Thread: Output Transformer

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  1. #1
    Join Date Nov 2006
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    Default Output Transformer

    There are 2 Valco amps here at nobby road, one is a Supro 1696TN and the other a Gretch 6160 "Chet Atkins".

    If a problem is super obvious, I can fix it myself. I got lucky with the Supro; a visual inspection showed that a ceramic resister right at the output stage was broken in half. I replaced that and the loose adjacent tube socket and it's been working perfectly ever since

    The 6160 was making a similar noise so I wondered if it might be the same problem, particularly since they were made in the same factory one year apart.

    The sound was kind of like...


    pssst... tit ... kik... fissssst... KRAK sit... tik... zzzissst KRAKAK!

    When I could clearly smell something burning, I turned it off and unplugged it. The larger Valco amps tend to have 2 chassis, one at the top with the preamp and controls, and the power amp on the bottom.

    I took out the power amp, didn't see any burnt components.
    I took out the preamp and powered it up. Without thr preamp in the way I could see the output xformer which is mounted on the top of the baffle between the 2 speakers.

    The plume of smoke that came out of it gave me a clue and there was an orange glow under the paper.

    Looking for a replacement, I noted that there were only 2 criteria, watts and ohms.

    I believe I found a match at 8 ohms and 35 watts. I'm curious as to whether there is a wattage range or if, for example, the same xformer would work in the Supro which is 24 watts. Not that it needs one right now, but it is 53 years old.

    http://www.classictone.net/40-18061.html
    Man! You have GOT to try a hit of this RANGE SUNSHINE!

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  2. #2
    Join Date Nov 2006
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    Default Re: Output Transformer

    Another thing. The input has 3 wires, presumably with the middle one tapping the center of the coil.

    I suppose if I wire the outside ones wrong the speaker will be out of phase?

    .
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    Man! You have GOT to try a hit of this RANGE SUNSHINE!

    IMTBO = In My Thoroughly Biased Opinion
    CMIIW = Correct Me If I'm Wrong
    Never underestimate the amount of contempt a failed musician has for those of us who are still trying.
    If the party's good enough, you can actually suck to a remarkable degree.

    Greedle
  3. #3
    Little River Band on The Run Internet Meme
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    Default Re: Output Transformer

    I didn't look too closely but maybe this is a good replacement. The place seems quite stocked:

    http://www.angela.com/fendervibrover...ansformer.aspx

    (edit) And I guess the variety of amps it can be used in adresses your power concern.
    Last edited by Knastratt; April 6th, 2016 at 08:54 PM. Reason: Ofloflo
  4. #4
    Join Date Nov 2006
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    Default Re: Output Transformer

    That looks like it would do the trick also

    If the one in the Supro goes, a Deluxe Reverb tranny would probably work since there's only a 2 watt difference in output.
    Man! You have GOT to try a hit of this RANGE SUNSHINE!

    IMTBO = In My Thoroughly Biased Opinion
    CMIIW = Correct Me If I'm Wrong
    Never underestimate the amount of contempt a failed musician has for those of us who are still trying.
    If the party's good enough, you can actually suck to a remarkable degree.

    Greedle
  5. #5
    Join Date Sep 2009
    Location Quake City
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    Default Re: Output Transformer

    There are 2 Valco amps here at nobby road, one is a Supro 1696TN and the other a Gretch 6160 "Chet Atkins".

    If a problem is super obvious, I can fix it myself. I got lucky with the Supro; a visual inspection showed that a ceramic resister right at the output stage was broken in half. I replaced that and the loose adjacent tube socket and it's been working perfectly ever since

    The 6160 was making a similar noise so I wondered if it might be the same problem, particularly since they were made in the same factory one year apart.

    The sound was kind of like...


    pssst... tit ... kik... fissssst... KRAK sit... tik... zzzissst KRAKAK!

    When I could clearly smell something burning, I turned it off and unplugged it. The larger Valco amps tend to have 2 chassis, one at the top with the preamp and controls, and the power amp on the bottom.

    I took out the power amp, didn't see any burnt components.
    I took out the preamp and powered it up. Without thr preamp in the way I could see the output xformer which is mounted on the top of the baffle between the 2 speakers.

    The plume of smoke that came out of it gave me a clue and there was an orange glow under the paper.

    Looking for a replacement, I noted that there were only 2 criteria, watts and ohms.

    I believe I found a match at 8 ohms and 35 watts. I'm curious as to whether there is a wattage range or if, for example, the same xformer would work in the Supro which is 24 watts. Not that it needs one right now, but it is 53 years old.

    http://www.classictone.net/40-18061.html
    You can always use a higher wattage transformer if the impedances are right (it's not just the output impedance, the input impedance should match the tube types in the amp, otherwise the impedance reflected to the output will be off.) Tube spec sheets will give the suggested load impedance for the tube type in various circuit configurations (Class A single ended, Class A push-pull, Class AB push-pull.)

    HOWEVER there may (probably will be) some change in tone, as the bigger transformer won't saturate as readily as the original and will also have better bass response.

    Another thing. The input has 3 wires, presumably with the middle one tapping the center of the coil.

    I suppose if I wire the outside ones wrong the speaker will be out of phase?

    .
    Not just that - the amp will almost certainly oscillate due to the negative feedback connection being on the wrong side of the trannie, converting it to positive feedback.

    The center tap wire goes to the B+ on the power supply.

    I recently did a conversion on a silverface bassman that had the hum balance circuit instead of a proper bias pot and the attendant output stage, which I changed to the AB763 blackface circuit. I missed the fact that one version has reverse phase from the other and first time I turned it on it went "WOOOOOOT!" before I shut it off, kicked myself, and reversed the wires on the output....
    http://www.johnnyoklahoma.com/

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  6. #6
    Join Date Nov 2006
    Location Long Island, NY, USA
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    Default Re: Output Transformer

    Interesting. Thanks for the details
    Man! You have GOT to try a hit of this RANGE SUNSHINE!

    IMTBO = In My Thoroughly Biased Opinion
    CMIIW = Correct Me If I'm Wrong
    Never underestimate the amount of contempt a failed musician has for those of us who are still trying.
    If the party's good enough, you can actually suck to a remarkable degree.

    Greedle

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