Kolster 60 Audio output question

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Kolster 60 Audio output question

Post by rdusel on Fri Apr 05, 2013 2:21 pm

Fellow radio restorers,
I'm restoring a 1931 Kolster K60 radio and I'm almost finished. Case came out nice with the original finish cleaned and recoated, all caps and resistors changed (first time I have seen a radio with all dogbone resistors off value), tubes tested and set re-aligned.
The problem I have is that the radio has a slight bit of distortion that did not go away with the re-alignment. Selectivity, Sensitivity are both good but the radio when tuned to the best always sounds like it is just off station a hair.
All coils check with the correct readings but the speaker sounds a bit off, weak on bass (it's an 8" field coil unit, 3 ohms VC) yet the bass returns if you slightly touch the back of the cone.
So I found an NOS Jensen 8" speaker that has the same coil ohms (3) and the same field coil ohms and fits the mounts perfectly.
My question has to do with the output transformer. The factory transformer is rated for a 650ohm primary and a .3 ohm secondary SE.
After reading up on audio output transformers I find that the #47 output tube in this set "likes" a 7K ohm impedance primary. So I bought an output transformer designed with a 7K primary impedance and a 4 ohm secondary impedance.
My question is which transformer to use? Stick with the stock unit that the set was designed for or use the newer unit that matches the specs I found for a #47 tube. I'm thinking the newer transformer may offer better sound from the closer impedance match.
Any and all opinions and advice would be great. If you have any questions let me know.
Schematic is here:
http://www.nostalgiaair.org/PagesByModel/107/M0040107.pdf

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Re: Kolster 60 Audio output question

Post by Bill Cahill on Fri Apr 05, 2013 8:22 pm

First, is it any better with new speaker? Could be original voice coil is rubbing.
Secondly, I'd stay with original transformer, if in good shape.
It was designed for it.
Bill Cahill

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Re: Kolster 60 Audio output question

Post by rdusel on Fri Apr 05, 2013 8:45 pm

So far I have only tested with a 4" test speaker. The sound quality was better and seemed to be clearer at low volumes. With moderate volumes it seemed that the test speaker was having issues with being overpowered.
It's difficult to test other speakers as the transformer is riveted to the speaker with flying output leads and then the fine wires from the coil are simply wrapped around the flying leads with no real support. No way to easily disconnect or alligator the connections. I guess I'll have to bite the bullet and grind off the transformer rivets so I can move it and test the new speaker.
I wasn't sure if the knowledge of what makes a good AF output circuit had advanced enough to make useing the newer transformer an improvement.
Or I just try it both ways.
Thanks for the help!
Rob


Last edited by rdusel on Fri Apr 05, 2013 8:46 pm; edited 1 time in total (Reason for editing : made a little clearer)

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Re: Kolster 60 Audio output question

Post by Bill Cahill on Fri Apr 05, 2013 8:51 pm

The transformer isn't the ptoblem. I'm even more sure it's the speaker itself.
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Re: Kolster 60 Audio output question

Post by rdusel on Fri Apr 05, 2013 8:58 pm

Thanks Bill, that is my gut feeling also. It's a shame as the original speaker looks perfect but I think the spider or cone stiffened up to the point that it distorts. I hope to finish it this weekend as it looks beautiful and has been on the bench way too long. It's backing up my old TV's..

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Re: Kolster 60 Audio output question

Post by Resistance is Futile on Sat Apr 06, 2013 1:01 pm

your going to have a problem, because the original speaker has an electromagnetic speaker. You will have to either keep the speaker to use the field coil or replace the field coil with either a choke of equivalent value in impedance or resistor. Or have the speaker re-coned.

You could replace the speaker with the same size permanent magnet type and make the modifications, For the field coil, because it is used in the power supply filtering circuit, and without it you will have no B+ voltage.
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Re: Kolster 60 Audio output question

Post by rdusel on Sat Apr 06, 2013 2:41 pm

Cliff,
I got lucky and found a NOS Jensen 8" speaker on Ebay with the same mounting holes and a e-m field coil that measures the same ohms as the original.
Output transformer on the Jensen was no where near close to the original unfortunately.
Now to move the original output transformer. Stay tuned, I hope to do it this weekend.
Rob

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Re: Kolster 60 Audio output question

Post by rdusel on Tue Apr 09, 2013 1:13 pm

Spent some time last night and was able to carefully remove the original output transformer, transfer it to the Jensen speaker and mount it. Looks good and looks correct for the radio as it is also a field coil speaker. During a test the new speaker played without the distortion present in the old speaker. Still lacking in bass and sounding somewhat flat but this should improve when mounted back in the case.
I also realize that I am used to listening to a 1937 Philco console with a 12" woofer and seperate tweeter, comparing a 1931 tabletop to the Philco may not be a good comparison.
Thanks for the advice. Now on to my next project, a magnificent Stromberg Carlson console (model 846). Wish me luck.

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