Finally I managed to build a stable version of the 2P29L preamp. The wooden modular version inspired on DHTRob designs paid off. Here it is:
The result is fantastic. The 2P29L is a superb DHT, quiet and it sounds amazing. The circuit is similar to the original one but with a slightly different operating point due to the valves themselves:
I’m still running them at 20mA. This time the anode voltage has to be increased to 150V to achieve this. Consistent across the 2 valves used.
C2 was modified to fit a gift I received from my friend Vyacheslav. Instead of the usual 220nF, I fit a bigger 470nF FT-3 version which improves the LF response.
The frequency response is very good, as I published before. For the curious ones, here are some further pictures of the build:
I can’t say more than what I said before. This is a fantastic DHT, it provides a great detailed sound and is quiet. No microphonic noise and the dynamics are great. One of my favourite clearly.
Question is Ale, which sounds better, this or the 4P1L?
A million dollar question. Well, they both sound fantastic. I have a 4P1L in screen mode which is sounding great at the moment. The G3 is meshed and performs brilliantly. The 2P29L is unique, very quiet and no microphonic.
I probably need more time to give an answer re preference.
Would you Day this sounds better than 4P1L?
I don’t like that question. The answer is it depends. I think both are very close. I don’t think I have an answer for it yet. haven’t listened to both and compared them carefully to highlight any differences.
The 2P29L has 2 clear advantages: no microphonic noise and low filament current requirements. I think is worth considering this given many are suffering on the 4P1L microphonic noise to a certain extent.
Hi Ale & Barry,
I’m wondering if the microphony of the 2P29L that Barry observed might be related to date of manufacture…could both of you please indicate the date codes of the valves you used, and whether or not they were microphonic?
Also, what does it take to remove the aluminum shield?
Thanks and Regards!
Hi Robert,
I can’t comment as haven’t tried many different 2P29L myself.
Removing the aluminium shield is just an aesthetic thing, no further impact in the audio band. It’s a shielding mechanism for RF.
Cheers
Ale
Hi Ale, I have just finished building the 2P29L have a couple of questions regarding it’s use according to data info found on this Valve, Anode current should be be 10mA, I have run them at 15mA/138V, 18.5mA/151V & 20mA/162V, there is very little difference in sound imho, I also tried coupling caps 220nF & 680nF, here there is a difference, although the main difference is in the top end. Also I do have problem with Microphony, more so in fact, than with the 4P1L, did this settle down after Valves were burned in? Rod Coleman also reckons that Filament Bias should be around the 17V mark, not sure that’s right though. I have built it in a single chassis with divider.
Hi Barry,
That is a surprise. I have to admit I only tried 6 valves for 2 different sources and all of them were dead quiet from the beginning. Not a sign of microphony, hence my comment first time around. Did you try other valves?
Ale
Hi Ale, I tried 2 pairs of Valves & got a couple that were reading within a couple 0.5 Volts at 15mA, 0.7 Volts at 20mA. They were both quite a bit worse than the 4P1L’s that are in my other build, incidentally the 4P1L in screen mode is the way I;ve decided to go, it truly isn’t far behind my 01a, I have found this to be a lot quieter that the 2P29L & more detailed. I did find th3 2P29L to be a warmer presentation, but it was also not as tight & I tried 2 different coupling caps, .22uF & .68uF, although the latter improved Bass Response, it dropped the Top End performance noticeably. If you aren’t getting the same results as I did, then I can only think it’s a batch problem with Valves! I did get the same problem with both Valve & to the same degree, I also had an initial problem with oscillation, which was triggered by microphony, this would disperse when holding the top of the valve, changing grid stopper to 200 ohm reduced it & the performance!
I had no such problems with 4P1L, which I really am quite enjoying at the moment. Valve vary unfortunately & this has bought that to the forefront, I have enjoyed the challenge though.
Hi, I have to connect a crossover per channel with a 10,000Ohm impedance and a subwoofer plate per channel with a 10,000Ohm impedance to the preamp. Can the preamplifier withstand this load? Should the value of the capacitors be changed?
Thanks, for the attention, greetings.
Hi, I have to connect to the DHT 2p29l preamplifier one crossover per channel with a 10,000Ohm impedance and a subwoofer plate per channel with a 10,000Ohm impedance. Can the preamplifier withstand this load? Should the value of the capacitors be changed?
Thanks, for the attention, greetings.