The Mule saga continues and it was time to modify the 3B7 preamp and to test the 2P29L valve. This was quite easy as they both have loctal sockets. I had to modify the Rod Coleman filament regulator to set the filament current down to 120mA. Then a bit of wiring work, and in less than an hour a new preamp was ready. Job done, this is why I built the Mule:
The circuit is quite close to my original design time ago. I modified the filament resistor to use an existing Russian NOS wirewound part I had in stock and suit this preamp quite well. Added grid and screen stoppers as well:
The rest stays the same. The gyrator board is based around the BSH111BK and the top MOSFET is IXTP08N100D (with its own heatsink).
I have a series of daughter PCB boards to adapt the BSH111BK to the gyrator PCB. I will be offering these shortly as pre soldered boards with the gyrator.
The 2P29L is run hot at is nearly maximum Pa. Well, I’ve got plenty of these so I won’t worry about it. It sounds better at 20mA in my opinion. If you can lower the HT a bit may help with power dissipation of the top MOSFET. I’d go for 200V to provide enough headroom.
Frequency response is really good, mind you there is no 100K load (I forgot to add it on my test) as in normal operation. When loaded it will reduce the response. However, this is really good. Gain about 18dB is as expected and similar to other DHTs I use. Enough to feed the output stage (4P1L PSE).
Distortion is about 0.045% for 5.5Vrms and 0.12% for 11Vrms output.
How does it sound?
Well, another great DHT in my opinion. Compared to the recent version of the 3B7, the 3B7 lacks of some warmth and or strong body sound I think. Hard to explain, I liked it but not as much as the other DHTs I have been using. This one has a powerful bass and clear treble. No coloration, great sound overall.
I wasn’t wrong when I said this was a good DHT to explore. Microphonic noise is minimal, surprising. No need of special arrangements.
Hi Ale,
I sent a couple of current feedback schematics to
Bartola
Didcha get ’em?
best
tim
Hi Tim, I had. I’ve been travelling this week to Singapore and back. Not much time to do anything else! Will look at it thanks
The 2P29L is one of the few good DHTs that has so far escaped my clutches. But I’ve now put this right with a preamp build using a Hammond 126C as plate choke and a couple of FT-2 caps giving me 0.2uF out. 150v A-K, 7v bias and a 62R Russian military cathode resistor in filament bias. Very simple build, and I wanted to use some of my 126C which have been on the shelf since they didn’t quite suit the 26. Well – they certainly suit the 2P29L at 15mA. Plenty of inductance at 106H. No hum, no microphony. Sound is great – extremely detailed with plenty of treble and bass. This tube is rather off the beaten track, but Ale has brought it to the table and I’m enjoying it! Thanks, Ale. This is a real contender, and 15mA is better for driving PSE 4P1L outputs than the 3mA or so out of the 01A, however wonderful that sounds. I’ve only listened for 15 minutes, but really liking what I’m hearing so far. Andy
Hi Andy,
I’m glad to hear you liked it. It was a great discovery in particular given the low requirements on filaments as well as its low microphony. I think is a great valve to use, in particular as a driver.
I’ve played it for several weeks and I love its sound!
Cheers
Ale
And if you try it with a gyrator load, it will sound much better 🙂
Ali
Have you tried tying the suppressor grid back to the Kathode with a .1uf cap? Steve deckard does this on his Russian el84 amps. I tried it it does some nice things. I am getting to the 2p29l over the next holiday and I’ll try there too
Hi David
No, I haven’t. Would be interested in hearing your results and feedback once you build the 2P29L preamp. Thanks Ale
Has anyone got the output impedance for this preamp (built according to the schematic above)? I have tried to do it myself using Ale’s blog on the subject but I’m no EE and maths isn’t one of my strengths – I used Ale’s ‘Proof of the PUdding’ example from his Zout blog to create a spreadsheet but I am just guessing with the values I need to substitute for the 2P29L use case.- I think Rp is 2K8 and Rs is 470R but Gm is a mystery to me?
Hi Ray, gm in that formula is actually the Gfs of the lower FET. If you have a BSH111BK running at 20mA you can estimate about 200mS.
Hi Ale,
I’m wondering … you mentioned changing Rod’s boards to accommodate the 2P29L filaments; what changes did you make?
Thanks and Regards,
Robert
Hi Robert,
The change as mentioned is to accommodate the current range, a simple current setting resistor as indicated by Rod. My “Mule” preamp has been bastardised after so many variations and tests. I hacked Rod’s boards to provide a simple way to connect different current resistor by plugging them with a 2mm gold connector (plug and PCB socket). That’s it.
Happy building!
cheers, Ale
Hi, I made the preamp with 2p29l as per the diagram but a problem appeared
I power the gyrator with a power supply with 2 6u4n valves per channel, anode and filament supplies all separate) I apply 280 volts. but I find 72 volts between ground and pins 3, 4, and 5 of the valve and on the 10 hom resistor I measure 1.5 milliamps. The gyrator components are all original, I tested both with IXTP08N0100D and with
AOT1N60 with the same results. If, however, I carry out the measurements at the same points as before but with the valve unthreaded (removed) I obtain a voltage of 170 volts. Could it be a valve problem? I got them new in the original box from eBay, I don’t want to have been deceived perhaps that they are used and sold as new.
This same problem also appeared a year ago with the 4P1L preamp abandoned due to microphonics; somehow it worked for several months with the standard measurements of both 152 volts and 25 milliamps for absorption then it started heating the Rmu from 470 hom and if I’m not mistaken I changed the M3 moffet but I no longer got the values standard. I changed both the LD150 fets and the IXtp08 mosfet…. and nothing like before. I measure 68 volts between the ground and the RMu 470 hom absorption on R5 2 milliamps I remove the valve and I get 148 volts on the Rmu from 470 hom I bought 2 other new 4p1l valves but when I inserted them the voltage dropped to 68 volts. and so I asked myself that it certainly won’t be a tube problem but I’m thinking of the mos fet combined with the bsh111 bk, so I abandoned the 4p1l preamp and moved on to building the preamp with the 2p29L but I find myself as I said at the beginning with the same problem.
Now to see if it was the tubes I excluded the gyrator by feeding directly with an 8.2 k resistor and I get 145 volts on the R-Mu and 20 milliamps as absorption but I’m not satisfied because there is a background noise in addition to the normal one.
so I ask what is the problem that causes the gyrator to lower the voltage when I insert the valve?
Hi Andreas,
Sorry to hear that experience. Unfortunately I get several emails asking for help in a similar way of describing an issue at a very high level without diagrams, pictures, measurements and exact documentation. It’s extremely hard to follow and advise without embarking on a lot of time, effort and hundreds of emails back and forth.
For example AOT1N60 isn’t a device suitable for the gyrator board so have no idea what you have been testing and changing.
If you cannot separate the source of the issue, I suggest dealing with that first. If you suspect it’s the valve, test the valve, socket and everything first separately before adding the gyrator board. Alternatively you can test the gyrator board with a well-known working valve which you trust and ensure it’s working well. Otherwise you have too many variables to explore at the same time, not good.
I will also ask you to email me instead of posting on the blog for support/troubleshooting as this isn’t the place nor the format to try to help out.
Cheers, Ale
Grazie infinito manderò in seguito documentazione con foto alla tua e-mail
Ciao Ale mi mandi l’email sulla mia e-mail così posso mandarti un po’ di foto delle schede ed evidenziare le varie misure