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    Building, troubleshooting and testing of these amplifiers should only be
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    the safety precautions around high voltages.

any benefit to screen-grid drive a 6V6?

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Parafeed813

Parafeed813

Member
Joined 2012
Probably not.
The screen grids of the 6V6 can be run as high as the anode, which means that the screens are not very sensitive. So you will need a very high drive voltage to change the anode current.
The best candidates for screen drive have sensitive screens, hence low voltage ratings. Typically these are 'TV tubes'.
 
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Parafeed813

Parafeed813

Member
Joined 2012
Another thing to remember, is that since the screen is positive, it will draw current. The driver needs to be capable to provide that.
Long time ago I had an experimental setup with an EL36 output valve as the driver, transformer coupled to a pair of screen driven EL38 outputs.
The 'modern' way would be to use direct coupled source follower mosfets to provide the current to the screens. Check out the Tubelab site for this.
 
Miles Prower

Miles Prower

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Joined 2005
dolphin-hsl.com
Bigun said:
I don't know anything about screen-driven tubes so this made it interesting and I happen to have a couple of Russian equiv. 6p6s in my stock....
Click to expand...

There'd be no real benefit to this. If you check the specs (Frank's) you'll see that the max cathode current is 100mA. If you have a look at the transfer characteristics, the screen voltage needed to pull 100mA is north of 200Vp.

The plate characteristic shows that you can pull the same current in grid drive mode with Vgk= 0V, for a grid swing an order of magnitude lower, and the same power output.

Screen drive is best reserved for types like the TV horizontal PA's, as screen drive requires similar screen grid voltage swings (somewhat less, usually) and gets you more plate current to increase output. The really big ones intended for deflection duty in big screen color TVs are some real beasts indeed, and a single pair in screen drive can easily get you north of 100W. Unforch, these types are getting rather rare since a lot disappeared into ham rigs (these could get you lotsawatts while staying in Class C1) and illegal CB linears (none too carefully designed, and these poofed lots of tubes pretty quickly) back during the big CB craze.

You might be able to find some of these big color TV HD's in odd heater voltages that are available and for not too much. A lot of color TV's, even premium ones, often used straight off the mains DC rails driven by voltage doublers. The heaters would be daisy chained across the AC mains, accounting for a wide variety of odd heater voltages. (One of the brags mentioned in spec sheets is high plate current with low plate voltages, this being necessary in order to get the HD system up and running to the point it could start making Vboost to power up fully the HD subsystem.)

I see, so essentially a screen driven 6V6 should be thought of as a 'plate follower' and although it may be quite linear in this mode the burden shifts to the driver and it's linearity. Well good to know all this, thanks for the quick replies.
Click to expand...

A screen driven pent is actually a pseudotriode operated in Class *2 with what would be a rather large control grid current. Back in "the day", this was accomplished with one power amp driving the screens through a coupling xfmr. These days, a source follower can slap the grids silly, and do it with much lower Zo for much improved sonic performance. Before power MOSFETs, screen drive was used for big watt PAs, plate modulators, or anywhere else efficiency and lotsawatts were needed, but not sonic performance.
 
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smoking-amp

smoking-amp

Member
Joined 2001
You can determine the screen transconductance, gm2, by dividing gm1 by the triode configured mu. 6V6 would give 4100/9.8 = 418.
While a typical TV sweep is around 2300 to 3800 for gm2.
Some suitable TV sweeps that aren't too expensive yet:
6HJ5
36LW6
21LG6
35LR6
EL509
42KN6
17JB6
17/22JF6
6/12/17JN6
12/17GE5
12/17GJ5
12/17GT5
17GV5
16/21GY5
6/21EX6
17/22KV6
22JU6
22JR6
21JV6
6/21JZ6
6/25CD6
6/25DQ6
6/26DQ5
13/27GB5
6CB5
 
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