Questions about output tubes

[hesamadman]

Ive read a great deal about preamp circuitry over the years but have found very little literature on power amps. Today I had an amp that seemed to work fine. It didnt seem to have any issues. I opened it up to just clean the pots and noticed a 470 ohm resistor on pin 4 of one of the four 6l6 had a broken lead and was not connected. I believe this is called the screen grid right? Had it been the grid resistor that was broken I most likely would have gotten some red plating. So what is the purpose of this screen grid? Even though the amp worked fine, thats one tube that wasnt getting any voltage on that pin. So what was happening that I couldnt see? Was this throwing something off somewhere? Id like to understand this and get some more education on power amps.

[PRR]

G2 Screen "pulls" electrons along.

Busted G2 feed makes a pentode very lame, nearly useless.

A *four* 6L6 amp can play pretty darn loud on three happy tubes. If it has been a while (or never) since it all worked, you may not have heard anything "wrong".

We once bought a used Plymouth "V-8". Came to learn that one cylinder was dead (I forget why). Was no big deal.

We have a 4-banger Miata with the strange 2-coil ignition. When one spark wire fails, two cylinders go dead. That's a big deal, though it will limp to the dealer. (We now know better, replace the wires early and often.)

A 4-banger Honda loosened one spark wire. Performance was a bit down, and maybe not obvious except this minivan is a slug even on all four, so it strained with a "Three". Even then I was not sure until it threw a Code: the dang ECU not only detected the bad wire, it tole me which one it was! (Be nice if tube amps did stuff like that; but then they would be as complex as recent cars.)

[sluckey]

The tube that had the broken resistor was not conducting. You could simulate this in a properly operating amp by pulling one of the four tubes. The amp will still work, but it is now unbalanced and cannot produce full power. Hum may increase. In most cases, you may never even know anything is wrong just by listening to the amp, unless you try to drive it very hard.

[hesamadman]

G2 Screen "pulls" electrons along.

Busted G2 feed makes a pentode very lame, nearly useless.

A *four* 6L6 amp can play pretty darn loud on three happy tubes. If it has been a while (or never) since it all worked, you may not have heard anything "wrong".

I assumed it rendered the tube useless. Im sure it was a great deal louder after I connected it. Why is it necessary for a pentode to have that screen grid? If the plate has a positive voltage, like on a 12ax7, why is that not sufficient for the electron flow?

[hesamadman]

Hum may increase.

It was much quieter once repaired. Makes sense.

[Willabe]

G2 Screen "pulls" electrons along.

Busted G2 feed makes a pentode very lame, nearly useless.

Why is it necessary for a pentode to have that screen grid? If the plate has a positive voltage, like on a 12ax7, why is that not sufficient for the electron flow?

Tubes developed from triode, to tetrode, to pentode, to get more power from them.

[PRR]

> If the plate has a positive voltage

In a POWER stage, the Plate should pull DOWN as far as possible.

In 6L6 amps the maximum current happens when the plate is under 100V. But electrons still flow because the +300V on Screen sucks them through.

Anyway if it didn't have a Screen it couldn't be a Pentode. (Or "beam power tetrode", which is the same except different enough to evade the old Power Pentode Patent.)

[HotBluePlates]

> If the plate has a positive voltage

... In 6L6 amps the maximum current happens when the plate is under 100V. But electrons still flow because the +300V on Screen sucks them through. ...

Expanding on this (without writing a book chapter; for the books, search "Pete Millett", go to his DIY site, follow the 1st link):

You're an electron and negatively charged. You just got spit out a hot cathode and are hanging out. But there's a VERY-positive screen which pulls you (and 10,000 of your closest buddies) very fast in its direction (opposite charges attract). Though some of your buddies run into the screen (and pass out the screen pin as screen current), you and about 9,000 of your buddies are going so fast you fly right through the wide open spaces between grid wires (like sifting flour with a barbed wire fence) and on towards the plate. The kinetic energy you built up by being accelerated towards the strong pull of the screen carries you on to the plate, even if it is a very low voltage with much less pull.


Now let's pretend the connection from power supply to screen is broken, so the screen is zero volts.

Same scenario, you're an electron hanging out around the cathode. You're looking for a place to go. Waaaaay far away is a plate you could go to, but you don't even see its high positive voltage because there's a tube element at zero volts between you & the plate. So you just stay where you are.

You could almost liken it to seeing your dream girl at a bar, but you hardly even glance her way because her bodybuilder boyfriend is standing between you two.