2P29L – Preamp and driver for 4P1L PSE Amp

A very interesting Russian directly-heated pentode related to 4P1L is the 2P29L. It has a similar mu (μ=9), much higher anode resistance 2.8-3KΩ and transconductance of 3mA/V when triode-strapped. The filament requirements are much lower at 120mA. I picked one valve from my collection to submit it to the mercy of the curve tracer:

2P29L test point (pentode)
2P29L test point (pentode)

The triode curves are really nice:

2P29L triode curves and model
2P29L triode curves and model

This valve is as linear as the 4P1L (hooray). As a preamp it can be easily implemented like the 4P1L Gen2 preamp using a gyrator PCB which simplifies the building process:

2P29L preamp
2P29L preamp

Running it at 15mA and slightly above the recommended 160V achieve its lowest distortion.

We could also use this valve as a driver for a 4P1L preamp, which comes very handy for filament bias:

2P29L-4P1L PSE Amp

2 Replies to “2P29L – Preamp and driver for 4P1L PSE Amp”

  1. Dear MOGLIAA,

    I guess someone has to ask the question …

    Interested in your sonic assessment of the 2P29L compared to the 4P1L.

    This “new” tube is certainly affordable!

    Do you leave the metal jacket on? I assume there is a glass envelope beneath this.

    Is that really a knob on top to easy extraction? That is a great idea if it is.

    Take care,

  2. Rick,

    Under the aluminium case there is indeed a beautiful glass bulb, like the one of 4П1Л but smaller. It is definitely worth the effort of removing the ugly case. I saved only the bottom (socket) part and glued it to the glass body.

    Happy DIY-ing,

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