Pentode vs triode in mic pre's

[HA_DA_JA]

What is the difference in Pentode and triode preamplifieres?
How does it each different method effect the sound of a mic?
There is some discussion about guitar amps but nothing in relation to mic preamps that I could come up with.
And why is it that some tube units run on a 100 volts and less( is this what they call starved plates design) and some on 250 volts?
Is it right to assume that the higher the volts e.g the 300 volt units are a better suited for high end audio applications or is this just another fallacy?

Thanks

[rob]

these are some deep questions, for which the answers require a lot of background understanding of how valves work.

so, it is tricky to answer without being superficial, inaccurate or assuming that have the required background knowledge.

information of many of the issues can be found on the www. Time to embark on a self education on tube theory....but start at the beginning. And be wary of discussions that focus on doing this or that affects the "sound". Talking about "sound" with words and mixing in technical facts can be a spiral into a pit with no return!

Rob

[HA_DA_JA]

Sounds like good advice.
Especially "the pit with no return" bit.
Sounds like a few marriages I know !!

[Milo]

As Rob says, it takes a lot of background. To answer one of your generalisations, ie about plate voltages: I'm running a preamp I built with 12AX7's on only 48 volts plate, which sounds fine, but you can overdrive it easily, and that's where the 'good sound' really kicks in. So its horses for courses regarding this factor alone. Good Luck

[Chinagraf]

Then to make it even more confusing, add pentodes wired as triodes. I just know the valves glow in my units.

[chris p]

Dale

Both are amplifiers, where a heated "cathode" spits electrons towards a collector "plate", in proportion to the signal voltage applied to a "grid". Hold on to this thought: for all the jargon, both basically do the same thing: they amplify.

In a triode, that's all there is. In a pentode, there are two more grids placed between the cathode and plate: called a "screen" and a "suppressor". The screen is there to stop capacitive coupling between the cathode and plate (and hence preventing feedback oscillations at high frequencies) while the suppressor is there to capture and return stray electrons emitted from the plate due to the high velocity impacts of electrons from the cathode.

Compared to a triode, a pentode has increased plate current and decreased screen current, meaning better, more linear gain and overall performance. In some instances it allowed for greater operating plate voltage (= headroom) as well.

That's the science of it. I would add to Rob's comments that in practical audio the sound of a valve is deep juju and depends very much on circuit design and construction.

Two brief additional comments - don't be mistaken in thinking that the tri- and pent- refer to the number of pins. A triode normally has 5 pins (one for each tube element and 2 for the DC voltage that goes to the cathode heater. A pentode can have up to seven pins depending on whether or not any of the grids are internally connected (eg screen to cathode is a fairly common one for 6 pin pentodes). A starved plate design is one where the voltage across the cathode and plate isn't intended to actually amplify the signal - its more a "tube-ness" effect that is applied to the signal at unity gain.

Try this site for info
[url]http://www.allaboutcircuits.com/vol_3/chpt_13/3.html[/url]

Regards