[Jordanbkc]

Chas - that's a good article. I'll be reading it tomorrow. Wore out working today.

[Jordanbkc]

Chas - that was an interesting article.

One thing I got from it. You could pick a modern 8" speaker of 4 or 8 ohms, and determine the output transformer ratio to match the tube. And fortunately it lists the 6F6 tube I have in my radio (assume 6F6G is the same?). 

One thing confuses me.  In the Philco library for the 32-7019 output transformer (57:1) it list tube single 47 or 45 under heading "tube".  That's different than the 6F6.  

Bryan

[Jordanbkc]

Chas -  another question is about the article & calculations. The 6F6 tube date sheets indicates 7,000 ohms.   Data for the 37-60 radio says an S-7 speaker with 1 ohm voice coil.

57:1 ratio for output transformer would equal 57 squared or 3,249:1 impedance ratio.  A 1ohm speaker would present a load of 3,249. That doesn't match 6F6 tube's 7,000 ohms.

Looks like the voice coil should be 2 ohms, or the transformer would be 83.6: 1 turns.  (Square root of 7,000).

Bryan

[Jordanbkc]

And last thing, I'll probably be out today.  Found this neat site later that verified my math.   

So what is going wrong?  S-7 is not 1 ohm as listed?   Variable impedance according to frequency?  32-7019 not 57:1 ratio?

[Jordanbkc]

Little more time and the wife hauls me off to shopping.  I tested the speaker voice coil and other connections with the speaker coil wire cut loose. Figured I'd be replacing the transformer anyway. Voice coil is 1 ohm.

[MrFixr55]

Hi Jordan,

The following Primary Impedances at a 250V plate supply for Class A Single End output (as opposed to Push-Pull) from the RCA Receiving Tube Manual RC13 (1937) may be helpful:
For 41 or 6K6 7,600 Ohm
For 42, 6F6 or 2A5 7,000 Ohm
For 6V6 5,000 Ohm (from RCA manual ver R24)
For 45 3,900 Ohm
For 47 7,000 Ohm

I would have to leave it to the Philco PhDs, but I am surprised that a 1936 or 1937 radio has a speaker with a voice coil impedance of only 1 Ohm. I am also kind of surprised that the same transformer is specified for a 45 or 47 tube as their load resistances are so different, unless the speaker used for a 47 had a somewhat higher impedance than the speaker used for the 45.

Voice coil (and transformer) resistance and impedance are 2 different things. Impedance is usually measured at a specific frequency, as it may differ for different frequencies.

[Jordanbkc]

Mr Fixr55

Thanks.  Sometimes I forget about the impedance vs resistance thing.  I did measure 1 ohm resistance at the voice coil itself with my meter.  The output transformer chart was from the Philco library.  In my radio is a 6F6G.  Would assume would be shooting for 7,000 impedance.

Just trying to find what ratio a new transformer would be.

Bryan

[Chas]

The impedance of the hum bucking coil must be considered as that is in series with the voice coil too.

As stated, do NOT separate the one voice coil to humbuck junction.

I would shoot for a primary impedance of 7 to 10k ohms with a secondary or 4 to 6 ohms impedance.

This would be a simple open frame transformer and should be available, still. If near that value is not available, multi-impedance one may cover the desired range too, but will be a somewhat more costly.

Settle for less than 5 watts, more likely three watts.

GL

Chas

[Jordanbkc]

Chas - Wasn't planning to run the speaker that way. I knew I'd have to at least replace the transformer, so I cut the voice coil wire at the terminal, then measured resistance with my meter. Got 1.1 ohms.

If I were to find an output transformer (single not push/pull type), it would have to be 7K impedance primary, 4 or 8 ohm (impedance) secondary rated 3-5W and hooked up to a NEW 4 or 8 ohm speaker (8 inches diameter mounting holes)??

Y'all keep it simple, new at this.

Bryan

[Jordanbkc]

Last question for the evening. Why are so many of these transformers bad? Pure age, or should have been a higher rating (wattage)?

[Chas]

There should be a capacitor across the primary of the OPT. This cap as OEM would be a metal foil/paper, often it fails. Then, the transformers windings are at risk from HV flash-over cause by a static burst received by the radio. During static, the plate current ceases for a tiny fraction of a second, the magnetic field in the transformer core caused by normal plate current collapses and a voltage pulse rises so high without the cap the winding flashes over through the glassine layers. Often the radio keeps working but the arc has set in motion by the creation of acids, that, slowly eats at the wire at the point of the arc until the wire fails and cannot pass current, the transformer has failed.

Often this cap is part of a high frequency tone control, combined.

The cap should be replaced with one with known dv/dt specification to absorb spikes without damage. Often that is not a concern just as the correct use of the hum bucking coil is often ignored.. The radio will play in either circumstance but with a low level hum and possible loss of a new transformer. Common metalized caps do not have a published dv/dt rating. I have seen these fail open when misapplied in a pulse situation, just one "hit" and mf value cut by 1/3 and the generation of faint ticking sound in an audio system as the broken metal film creates spikes of its own.

