Why directly ground cathode on standard triode gain stage?

[eddie25]

I'm checking out this amp where the gain channel seems to go into a 12ax7 stage, which is normal except the cathode is directly grounded, no resistor or cap. The grid does not appear to be negatively biased in any way. The signal is continuing through the plate as normal. My instinct is to assume this wouldn't work, but it does. I'm just really curious, why do this?

[es345]

I assume that the grid resistor is something like 5 or 10 MegOhm. This mode is bias via grid resistor.
As an example have a look at this pdf, page 2 http://www.mif.pg.gda.pl/homepages/fran ... /12AX7.pdf.
Look at the table, the last 4 rows show this mode.

[eddie25]

I will double check, but it looked like it went directly from the tip of the jack to a .1uF cap in series with a 68k right into the grid.

[es345]

The 68K is the grid stopper. In addition there should be a resistor from grid to ground - before or after the grid stopper.

[eddie25]

Yes, I was just gonna add that there was a 1M to ground from the grid. Thought that was just a standard grid leak, but I will check it out again, maybe its higher than 1M. Thanks.

[Leo_Gnardo]

I will double check, but it looked like it went directly from the tip of the jack to a .1uF cap in series with a 68k right into the grid.

So far so good, there would have to be a DC block - the 0.1 uF cap - between input jack and grid. But the grid must have a path to ground also, typically somewhere around 220K up to several megohms. If not the grid will "float" to who knows where, with negative consequences. Any "stopper" R like the 68K, would be optional presumably selected to promote stability.

Grounded-cathode circuit generally exhibits a compression effect which can be a nice enhancement to guitar tone.

edit: whups, eddie ya beat me to it. 1M gridleak is just fine. All seems good.

[eddie25]

Cool thanks. I remember reading about this now a while back, just haven't actually seen it in practice so I forgot.

[RockinRocket]

Bad Cat and a bunch of Supros do this. Not using a cathode resistor does this afect tube life?

[Leo_Gnardo]

Bad Cat and a bunch of Supros do this. Not using a cathode resistor does this afect tube life?

Typically I see plate resistors on the high side in grounded-cathode preamps, say 220K - 470K, and sourced from a fairly low B+ node. As long as the circuit is made correctly you should not see a reduction in tube life, nor hear a DC crackle when you turn the volume control on your guitar. It was a popular circuit @ 60 years ago more or less. Good to see Bad Cat has picked it up. Many players like a little compression. Hobby/custom builders get it for free ; Bad Cat buyers pay a big premium.

[tubeswell]

Bad Cat and a bunch of Supros do this. Not using a cathode resistor does this afect tube life?

Bias deteriorates over the life of the tube with grid leak bias (whereas its much more stable with cathode biasing)

[eddie25]

It was a Bad Cat, that's also why it was hard to see the values cause there was a relay board over the tube and components. The amp was indeed very compressed.

[pdf64]

There was a thread on the Trainwreck Discussion forum regarding how the low plate voltage of the input stage resulted in guitar volume controls being 'scratchy'.
This was thought to be due to grid current, tending to increase as the plate voltage decreases.
Different brands of 12AX7 were identified as being better / worse.
Sorry, I can't locate that thread at the moment.
But yes, a blocking cap at the input is necessary for proper 'grid leak' operation.

[Andy Le Blanc]

oh ya... it's grid leak or sometimes call "zero bias", theres R-C gain tables for it
in the sylvania tube book 1951... it maximizes the gain from the tube stage...
The tube data is represented with a 5% harmonic distortion figure, a lot of tone color.
The tone color is the reason to go that way, usually best a a pre,
a zero bias stage can literally run out of bias when its presented with a very large signal.