simple, cheap, 6L6 amp? help

[Sch3mat1c]

Yeah, that schematic will work.

First of all, think of a tube as a P-channel(?) FET.
As grid goes negative, the tube cuts off (as the electrical charge around the grid wires repels more electrons), and as it goes positive, more are allowed out.
As the plate goes more positive, its charge draws more electrons from the cathode, whatever the grid voltage - this is plate resistance. (Note that a tube will never cut off entirely, there will always be a few microamperes as long as there's plate voltage.)
If you replace the physical plate with a 'transparent' grid (known as the screen grid), two things happen: the physical plate is electrostatically isolated from the grid-cathode area, increasing plate resistance dramatically; and the screen presents a constant electrostatic 'pull' to the electrons, pulling them from the G-K area as in a triode, but since its potential is constant, it pulls a constant current whatever the plate voltage; again, increasing the plate resistance. These two effects combine to create a low-Rp region at low plate voltages, rising to a knee where the Rp flattens out to 10 to 200kohms in power tubes, to over 2M in small signal types.
At low plate voltages, above the knee, but below the screen voltage, electrons aren't pulled strongly to the plate and many bound off and are absorbed by the screen. (This produces a negative-resistance response in the plate, where increasing the voltage actually decreases current!) A third grid, the suppressor grid, goes between the screen and plate to restrain the secondary emission.
A cheap alternative to the supressor grid is using two beam-deflection plates either side of the screen, which focus the electrons into two denser beams. The space charge created by this increased density (combined with an increased distance between the screen and plate) repels the secondary-emission electrons, keeping them at the plate. Nonetheless, at low currents, such beam tetrodes still experience a small negative-resistance curve.

Enough jist?

Tim

[Morse]

Hi Trespasser_Guy;

>>>For my high-z headphone situation, would it be best to use 6SN7 or 6SL7? I am guessing u is the amplification factor. <<<

Yes. For a common cathode amp use the formula:

Voltage Amplification = u * Load Resistance/(Load Resistance + Plate Resistance).

As far as "which is best", it really comes down to voltage gain and allowable plate current. If you can supply enough of both for your 'phones then that's all that counts. You might consider using a single 6SN7 as a cathode follower to impedence match your phones to the 6SL7 voltage gain stage. It's just an idea though....

>>>does anyone know of a site that explains tube stuff in terms of SS?<<<

As an aside, I found it much easier to understand SS in terms of valves! LOL!

All the best,
Morse

[ppl]

This must be in the Air. I have been also thinking about a similar design and lot's of these designs use what i consider Wimpy 12AU7 types. Now I have a bunch of TV sweep tubes i got Dirt cheep in the late 1970's when the shop i worked for at the time stoped doing service on Tube type TV's. I got like three of four tube caddy's full of RCA TV tubes still new in the Box. I never considered these useable for audio. Any Idea's on types to look for or is that just a waist of time?

[fdegrove]

Hi,

Any Idea's on types to look for or is that just a waist of time?

With a bit of imagination you can even put the oddest of odd tubes to good use for audio.

Much depends on where you set the goalposts for your audio nirvana but technically it sure is possible.

Cheers,

[SHiFTY]

Try downloading the tube database software at www.duncanamps.com

It will tell you what the tubes are...

Some really good ones are the odd voltage tubes, like 12SL7 or 12SN7, 50L6, etc etc.

If you can, post a list of the numbers (start a new thread) and I am sure the people here can tell you what is cool!