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Old 17th January 2008, 10:02 PM   #1
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Default 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.
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Old 17th January 2008, 10:59 PM   #2
cerrem is offline cerrem  United States
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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
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Old 17th January 2008, 11:10 PM   #3
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"Hard to drive" usually means low transconductance so more of voltage swing is required.
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Old 18th January 2008, 01:38 AM   #4
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The "hard to drive" reputation is usually earned by large drive voltage requirements, large C miller or both.

Quote:
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.
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Old 18th January 2008, 06:14 AM   #5
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Tubelab, is it true, in your experience, that tetrodes/pentodes strapped as triodes do not sound as good as IDHT power triodes? If that's true, where does the difference lie?
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Old 18th January 2008, 08:01 PM   #6
Jeb-D. is offline Jeb-D.  United States
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2 more IDHT's 6c33c an 6c41c
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Old 18th January 2008, 08:05 PM   #7
ilimzn is offline ilimzn  Croatia
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Quote:
Originally posted by ray_moth
Tubelab, is it true, in your experience, that tetrodes/pentodes strapped as triodes do not sound as good as IDHT power triodes? If that's true, where does the difference lie?
How would you actually compare such a thing, save for finding an IDHT that corresponds exactly in all characteristics to a triode strapped IDHP?
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Old 18th January 2008, 08:30 PM   #8
Yvesm is offline Yvesm  France
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Quote:
Originally posted by ilimzn


How would you actually compare such a thing, save for finding an IDHT that corresponds exactly in all characteristics to a triode strapped IDHP?
Well, not exactly like a 2A3 but not so far away

Yves.
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File Type: pdf 6dq6a_triode.pdf (28.7 KB, 69 views)
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Old 19th January 2008, 01:23 AM   #9
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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?

mike
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Old 19th January 2008, 03:38 AM   #10
JLH is offline JLH  United States
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Why are you guys only listing whimpy tubes like the 6336 and 6AS7? Real men run 7241 power triodes. Now that is a tough tube to drive. 100W Zirconium coated graphite plates and a 6.3V heater that draws 7.5 amps. Oh yeah, that's the one.

http://www.mif.pg.gda.pl/homepages/f...127/7/7241.pdf
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