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In Reply to: rectified cathodes and "ground" posted by hifitom on October 31, 2001 at 22:56:18:
As long as the driver and output tubes are cap or IT coupled, having their cathodes at different potentials won't matter. The grid on the output tube is only interested in the relative diff between itself and the output cathode. Same for the driver. I haven't thought it through enough to answer for direct coupled.
If you "ground" the driver at the cathode of the output tube you will need to increase the B+ the driver sees by the same amount as the drop across the output tube cathode. For example, if you drop 60V across the output tube cathode resistor (or whatever), then you will need to boost the driver B+ by 60VDC to make up the loss between intermediate ground and ultimate ground (which is all relative - woo).
Now for the opinion part - by grounding the driver at the cathode of the output tube you're effectively putting the output cathode resistor and cap in the driver's return to ground - adding stuff in that path is usually not a good thing sonically. Most folks are trying ways to reduce the parts count twixt cathode and ground.
Others may have different views,
Ah yes...I drew myself a little schematic of what I had proposed and now see what you are referring to. It was seeming more complex in my head...I was driving at the time.
I my case, since I'm using the voltage drop of the rectifier to bring the cathode to a higher potential, there might be some benefit to moving the "ground" of the driver section up too, although I really can't think what/why right now. I would still avoid the consequences of the usual resistor/cap cathode combo that you speak of, but then the rectifier may bring it's own sonic problems to the table. That may be even more pronounced because it's now part of the driver section too.
I'm still unclear about how this might affect the AC loading of the driver tube. Since I have no cap from the cathode of the output tube, there is no direct return for AC. This is where the input tubes grid and cathode would rest...elevated at a DC potential via the rectifier tube. Would the rectifier combine the positive and neg swings of the audio waveform as it does in a 60HZ powersupply, giving you only positive swinging ripple? The bigger question for me is if that would make a difference since it's then connected to the powersupply ground?
BTW, does any know how much of a voltage a 6D22 or 6DM4 drops?
Thanks for your input,
The rectifier will pass the signal AC because its all positive varying voltage, not positive/negative like the stuff coming out of the wall. Tubes are positive voltage only devices, thus you are generating a varying "AC" signal by going from +10V to +20V and back to +10V (or whatever).
The cap is only in the cathode return circuit to reduce neg feedback, not to provide a path for the signal to ground - the signal will pass thru the cathode resistor (or rectifier/diode/etc) just fine. But it changes the DC bias point if its not shorted by a cap. That +10/+20/+10 cycle I mentioned above will affect the operating point if its not shunted thru the cap. I think this is why some folks are trying regulator tubes - they maintain a constant voltage within a reasonable range which is exactly what the cathode bypass cap does.
Your question about the effects of connecting the cathode return directly to the PS ground is good - and is addressed by designs like Jack Elliano's Ultrapath, and some variations of parafeed where they connect the parafeed cap directly to the cathode of the tube which eliminates the PS from the signal return loop completely. I know my Ultrapath preamp sounds very clean with the signal shunted straight to the cathode. Unfortunately, it doesn't help with DC powersupply noise.
Sorry, I don't know the voltage drops of the tubes you mentioned.
Hope this helps. I've been following your experiments with rectifier tubes so keep the posts comin'. You wouldn't believe the combinations of caps I've tried to minimize the cathode bypass cap "sound".
Thanks guys for shedding some light on this for me. It really helps answer some questions and brings about some new ones too. I consider this experiment a success, in part thanks to you guys, because I am learning something and improving the sound of what I had already built. I really wish I had more time to devote to reading and experimenting these days...and I have one of the most tolerant and supportive wives you could hope for.
I was talking with "jcoe" on the phone this morning and we both are eager to see what the next original "breakthrough" idea in tube audio might be or if there is something out there that someone will uncover from a long since forgotten text. I'm curious if it will be one of the "elders" in this or another group, or possibly someone coming out of left field that has a completely different perspective.
Hmmm..."Bypass UP"...now I'm going to be up all night thinking about that concept.
Keep your iron hot,
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