Chas

[MrFixr55]

Chas, great info on the hf bypass cap. Jordan, I agree with Chas, that if you think that the speaker impedance is really that low, shoot between 7 and 10K for the transformer primary, more toward the 10K.

One other thing. If the radio was operated with an open output transformer primary, the 6F6 may be trashed. With the primary open, the screen takes all the current, and it was not designed to do so. The screen will glow bright yellow to almost white hot and eventually short to the suppressor or to the control grid, both being really BAD (burning out the cathode resistor, the field coil, the power supply, etc.). I saw this on a Philco 89 with a nice Globe 42. Fortunately, I caught it in time.

[Jordanbkc]

Questions: 
1. Is part 40 the capacitor across the primary?  I have a new .03 (1.6KV) cap.

2.  If I find an exact replacement 32-7019 output trans I know to just wire it EXACTLY like the original. I know the original speaker voice coil is 1 ohm RESISTANCE.
If I buy a replacement output transformer of the same dimensions and KEEP the original speaker (1 ohm), how would I choose the primary/secondary values?   I cannot use 7K/4 or 8 ohms if I'm keeping the original 1 ohm speaker. 

3.  I see the 1140 ohm field coil (hum buck, etc) in series with the output T (open 400ohm terminal 1 & 2).  How would I take this into account when choosing/connecting a NEW replacement output transformer? Piggyback the new one on top of the 32-7019 and series connect the 1140 ohm section?       I don't have a deep understanding of the theory.   I need some answer like "choose this XX value of output transformer instead, and connect wires 1,2 & 3 like this ____.  Or buy 2 different parts, one XX impedance to replace part 37, and one output transformer of XX value. 

4. The output tube is 6F6G, it won't change. Capacitor 40 is .03 (will it need to change if I use a different transformer & speaker?)

Assume I have no knowledge of theory.   Appreciate the responses but some is above my head.

Bryan

[Chas]

Hmm,

Seems I am guilty of allowing overthinking of this project  

I have posted this snip of the output area, I want to post more from the power supply for reference but I don't want solutions to get carried away...

   

Look to the top/right corner, see the cone icon, immediately below represents the voice coil, below that is the hum bucking coil... It is that common junction that must be restored to "help" reduce the likelihood of having to swap "voice connections" to reduce hum. In other words this is a proper connection leave it alone. The hum buck coil is mounted on the same pole piece next to the field coil. Therefore, for this speaker, the "voice-coil" impedance is the sum of both coils for determining OPT output impedance

IMHO an output transformer for 7 to 10K plate to 4 to 8 ohms impedance at roughly 5 watts should be well within the "ballpark" for this radio.

Any "precise engineering" will give precise headaches, this is a technicians repair job no more...

The plate/B+ wires on  new OPT will be identified, connect as shown in docs for the OPT. The "voice-coil" wires will be either all black or coded, none the less make temporary connections to the series combined voice-coil/hum-buck ends, then operate the radio, if there is residual low level hum with volume control turned down, then swap those two transformer secondary connections. If the hum decreases keep and make permanent, if not the original connections are the best it can be...

This radio uses voltage drop from a tapped or separate string of resistor from the center-tap of the power transformer to chassis/ground connection. This group of resistors must be intact, not shorted or seriously changed in resistance.

Look and see that next to the power transformer center tap is a wire connection, this is the most negative voltage source and via a 1 meg resistor provide bias to the 6F6. If this bias is too low to zero the tube will conduct heavily, meaning an excess of plate current, it could have been a cause of OPT failure...

Another loss of negative grid voltage for the 6F6 is the signal coupling capacitor from the plate of the previous stage to the grid of the 6F6, if this began to leak B+ it would allow the 6F6 grid to become more "positive" and again conduct heavily.

So, the OPT gave up the ghost, despite theoretical analytics of mine there are other external causes that a technician must look for a successful and robust repair.

You have a copy of the complete schematic so you know the areas of the power supply I am referring to.

As for the combined tone capacitors, use at least the voltage rating specified but know that increasing the voltage rating of this capacitor in a common metalized unit, will not prevent its ruin from spikes, as it is the metalizing that fails and not from a puncture of the dielectric.
FWIW use a common metalized in that tone position to get the radio up and running and plan to replace the cap with a dv/dt rate cap in the future, avoid listening during a thunderstorm until the cap is replaced. Though a new OPT will be much more robust, better insulated, why chance?
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It is always good to retain the original field coil speaker along with the close approximation of the filter cap values.
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Do, have a replacement 6F6 handy as MrFixr55 has stated they are easily roasted, I have been guilty of such cooking too.
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Ron's suggestion is also an ideal solution for an OPT because the manufacturing of such audio distribution transformers is still very competitive, the prices are quite low when compared to a conventional OPT of limited manufacture.

GL

Chas

[Jordanbkc]

Chas - still reading, had a long day of work. I reckon it would be sacrilege to add some modern device like a varistor to absorb voltage spikes. Hide it in a Bakelite container.