Obviously it was time to test the little brother of the “Ba” DHT. In particular, as I have such a low DAC in place now, I need the gain. With nearly a gain of 30, it’s an attractive fellow to work with.
First good sign is that it doesn’t pick up as much hum as the Ba. That’s good, nevertheless I placed a back copper plate which acts as enough earth shielding to keep the Ba quiet.
I worked with my breadboard to find a good “sweetspot”( at least electrically) for this valve. Given the low signal source level, I aimed for a low bias. I played with my fixed bias setup before replacing it with the SiC bias board. I found that 2V/160V was very good in terms of keeping the distortion profile to minimum. Here is what I ended up with:
Only 2 SiC diodes are enough. The source follower PCB is mandatory given the low anode current. I run it at 20mA (hot) to get best results of the stage. Rest of the circuit is very simple, achieving a gain of about 30. Here is the distortion profile:
There is no shielding and you can see some minor IM distortion with mains hum. Harmonic decay is nice with H2 being strongest. THD is very low at 4Vrms which is good sign. Let’s see the frequency response now:
Nearly 140kHz of bandwidth which is plenty for the stage. Great response. I just need to plug this one in and listen to this promising German DHT.
Ok. I plugged these on my “Mule”. Simple tweak as needed to adjust the filament regulators (these are not CCS based) and move the SiC bias PCB jumper to “2V” mark. The gyrator voltage was increased from 150 to 160V as per above tests. Great, 5 minute job and have the Aa preamp instead of the Ba.
Below picture shows the Mule with the Aa.
It sounds amazing, what can I say. As mentioned previously, my system needs the gain now as have an oDAC which has an appallingly low output level. Taking the gain of 30 aside, I can say that the tone clarity, detail and dynamic of the Aa is at top level. Probably one of the best DHTs I have heard so far in this setup. It’s not microphonic and with the existing copper plate (grounded) on the back, is dead quiet. I should try later to remove the plate and see if some hum is picked up. Either way, is not like the Ba which is a live antenna. I like this.
My pair of NOS “Valvo”are breaking in now, what a joy to my ears.
Compared Ba, which valve has better sound?
That’s never an easy question to answer as it’s all relative. In my case, I preferred the Aa to the Ba. Both are amazing DHTs and challenging to implement. You need the “back screen” plate to null the hum.
Also, mind you that with a gain of 30, it may be too much for most of the system out there. I’d use it if you have a low mu driver stage (e.g. 4P1L, 801a, 26, etc.) driving a transmitting DHT or a DHT which needs a large voltage swing signal (e.g. 45, 2A3, 300B, etc.). Otherwise gain will be far too much.
Thanks for your opinion, my plans is to use it as preamp after my phono stage. even with MC Step up and I crank up the volume pot, the sound is still low to my liking.
Hi Alle,
I like your articles very much and I really appreciated your inputs.
I am convinced about Aa tube so I’ve already got a pair via eBay.
I am going to use them in a two-stage s.e. schema driving the D.H.T.
6B4G (the Russian type 6c3c) Do you think is in off the gain to drive 6b4g directly,
-or- should I add a) an input step up in front (like LL7903)?
-or- b) maybe add another stage?
Please tell me your opinion
All the best!
Hi Angelos,
Thank you for your feedback, appreciated. The Aa has a gain of 30. You need 120Vpp to push the 6C4C to maximum level. Input level should be 4Vpp maximum. So you can get away with it depending on the source signal. Alternatively you can add an LL7901/02 for 1:2 step up. Two challenges though: miller capacitance reflected to source and also whether the Aa you have has good electrode alignment to ensure can swing 120Vpp at very low distortion, which I think it should be ok if its in good state.
You will have to use an AVC/TVC for the input as volume control if you add a SUT though.
Good luck
Ale
Hello
Thank you for your tube information.
If possible, I want to receive Aa
Spice model.
I had built Ba preamplifier. And
I will attempt to build Aa preamplifier.
Best regards.