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EL34 output transformer replacement
EL34 output transformer replacement
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Old 13th January 2018, 05:16 AM   #11
samitha10555 is offline samitha10555
diyAudio Member
Join Date: Sep 2010
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,
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Old 15th January 2018, 03:12 AM   #12
wdecho is offline wdecho  United States
diyAudio Member
Join Date: Oct 2014
EL34 output transformer replacement
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.
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Old 15th January 2018, 04:01 AM   #13
Johan Potgieter is offline Johan Potgieter  South Africa
diyAudio Member
Join Date: May 2005
Location: Pretoria, South Africa
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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