I’ve been posting not very frequently lately. This is mainly due to lack of time and the level of business travel which reduced to nearly none the time available for DIY audio.
Nevertheless, the scarce time always pays off. It’s incredible how selective I have to be in order to prioritise which project I should work on. The list is long though.
Last time I did a quick exercise on the Ba DHT based on the curves I traced and the LTSpice simulation. Well, you always need to build and test in order to check against simulations. The result is, that you may need to adjust and learn from your practical experiences.
The Ba (like the Aa) are tricky to use. They pick up any electrostatic induced noise. You don’t need even to place your hand close, the mains noise is induced already in its plate. This force you to shield these valves if you want to use them. Am afraid, that is what it is. My friend Rob (DHTRob) warned me, thank you.
The circuit I posted here, had to be readjusted. Distortion was way too high. The operating point wasn’t good enough. You’d normally get inclined to run the valve as hot as you can, but I was wrong here with this one.
The distortion is about 0.09% for 2Vrms and 0.1% for 4Vrms. It’s about 0.15% for 10Vrms or less at this operating point. What is interesting to see below is the intermodulation of the 50Hz and 100Hz byproducts of the noise picked up on my workbench:
The 115kHz bandwidth of this stage can only be achieved thanks to the source follower.
If you’re planning to build this stage, get a nice shield screen for the valves. Either a mesh or enclose them under any sort of faraday cage. It won’t be practical otherwise.
The circuit is very easy to implement if you use a gyrator PCB and a Source Follower PCB. This reduces the complexity enormously.
Now it’s time for listening to this stage after modding the Mule….
Do you have any examples or advice for making mesh tube shields? I think I could use shields on my 01a preamp.
Hi John,
Probably there are some people out there who can better advise on this. I never had to build a shielding mesh.
For the 01a, I’d strongly suggest finding other valves which may work well. In particular the ST versions are really good. You can get a dead quiet system with the 01a without a shielding mesh. I use my UV-201a which are really old valves perfectly well.
I’m sure you can find a thin bronze mesh which can be easily shaped in a cylindrical shape. Also something like this would work
Cheers
Ale
Hi Ale,
Hope ya family are all well and growing healthy.
Argentine rugby is improving with the increased international competition.
Have you simmed your circuit with R4 bypassed with a cap?
best
tim
Hi Tim
All good thank you, and yours?
Rugby is a bit challenging, Pumas haven’t had a great season, but getting better I hope.
Bypassing R4 can create a null at desired frequency to improve PSRR. Is already good without about 70dB. You can get it down to 100dB. For example with a 220nF and R4 being 220k you can create the null at 50Hz.
Worth trying, I will…
Hi Ale, I’m very interested in how this develops and sounds! For curiosity – are the Aa/Ba thoriated? Are they bright?
nice to meet you. I am learning a lot from your site and I appreciate it. I am making a preamplifier with a Ba tube, so I am looking for a Ba spice model. If you have any information, please help. Thank you.