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  • Tektronix preamplifier

    Tektronix type 122 preamplifier. Picked this up at the local flea market along with some tubed medical equipment. Recapped the power supply and it works. Going to use for guitar, or a conversation piece anyway. It's a differential amp so it works if i ground one side of the input. I made a simple pnp phase splitter (filaments are -6vdc so i used that) for input to the preamp. Here are my 3 questions, the 10meg resistors on the V1 (12ax7) input, very high, should i reduce to 1meg so the tube doesn't run away? V2 (12au7)also has 4.7 meg, should i drop that too? Also, what cap should i use for the output? I have a 10uf soldered there now because it was on the bench. Will a 0.1uf be ok? And yeah, i can get 15vrms out of it, i think the spec was 20mv for full output.Click image for larger version

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  • #2
    Replying to my own thread. Since there is a negative voltage on the cathode, these are set up as grid leak bias? So, the high value resistors are needed. I think anyway. Data sheet for 12ax7 and 12au7 shows 10meg when cathode is at 0.

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    • #3
      Drop it so tube doesn't run away? Wha??? I doubt the Tektronix folks had a problem with runaway 12AX7s.

      If you plan to turn it into a guitar amp, then change whatever you like.
      Education is what you're left with after you have forgotten what you have learned.

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      • #4
        This is very similar to the Type E Plug-in that serviced the 530 & 540 series scopes of that era. I've hung onto my Type 132 Power Supply, which allows me to use any Letter Series or 1-series plugins from those scopes. The 132 supply allows differential output, while the 131 supply, accepting the same plug-ins, only yields single-ended outputs. Those early plug-ins are a thing of beauty in construction, and lend themselves nicely to mods for audio use. The Type E has a lot more gain available than the 122. I also have one of the coveted blank plug-ins for that series so when the desire motivates me, I can build what I want into that plug-in and have it for use in the 132 Power Supply. These plugins sell for peanuts, and have for years, as the demand has long since dropped off. But, are a great place to start for building a nice preamp.

        Among the vintage plugins I've kept are: 1A7, 1A5, Type W & Type E...all differential single channel circuits. The 1A7 was the first all-solid-state high gain differential plugin that has been packaged as the front end of 5030 & 5031 dual beam scopes (10uV thru 10V/Div), 7A22, 5A22, AM502A (TM 500 series plugin), and on into the 11000 series plugins. The 1A5 and Type W have a DC offset generator to allow DC coupling at high gain in the presence of high voltage. The Type E had a sensitivity of 50uV/Div in those early scopes, with the same LF/HF response filters (single-pole rolloff).
        Last edited by nevetslab; 04-22-2019, 03:59 AM.
        Logic is an organized way of going wrong with confidence

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        • #5
          This was part of the deal also, very sharp trace compared to other scopes.

          Click image for larger version

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          • #6
            5000 series scope. I don't recall those particular vertical amp plugins, though recognize the time base plugin. BGW Systems had bought a number of 5000 series scopes in the late 70's, though that model (don't recall which) never did have a sharp CRT trace. This one you picked up looks far sharper.
            Logic is an organized way of going wrong with confidence

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            • #7
              Vintage Tektronix Letter & 1-series plugins & Power Supply for Preamp projects?

              I was able to pull up some images on the Letter Series Plugins from Tektronix, along with the service manual on the Type E Differential Plugin, which is very similar to your Type 122.

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              Type E.pdf

              I forgot about the Type O Dual Differential Plug-in shown installed in the Type 132 Power Supply. I also picked one of those up over the years, waiting for one of those periods of time to turn it into something. If memory serves, it's an all-tube design.
              Logic is an organized way of going wrong with confidence

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              • #8
                Yes, the trace is very sharp. Plug ins 5a24n, 5a24n, 5b10n. Just sold a B&K and need to get rid of a few more but the 5103 was part of a haul. I could always leave it in the garage for a backup to my backup. Going to a hamfest this week so I hope more plugins don't follow me home Except maybe a spectrum snalyzer. I have a 2465a and a 2430 I need to recap but keep putting that project off, wonder why.

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