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Hello again! I have a Philco Model 37-670. I have restuffed block caps, replaced paper caps, replaced all out of range resistors, checked the tubes, checked the volume and tone controls, checked the field coil, filter choke and output transformer. The radio will not make a sound. The input transformer has four sections. the two 30+ ohm sections are OK. The 217 ohm section measures 219 ohms. The 200 ohm section that appears to handle the plate voltage for two 6J5 "driver" tubes is not generating a reading. Does that mean it is open/bad and what may I do to rectify the situation? Many thanks for any assistance!
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>The 200 ohm section that appears to handle the plate voltage for two 6J5 "driver" tubes is not generating a reading. Does that mean it is open/bad and what may I do to rectify the situation?
If there is no plate voltage on the 6J5 then the answer is yes. Well there is two answers. One you could replace the driver transformer. Or connect a resistor across the primary(like 70k or so ) and then connect the cap(like a '01 @630v) from the plate of the 6J5 to one of the grid connection on the transformer secondary.
Replacing the transformer is more efficient but adding the resistor and cap is much easier/cheaper.
Terry
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(03-10-2016, 04:04 PM)Radioroslyn Wrote: >The 200 ohm section that appears to handle the plate voltage for two 6J5 "driver" tubes is not generating a reading. Does that mean it is open/bad and what may I do to rectify the situation?
If there is no plate voltage on the 6J5 then the answer is yes. Well there is two answers. One you could replace the driver transformer. Or connect a resistor across the primary(like 70k or so ) and then connect the cap(like a '01 @630v) from the plate of the 6J5 to one of the grid connection on the transformer secondary.
Replacing the transformer is more efficient but adding the resistor and cap is much easier/cheaper.
Terry What wattage on the 70K resistor? 1 watt or higher? And the grid connection would be the low ohm sections that go to the input grids on the 6F6's? I have never, ever fooled with one of these so pardon all of the questions. Terry, you have consistently come to my rescue here and I am very grateful! I sure wish i could return the favor somehow...thanks again!
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(03-10-2016, 05:17 PM)dfields69 Wrote: (03-10-2016, 04:04 PM)Radioroslyn Wrote: >The 200 ohm section that appears to handle the plate voltage for two 6J5 "driver" tubes is not generating a reading. Does that mean it is open/bad and what may I do to rectify the situation?
If there is no plate voltage on the 6J5 then the answer is yes. Well there is two answers. One you could replace the driver transformer. Or connect a resistor across the primary(like 70k or so ) and then connect the cap(like a '01 @630v) from the plate of the 6J5 to one of the grid connection on the transformer secondary.
Replacing the transformer is more efficient but adding the resistor and cap is much easier/cheaper.
Terry What wattage on the 70K resistor? 1 watt or higher? And the grid connection would be the low ohm sections that go to the input grids on the 6F6's? I have never, ever fooled with one of these so pardon all of the questions. Terry, you have consistently come to my rescue here and I am very grateful! I sure wish i could return the favor somehow...thanks again!
And Radio Daze as well sells a Hammond 124 series interstage transformer. Were I to replace it, which of those would I go with?? The 124D looks close as a novice, but I am very uncertain.
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People who do not drink, do not smoke, do not eat red meat will one day feel really stupid lying there and dying from nothing.
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(03-10-2016, 05:17 PM)dfields69 Wrote: (03-10-2016, 04:04 PM)Radioroslyn Wrote: >The 200 ohm section that appears to handle the plate voltage for two 6J5 "driver" tubes is not generating a reading. Does that mean it is open/bad and what may I do to rectify the situation?
If there is no plate voltage on the 6J5 then the answer is yes. Well there is two answers. One you could replace the driver transformer. Or connect a resistor across the primary(like 70k or so ) and then connect the cap(like a '01 @630v) from the plate of the 6J5 to one of the grid connection on the transformer secondary.
Replacing the transformer is more efficient but adding the resistor and cap is much easier/cheaper.
Terry What wattage on the 70K resistor? 1 watt or higher? And the grid connection would be the low ohm sections that go to the input grids on the 6F6's? I have never, ever fooled with one of these so pardon all of the questions. Terry, you have consistently come to my rescue here and I am very grateful! I sure wish i could return the favor somehow...thanks.
OOphsss!!!
I just looked that the schematic, I was thinking that it used a single driver tube but it doesn't it's push/pull. So you'll need two resistor one for each primary and two caps to go from each the driver tube plate (pin3) to the 6F6 grid (pin5). 1/2w should be ok.
Glad to help out!
Terry
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(03-10-2016, 05:20 PM)dfields69 Wrote: (03-10-2016, 05:17 PM)dfields69 Wrote: (03-10-2016, 04:04 PM)Radioroslyn Wrote: >The 200 ohm section that appears to handle the plate voltage for two 6J5 "driver" tubes is not generating a reading. Does that mean it is open/bad and what may I do to rectify the situation?
If there is no plate voltage on the 6J5 then the answer is yes. Well there is two answers. One you could replace the driver transformer. Or connect a resistor across the primary(like 70k or so ) and then connect the cap(like a '01 @630v) from the plate of the 6J5 to one of the grid connection on the transformer secondary.
Replacing the transformer is more efficient but adding the resistor and cap is much easier/cheaper.
