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Magnavox amp101b output transformer

Edbrody2

Edbrody2

New Member
I’m currently working on a pair of Maggie amp101b’s which I am rewiring as amp142’s. First one went smoothly sounds great. Second one has an open winding on output transformer secondary. Transformer number is 330024(61 or G1) I can’t make out last 2 characters. Amp is a PPP 6v6.
I have an Amp179AA (also PPP 6v6) that has output transformer numbered
xc330024-1. Does anyone know These are the same as I would like to use the 179aa transformer in my 101b/142 project. Thanks in advance for any help.
 
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Might be worth a quick check to see if the impedance rings out the same but if the base part number and the tube lineup are the same the transformers are probably interchangeable.
 
i was thinking more of a ratio check. Voltage from plate lead to plate lead / output voltage would give the transformer ratio. If its the same the impedance will be the same.
 
I’ll be attempting to measure the impedance ratio today. It will be a good learning experience for me. As far as the winding on the original transformer being open, it looked that way when I was measuring resistances. I am going to double check everything today. When it was in circuit I traced an input 1khz signal to amp all the way thru to primary side of transformer, but got no output signal on secondary. I did hear the tone, but it seemed to be coming from transformer itself??

Thanks for the help!
 
ohm check on the secondary would show if its open or not. I'd expect maybe an ohm or so at most. Primary side might be 50 ohms or so from center to either end. The secondary rarely fails though. If it is open you may find its just broken right inside the transformer cover rather than the winding itself.
 
^^ I was also thinking this. If you can hear tones or music coming from the transformer itself, this is called singing and will happen either when no speaker is attached or a wire is broken internally. You can take it apart and sometimes find the problem or break on the first layer.

I inadvertently blew an output transformer to one of my tube amps a year or so ago. A wire had become disconnected from the speaker itself and when I cranked it up, I saw a wisp of smoke come from a corner of the end bell cover. My heart dropped.

I decided to dig into it and remarkably it just melted the incoming wire to the winding itself right on the first layer. Phew! :D Soldered it back on and am still in business today.
 
Okay, I believe I found the problem with the transformer. Besides the 2 leads that go to speaker, there is a cloth insulated sleeve that contains 2 bare wires with a black coating on them. I remember one of them being hooked up to a contact labeled as a speaker switch on the original amp101b. Got me thinking and I decided to try soldering these wires together. This completed the secondary winding and no more open winding. Put it back in and seems to be working.
On this amps twin, which is working, I never touched those 2 wires in the sleeve. I guess they were making contact. I went back and soldered them also.
While I was into this I decided to measure impedance ratios of the three transformers.
(Thank you Uncle Doug and Blueglow for the videos on doing this)
For the 2 transformers that were from the amp101b’s I got a turns ratio of 39.4:1 . I squared this and get 1552:1 for the impedance ratio.
For the transformer from the amp179aa, I got a 53:1 turns ratio and 2809:1 impedance ratio.
So I don’t think they are the same even though they have the same base number.
Do these numbers sound legit or did I screw something up?
According to Sams for amp142a the primary impedance is 3.7k ohm and secondary is 4.3 ohm. 3700/4.3 = 860. So none of my transformers are the same.

At any rate it seems to be working, I appreciate the help from everyone. Thanks
 
My AMP-150 has an impedance ratio of 1588:1 so that sounds pretty similar to whats on the 101b.
SAMS trafo number on that is 330024-1. Says 5.6k primary, 4 ohm secondary, tap@ 3 ohms.
 
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