping resistor should work. An old timer who REALLY knew his stuff once advised me that if I encounter an open field coil that I could use a similar sized PM speaker with a power resistor of approximately the same value as the field coil to replace the field coil. So logically a resistor in series with a lower resistance field coil should also work.

The difference in winding value was probably as much or more so a function of overall power supply design(ie. voltage, load current, and filtering) specs than of audio output power....Unless the two sets are identical in all but the PS and audio stages, but that is unlikely.

[dieseljeep]

Originally Posted by Electronic M

A dropping resistor should work. An old timer who REALLY knew his stuff once advised me that if I encounter an open field coil that I could use a similar sized PM speaker with a power resistor of approximately the same value as the field coil to replace the field coil. So logically a resistor in series with a lower resistance field coil should also work.

The difference in winding value was probably as much or more so a function of overall power supply design(ie. voltage, load current, and filtering) specs than of audio output power....Unless the two sets are identical in all but the PS and audio stages, but that is unlikely.

I've subbed a P.M. speaker for a field coil speaker, but had to increase the value of the filter caps. An R/C filter circuit, can never equal a L/C circuit, as far as hum reduction is concerned.

[Reece]

Before you give up on that speaker: get some fabric glue from a ...fabric store! This looks like ordinary white glue but remains somewhat flexible after drying, and becomes at that time waterproof, although it is soluble in water out of the bottle. Squirt some in a container and thin with a little water, enough that it is brushable but still thick. Brush it on the speaker cone all over, outer surround also, use it to reglue down your repairs, too. Keep it out of the voice coil, of course. Leave the speaker facing upwards to dry overnight. I have had speakers with deteriorating cones where there were myriad little cuts and tears invisible unless you pushed on the paper, and that rattled in operation, become cured with this treatment.