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6CA7 in AB2.



 
 
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  #1  
Old August 13th 03, 10:47 AM
Patrick Turner
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Default 6CA7 in AB2.

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.

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  #2  
Old August 13th 03, 12:32 PM
john stewart
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Patrick Turner wrote:

> 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.


See ABPR for the attachments referenced in the following.

Hi Pat- Never ran a 6CA7 / EL34 in AB2 but last year built an
experimental 6V6GT rig running AB2. It managed 26 watts in burst
mode & 19 watts continuous after the PS sagged to steady state.
The 6V6 grids are driven by a PP 6BQ7 CF. That helps to minimize
distortion as a result of G1 current.

IMO should be OK for a music amp since the grids will be driven
+ve only on peaks which are but a small percentage of the time.
See the attachment for test results. This all part of an article on
Amplifier Burst Testing to appear in the November issue of
AudioXpress magazine.

Also built an AB2 amp using a PP pair of 33 two volt pentodes.
It runs triode or UL. The driver is a triode connected 33, which
in turn is transformer coupled to the following grids.
That one is published in the "Glass Audio Projects" book from
AudioXpress.com.

The attached graph is a composite
shewing the drive signal to the grids (LHS with X10 Probe) &
the resulting G1 current (RHS) measured across a 10R sampling
resistor in the grid circuit. This one manages 4.85 W in UL &
2.5 W in Triode mode.

Cheers, John Stewart
  #3  
Old August 13th 03, 01:41 PM
Patrick Turner
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john stewart wrote:

> Patrick Turner wrote:
>
> > 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.

>
> See ABPR for the attachments referenced in the following.
>
> Hi Pat- Never ran a 6CA7 / EL34 in AB2 but last year built an
> experimental 6V6GT rig running AB2. It managed 26 watts in burst
> mode & 19 watts continuous after the PS sagged to steady state.
> The 6V6 grids are driven by a PP 6BQ7 CF. That helps to minimize
> distortion as a result of G1 current.


Indeed, I know about the drive requirements needing to be CF, and low
impedance,
because the grid I reduces R in to the output tubes.

>
> IMO should be OK for a music amp since the grids will be driven
> +ve only on peaks which are but a small percentage of the time.
> See the attachment for test results. This all part of an article on
> Amplifier Burst Testing to appear in the November issue of
> AudioXpress magazine.


I wanted to try 6CA7 in a circuit meant for KT88.
The limitation, especially in triode mode, is the Vg = 0V line.
With say 807, you often see the data curves including for a grid swing
up to +15v, allowing 30 watts AB2.

>
>
> Also built an AB2 amp using a PP pair of 33 two volt pentodes.
> It runs triode or UL. The driver is a triode connected 33, which
> in turn is transformer coupled to the following grids.
> That one is published in the "Glass Audio Projects" book from
> AudioXpress.com.
>
> The attached graph is a composite
> shewing the drive signal to the grids (LHS with X10 Probe) &
> the resulting G1 current (RHS) measured across a 10R sampling
> resistor in the grid circuit. This one manages 4.85 W in UL &
> 2.5 W in Triode mode.
>
> Cheers, John Stewart


I couldn't see any item under your name at abpr,
maybe the currawongs and magpies haven't flown in yet with the info.

The data curves for 6CA7 are usually given as EL34/6CA7,
and there is a difference, the former is a beam tetrode, the latter is a
real
pentode, and the couple of 6CA7 I do have in operation ARE different to
EL34,
with lower Ra and gain, but higher Pd.
I like the 6CA7 more than the EL34, and they sound well.

If I buy 50 of these now from New Sensor, they will only cost US $280.

The driver for the AB2 amp will have a choke with CT grounded,
and each end going to the grids of the output tubes.
The cathode circuits might be a new AC regulated cathode bias type,
which has all the benefits of excellent self regulation of the cathode
bias R,
but when going into class AB, the cathode cap won't charge up
due to the presence of one transistor bypassing the high positive peak
currents
seen in the cathode circuit, in all AB amps.
The choke won't allow the grid bias to change, as it does with a normal
RC coupled stage driven into grid current, because of the charge up in
the
coupling caps.
The charging up of cathode biasing and the grid coupling cap
both cause a tube amp to become paralysed when pushed a lot.

The output stage will be acoustical, so the CFB will INCREASE
the input impedance, despite the grid current, in the same way as in a
McIntosh.
Thus the AB2 amp is possible, and the extra headroom good for dynamics
such as loud drumbeats, where the slightly higher distortion after going
from
class AB1 into AB2 don't matter.
The acoustical prevents severe distortion developing in the driver LTP
stage,
because the voltage demanded still isn't too high for RC coupling.
Probably, I will have a direct coupled CF driving the choke, and then
the gain LTP can be a 6CG7 which is RC coupled to the CF.
Then the usual SE input stage.

