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Meaning of "hard to drive"

I have seen reference to different output tubes as being hard to drive or harder to drive than some other tube. Am I correct in deducing that the factors that make a tube harder to drive are

1. Required voltage swing for full output.
2. Input capacitance (miller) requiring higher current driver.

Are there any other factors that I am missing? To determine whether your driver circuit is adequate for driving a certain capacitance can you calculate it based on the time constant of the grid resistor and the miller or is there another method?

mike

P.S. There are not many IDHTs out there are there. I have found only 6S4 and 6AS7.
 
Another consideration is if you are driving the grid into Class A2...if this is the case then the input impedance looking into the grid will drop significantly....
If you are not drawing grid current...then essentially you can easily calculate the required source resistance needed to appropriatly drive the worst-case input impedance(r-jw), of your output tube....
Idealy you want the lowest possible source resistance as possible to make a good driver into a capacitive load such as the grid...
But practically you need to design sensibly and also need to know where you stand....
I typically set the driver resistance so that it is equal to or better than having a -3dB POLE at no lower than 70kHz.....
The reason for placing the -3dB far above the audio band is to maintain as best phase linearity as possible in the upper audio frequencies above 10kHz....
For example, if you place your -3dB POLE at 162kHz...you would get a -7 degree phase shift at 20kHz..... which is pretty darn good..
Keep in mind that the ears are sensitive to phase shift at the higher audio frequencies....

Chris
 
The "hard to drive" reputation is usually earned by large drive voltage requirements, large C miller or both.

P.S. There are not many IDHTs out there are there. I have found only 6S4 and 6AS7.

There are plenty. I am assuming that you are referring to output tubes, so I will list some of my favorites.

The mentioned 6AS7 has an identical twin the 6080. It also has several brothers, all of whom have the same pinout but different characteristics, listed in order of brawn. 6SN7, 6BX7, 6BL7, (all smaller than the 6AS7), the 7236, and the 5998A are "easier to drive" versions of the 6AS7, and the 6336A and 6528 are 6AS7 types on steroids.

The 6EM7, 6EA7 and the 6DN7 are dissimilar triodes containing a driver and output tube in one envelope. A 2 watt SE amp can be made with one tube per channel, again these have the same pinout as the 6AS7. The 6EW7, 6DR7, and the 6CM7 are dissimilar triodes in a 9 pin miniature package. The 6GF7 and 6FM7 are compactron versions.

The 6S4 also has relatives. The 12B4 is similar and the rare 7233 is like a 12B4 on roids. All are in 9 pin miniature envelopes. The 6CK4 and the 6AH4 are similar in an octal package.

There are also come dual triodes made for computer use that are suitable for low power output stages. Wire both sections in parallel. The 7119, 7044 and 6463 are examples. These make good drivers for those "hard to drive" tubes as does the 5687.
 
Very interesting stuff guys. I have a bunch of data sheets to look up now.

Yes Tubelab I was speaking of power triodes. I think of the 6SN7 as a small signal device so didn't list it.

That 6AS7 is intriguing though. In addition to power amp duties it seems like the low mu, tiny Rp and high gm could make it useful in preamp use. I am thinking that after a normal inverting VAS stage one could use a 6AS7 in CC form as the output instead of a CF or transformer output. With the high plate current capabilities one should be able to bias it with enough grunt to drive longer cables (capacitance) and the low Rp means you should be able to have a reasonably low Zout for matching to even a SS amp if one were so inclined. As a bonus you get absolute phase relative to your source. What do you think?

So many interesting old tubes out there to study and so little time. :) You know I keep hoping that when they force us to use Al Gore light bulbs that the old light bulb factories might start churning out tubes for us. Ok, so I can dream can't I? :D

mike
 
Real men run 7241 power triodes. Now that is a tough tube to drive.

Yes, but I don't have any and they are ridiculously expensive. I have boxes full of used 6AS7 and 6080 although many seem to be used up. I have been able to collect about 20 6336A's (also used) without spending any money. Most seem to be good. I have spent what little tinkering time that I had last year experimenting with all of these tubes in cathode follower mode. Think about the drive requirements for a 20 watt cathode follower, 500 volts P-P but no Miller C. The 6EM7 can do it. Details will be posted once I get it all sorted out.

Tubelab, is it true, in your experience, that tetrodes/pentodes strapped as triodes do not sound as good as IDHT power triodes?

