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.
Can I ask a couple of questions about removing the aluminium can please:
Hi Ray, you can remove the metal can without any issues. You can then cut the base with a Dremel. Glue should be whatever you have at hand that will glue metal and glass. You will find it very easy to do.
Hello Moglia
I’m planning a 2P29L PSE for my HD600 & DT990. Have you ever tried 2P29L in power stages?
I don’t think you need to go down the PSE route. You can drive the headphones with the right output transformer in the way I did my D3a/6e6p or E282F headphone amplifier: https://www.bartola.co.uk/valves/2019/12/25/e282f-hp-amp/
Hello Moglia
I’m excited to learn that 2P29L can produce fansastic sound as preamps. It’s Chinese equivalent is 2P29S, which is even cheaper than the Russian one… Highly accessible in mainland China.
Thanks!
I don’t have experience with the Chinese version of it am afraid. The Russian is also cheap and available.