MagneQuest/Peerless Forum

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EXO-173 progress

Posted by keto on May 8, 2004 at 10:30:39:
Thinking it might be interesting to modify the Pimm 120mA CCS in the output cathode of a PP2A3 circuit, I added another dial on top of the chassis, to vary (balance???) current between the two output halves.
"Is this the volume control?" "Nooooo!!!!!!"
This scenario is avoided by locating the trimmer between the 2 output tubes and a VR tube, and, should someone still think its the volume control by mistake, no problem, because its really a 250R pot with a 100R across each leg. I think a one-turn pot is good for this application, because it gives a good visual of nominal "center".
Though the 250R (really about 100R) is a nasty pot, which I'm assuming is adding a funky quality to the sound, it does the job: once warmed up, a stable null of 0.7mV humm (TJ2.5V300Bs) is within its range.
It seems like a really good idea to have separate 2.5V/2.5A filament windings, so that the output cathode can be coupled or de-coupled to a specific degree. The immediate benefit is a reduction in noise. It may be that a coarser and finer pot in series (a la Pimm PP47), which would de-couple via a higher impedance, might be even better. There might be a turning point, where the cancellation of the filament noise becomes less effective, or there might not. A 100R effective pot seems like a good minimum, though.
In addition to humm being louder with just the 120mA sink (connected via a 25R resistor from each of the four filament pins) and wanting to be able to adjust the current between halves, I've also been wondering about reducing the "intrinsic compact circuit".
This where my slope gets really slippery.
With an EXO-173 connected to a PP output with separate cathode bias for each half, the phase-inversion is done entirely by the EXO. When the cathode biases are connected, there's a coincident phase-inversion: that of the "compact" circuit (see link).
My first thought was that it would be interesting to reduce the "compact" effect, by decoupling the two halves a little. Not sure if this is what really happened, but the amp sounds even better now, somewhat decoupled. And is quieter.
Since my other amp is set up with the "compact" circuit, direct-coupled between SE and PP stages, I've been staring at that configuation a little. My latest pseudo-observations are that
1) such a circuit has a hugely increased and assymetric input capacitance; and
2) the "second" half will have a quiescent operating point that is Vg volts high and cool with respect to the "first" half, and this along the nominal plate load slope. Think 20% less output and no deeper into class A.
If true, two strikes against using it in the output of a PP2A3. Better done in the previous stage with 1) a lower input capacitance and 2) plate CCSs; these, to minimize the two effects, respectively. I've got a 6SN7 driver lined up to give this a try, in amp number two.
That's the latest anecdotal report from the tropics, where mango season is in full swing.
--keto