6080 audio amp?
Engineer wrote:
Hi, Vacuumlanders.
Just a speculation but has anyone built a P-P audio ampliifier with
the 6080 (6AS7) tube? (twin power triodes used as the series element
in voltage regulators)
OK, why would you want to?... it's low voltage, high current, high
bias, likely difficult to drive, you don't need to since there are
plenty of better alternatives, etc...
My answer: I have several 6080's (mostly NOS); getting bored with
designing for 6V6, 6L6, EL84, EL34, etc (well, not really!); you get P-
P in one envelope; I have a couple of spare universal replacement
OPT's (the Hammond 125E) so a choice of plate-to-plate impedances from
3000 ohm up (low enough?) is easily obtained; two O/P tubes for
stereo; and, finally, the 6080 sort of looks nice!
So, any opinions on this?... including don't bother! g
Cheers,
Roger
You can set up a PP pair of class AB1 6080 or aka 6AS7 with
RL about 1k, a-a and Ea = 200V and if the Pda of the two tubes = 40W,
then you get about 10Watts of clean power, or about 100Vrms a-a into the
1k,
and gain at about 1.8, so you need about 30Vrms at the grid which is
none more than for 6550 in triode.
3k is to high, and the Vswing will be limited by the Ea, which cannot be
much above 200V because the tube
just doesn't like it, it tends to run away with current if the Ea is too
high
no matter what the applied grid bias voltage is.
The peak Ia available is VERY high for a tube, and ppl have used a quad
of 6080 for
a basic OTL amp, and Atmasphere makes OTL with lots of 6AS7 as a
commercial example.
Its also possible to series connect the 6AS7 and use a cap coupled OPT
which is a line
voltage speaker tranny, Altronics of Western Aust make a toroidal one
which I thought would work
with the series PP.
But a good use is the circlotron circuit with a low Z load on the
primary.
Two 200V floating supplies are needed.
Some toroidal mains transformers are OK to use because with say 240V : 2
x 35V windings
you have 240 : 35 with both secs paralleled, giving TR = 6.86, ZR = 47:1
= 376 ohms to 8 ohms,
and about right for a suitable tube circuit if the Ra-a is about 90
ohms.
1/2 a 6080 Ra = 280 ohms, and a whole one has 140 ohms,
a pair have 280 ohms a-a, and a six pack have Ra-a = 93 ohms, just what
you want
and you could get about 50 watts AB1 easily. They'l also go AB2,
but maybe not much is gained except more thd, which isn't wanted.
A 150Watt mains toroidal tranny could be fine, and is worth a try.
Such mains trannies are dead useless for other large octal tubes, 6550
etc, because the
AB load for a pair is typically 5k for 50 watts AB1 into 4 ohms, and
7k for 35 watts into 8 ohms, with most of the power class A.
"Universal" mains toroidal trannies with two 120V primary windings are
needed, and the secondary voltages of say 35V are a guide only, and
could vary
down to say 18V + 18V for a transistor amp PS.
240V : 18V gives 1.42k : 8 ohms.
The 240V across the primary gives a Bmax of about 1.2Tesla at 50Hz.
For good low bass you want 1.2T at say 20Hz, so the voltage across the
primary
should be no more than 240V / ( 50/20 ) = 100V only.
240Vrms into 1.42k = 40 watts only.
For a guitar amp the mains toroidal is usable with 6080 but for hi-fi
the available
turns per volt are simply not high enough, no matter what the VA of the
tranny is,
because it will saturate at too high an F, especially when used
with the high Z of bass speakers below 100Hz.
Have fun, but I prefer the 6550, KT88, KT90, EL34, 6AC7, 2A3, 300B etc.
Patrick Turner.
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