"Fred Nachbaur" wrote in message
news:NwC_a.8048$zE1.6670@edtnps84...
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.
Likewise. The EL34/6CA7 is an unknown quantity to me. We're not really
answering Patrick's question here, but with a variable PSU. I've run UL
Reflektor 6L6GC pairs up to 550v with idle dissippation of 24W into loads
between 4 and 6k and like the way they sound.
(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).
This looks kind of elegant if you draw it out. It looks so simple, I put
one together a couple of years ago. As I remember ...... 550v B+... a pair
of FB UL KT88s..... 2 x 6SN7 CFs off the same 550v supply, direct coupled
to the KT88 grids ..... 22K loads off a fixed 150v low imp line and
variable -ve bias on the 6SN7 grids to set the KT88 idle current. Up front
was a 6SL7 LTP. I could swing 170V pk-pk on the KT88 grids before it cut
the tops off a sine wave input. The PSU would start a broken down truck.
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.
Up one paragraph. Did it sound nice ?? It didn't actually sound any better
or worse
than the same KT88s, without the CFs, with either a LTP or Williamson front
end up to the sound level where you had to hide behind something.
Such a layout may be useful for getting 100W out of a pair of EL84s whilst
staying within the stated max anode dissippation by driving them hard enough
to cut them off for over 50% of the input cycle, but this is not really
hi-fi.
I just realized that big noise and low distortion starts with a low
component count, big volts, big toobs and big OPTs.
Patrick has a man who insists on AB2 This is hard work
regards
jim
Cheers,
Fred
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