Eric, you can try the transformer you have, but I don't think it will work connecting the low impedance secondary to the 2500 ohm speaker.
What might work better is if you use that transformer, but do not connect anything to the secondary speaker winding. Connect the primary as usual to the plate of the 71A and B+. Then connect a .47 uF film cap from the plate to one speaker terminal, the other speaker terminal to ground. The cap will block DC but allow the high impedance audio signal to pass to the speaker. This would effectively be the same as the choke coupled speaker output used in the other radios i mentioned.
The output transformer has a high inductance, and if the secondary is left disconnected it makes a very good audio choke.
I will give that a try and I understand how you said to connect it, but first I have to do is finish the recap and test out the Power transformer. Hope to do that over the next couple of days then I'll be gone for a week. Thanks for the help guy's.
The power transformer is good I get 385ac on the 280 and 265vdc at the filter caps 5vac on the filament of the 280
I have a 10 Henry choke would that be better I measured the new output and it is the same 10H over the full coil but the choke is much heavier ?
the original primary was connected one lead to the plate on the 71 the other went to the chassis
then connect one speaker lead to chassis
the other with a .47 cap in series connected to the B+ my question would the + end of the filter cap be the B+?
(This post was last modified: 09-03-2015, 11:41 AM by Eric.)
Are you sure one end of the original primary went to the chassis? It doesn't make sense because there would then be no plate voltage on the 71A. How many leads were on the original transformer?
I would think that the primary winding would connect one end to the 71A plate and the other to B+, which is the
+side of the filter cap. Then the secondary would connect to the speaker coil and ground, provided one side of the speaker coil was also grounded.
In any case, either the choke or the output transformer will work, but if the transformer is smaller it might be more convenient. Connect one end of the winding to the plate of the 71A, the other to the
+ siide of the filter cap. Then connect one end of the .47 uF cap to the 71A plate, the other to the speaker. The other side of the speaker would connect to chassis or the - end of the filter cap.
You want to make sure the cap is in series with the speaker, so it will block the DC voltage on the plate from passing through the speaker coil.
It really sucks not having a print! The output has 4 leads in the picture on the bottom side of the coil on the one lead goes to the middle yellow resistor on the end which is tied to ground. the other lead goes to the plate of the 71
On top one lead goes to the speaker and the other goes to the 26 pin #2 the 4th tube in line. This lead now goes nowhere?"
I powered it up and it draws .34a I get a very slight hum from the speaker
Yes, its very difficult without a print, especially if the radio has been modified. It looks like that transformer at the bottom left in the photo is not original, so who knows if it was wired in correctly. If it was connected to the plate (pin 2) of a 226 tube, it might have been a interstage transformer driving the grid of the 71A, not an output transformer, but then why was the speaker connected to it?
What is the transformer in the upper right? is it connected to the 227 detector plate? What does the grid of the 71A (pin 3) connect to?
(09-03-2015, 03:55 PM)Mondial Wrote: Yes, its very difficult without a print, especially if the radio has been modified. It looks like that transformer at the bottom left in the photo is not original, so who knows if it was wired in correctly. If it was connected to the plate (pin 2) of a 226 tube, it might have been a interstage transformer driving the grid of the 71A, not an output transformer, but then why was the speaker connected to it?
What is the transformer in the upper right? is it connected to the 227 detector plate? What does the grid of the 71A (pin 3) connect to?
The one in the upper right has 2 leads so that is probably a choke?
one lead goes to the 227 pin 2
one lead goes to the 26 the 4th tube pin 3
the grid of the 71 pin 3 went to the one side oh the output that is bad and the other lead of that coil goes to chassis
Quote:The output has 4 leads in the picture on the bottom side of the coil on the one lead goes to the middle yellow resistor on the end which is tied to (chassis) ground. the other lead goes to the plate of the 71
Quote:the grid of the 71 pin 3 went to the one side oh the output that is bad and the other lead of that coil goes to chassis
Now you have me really confused. First you said the output transformer was connected to the plate of the 71A, but your last post says its connected to the grid?
If the transformer is really connected to the grid and ground, it is an interstage transformer and not an output. This makes sense because you said the other winding connected to the plate of the 226, which is probably the first audio amp.
This radio may never have had an output transformer at all. Sometimes the high impedance speaker was driven directly from the plate of the 71A, with no transformer.
At this point, I really think you need more info on the radio from someone who has one, or an actual schematic. It does not help that someone replaced the transformer and possibly wired in in wrong. The only other alternative is to trace out all the wiring yourself and draw up an accurate schematic.
Quote:The output has 4 leads in the picture on the bottom side of the coil on the one lead goes to the middle yellow resistor on the end which is tied to (chassis) ground. the other lead goes to the plate of the 71
Quote:the grid of the 71 pin 3 went to the one side oh the output that is bad and the other lead of that coil goes to chassis
Now you have me really confused. First you said the output transformer was connected to the plate of the 71A, but your last post says its connected to the grid?
