Tube DIY Asylum

Do It Yourself (DIY) paradise for tube and SET project builders.

Return to Tube DIY Asylum


Message Sort: Post Order or Asylum Reverse Threaded

rectified cathodes and "ground"

24.2.241.146

Posted on October 31, 2001 at 22:56:18
hifitom


 
Hi All,

As I have been playing with this rectifier (a 5Y3) in place of the resistor & cap from the cathodes (actually the CT of the filament transformer)of the output tubes (2A3s), I have been thinking about what is considered "ground". Taking what I have a step further, would there be any benefit to moving all ground points in the driver section to the cathode (the CT of the filament transformer, also the plate/s the rectifier), leaving only the grid resistor of the output tube and the power supply caps connected to the CT of the power transformer, a point formerly known as "ground"? How would this affect the load that the driver would see, as the grid resistor on the power tube is not terminated at the same potential as the cathode resistor of the driver? Also, would this lead to problems with the coupling between the 2 tubes?

You can probably tell that I don't get enough sleep and my mind wanders...

Tom

 

Hide full thread outline!
    ...
Re: rectified cathodes and "ground", posted on November 1, 2001 at 11:25:26
petef


 
Interesting questions...

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,

Pete

 

BTW...great answer to the guy (below)..., posted on November 1, 2001 at 11:42:55
petef


 
...who just "wanna tube". You pointed out some big gotcha's with buying used gear, etc. that most newbies can't imagine growing up in the digital age where everything just plugs 'n plays. I wonder how many old ST70's, Fishers, etc. are laying in the closet with blown power supply caps, xformers, etc. from being turned on and run without any checkout beforehand? 'Taint like a PC...

Myhatsofftoyou,

Pete

 

Re: rectified cathodes and "ground", posted on November 1, 2001 at 15:31:29
hifitom


 
Hi Pete,

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,
Tom

 

Re: rectified cathodes and "ground", posted on November 2, 2001 at 08:09:33
petef


 
Yo Tom,

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".

Cheers,

Pete

 

Re: rectified cathodes and "ground", posted on November 2, 2001 at 12:26:57
J Epstein
Reviewer

Posts: 1041
Joined: April 6, 2000
This subject strikes at a concept that has been the source of some of my real "Eureka" moments in understanding what is going on in a circuit. Namely: An AC ground does not have to be located at the 0 V DC reference.

Take a typical implementation of a bypassed bias-setting resistor. The cathode of the tube is bypassed for AC to ground, so the cathode is an AC ground. The DC voltage at that spot is not at 0V DC.

With the diode providing your bias, yu now have an interesting hybrid - the impedance of the diode is different for AC than it is for DC, so you have a DC voltage at the plate of the diode tube, but the AC impedance is much, much lower than the "DC resistance" given by the voltage across the tube divided by the current. The plate of the diode is a quasi-AC ground here.

Now you can take a look at everything else in your circuit that is "grounded" and decide - what DC voltage is useful here? And you can listen to what using different sorts of bypass components do to the sound, the small AC impedance of the diode sounds different from the small AC impedance of a cap, and the resulting cathode degeneration from each approach sounds different. Another way to play with your sound.

You can also bypass UP - B+ is another AC ground.

This is the exact thought process that got me working on the Free Lunch amp I designed and - you know what? - I like what I was able to do with the sound using this approach.

Other examples of this type of exploration are the Ultrapath bypass cap, the Monkey amps, and Steve Bench's No R No C amps.

-j

 

Thanks Pete & Jeremy!, posted on November 2, 2001 at 19:38:01
hifitom


 
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,
Tom

 

Page processed in 0.023 seconds.