4. Preamp

First Stage

From the input jacks, the signal is first amplified by a 2N1273-5 transistor, which research suggests is a germanium PNP.  These are not used in anything anymore, and data on the transistor is scarce.  There may be some old stock available to purchase, but I can’t be sure about this.

Both channels use the same basic circuitry, but there are minor differences in a few resistor values.   I believe this is because someone replaced the stock resistors with the closest value they had at hand.

—————————————————————————————————————————————————————

Top side of the first stage board for one channel:

—————————————————————————————————————————————————————

Connections on the bottom side of the first stage boards (both channels):

—————————————————————————————————————————————————————

Note that the boards are pried apart in the above photo.  This wasn’t possible with the other two boards, as they are “short-wired” to the tube sockets, and each other.  Yep, still complaining.

Electronically, the first stage circuit looks like this:

I’m no transistor guru, but this circuit ought to work.  However, given the nature of germanium transistors, I’ve been told this may not be the most temperature-stable circuit in the world, and it’s sitting in a small box with a lot of tubes.

As indicated in the schematic, the transistor is powered with negative DC.  This (V-) is wired directly to the bias voltage point (grid leak resistors) for the power tubes, which is how I know it is powered with negative DC (plus the fact that nothing else would make this circuit work).  From the power tube bias point, I would assume a wire originally ran to a negative voltage point on the rectifier board, but there was no connection in the amp as I received it.

In case anyone cares, here’s the data I have on 2N1273:

  • Ge PNP Lo-Pwr BJT
  • Various
  • V(BR)CEO (V)=15
  • V(BR)CBO (V)=15
  • I(C) Abs.(A) Collector Current=200m
  • Absolute Max. Power Diss. (W)=250m
  • I(CBO) Max. (A)=14u
  • @V(CBO) (V) (Test Condition)=12
  • h(FE) Min. Static Current Gain=27
  • h(FE) Max. Current gain.=165
  • @I(C) (A) (Test Condition)=50m
  • @V(CE) (V) (Test Condition)=1.0
  • h(fe) Min. SS Current gain.=18
  • @I(C) (A) (Test Condition)=1.0m
  • @V(CE) (V) (Test Condition)=5.0
  • @Freq. (Hz) (Test Condition)=1.0k
  • Package=TO-5
  • Military=N

Second Stage

Both channels have an identical transistor-driven first stage, described above.  Both channels then send the signal through a typical 12AX7 triode gain stage.  The reverb/tremolo channel is shown here:

S950 Preamp Stage 1 and Stage 2

Nothing odd about that.  Any circle with the letter “V” in it represents supply voltage coming from the power supply.  The 100K resistor from V7A grid to ground does seem to be dumping a bit of gain.

What happens next is a bit odd.  See the Tremolo page for that.

One Response to 4. Preamp

  1. Bill Takatsuki says:

    Great story ( and site), I can’t wait to see the next installment.

    The transistor stage that you have drawn is a very typical “common emitter” design. Common convention would have the transistor drawn with the emitter and ground connection going down, and the collector and power supply at the top.

    Because the transistor is a PNP, the supply voltages have to be negative. If the transistor was an NPN the voltages would be positive. If you are a Fuzz Face fan, this is why Germanium FF’s need a different power supply than the Silicon versions do.

    Also, the photo of the board shows both input isolation resistors as 4K7. Does the second channel board have one 4K7 and one 3K9?
    [Ed. Note: Yes, someone put in a 3K9 at some time]

    And here’s something to tease you with, did you know that Lectrolab made a copy of the Gibson Maestro Fuzz pedal?
    [Ed. Note: More! More! We must know more!]

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