Thread: 6CA7 in AB2.
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Fred Nachbaur
 
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Patrick Turner wrote:

[...]

Pentode operation allows for up to 800v for the plate supply, and say 400v
on the screens, and a 12k a-a RL, and grid current never occurs.
But if we reduced the screen volts to say 300v, then
we would maybe run out of grid drive headroom.

UL and trioded tubes cannot be run at such high voltages, because the grid 1
would have to be biased at impossibly low -ve bias voltages.
[...]


Well, I wouldn't say it's impossibly low. My favourite hi-fi amp uses
6L6GC's at about 620 volts B+ in UL. Grid bias for a reasonable plate +
screen dissipation (cathode current 35 mA) is about -78 volts. I don't
think that it would be very much different for EL34 or 6CA7, but haven't
tried it. (If anyone would care to donate a set, I'd be happy to try
them out and report back!)

This means, though, that the driver needs lots of headroom; about 160
volts peak-to-peak. The usual concertinas are therefore rather
impractical. But what a great application for a long-tail pair with
pentode (or transistor) CCS! Lots of output swing -- and gain as a bonus.

The net result is lots of power, good clean sound even at full power due
to the UL topology, using relatively common-place tubes. I've found AB2
to be a real nuisance for hi-fi use, but it's great for guitar amps
because it give a more graduated distortion characteristic, passing from
class A to class AB1 to class AB2 to full clipping. It's absolutely
*wonderful* for that, the louder you play the "harder" you play, without
suddenly crunching into the ceiling like you do with SS amps.

But for hi-fi, unless you're a glutton for punishment and want a real
challenge to get a low-impedance driver, and then maybe still have to
try to iron out the added distortion by using NFB tricks, I wouldn't
recommend AB2 using "conventional" valves like the 807, 6L6, EL34, etc.
I think you'd be better off with a lower AB1, even approaching B1, than
to cope with the difficulty of trying to stay clean whilst drawing grid
current.

I too am interested in AB2 for hi-fi, but would use a pair of
high-voltage TX triodes such as 809 or 811A, in a "darlington" kind of
configuration (direct-coupled cathode followers on the control grids).
From reports I've seen, and from a very crude mock-up I did years ago,
this has the potential of sounding very nice, and have lots of available
power.

Cheers,
Fred
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