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In the Forum: Melquiades Amplifier
In the Thread: Single-stage Melquiades vs. DHT amps
Post Subject: Driver tube and biasingPosted by cv on: 6/17/2009
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Allo
On the matter of the DHT having a plate resistance of 500-700... you do realise that running the 6E5p with an unbypassed cathode resistor is giving you an effective Rp of about 12k, right? The formula is, IIRC, Rp + (mu+1)Rk.
Given that 12k seems to be ok for driving the output stage, I had in mind the LP2 which has a mu of 15, an Rp of about 5k, 2V heater. Alternatively, there's the P2, which is half the mu and Rp.
The idea was to use filament bias - this is where you put the heater and cathode bias resistor in series, so that the bias is derived from the heater current. It's wasteful of power, but it does mean you can use a very, very small cathode resistor. Eg if the tube needs -6V of bias and has a 2V / 0.2A heater, then you have an 8V heater supply and a 30 ohm cathode resistor...
Regarding the inductive attenuator - in using an LR filter at the input, the series R will give the inductor a chance to express any non-linearities in voltage terms. With a 6db attenuator, the signal is tapped half-way through the core, so I can almost visualise cancellation of non-linearities in each half...whatever they might be. Then there is also the benefit of the driver tube seeing 1/4 of the filter R.
In any case, just some food for thought, lest you do identify something lacking in the sound down the line.
cheers
cv
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