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:
The triode curves are really nice:
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:
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:
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,
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,
M.
Why save the bottom part? And how much of it (along the tube axis) are you saving?
Cheero!
RCC
I saved the bottom part for convenience only, so I know where the key is when I insert the tube in the socket. I cut the envelope down to the flat bottom part, leaving only the “disk” and the key.
M.