At your risk.
Today two Pleiades V6 amplifiers drove again the Tascam Porta 03 cassette head. Cassete tape at normal speed.
This time, grids of EF183 at space potential was tried together with other variations.
Summary:
Pleiades V6, EF183 with Rag=1M5 Pleiades bias, Vb=12V, 10K between anode and head. Ia=head bias =350μA.
Not very good.
Although sound was easy, modulation low, much hiss.
Pleiades V6, EF183 Rag=oo ie grid at space potential, Vb=21V, Ia ie head bias 500μA.
Not as good as expected but nice. Distortion. Pleiades V6 does not seem to operate well as a power amp at these conditions. The signal sent to head should be distorted. Treble is somewhat better. Sounds better when electron tube underheated, more bright.
Pleiades V6, EF183 Rag changed to 4MΩ, Vb=5.5V, Ia=250μA-350μA
Possibly best sound so far. Modulation at playback above 0VU. There is more HF response. Sound improves with little cathode under heating. Some distortion.
Pleiades V6 EF183, Rag=4MΩ, Vb=6.3V, Ia=400μA-500μA
Modulation at playback above 0VU. Less treble, duller, less distortion.
Additional comments.
At some of these configurations the Sennheiser HD580 headphones were connected to the anodes. The sound was not clear and even worse distorted when the heads were connected too,
It seems the heads are not driven well with an undistorted signal in the first place.
So there is much more potential to the sound quality.
There seems to be incompatibility of having just 400μA anode current so that the heads are biased at 400μA too and at the same time the amplifier not distorting while delivering a high music AC voltage to drive the heads.
A different electron tube may be more suitable? ECC82, or a higher μ ECC83?
Or different values of Pleiades Rag pull up (electron tube grid bias) resitor and anode supply voltage may solve the problem even with EF183.
Perhaps looking at load lines and anode characteristics. Operating point at say 350μA yet grid potential at mid excursion point.
More experiments needed.
No comments:
Post a Comment