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Vibro Champ w/ 8ohm Transformer Capable of 6L6?

Discussion in 'Shock Brother's DIY Amps' started by markesquire, Nov 28, 2020.

  1. markesquire

    markesquire Tele-Afflicted

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    I own a Victoria 518 tweed champ, which comes with a 4 ohm extra large (tweed super) transformer capable of taking both 6V6 and 6L6 tubes without rebiasing.

    I would love to build a 60's Vibro Champ replica, but with a transformer (1) big enough to handle both 6V6 and 6L6 tubes without rebiasing; and (2) 8 ohms for easier use as a head with extention speakers/cabinets.

    Is that possible?

    What transformer would you recommend?

    Would such a transformer require any other changes in the circuit? This would be my first build, so I just don't know this stuff.
     
  2. Wally

    Wally Telefied Ad Free Member

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    Mark, I’ll bump so others with more expertise might advise you. First, a tweed Super OT is a push/pull OT and is not suitable for single-ended output. There must be some confusion on that OT in your Victoria. Secondly, it is not the size of thenOT but rather the
    Impedance of the primary that makes it suitable for more than one type of tube. The size might affect the output power and sonic content. A 6V6 wants to see 8200 ohms on the OT primary impedance whereas the 6L6 wants to see 4200 ohms. These n7mbers are for single-ended applications. For the use of either tube, compromise might be made???? 6000 ohm primary??? This is not my area of expertise. I have a little Jet City Pico Valve amp that can run a wide variety of tubes in its single-ended output...6K6, 6V6, 6L6, EL34/6CA7, 6550, KT88, KT66. This amp is the made in China version of the THD Univalve amp. The OT has 4,8, and 16 ohm secondaries. If you could access one of those, it would do what you want.
     
  3. kleydejong

    kleydejong Tele-Holic

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    I have done some limited experimenting with different OT's.



    The stock OT was from a Hammond AO-29 designed for vintage style 6V6's in push pull. I rebuilt the amp as a 6V6 Plexi using JJ 6V6's. My voltage was pretty hot and I believe JJ 6V6's are a bit beefier than vintage style, so I concluded that I was pushing the OT pretty hard. I found it compressed and even saturated the signal a bit in a way that I did not like. It was fine for a bit, but as I got to 6-7-8 it would gain up excessively. It was a bit harsh and crispy. I suspected it was an under powered OT. I upgraded to a significantly higher powered and larger OT from Classic Tone meant for 6L6's. I found the new OT was more able to handle pushing the amp better all the way up the volume range.

    I do believe some amps utilize the right amount of OT saturation in a positive way. The 5E3 or Blackface Princeton come to mind. I think if you run a vintage 5E3 to its limits and smash it with a humbucker you're going to saturate the OT a bit.

    On the other end of the spectrum I've heard an oversized OT can be sterile. So all together I think you'd be best served with an OT that is appropriately rated for the beefier power tube you may use.

    I believe classic tone has a 5w-15w OT for beefy Champs. I have that in my SE El34 based plexi build. Sounds very good to me.

    Then you also should consider primary impedance. Splitting the difference is probably good enough for rock and roll. But you're going to make a compromise by accommodating different output tubes.

    Might be worth exploring if there is a way to tinker with your OT through switchable circuitry that changes impedance. Zobel network? I'm not very confident on this, just a thought.
     
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  4. Wally

    Wally Telefied Ad Free Member

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    Small OT saturation compared to larger OT....
    5E7/5F4/5E5A versus 5F6A.
    BF Bandmaster, Tremolux, Vibroverb, Pro, Pro reverb versus Super Reverb and Bassman.
    Yes, the size of the OT has something to say about the output.
     
  5. Mongo Park

    Mongo Park Tele-Afflicted

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    This is an interesting idea to work through
    Uncle Doug has a great video that might help



    I think this is the one, kudos to uncle Doug
    I Am no expert so more detailed info can come from others. I think the video explains it well So you want to decide what impedance your tubes need and what watts output you want, as in how loud it is going to be. A low watt op transformer will not play a 6L6 any louder than a 6V6, louder cost more money as in it goes up quite fast like a audio log pot.
    For me I would build it with two output tube sockets one for 6V6 and one for 6L6 with a a caution that both tubes going at once might be too much
    This way each tube can be run to its best advantage. I am not smart enough to figure one socket and get glorious tones from each different tube.
    Impedance can be changed with different speaker tap 4ohm vs 8...16. So this would require having a speaker Jack for each tube, otherwise some overlap impedance should be picked, they do have a range. About 6k would be the point where both tubes overlap but not ideal for either.
    I am curious to see how other would do it.
    Cheers Ron
     
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