It is my general experience that this is true, but the differences are often small. It should also be stated that there are no direct comparisons. Do you compare tubes with similar ratings like a 45 and a trioded 6V6 in exactly the same circuit (TubelabSE)? If so the 45 seems to have a slight edge in the detail and image department. If cost is added into the equation, the 6V6 wins. In a similar comparison I have compared trioded Russian KT88 to Russian and Chinese 300B in SE and P-P amps. I like the sound of the 300B better for most music. The amps were not identical though, and most people (myself included) tend to use better parts on DHT amps since the overall budget is higher. On the other hand I have tested trioded 6AV5's against NOS 6B4's (in the same amp) and I like the sound of the 6AV5 better. It is more dynamic with better bass.

I plan to test some more trioded sweep tubes in the future, but time is very limited right now.
 
Transmitting tubes?

7241?..With an amplification factor of only 2.7 is that really usefull?
I was looking up the "Eight" series of transmitter tubes..811, 812 or 813 'sound' better with truely useful 'factors'.
812 sounds ideal, with a 300B drive?
Plan on a second mortgage for an OPT though!
_______________________________________Rick..........
 
7241?..With an amplification factor of only 2.7 is that really usefull?

Its brother the 7242 has a Mu of 9 and a transconductance of 111,000. I could dream up all sorts of uses for those, but since I will likely never see any, they are just dreams. The 6AS7 has a Mu of 2 and people have found plenty of use for them.

I suppose those twin triodes for computer use and the dissimilar TV tubes are only available used and NOS now.

The 6AS7 is the only tube mentioned that was recently made. I am not sure if they are still being produced. The TV triodes were produced in relatively large numbers and are still available NOS from the usual sources. I have been buying 6EM7's at hamfests for years the price has always been $4 each, sometimes $3. They do make a nice 2 watt amp and are useful for drivers. 6CK4 and 6AH4 are getting a little scarcer. 7233 can only be found in old HP sweep generators.

The computer triodes were only common for a few years but can be found surplus for cheap. The big lot of tubes that I got a few years ago had plenty of them. Some are really obscure numbers. I am still evaluating some of them for audio use. 7236 and 5998 are like the 6AS7 with a Mu of 5, so they are easier to drive. They sound good too.

6528 and 6336A were often found in military and industrial power supplies. They used to be common and cheap, but supplies have dwindled and prices have gone up a lot. I always check out the large boat anchor like chassis seen at hamfests. Sometimes there are good tubes to be found.

A friend once let me behind the wheel of his 1940's Chevy pickup...no synchromesh, that was hard to drive

My first car was a 1949 Plymouth. Had it for 25 years. It came with a vacuum tube radio that used 2 X 6V6's in push pull. I found a similar radio in a junkyard, and used the power amplifier sections of both (mounted in the trunk) to make a vacuum tube stereo amp (in 1970). Drove that with an 8 track unit (it was 1970) which was powered by a home made voltage booster off of the 6 volt electrical system. As I remember it (again it was 1970) it sounded pretty good. It was loud though, read loud compared to the usual 4 WPC from the 8 track decks.
 
tubelab.com said:
My first car was a 1949 Plymouth. Had it for 25 years. It came with a vacuum tube radio that used 2 X 6V6's in push pull. I found a similar radio in a junkyard, and used the power amplifier sections of both (mounted in the trunk) to make a vacuum tube stereo amp (in 1970). Drove that with an 8 track unit (it was 1970) which was powered by a home made voltage booster off of the 6 volt electrical system. As I remember it (again it was 1970) it sounded pretty good. It was loud though, read loud compared to the usual 4 WPC from the 8 track decks.

I know you get this all the time, but you rule dude! I was born in 71 and I like to tell the story that I was conceived on the Rolling Stones 71 world tour... my folks were hippies spaced out in the back of the car on lucy in the sky with diamonds after the show listening to the eight track really loud.. Now all I have to do is add plymouth and tube 8 track and the story is even better! Which, BTW, is a complete fabrication, but makes for great conversation none-the-less. You rule!
 
Glowbug said:
A friend once let me behind the wheel of his 1940's Chevy pickup...no synchromesh, that was hard to drive :D


You obviously haven't learn't to double-de-clutch to master the art of changing gear. Once youv'e mastered it you don't need the clutch at all.
First and third in Willy's jeeps without inter-diff was hi-fi.

The radio had a tube vibrator rectifier 0Z4 (could be wrong) radiated so much that it was often picked up by the enemy miles away.

richj..ex services
 
The radio had a tube vibrator rectifier 0Z4 (could be wrong)

Yes, a mechanical vibrator converted the 6 volt DC into AC that resembled square waves which was applied to the transformer. The rectifier was an 0Z4.

hippies spaced out in the back of the car on lucy in the sky with diamonds after the show listening to the eight track really loud..

That type of activity might have occurred in that car. I was more into the Moody Blues than the Beatles then though.