Well I need a outpit transformer, but not sure what to get?

4/2/2008 4:19:58 AMSean

Hello, I need an output transformer but dont know what to get. Zenith 6s147 chassis. Tube chart shows load resistance to be 6 or 7 k ohms. I found a hammond on the AES site but I dont know how close it comes to 7k and Im wondering what the secondary impedence should be. The transformer I am looking at is the P-T125A. I have a p-t291 but will that be a good replacement or not? Im not really sure what alls involved in getting a close as possable replacement. I measured the primary leads and it measures 500 ohms yet the primary impeadance is 4 to 9000 ohms as stated. Maybe I really dont need an output transformer? I just know that the radio distorts a lot when turned up any amout of volume.

4/2/2008 11:27:22 AMDoug Criner

Sean: Usually, audio output xmfrs don't just start causing distortion - that's not their typical failure mode. I seriously doubt that your problem is due to the xfmr. Have you recapped the radio?

But, to address your question, I suspect that you are confusing the DC resistance with the AC impedance. The important thing is the xfmr's primary impedance seen by the final audio tube's plate circuit. That impedance will be equal to the speaker impedance multiplied by the square of the xfmr turns radio. But it isn't all that critical. "Universal" audio xmfrs are available with several different windings to play with, if you wish.
Doug

:Hello, I need an output transformer but dont know what to get. Zenith 6s147 chassis. Tube chart shows load resistance to be 6 or 7 k ohms. I found a hammond on the AES site but I dont know how close it comes to 7k and Im wondering what the secondary impedence should be. The transformer I am looking at is the P-T125A. I have a p-t291 but will that be a good replacement or not? Im not really sure what alls involved in getting a close as possable replacement. I measured the primary leads and it measures 500 ohms yet the primary impeadance is 4 to 9000 ohms as stated. Maybe I really dont need an output transformer? I just know that the radio distorts a lot when turned up any amout of volume.

4/2/2008 12:21:51 PMDoug Criner

More...

This set has a 6F6 final audio tube. According to my RCA Tube Manual (R-14), a 6F6 (pentode connection) wants to see 7K-ohm load. But, I think anything within +/- 20% of that should be OK. Much too low, and distortion would increase. Much too high, and power would drop off. It's not critical.

But I still doubt the xfmr is your problem.
Doug

4/2/2008 3:19:59 PMseaN

:Yea I thought of that too, impeadance. Well this transformer has been replaced so I do want to get the right one if this isnt close enough but how do I tell if its close enough? No I havnt recapped it yet other than the .005 cap across the output transformer.
:More...
:
:This set has a 6F6 final audio tube. According to my RCA Tube Manual (R-14), a 6F6 (pentode connection) wants to see 7K-ohm load. But, I think anything within +/- 20% of that should be OK. Much too low, and distortion would increase. Much too high, and power would drop off. It's not critical.
:
:But I still doubt the xfmr is your problem.
:Doug

4/2/2008 4:43:17 PMDoug Criner

Well, it's possible that the xfmr, which you say has been previously replaced, is the wrong ratio, causing distortion. But until you recap the set, no sense worrying about it or wasting money on a new xfmr.

If you do ultimately need to replace the xfmr, first determine the speaker impedance - you can roughly estimate it by disconnecting the speaker coil, measure the DC resistance, and double it. Then pick an xfmr with a turns ratio = sqrt (7000/spkr imp). Or pick an xfmr with rated impedance ratio = 7000/spkr imp.

But until you replace the caps, I think you are getting the cart before the horse.
Doug

::Yea I thought of that too, impeadance. Well this transformer has been replaced so I do want to get the right one if this isnt close enough but how do I tell if its close enough? No I havnt recapped it yet other than the .005 cap across the output transformer.
::More...
::
::This set has a 6F6 final audio tube. According to my RCA Tube Manual (R-14), a 6F6 (pentode connection) wants to see 7K-ohm load. But, I think anything within +/- 20% of that should be OK. Much too low, and distortion would increase. Much too high, and power would drop off. It's not critical.
::
::But I still doubt the xfmr is your problem.
::Doug

4/2/2008 5:00:40 PMEdd

With your initial description of the malady, if that was my unit and with ONLY one component to take an initial shot at, I would zero in on the grid coupling capacitor to the AF output stage, item C 16 on the sets schema, with it exhibiting leakage and thus shifting the class of operation of the output stage from an AB…probably well onto the B-C category as you run up the volume of the set.

Lift the right lead of the cap free of circuit connection, power up the set, and meter it for ANY DC voltage as referenced to ground . Then short that same connection to ground it self and see if any voltage shows up. If so, replace that capacitor with one that exhibits no DC leakage, so that your output stage can be grid biased in the proper manner and give its normally designed audio reproduction.

73's de Edd