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Thread: Test New Output Transformer

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    Member HaroldBrooks's Avatar
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    Test New Output Transformer

    Just got a spare 20 watt output transformer today that I think might help an amp of mine (A Gretsch 6162) to sound a bit better (more low end). The amp is running two 6973 tubes at 392vdc plate and 383vdc grids, and twin 10" speakers.

    This amp has a very big massive power transformer and a 5U4 rectifier, and is in basically the same configuration as the 35watt 6L6 twin amps Valco was making at the same time, but it seems they might have skimped on this one model by using a smallish output transformer, while still using the massive power transformer and bigger recitifier.

    My question is : As a test, can I just hook up the new output transformer without removing the old one, but just disconnect the speaker leads from the old transformer and use the speaker leads from the new temporary fitted one ? I realize I'm just being a bit lazy not disconnecting the existing transformer from the plates.

    Sorry if this is real basic stuff I should know by now, but I have never had formal electronics training, so I only know what I know.

    I would like to see the net effect of running a new larger output transformer, without fully swapping out the current transformer, and if I can leave the old one soldered in place, that will save some time.

    My gut feeling is that if the old transformer is only connected at the tube end and not put under a load, it shouldn't matter... (famous last words...)

    Thanks for your responses !!!

    gretsch 6162.pdf

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    Last edited by HaroldBrooks; 09-05-2019 at 10:20 PM.

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    My question is : As a test, can I just hook up the new output transformer without removing the old one, but just disconnect the speaker leads from the old transformer and use the speaker leads from the new temporary fitted one ? I realize I'm just being a bit lazy not disconnecting the existing transformer from the plates.
    Sorry, no.

    If as you report, the old OT suffers from weak low end, the reason is low primary inductance. Now if you parallel the 2 primaries, inductance will be even lower.
    It would work in principle, though.

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    Quote Originally Posted by Helmholtz View Post
    Sorry, no.

    If as you report, the old OT suffers from weak low end, the reason is low primary inductance. Now if you parallel the 2 primaries, inductance will be even lower.
    It would work in principle, though.
    Thanks Helmholtz, I have to read up on transformers a bit more and understand inductance. Now in the paralleling of the two transformers, only one will actually be connected to the speakers at the time, is the change in inductance still an issue ?

    Not too much effort to disconnect the original transformer, but now that I asked then question, I would still like to understand. Thanks for your patience.

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    Now in the paralleling of the two transformers, only one will actually be connected to the speakers at the time, is the change in inductance still an issue ?
    Yes, primary inductance aka magnetizing inductance does not depend on a load. Without a load any transformer behaves just as an inductor (choke). Total primary inductance determines bass cut-off. An ideal transformer would have infinite primary inductance and would not draw any current without a load. The non-ideal original transformer steals current and loads the tubes especially at low frequencies as its primary impedance drops as frequency decreases.

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    Last edited by Helmholtz; 09-05-2019 at 10:58 PM.
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    Don't forget the joker g1's Avatar
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    Leaving the primary of the original wired up is like running it with no load, very bad.
    (edit: not true as primary is paralleled with loaded OT)

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    Last edited by g1; 09-06-2019 at 02:02 AM.
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    Leaving the primary of the original wired up is like running it with no load, very bad.
    A long as the paralleled new transformer has a load, I don't see a risk of excessive primary voltage as there will always be a path for demagnetizing current. In other words the loaded OT shunts and loads the unloaded one.

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    Last edited by Helmholtz; 09-06-2019 at 12:37 AM.
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    ^^^^^^ Agree. The problem is that the entire reason for the test is to see how the new transformer sounds. Without disconnecting the old primary, you won't get an accurate sense of this because of the additional inductance.

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    Quote Originally Posted by Helmholtz View Post
    A long as the paralleled new transformer has a load, I don't see a risk of excessive primary voltage as there will always be a path for demagnetizing current. In other words the loaded OT shunts and loads the unloaded one.
    Thanks. I didn't think too long about it as I was worried about damage, but it makes sense.

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    Quote Originally Posted by The Dude View Post
    ^^^^^^ Agree. The problem is that the entire reason for the test is to see how the new transformer sounds. Without disconnecting the old primary, you won't get an accurate sense of this because of the additional inductance.
    Yeah, that's what I meant in post #2.

    additional inductance
    Maybe I should have explained a little better. The primary of the original OT means an additional inductor. But as both primaries are wired in parallel, total primary inductance will not increase but drop (-> paralleling of inductors). Lower primary inductance means increased reactive primary current. This in turn means reduced available load current. As the current drawn by inductors increases at low frequencies, bass response suffers.

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    Thanks for all the help so far. My laziness has passed, and I will remove the old transformer connections when I test the new one, and now I know a bit more about the role of inductance being lowered by two parallel transformers connected to the plates, regardless of a load being applied.

    I will let everyone know about what I find when I hook it up.

    I had developed a mental bias toward leaving my vintage transformers put regardless of if I thought they were doing a good job, but since I've implimented so many other fairly successful mods to my vintage amps, I thought it was time to see if I could make any OT improvements (what I think is an improvement that is).

    What got me started on this was using the same type of replacement 20 watt OT in my National amp. Adding that transformer in place of the original I believe made the amp a bit louder overall (a good thing in this case), and definitely added some clean lower bass as well.

    I often state that Bass is not important in a guitar amp, but it is necessary in any amp to a point, at least when you are playing without a full band and to achieve powerful sounding chords with the available sub-harmonics, at just the right amount. It's one of the fundamental reasons I believe many rock guitarists prefer huge stack amps, even playing at low volumes with less than optimal preamp distortion only. They sound big, and you may need that from time to time depending on what you are playing. It's always easier to back off from Bass in the input signal, rather than trying to add it at the beginning of the amplification chain, as in a high gain amp this will distort and modulate the other frequencies from the presence of a large bass amplitude.

    Thanks for listening !

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