Sunday, August 12, 2012

JE Labs SRPP 45/2A3

SRPP 45 and 2A3

In general I prefer the simplicity and purity of a plate loaded voltage amplifier [R-C couple], however i found SRPP [shunt regulated push pull] sonically satisfying using low-gm, hi-mu tube like the 6SL7/5691. if you find the Simple 45/2A3 a bit slow or lacking in definition, then this may be your solution.

I do not have the technical background to fully explain how SRPP works however through pure logic, the top half of the triode acts like a constant current load to the plate of the bottom triode and the drive signal is basically a combination of the low impedance nature of the top half's cathode and voltage amplification ability of the lower half, Giving this input/driver circuit greater potential drive signal to the grid of the output power tube.

This is a simple and straightforward design and the only thing I need to point out is the voltage divider network in the power supply that lifts the cathode potential of the 6SL7/5691 around 70-90 volts above ground. Thus, the +/- 90V heater [filament] heater to cathode rating is not exceeded as per the RCA manual's recommendation.


  1. How did you find the srpp 45 version vs the simple 45?

    1. Back in the late 90s, I felt that the Simple 45 played more LPs and CDs whereas the SRPP was more audiophile oriented presenting more detail which on long term listening was not as pleasant. However, I had a chance to hear a Morikawa SE 2A3 owned by a friend - - I enjoyed listening to this similar hi-mu triode/SRPP input amp! So YMMV!


    2. Thanks JE, I'll go with the simple 45 for now as I've been wanting to do it for awhile. I'd like to use a 6sf5 as a driver as I have them in my junk box. Would the existing circuit be ok or should I change the parameters to better match the 6sf5?


    3. The original circuit works well as-is with a 6SF5. But if you want to compensate for the slightly higher Rp, use a 150k plate load resistor instead of the 100k.

    4. Sounds good.
      Thanks very much.

    5. I am using 105db sensitive horn speakers. Do you think ac on the filaments would have low enough hum to not be heard on this sensitive of a speaker?

    6. My Altec 2-way speakers are about 99dB/1W/1M and 0.2mV of hum from my SE45, 46 and 2A3 (all 2.5VAC filaments) is hardly audible. If you get hum levels greater than that, there might be wiring/grounding issues.

      I just don't know how that translates to a 105dB speakers.

    7. I'll try the ac first, a lot easier to implement.
      Thanks again.