Thread: 6CA7 in AB2.
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Patrick Turner
 
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jim wrote:

"Patrick Turner" wrote in message
...
Anyone have any experience in driving 6CA7
in either tetrode, triode, or UL, into class AB2?
The 6CA7 is supposed to be equal to EL34.
EL34 don't much like AB2; but since the
6CA7 is a tetrode, not a pentode, perhaps, like the 6L6,
AB2 operation is perhaps fine.

What sayest thou, and they of years challenged,
and who accompanied history's beginning, and who must
have memory of a bygone age?
What noble efforts at AB2 were sustained,
leading to much music well listened to?

Patrick Turner.


I'm certainly 'of years challenged' Why do you want to run EL34s in AB2 ??
There are easier ways of achieving the same result with 6550/KT88.. The
raison d'etre would be helpful
regards
jim


I have a client who has an amp where he'd like the high power ability,
but most days he sues low power.

This is where class AB2 operation fulfills a niche.
Jolida manage 66 watts from their UL amps with 530 for the B+,
and that's because the mains inputs are rated for 220v, yet we have 250v most
days here,
so hence the B+ goes from the design value of 470v to 530v wkg.
I know, becuase I tested one.
KT88/6550 don't give you much more power.

The high screen voltage means the grid bias voltage needs to be more -ve,
therefore you get more load swing without running into grid current, with either

UL or pentode, ie, all the power is had with AB1 operation.


But let us concentrate on pentode operation.
If one reduces EG2 to say 300v, then the grid1 bias can be reduced, to have the
same
idle plate current.
But it also means you can't have the same high input voltage to the grid1
because the input voltage will go +ve sooner with respect to the cathode.

The lower Eg2 supply relaxes the load on the screen, and gives lower thd at
the first 20 watts, because the anode signal voltage spends most of the time
at a potential above the screen potential, therefore screen current variations
are
minimised, and screen current draw during the cycle isn't too linear, which
contributes to anode current distortions.

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.

Triode operation results in a plate curve line at Vg1 = 0.0v which
is a boundary for load voltage swing over which you normally cannot step,
because the grid draws so much current itself, that it stalls the driving amp.
Some tubes, like the 6L6, 807, and others, don't have so much grid current
that they cannot still be driven, although it does take something like a cathode
follower to do it.
With 6L6,807,KT66, Triode AB2 operation is desirable, if its possible, because
we have all the finesse of the triodes during the normal part of the class A,
and AB1, that you'd get with any other amp, plus the bonus of the extra 15 watts
in AB2.

Sure I am a big fan of the EH and Sovtek 6550, and KT88.
All four tubes are, afaik, exactly the same electronically, with the same
internal
electrode structure, but they just have different glass envelopes, and writing
on the outside,
with the EHKT88 looking the prettiest.

These slightly more capable large octals are 3 times the price of the 6CA7.

The 6CA7 seems to me to be the best watts/$ value of output tubes at this
present time.
Sovtek made copies of the Sylvania 6CA7 up to about 1996, and then
I never seen any more until EH announced their return, along with
a few other tubes which havent been made for 35 years,
and including a russian KT90, which will be available later this year.
The tube business seems to be growing and diversifying.
Anyways, mild reliance on AB2 operation is not evil, and seems a sensible
option if done well, it seems to me,
so hence my query, how easy is it to drive 6CA7 into AB2?

Patrick Turner.