If no transformer crossover manual available, a quick look through the RCA Receiving Tube Manual shows a typical Class A amp 6K6GT with plate voltage of 250v will need a load resistance of 7600 ohms. So that's the primary resistance of the output transformer.
He Bob, I do appreciate these types of videos with trouble shooting . You could save yourself some time by just going in and repace all the caps but I would not get any smarter.
That audio output transformer is not critical. Almost any tube audio output transformer will work. As long as it transforms impedance from 4 or 8 ohms up to 5k to 15k ohm range it is fine.
Output transformer specs are fairly easy to figure out for modern replacements. Look up the last audio stage's tube type in an RCA tube manual, e.g. let's say its a 6F6. The RCA book RC-13 (1937, they made newer editions every few years for decades) on pg76 gives the "Load Resistance" of 7,000 ohms for SE (single ended) audio stages; for PP (push-pull) its 2500 per-tube. So go over to Hammond or AES and look at output transformers and look for one where the primary is 7,000 ohms (if SE), or 2500-O-2500 for PP. With output transformers -especially- in nonaudiophile stuff like vintage radios & tvs, there is a surprising amount of wiggle room in replacements. Anything from 5,000 to 10,000 primaries would -probably- work for a SE and noone would be any wiser, though you want to buy one as close as possible to 7,000.
@@bandersentv There are cheaper brand-new options. AES has some generic options in the $20 range (e.g. P-T31), for PP there is a telecom line matching transformer that can be had sometimes as cheap as $10 which is fine for PP-45s & 71As (p/n 300-040). All depends on what you need.
That transformer from the Motorola should work, but the output impedance of a 25L6 is only about 2000 or 2500 ohms, and a 6K6, which I believe the 721TS uses, is 8,000 ohms. So, if you can find one that is low wattage for an old radio that uses a 6F6, 6K6 or even 6V6 output tube, that should be a better match. There can be beat up speakers online or at swap meets that have good output transformers on them, that may be a cheap way to find a good, used output transformer.
![[UA] Eternal Fire проти NAVI | EPL Season 20 Malta](http://i.ytimg.com/vi/chvlsDygrZM/mqdefault.jpg)
Something else to know about those adjustable power resistors is that if you tighten the tap clamp too much you can crack the resistor in two!
GReat find on audio xformer. Nice video and wow it is a work of art love this channel thanks mike
If no transformer crossover manual available, a quick look through the RCA Receiving Tube Manual shows a typical Class A amp 6K6GT with plate voltage of 250v will need a load resistance of 7600 ohms. So that's the primary resistance of the output transformer.
You should restuff the capacitor to keep the vintage look.
He Bob, I do appreciate these types of videos with trouble shooting . You could save yourself some time by just going in and repace all the caps but I would not get any smarter.
Shango is 'god" when it comes to the SAMS photo facts "You can't always get what you want But if you try sometimes, you'll find You get what you need"
Well he just signed up for full access with Sans. Any of us could do it.
@@bandersentvI have all the paper copies and scan them myself. The website customer service is atrocious And the resolution it's pretty minimal
That audio output transformer is not critical. Almost any tube audio output transformer will work. As long as it transforms impedance from 4 or 8 ohms up to 5k to 15k ohm range it is fine.
Yes. That's what I did
Output transformer specs are fairly easy to figure out for modern replacements. Look up the last audio stage's tube type in an RCA tube manual, e.g. let's say its a 6F6. The RCA book RC-13 (1937, they made newer editions every few years for decades) on pg76 gives the "Load Resistance" of 7,000 ohms for SE (single ended) audio stages; for PP (push-pull) its 2500 per-tube. So go over to Hammond or AES and look at output transformers and look for one where the primary is 7,000 ohms (if SE), or 2500-O-2500 for PP. With output transformers -especially- in nonaudiophile stuff like vintage radios & tvs, there is a surprising amount of wiggle room in replacements. Anything from 5,000 to 10,000 primaries would -probably- work for a SE and noone would be any wiser, though you want to buy one as close as possible to 7,000.
Sure only problem is the cost. 3W Hammond 125ASE is $60! That's why I scavenged one from another TV and it worked fine.
@@bandersentv There are cheaper brand-new options. AES has some generic options in the $20 range (e.g. P-T31), for PP there is a telecom line matching transformer that can be had sometimes as cheap as $10 which is fine for PP-45s & 71As (p/n 300-040). All depends on what you need.
That transformer from the Motorola should work, but the output impedance of a 25L6 is only about 2000 or 2500 ohms, and a 6K6, which I believe the 721TS uses, is 8,000 ohms. So, if you can find one that is low wattage for an old radio that uses a 6F6, 6K6 or even 6V6 output tube, that should be a better match. There can be beat up speakers online or at swap meets that have good output transformers on them, that may be a cheap way to find a good, used output transformer.
Definitely try the Motorola one, though, and if it sounds decent, leave it in.
It does and I did 🙂
Couldn't you test that output transformer using signal injection through a capacitor ? Here comes the dog down the steps !
Yes I sure could