Two 6CA7 work OK into a 6.6 k RL, without much AB2,
with B+ = 450v, and 10k is really nice.
But if the load has a dip to 3.3k, then the voltage
swing is limited, even in tetrode mode.
I was just after a little bit more performance.

Patrick Turner.

  #4  
Old August 13th 03, 04:24 PM
John Byrns
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Default

In article >, Patrick Turner
> wrote:

> 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.


Why is it that the EL34 doesn't like AB2 operation, what happens that is bad?


Regards,

John Byrns


Surf my web pages at, http://users.rcn.com/jbyrns/
  #5  
Old August 14th 03, 12:37 AM
Patrick Turner
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Default



John Byrns wrote:

> In article >, Patrick Turner
> > wrote:
>
> > 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.

>
> Why is it that the EL34 doesn't like AB2 operation, what happens that is bad?


Good question.
I have never really tried to push an EL34 hard into grid current.
But perhaps the 6CA7 is different, since it is a beam tetrode, not a pentode.
I was wondering who had experience of this, or whether
I will have to try it out myself to see if AB2 works as well as it does say with
807, or 6L6.

Patrick Turner.

>
>
> Regards,
>
> John Byrns
>
> Surf my web pages at, http://users.rcn.com/jbyrns/


  #6  
Old August 14th 03, 01:03 AM
jim
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Default


"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


  #7  
Old August 14th 03, 01:28 AM
Patrick Turner
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Default



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.




  #8  
Old August 14th 03, 03:32 AM
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
--
+--------------------------------------------+
| Music: http://www3.telus.net/dogstarmusic/ |
| Projects, Vacuum Tubes & other stuff: |
| http://www.dogstar.dantimax.dk |
+--------------------------------------------+

  #9  
Old August 14th 03, 03:47 AM
Tim Williams
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"Patrick Turner" > wrote in message
...
> The lower Eg2 supply relaxes the load on the screen, and gives lower
> thd at the first 20 watts ...


- Even though the load impedance is lower, relatively speaking?

Tim

--
In the immortal words of Ned Flanders: "No foot longs!"
Website @ http://webpages.charter.net/dawill/tmoranwms


  #10  
Old August 14th 03, 10:07 AM
Patrick Turner
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Fred Nachbaur wrote:

> 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!)


True, some folks do use really high voltages for UL or triode.
Its sudden dearth territory, and many older tubes just can't be biased
down to be stable, 6CM5 is a classic.
I don't like such high voltages for trioded or UL amps,
because it means the idle current has to be low, to get the tubes
to be biased a LOT below the rated max.
This means the tubes are not working in the most linear region
for the first few watts, which are the most important, and the amp mainly class B.

+500v is as high as I like to go.

Acoustical goes really well at all voltages, and is a better way to
do UL, and the screens are given a reprieve from high screen currents,
since the screen voltage can be a lot lower than the anode supply.



> This means, though, that the driver needs lots of headroom; about 160
> volts peak-to-peak.


That's only around 55 vrms, per each input, and that's easy to do,
and a heck of a lot easier than a McIntosh, where the drive to each output tube is

maybe 140 vrms.
Same for EAR509.


> 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.


Well exactly.

>
>
> 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.


One might have to have some limiting R between a CF driver and the output grids,
say 1k, maybe 2k, to give a nice squishy overdrive, ie, a natural compressor
effect.
Also, grid current needs to be limited somewhat in amps
subjected to continual overdrive all night.



>
>
> 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 would still have the same highish value load I normally use,
so the first 25 watts are still the same low thd as the AB1 amp.
If the load is halved, the power is lower, due to grid voltage excursions
needing to exceed the bias voltage, to get the preferable extra load voltage
swing.
The extra thd above 25 watts isn't going to be huge,
and is for the occasional drumbeats.

>
>
> 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.


!/2 a 6SN7 is a fine CF driver for 6L6 in AB2, and I have a Brimar circuit
in an old tube manual of theirs with this set up, using 807, with +600v
for B+, and +300v for EG2, and it makes 80 watts, and that's what folks used for
PA, back in 1953, complete with hardly any FB, and 10% thd max.
At 10 watts, they weren't too bad.
The 807 meant you didn't get arcing on the tube socket, and the Eg2
voltage meant the screens didn't fry every time some galoot ran a shorted speaker
lead,
and turned up the volume.

Patrick Turner.

>
> Cheers,
> Fred
> --
> +--------------------------------------------+
> | Music: http://www3.telus.net/dogstarmusic/ |
> | Projects, Vacuum Tubes & other stuff: |
> | http://www.dogstar.dantimax.dk |
> +--------------------------------------------+


 




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