09-21-2022, 07:42 PM
Hi Cry42,
You can "sleeve" the wiring to the OPT (Output Transformer) with "spaghetti" or heat shrink tubing.
Easy test for the OPT is to take a 9V battery and momentarily connect across the OPT Primary. You should get a pretty good click from the speaker. If not, it is open. OPT secondaries usually don't open. +1 on Terry's comment that you are not gonna get an OPT on one of these sets to get so hot that the tar melts on just current from a 90V Battery, even if the 1A5 shorted! Now with the radio in a hot attic, that may be a different story.
As to what to replace the transformer with if it is bad, there are not many options. OPTs from AA5s have a primary impedance of about 2.5K Ohm. OPTs from radios using a single 41, 42, 6F6, 6K6 or 6V6 usually have a primary impedance of 7K Ohm - 8K Ohm. Battery Radios often have higher values. The Plate Load impedance for a 1A5 is 25K Ohm! A 1C5 has the same basing diagram, needs -7.5V bias vs the -4.5V bias voltage for the 1A5, so R25 may have to be changed to get the correct bias. However, the Plate Load Impedance is 8K Ohm, so the OPT from an AC Radio with a power transformer and 41, 42, 6K6, etc. type output tube will work. A "plus" is that the output with this tube is 0.2W, twice that of the 1A5. A 1Q5 also has a plate load impedance of 8K Ohm. Per the RCA Receiving tube Manual, the basing diagrams have different numbers, but the pinouts are the same. You may have to change R25 to get the right bias (-6.5V for the 1Q5 vs -4.5V for the 1A5). If fed with 110VDC, this tube puts out 0.4W.
As to matching the transformer secondary to the speaker, there is quite a bit of science to that. It is better if the speaker impedance is a little higher than the OPT impedance if a perfect match can't be found. Less power but possibly less distortion. A speaker with a lower impedance than the OPT secondary affects the primary impedance (or something like that. See what the advice of Phorum Experts is.
Hope this helps, hope that OPT on your set turns out to be good!
You can "sleeve" the wiring to the OPT (Output Transformer) with "spaghetti" or heat shrink tubing.
Easy test for the OPT is to take a 9V battery and momentarily connect across the OPT Primary. You should get a pretty good click from the speaker. If not, it is open. OPT secondaries usually don't open. +1 on Terry's comment that you are not gonna get an OPT on one of these sets to get so hot that the tar melts on just current from a 90V Battery, even if the 1A5 shorted! Now with the radio in a hot attic, that may be a different story.
As to what to replace the transformer with if it is bad, there are not many options. OPTs from AA5s have a primary impedance of about 2.5K Ohm. OPTs from radios using a single 41, 42, 6F6, 6K6 or 6V6 usually have a primary impedance of 7K Ohm - 8K Ohm. Battery Radios often have higher values. The Plate Load impedance for a 1A5 is 25K Ohm! A 1C5 has the same basing diagram, needs -7.5V bias vs the -4.5V bias voltage for the 1A5, so R25 may have to be changed to get the correct bias. However, the Plate Load Impedance is 8K Ohm, so the OPT from an AC Radio with a power transformer and 41, 42, 6K6, etc. type output tube will work. A "plus" is that the output with this tube is 0.2W, twice that of the 1A5. A 1Q5 also has a plate load impedance of 8K Ohm. Per the RCA Receiving tube Manual, the basing diagrams have different numbers, but the pinouts are the same. You may have to change R25 to get the right bias (-6.5V for the 1Q5 vs -4.5V for the 1A5). If fed with 110VDC, this tube puts out 0.4W.
As to matching the transformer secondary to the speaker, there is quite a bit of science to that. It is better if the speaker impedance is a little higher than the OPT impedance if a perfect match can't be found. Less power but possibly less distortion. A speaker with a lower impedance than the OPT secondary affects the primary impedance (or something like that. See what the advice of Phorum Experts is.
Hope this helps, hope that OPT on your set turns out to be good!
"Do Justly, love Mercy and walk humbly with your God"- Micah 6:8
Best Regards,
MrFixr55
