Power versus driver tubes question.

There are others who know more than I do, and I hope they also contribute to this thread.

First, a lot of it depends on the circuit. The driver can perform a variety of functions. In the simplest sense, it boosts the signal up to a level where the output tubes become more linear. Also, you can use this configuration to apply feedback to the circuit. There are other tricks and designs, but that's the most I feel comforable explaining.

Also, it isn't necessary to have a driver and an output tube. I've seen some single-stage designs, and it is possible to use an input transformer. Though a quality input transformer will add substantially to the cost. I imagine the lack of input transformers in gear has more to do with keeping down prices.

In Sound Practices magazine, they have a very simple single stage preamp design using a 417A/5842. It is very similar to Ciuffoli's SESS amp (see the plans at HeadWize). The article says that it could be adapted to a 2A3, 45, etc. if you use an input transformer. Still a bit beyond my skills, but I've love to put together a single stage headphone amp with a 2A3 (or other DHT) using an input transformer and a headphone-appropriate output transformer.

 

A tube has three main characteristics: mu, gm, and rp.  Roughly speaking, mu is the voltage amplification, gm, or transconductance, is current amplification, and rp is output impedance.  These are related by mu = gm * rp  Thus, holding gm constant, as mu increases, so too does rp.

 

A driver tube is usually one with a high mu.  It provides the voltage gain of the circuit.  But, because a high mu tube tends to have a high rp, it can only drive a high impedance load such as the grid of the next tube.  An output tube generally has a lowish mu and highish gm, meaning a low rp and is thus able to drive a more difficult load, such as an output transformer.

 

There are tubes with both a high rp and high enough gm to allow them to provide both voltage and current, such as the 6C45 of 5842.  They can be difficult to work with as the very high gm can lead to oscillation unless care is taken.

 

Finally, an output tube must also have a high enough wattage capability to work as an output tube.  For instance, a 71A has a mu of 3, a gm of 1700 Micromhos, and thus an rp of ~1700 ohms.  However, it can only dissipate about 3W meaning that it can only output about 800mW.  Compare that to a 300B with mu of 4, gm of 5000 micromhos and thus an rp of ~800 ohms.  It can dissipate over 30W allowing it to provide ~8W of power.

 

This might be helpful for some tube basics: http://www.ecpaudio.com/pdf/parafeed_basics.pdf

For most audio amps:

 

Output tube = power tube

Input tube = gain tube = driver tube 

Driver tube sometimes=driver tube :p

 

Are the common uses of the names. They are used fairly interchangeably in many descriptions.

 

Output tube is used in place of power tube for a few reasons. Output tube means the last tube in the amp. In a speaker amp this is almost always a fairly high power tube. BUT not all headphone amps use "traditional" power tubes as output tubes - many use what speaker amps would use as drivers!

 

Driver tubes are a really foggy category. When you start looking for every last watt out of a power tube it helps if the tube before it can supply a bit of power - you kind of smack them around a bit (despite peoples love for fairly large power tubes in headphone amps, headphone amps almost never get to this level). The gain/input tubes cant do this because they are generally designed for better linearity rather than more power so the drivers supply medium amounts of power to drive the output/power tubes when they throw a fit but are easy enough for the gain tube to drive.

 

The input/gain tube is the tube at the input of the amp that is generally considered the key tube for providing gain. They are generally small little guys that cant handle much power, but have nice low distortion & low noise.

 

All of these designations are somewhat foggy because every circuit is different. 

 

(this is where dsavitsk normally corrects me)