Terry What wattage on the 70K resistor? 1 watt or higher? And the grid connection would be the low ohm sections that go to the input grids on the 6F6's? I have never, ever fooled with one of these so pardon all of the questions. Terry, you have consistently come to my rescue here and I am very grateful! I sure wish i could return the favor somehow...thanks again!
And Radio Daze as well sells a Hammond 124 series interstage transformer. Were I to replace it, which of those would I go with?? The 124D looks close as a novice, but I am very uncertain. I think the 124D is a good choice but I not quite sure what sort to turns ratio we need. It's at 1 to 1:5. The higher the ratio the higher the ac signal will be at the 6F6 grids. The 6F6's are triode connected (plate and screen grid are connected together) which lower the gain of the tubes. So typically it requires more driving signal.
The transformer Mike suggested I think is a single plate to P/P grids. Won't work no tap on the primary. Yours is P/P plates to P/P grids.
Terry
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Yep, did not look at the sch, pushpull drive....serious stuff.
People who do not drink, do not smoke, do not eat red meat will one day feel really stupid lying there and dying from nothing.
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OK...I'm coming clean here. I have no idea which interstage transformer to go with. I'd like to replace it. I do not have the tech know how to discern which one I should purchase. I hate to lean on y'all like this, but which should i get? The higher ohm side that goes to the 6J5's is 215 ohms/200 ohms. Only the 200 ohm section is dead. The other side links to the input grids on the 6F6's. That side is 38 ohms and 34 ohms. I just don't want to fry this thing. I also re-stuffed the 3 tap can electrolytic 1, 2, 3 MFD located on the floating subchassis with 10 mfd's. Didn't think that would be harmful. And were those wires a PAIN to get at then hook back up...ugh.
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>The higher ohm side that goes to the 6J5's is 215 ohms/200 ohms. Only the 200 ohm section is dead. The other side links to the input grids on the 6F6's. That side is 38 ohms and 34 ohms. >
The ohm values of the old transformer don't really tell anything other than whether it is open or not. What is important is the impedance and turns ratio. Impedance will determine how good of a match to the plates of the 6J5's it will have and turns ratio determines the amount of signal that is applied to the grids of the 6F6's.
I'd go ahead and grab the 124D.
Terry
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(03-10-2016, 11:38 PM)Radioroslyn Wrote: >The higher ohm side that goes to the 6J5's is 215 ohms/200 ohms. Only the 200 ohm section is dead. The other side links to the input grids on the 6F6's. That side is 38 ohms and 34 ohms. >
The ohm values of the old transformer don't really tell anything other than whether it is open or not. What is important is the impedance and turns ratio. Impedance will determine how good of a match to the plates of the 6J5's it will have and turns ratio determines the amount of signal that is applied to the grids of the 6F6's.
I'd go ahead and grab the 124D.
Terry
Done! I just ordered it from Radio Daze. As well, just picked up a make of radio I usually stay away from. I low bid offered a guy for a 1935 Grunow All Wave tombstone. Bought it this evening. Got my fingers crossed. The last Grunow I worked on was a huge Tele-Dial console way back in the 90's and it never worked right. We'll see...
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One thing to remember is that the 37-670 has a triode class A-B output stage. That's why the old driver transformer has a step down turns ratio. As you increase the volume, the 6F6 grids begin to draw grid current on the positive audio peaks and go into Class B. The grid current loads the driver stage, so a step down driver transformer provides a low impedance drive signal needed for low distortion.
The winding resistance readings do give some indication of the turns ratio of the old transformer. Assuming similar wire gauge on both primary and secondary, the fact that the secondary has a much lower DC resistance implies that there are less turns than the primary, (34 ohms vs 200 ohms) which would indicate a step down ratio.
Step down driver transformers are no longer made, but you can try connecting the 124D in reverse ( using the primary as the secondary and vice versa. The will give you a 1.5 to 1 step down ratio, which would be closer to the original.
It will work both normally connected or with the windings reversed. So you can really try connecting it both ways and see which way sounds better.
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(03-11-2016, 08:42 AM)Mondial Wrote: One thing to remember is that the 37-670 has a triode class A-B output stage. That's why the old driver transformer has a step down turns ratio. As you increase the volume, the 6F6 grids begin to draw grid current on the positive audio peaks and go into Class B. The grid current loads the driver stage, so a step down driver transformer provides a low impedance drive signal needed for low distortion.
The winding resistance readings do give some indication of the turns ratio of the old transformer. Assuming similar wire gauge on both primary and secondary, the fact that the secondary has a much lower DC resistance implies that there are less turns than the primary, (34 ohms vs 200 ohms) which would indicate a step down ratio.
Step down driver transformers are no longer made, but you can try connecting the 124D in reverse ( using the primary as the secondary and vice versa. The will give you a 1.5 to 1 step down ratio, which would be closer to the original.
It will work both normally connected or with the windings reversed. So you can really try connecting it both ways and see which way sounds better.
Thank you! The 124D interstage is on it's way.
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Also keep in mind that the open driver transformer primary winding may not be the cause of you having no sound at all. If the other half of the primary is still good, then one of the 6J5 drivers will have plate voltage and will provide some audio signal to the output stage. The radio should still play to some extent, but maybe not at full volume.
Do you have voltage on the 6J5 plate connected to the good winding which measures 217 ohms?
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No I did not check. Let me Pull it out over the next 2 dys and I'll post after checking all plate voltages.
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