Need an output transformer recommendation

Discussion in 'Amps and Cabs [BG]' started by Pill Bass, Jun 9, 2019.

  1. Pill Bass

    Pill Bass

    Feb 1, 2005
    Alright all you tube amp gurus out there, here's a interesting one for you:

    So the output transformer on my Carvin VTR-2800 amp got slagged (long, face-palm-worthy story), and I need to find a replacement. The original is marked OT543A, which I believe is a Carvin part number, because if there is any information or specs on that transformer out there, my Google-fu is not good enough to find it. I guess I'm stuck having to buy a modern transformer with similar characteristics as a replacement.

    The specs on the VTR-2800 are: solid-state preamp, tube power section with 4 EL34s in a push-pull configuration. The OT543A has 3 primary taps, with the center tap running at -480V (I think; Carvin's schematic is a bit difficult to read) and secondary has taps for 8-ohm, 4-ohm, and 2-ohm speaker loads. What I do not know is the impedance of the primary coil.

    I've found what I think is a suitable replacement in the MojoTone model 774. My concern is that the 774 shows 2250 Ohms as the impedance between the center tap and each tube plate leg.

    So the end result of all of this rambling is: Is 2250 ohms a suitable impedance for a 4xEL34 amp running at 480V or should I be looking for something with a different (higher?) impedance?

    Any and all input and advice accepted!

    Last edited: Jun 9, 2019
  2. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
    So this transformer isn't going to be a good match for your needs because it was designed for a pair of 6L6's. Regardless of the primary impedance, it doesn't have enough core area to support 100 watts and 4 tubes. The giveaway is it's size, weight and intended models.

    PM me, I have an output transformer on the shelf that will work for your application, specifically 4 x EL34's.
  3. BogeyBass

    BogeyBass Inactive

    Sep 14, 2010
    1.8k to 2k being common load line
    For 480 volts

    Marshall JCM 800 or 900 style output transformer will work fine.

    800 was little smaller. Saturated krap outta it, 900 was larger usually more taps.

    Hammond 1750x is 900 style transformer with 16 8 4 and 2 ohm taps.

    I'd double check your schematic. And of course use correct tap for negative feedback. I'd assume either 8 or 16 ohm tap

    Some designs will use 50 watt transformer. Use smaller power transformers. So 4k primary magically becomes 2k with more tubes. 16 8 4 becomes 8 4 2 . Use 4 tubes but power transformer only good for 70/85 ish.
    Super saturated early breakup with wimp transformer and small output transformer.

    Intentionally design for distortion

    Transformer set to a ratio not fixed impedance. So you can double or half the ratio depending on secondary load and needed primary/ tube count etc. Giving the ohh so classic spongy 2 ohm tap. Lol
    Last edited: Jun 10, 2019
  4. catalin gramada

    catalin gramada

    Dec 25, 2014
    Hammond 1650T or TA. Get in my amps 120w clear sine with a quad of EL34 at 480v down to 30 cps. in pentode mode. I can post my measurements if You really interested. Bonus, it have ultralinear taps so can be connected in ultralinear mode very easy to try if is more convenient for You bass application. (Edit: You can test play same circuit Bassman 100 vs 135 to see how differences between pentode and ultralinear connection works in low freq aplication).
    Last edited: Jun 10, 2019
  5. BassmanPaul

    BassmanPaul Inactive

    According to mullard the correct P-P load for a pair should be 3.8KΩ. For a quartet 1.9KΩ. Rounded up to 2K Canada’s Hammond make a whole range of OPT’s so you can select from their list. As @Catalin gramada posted, the 1650 should suit your application admirably.
    Last edited: Jun 14, 2019