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.


8 comments:

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

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    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 - https://jelabs.blogspot.com/2014/01/altec-2-way-horn-system-redux.html - I enjoyed listening to this similar hi-mu triode/SRPP input amp! So YMMV!

      JE

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    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?

      Ritchie

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    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.

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    4. Sounds good.
      Thanks very much.

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    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?

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    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.

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

      Ritchie

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