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EL34 output transformer replacement

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Hi, recently an damaged output transformer was rewound and reinstalled in my Muse EL 34 based amplifier. Now there is a motorboating noise coming from the channel that the transformer was repaired. Further it was noted that this noise goes off when the Negative feedback loop is removed.
Much appreciate if I could get advise to sort out this issue.
Many thanks
 
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If you can swap the O/P transformers over, the process may indicate if the fault is with the rewound transformer or it is specific to the one channel. Swapping the tubes/valves over may also be a worthwhile procedure to follow as a faulty one could be the cause of the problem. I trust that the transformer rewinder carefully followed the 'recipe' of how the original sectioning of the primary and secondary wirings were layered. If not you have an expensive repair on your hands.
 
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May be as simple as the plate/screen leads are swapped. Assuming that power supply decoupling is in order (and not shared between channels) it could be swapped leads or the transformer has more LF phase shift than was originally case and this is particularly critical if the design is based on the Williamson.
 
Thanks VaNArn, I did swap the two transformers and now fault has got transferred. As you mentioned looks as if there is an issue with the re-wound transformer but all works fine when negative feedback loop is removed. No distortions. Sounds good but volume level has gone up as a effect of NFB removal. Is there a way to adjust the NFB to suit the rewound transformer?
 
There might be excessive large caps in the amp. Transmitting and amplifying frequency's outside the transformers ability is a sure way to get feedback problems.

As the problems occurs after rewinding, the rewinding have not reproduced the transformer as it was. A repeated rewind might be a waste om money, i would buy a new transformer from some well-known source.
 
Dear bandol83, JonSnell Electronic, wdecho,

Thank you for the support.
I swapped the plate and B+ wires and it started working. Volume was low when compared with the other side. Then it was noted that there is a tuning capacitor parallel to the NFB capacitor/resistor. This was damaged. The same was removed and volume became ok.
I cannot find the value of this tuning cap. so I cannot replace it. Wondering whether to remove the one on the other side?? ;-)

Thank you for all the help,
Regards,
Sam
 
Just remove it. You could measure the value of the good one and just install the same value on the other side but the amp should work just fine without the adjustable cap. If I am not mistaken it is just used for some fine tuning. Really need to see the schematic to be sure but I would not fret over a parallel cap on the feedback network.
 
I must respectfully disagree with the previous post. Absence of a phase lead capacitor across the feedback resistor might well lead to h.f. instability, manifesting in some sort of relaxation oscillation and causing motorboating. It depends on the circuit and what the value of said cap is.

'Curing' the malady by removal of NFB points to such instability. It might also come from the replacement OPT not having the same primary inductance as the original.

But you mention that you swapped the OPT primary leads and things were normal. That meant that phase was reversed initially? In that case you would not have had mere motorboating, you could very well have had a woofer come being dislodged from its frame! Was the replacement transformer connections not identical with the original?

Barring the above it is quite possible that the repalcement's primary inductance is different from the original. That parameter also plays a role in NFB stability! (Not accusing, but is the replacement transformer at all equivalent to the original? Was your rewinder aware that it was an OPT of spesific design and not merely some power transformer?)

[In general I would caution against determining NFB phase by trial end error. Positive feedback could damage a tube amplifier in this way. The proper way is to connect up without feedback resistor, then put a resistor of some >50x the feedback resistor in place, and noticing whether the output increases or decreases slightly. This would indicate feedback phase without risking oscillation. Old hat to some, but rather unnecessary advice than a broken loudspeaker or worse.]
 
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