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In the Forum: Melquiades Amplifier
In the Thread: The 6E5P tube data.
Post Subject: 6E5P BiasPosted by hagtech on: 2/20/2008
I run the driver tube at 170V @20mA. I don't think you need to run this tube at high current. It is very linear with wide signal swings at this bias. I can get roughly 70Vrms out of it. Output stage clips first, driver continues with perfect waveforms well into clipping of output. Oh, using transistor constant current source on driver plate. Output is push/pull parallel SE. I can set the ratio between single-ended and differential modes via a resistor value. Suprisingly, best performance was with almost all SE parallel. I figured more of the differential mode would be needed, as least per WE 93A schematics (their idea).
I run the driver through cap to SE/PP interstage tranny. The tranny is required for my new topology, which has CCS on both output tube cathodes, hence allowing good operation even without matched tubes. No bias adjustments are necessary, both 2A3 always run at 50mA. If both working, then voltage across 2A3 is about 235V. I don't get much power, but this is more of a sonic sweet spot. Better to give up a watt or two. Output stage continues to run even if one of the 2A3 is pulled.
All heaters are AC (I think DC is bad for a DHT). Output noise is 0.16mV A-weighted. Measurements are with the cheap Sovteks. The main drawback of this design is a low damping factor. Output impedance from the 9 ohm tap is 3.5 ohms. That's pretty dang high. It is because the rp of 2A3 at this bias is about 1k ohms. It is not the most powerful output tube. More elegant than robust.
At the moment, my listening setup is in total disarray. I sold everything I had and am starting over. Difficult to ascertain sonics as I have no other working amplifier to compare it to. The speakers I have are a bad fit, and tweeters are new with only an hour of break-in so far. Plus I did crossover mods. Basically, everything is an unknown. I have only one channel of amp, which make it difficult for me to listen. So far, it sounds thin and weak to me. Nice and clear on vocals, but thin. The treble coming out of the speakers is slightly harsh. I don't think it is the amps, but I cannot prove it yet. Should take me about a month or two to get parts to make up a matched pair of amps. Then maybe I can get more serious with sound.
Oh, bandwidth is 6Hz to 25.5kHz. Not very high, and this is purely a limitation of my interstage tranny. The driver alone runs to perhaps a megahertz. The 10kHz waveform is funny, so I made a 5kHz one, which you can see the nice response. I underdamp it a little in an attempt to maximize bandwidth. That overshoot is not from feedback, but by the ringing between secondary leakage inductance and grid capacitance (and circuit strays). I spent a lot of time optimizing this tuning.
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