Solid state power transformers

Discussion in 'Shock Brother's DIY Amps' started by Rock_Glenn, May 5, 2018.

  1. Rock_Glenn

    Rock_Glenn TDPRI Member

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    I've built six tube amps, four of which started their lives as crappy solid state amps i got for nothing. It's absolutely killing me that I have three low voltage power transformers kicking around with no project for them.

    One is a approx 150kva 120 x 50vac 2wire secondary from a peavey 100w head.

    Every solid state schematic shows a 3 wire secondary, ie. +50v, -50v with a center tap. Can I create an artificial center tap on the 2 wire secondary to give me +25, -25 and a reference of 0 volts? Is it safe?

    I'm really digging some of the rickenbacher ss amps and would love to build.

    I also have two power transformers with various taps 24v 36v 50v and 6v... I can see using the 6v for a heater on a 12ax7 preamp, but the other voltages are probably too low for a tube pre. Any ideas, or existing circuits for me to play with using these pt's?

    Am I crazy, and should scrap these and build an ac30 instead?
     
  2. richa

    richa Tele-Afflicted

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    You could just use +/- 50 and limit the sensitivity of the power amp so that it doesn't swing to the rails. In effect make it so you can't turn it up that loud. That said yes I think you could choose to use half the coil with a virtual ground for the circuit ground. But you have to keep it balanced and that can be problematic. Look up virtual ground circuits (Elliot Sound has good discussions of that).

    And if you've got transformers to burn you could consider a voltage doubler circuit to get a useful b+. I think uncle doug has a video this. Again the Elliot site has some cautions about doing this.

    All that said if you want to try something SS look at doing a current drive SS amp. I built one and love it.
     
  3. wanderin kind

    wanderin kind Tele-Meister

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    you could use the xfmr with the 6 volt heater backwards as a tube output,

    120/6 = 20:1 ratio = 400:1 Z = 1600:4 Ohms or 3200:8 Ohms or 6400:16 Ohms,
     
  4. Rock_Glenn

    Rock_Glenn TDPRI Member

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    Richa balancing the virtual ground is my concern, I'll read up at the Elliott sound site and make a decision. The class a current driven amp seems perfect, though my iron is going to be a bit hefty. I dismissed a voltage doubler for a few reasons.


    Wandering kind, is it common for people to use pt' s as ot's? The math makes sense, I have a box of various primary x 6,10,12,24 sec that are suddenly useful if that's the case. Though these are more of the industrial control variety as opposed to audio, would I run into any issues based off that labelling?
     
  5. Finck

    Finck Tele-Afflicted

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    Last edited: May 6, 2018
  6. OneHenry

    OneHenry Tele-Holic

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    Excellent article, and Google Translate works well for translating it. The takeaway is that a power transformer can work as an output transformer and the result can be listened to.
     
    Finck likes this.
  7. Finck

    Finck Tele-Afflicted

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    Yes, it works!

    The only concern is about the "gap" in the iron of SE transformers, which a PT doesn't have, since the E's and I's are interlaced. It's recommend to disassemble the iron and rebuild it the "right" way.

    Not a difficult task, but requires patience.

    Just as information, I have tried an audio line transformer (an impedance matcher used in ambient sound, that has a 5K primary and 4/8/16 ohms secondary). The iron has the same issue, interlaced E's and I's, but I had no problem to fix it. It was very cheap and worked like a charm with a 6V6 (actually a 6AQ5/EL90) small SE amp, better than the specific OT.
     
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