jim wrote:
"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 you had a 4 k load, and used the amp at elevated levels, perhaps the
Pd might exceed the Pd limit. Load line analysis would show that,
but one can get away with a lot with tubes, and music.
(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.
The "sound" is mainly determined by the output stage operation,
since it usually makes far more thd than the input stages,
with or without CF drivers to the OPV grids..
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 have seen EL34 used with B+ at 900v, and Eg2 at 450v,
and Po = 125 watts.
It makes for a very fragile amp, if ever the load becomes lower than
rated for 125 watts.
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
In pentode/tetrode, using 2 x EL34, or 6CA7, with RL = 8ka-a,
with Ea at 500v, and Eg2 at say 400v, you get 636 vrms into 8k, ie,51 watts AB1,
with about 8 watts of class A.
Pd is OK, and biasing at 25 watts is possible.
Using KT88, you get no more than this, with the same RL.
trying to drive AB2 would be pointless, since the Emin swing
is already close to the vertical axis on a loadline graph.
Say you reduce RL to 4ka-a.
6CA7 will have Emin swing cut off by the Eg = 0.0v line,
so you get 462vrms into 4 k, ie, 53 watts, class AB1,
and prolonged use at such Po will turn the plates red.
Class A power is only 5 watts.
With KT88, the AB1 swing will be 560vrms into 4k, ie, 78 watts,
but the extra Pd ability allow continuous duty cycle.
But if we could push the EL34, or 6CA7 into some grid current,
then we may be able to force the voltswing on the load to very nearly that
of the KT88.
We have to accept we won't be asking the amp to make say 70 watts into
4k for 100% of the time, not in a hi-fi amp at least, but we do
garnish our construction with three things,
low cost output tubes, nice high peak power ability, and greater load tolerance.
Waht if we had six output tubes? we could thus get 210 watts reliably
from 6 x 6CA7, although aiming for 150 would be better.
Who uses more than 150 watts for hi-fi?
Not many, although I know folks who like the utterly effortless
sound of a huge but agile tube amp, working well away from
areas of high thd, except for the occasional huge crescendo,
or cannon shots.
I might add that one could get a pair of 6L6 or 807 to make 80
watts with a 600v B+ supply, using simple AB2 techniques.
Power isn't everything, and a plain old trioded williamson
with 16 watts does sound a treat, and can offer superlative listening,
but I like to explore the world beyond 16 watts, and a land where
speakers of 80 dB efficiency have to be driven with tube amps.
We should be able to make the best of whatever mode we chose, A1, A2, AB1, AB2.
Musical Reference get 36 watts from a pair of EL84, AB1, acoustical.
There is a 200 mA fuse in the cathode circuit, which is large enough
to prevent nuisance fuse blows, but small enough to prevent tubes
from bias failures. A bloke I know has been using the same tubes since he got
the amp
in 1996.
Patrick Turner.
Cheers,
Fred
--
+--------------------------------------------+
| Music: http://www3.telus.net/dogstarmusic/ |
| Projects, Vacuum Tubes & other stuff: |
| http://www.dogstar.dantimax.dk |
+--------------------------------------------+