If the transformer is really connected to the grid and ground, it is an interstage transformer and not an output. This makes sense because you said the other winding connected to the plate of the 226, which is probably the first audio amp.
This radio may never have had an output transformer at all. Sometimes the high impedance speaker was driven directly from the plate of the 71A, with no transformer.
At this point, I really think you need more info on the radio from someone who has one, or an actual schematic. It does not help that someone replaced the transformer and possibly wired in in wrong. The only other alternative is to trace out all the wiring yourself and draw up an accurate schematic.
My apologies Mondial
Let me try this
Coil 1- One lead goes to the 71 grid pin 3 the other goes to the chassis
Coil 2 one lead goes to the 226 pin 2 the other goes to the speaker
Does this layout make it an interstage transformer?
Yes, it seems like an interstage transformer. The connection to the speaker is a bit strange, unless it is the common B+ supply to both the 226 audio stage and the 71A output.
What does pin 2 of the 71A connect to? The other terminal of the speaker seems like it should connect there somehow.
(09-04-2015, 03:15 PM)Mondial Wrote: Yes, it seems like an interstage transformer. The connection to the speaker is a bit strange, unless it is the common B+ supply to both the 226 audio stage and the 71A output.
What does pin 2 of the 71A connect to? The other terminal of the speaker seems like it should connect there somehow.
Pin 2 of the 71A goes to the speaker.
the other side of the speaker goes to the interstage and the center coil in the picture. the other end of the interstage coil goes to the plate of the 226
Yes, the drawing is very helpful. Now it makes sense. The transformer is the interstage between the 226 fiirst audio and the 71A output stage. The common connection between the speaker and the interstage transformer primary is the B+ power supply, fed from the filter cap and 280 rectifier.
There is no output transformer. The speaker is driven directly by the plate of the 71A.
This would be a better choice for the interstage transformer, as it has a higher turns ratio (and is less expensive!)
(09-04-2015, 06:11 PM)Mondial Wrote: Yes, the drawing is very helpful. Now it makes sense. The transformer is the interstage between the 226 fiirst audio and the 71A output stage. The common connection between the speaker and the interstage transformer primary is the B+ power supply, fed from the filter cap and 280 rectifier.
There is no output transformer. The speaker is driven directly by the plate of the 71A.
This would be a better choice for the interstage transformer, as it has a higher turns ratio (and is less expensive!)
After some more poking around I think the P-T124 might be a better choice because the one side that was not open I measured and I got 8.6k ohms which I call the primary it was the connection to the 71A pin3 and the chassis and the primary on the 124 is 7k and it is rated 5w https://www.tubesandmore.com/products/tr...Interstage
Thanks Eric
(This post was last modified: 09-04-2015, 07:48 PM by Eric.)
Eric, you are confusing AC impedance with DC resistance. The PT-124 has a 7000 ohm impedance primary, but this is not what you will read with an ohmmeter. If you look at the specs, the DC resistance of the primary is only about 450 ohms ( 218 +236 ). So if you measured the existing transformer and measured 8.6K, the PT-154 would only read 450 ohms when measured the same way.
Also keep in mind that what you call the primary is really the secondary which is connected to the 71A grid. The primary was connected to the 226 plate and is the open winding.
In any case, the resistance readings and 5 watt power ratings are immaterial. What you want is a step up interstage transformer with a 1 to 3 ratio, which is the standard transformer for driving a 71A output from a 226. While the PT-154 will work with a 1 to 1.5 ratio, the PT-156 is a better choice and will provide more volume.
(This post was last modified: 09-04-2015, 08:49 PM by Mondial.)
(09-04-2015, 08:49 PM)Mondial Wrote: Eric, you are confusing AC impedance with DC resistance. The PT-124 has a 7000 ohm impedance primary, but this is not what you will read with an ohmmeter. If you look at the specs, the DC resistance of the primary is only about 450 ohms ( 218 +236 ). So if you measured the existing transformer and measured 8.6K, the PT-154 would only read 450 ohms when measured the same way.
Also keep in mind that what you call the primary is really the secondary which is connected to the 71A grid. The primary was connected to the 226 plate and is the open winding.
In any case, the resistance readings and 5 watt power ratings are immaterial. What you want is a step up interstage transformer with a 1 to 3 ratio, which is the standard transformer for driving a 71A output from a 226. While the PT-154 will work with a 1 to 1.5 ratio, the PT-156 is a better choice and will provide more volume.
Yes have to remember that were talking about impedance and not just resistance. I've done a little reading and the 1:3 appears to be the standard. I ordered the PT-156 and will now do the rest of the caps I think I will hang on to these and keep them with the radio as I've never seen this construction before.
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