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FDP Forum / Amp Mods, Repairs, and Projects / Output transformer question. 6v6 push pull 18 watts.



Jan 4th, 2017 07:42 PM   Edit   Profile  

It's an old Magnatone 113 I'm overhauling. Anyway simple question:

I'm measuring DC resistance across the o.t. secondaries for a simple check to see if it's good.

It reads 918 ohms across the entire secondary.

532 ohms from one end to the center tap

386 ohms from the other end to center tap

Is this off too much?
Think there's a short?
Replace it?

I'm not worried about originality. I've replaced the power transformer with a Magnatone 213. Also replaced the tube sockets with ceramic and all the wiring. The tremolo is being replaced with 213 vibrato while I'm at it. The only thing original will be the cab, speaker, chassis and knobs and switches.

I have a really nice new 6V6 OT that brings out killer bass I am considering. Thoughts?



Jan 5th, 2017 01:50 AM   Edit   Profile  

It's a higher resistance than I've seen before but if it works and sounds ok then no reason to change it.
In a regular beam tetrode/pentode amp, the resistance discrepancy is immaterial; a turn of wire at the inside of a coil will be shorter (so lower resistance) than a turn on the outside of the coil. Transformer action is determined by the number of turns of wire, rather than their resistance.
OT primary resistance does matter with very high power amps, eg >100watt, as it acts to limit current flow and so power output.

(This message was last edited by pdf64 at 07:51 AM, Jan 5th, 2017)



Jan 5th, 2017 10:31 AM   Edit   Profile  

It did work but sounded a bit weak compared to other amps where I've changed out the OT so I'm thinking of going for it.

I know people value originality when it comes to things like the output transformer but I've found that replacing with a new unit often improves the sound. Namely more bass and definition/clarity. This one having seemingly unbalanced secondaries makes me wonder if this might be an improvement.

Here's what I usually replace them with because I never want to spend money for Mercury Magnetics stuff.

6V6 output transformer



Jan 6th, 2017 10:58 AM   Edit   Profile  

That looks a nice OT for a very reasonable price.
I checked an SF DR OT that I remember as having a higher than expected primary winding resistance, its resistances CT to plate leg are 215 and 217 ohms.



Jan 6th, 2017 11:37 AM   Edit   Profile  

I guess it was the imbalance more than the actual values that concerned me.



Jan 6th, 2017 12:20 PM   Edit   Profile  

Yes, it seems a larger than normal difference but it shouldn't, in itself, cause a problem.
It could just be the way it was wound, eg due to an interleaving arrangement.
The concern is that a high resistance has somehow developed, eg due to a failing connection between 2 sections of the primary.
But I think that would cause other symptoms, eg bad sound, crackles, heat.
Hence if the amp is working ok then don't be concerned about it.



Jan 6th, 2017 12:57 PM   Edit   Profile  

Since the amp is already far from original I plan on installing the new one and seeing if there's a difference. The new it has a 4 ohm secondary tap as well so that is another point in its favor.

FDP Forum / Amp Mods, Repairs, and Projects / Output transformer question. 6v6 push pull 18 watts.

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