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tape recorder to amp conversion

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  • #16
    R1 & R2 are connected to the cathode of the power tube instead of ground as this will raise the heater voltage by the 14VDC at the cathode, which will, in turn, reduce hum.

    Yes, you'll need to figure out what the power transformer is putting out. You can measure it unloaded and then use Duncan's Power Supply designer app to figure out how to get the voltages to where you need (assuming it's high enough. I suspect it'll be good.)

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    • #17
      Thanks, I measured 562v, and downloaded the calculator. Not really understanding it all right now, so I will read the help file, probably have some questions, maybe learn something.
      Vote like your future depends on it.

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      • #18
        Okay, so I'm not sure I'm doing it right, but I measured 277v from ea. secondary to ct. and 230 ohms resistance. I plugged those numbers into the calculator. Am I right in assuming that the results I have to pay attention to are v(c1) v(c2) and v(c3) If I did the calculator right i have 374, 370 and 366 volts. That's too much for the 6aq5, right? So I need to plug in different resistors 'till I get the appropriate voltages? also, that last resistor (load) I entered 100k, the value of R9 in the schematic I posted earlier.

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        Last edited by dmartn149; 10-08-2015, 04:05 AM.
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        • #19
          I had it in my head that it was a 6bq5. You could run a 6BQ5 at those voltages (although they're beyond the maximums on the datasheets, but the Heathkit AA-161 ran the plates at 370V). I doubt you'd want to run a 6AQ5 at those voltages, though. The microchamp's transformer is about 80 volts lowers. That's a lot of voltage to bleed off with resistors, but you can, I suppose. I think I'll let someone more experienced than I chime in on this.

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          • #20
            Ignore these high voltages you got with calculator. Your load isn't right ( 100k is almost no load).
            If your tape recorder PT was configured to work with 6AQ5, you're good to go.
            I would recommand to fallow completely 5F1 diagram and use same resistors values for B+ string for less hum and better screen protection for 6AQ5.
            For cathode resistor use the value from tape recorder to start with.

            Cheers, Damir

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            • #21
              Thanks Damir. It is the PT and tubes that were in the recorder, so I'm sure you're right that it will work. I just didn't think to record the resistor values when I removed all the components from the chassis. I might just do as you recommend (build it like a champ) then check voltages.
              Vote like your future depends on it.

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              • #22
                You're welcome 470 Ohm (as in 5F1) for cathode resistor is good starting point, but, most likely you'll have to settle down with 250 Ohm for 6aq5.
                Just measure voltage drop on it , do calculation of bias current and power dissipation and adjust it as needed.

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                • #23
                  Originally posted by epis View Post
                  You're welcome 470 Ohm (as in 5F1) for cathode resistor is good starting point, but, most likely you'll have to settle down with 250 Ohm for 6aq5.
                  Just measure voltage drop on it , do calculation of bias current and power dissipation and adjust it as needed.
                  The schem I'm using has that resistor @ 470 ohm. (see post #15) It's Adam Alpern's Micro Champ. I figured he's already done all the heavy lifting, so I planed to build it (mostly) like the schem.
                  There is more to re-purposing this old amp than I initially realized. Just the power supply kind of has me baffled.
                  Vote like your future depends on it.

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                  • #24
                    Okay it's finished and working great, if anyone is interested. I mostly followed Adam Alpern's Micro Champ schem. I deleted the cathode bypass cap on the second half of the 12ax7 and didn't include a triode switch for the 6aq5. I also added a second channel that uses a 5879 for the first stage then feeds into the second half of the 12ax7. I used the schem from the 2nd channel of a Gibson ga90 as a guide for wiring the 5879.
                    It sounds really good. I think even better than my Epiphone Ea50 Pacemaker.


                    thanks again to all who offered advice.
                    Vote like your future depends on it.

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