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Old 9th June 2008, 06:53 AM   #1
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Default Direct-coupled cathode/source follower driver in PP?

If not using transformer coupling, a direct-coupled CF or SF driver for a PP output stage would seem to offer a number of advantages. It can be used to prevent blocking distortion of pentodes, to supply sufficient drive current to overcome miller capacitance of triodes, or to serve as a low impedance source to drive Class AB2.

Seems like a good idea to use this type of driver in every PP design, but is there any downside to doing this?
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Old 9th June 2008, 07:22 AM   #2
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Take a look at the McIntosh amp schematics...

Chris
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Old 9th June 2008, 10:51 AM   #3
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hey-Hey!!!,
Whatinthehell is, 'blocking distortion of pentodes'?

On the rest, the CF/SF will add another stage. If it isn't enclosed in a FB loop, there is no risk. What it might sound like is going to depend on the implementation.
cheers,
Douglas
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Old 9th June 2008, 12:01 PM   #4
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Quote:
Whatinthehell is, 'blocking distortion of pentodes'?
Blocking distortion AFAIK is caused when the grid of the OP tube (in this case) is fed by a coupling cap and goes positive, becoming effectively a diode. It can happen on transients and it takes a large current from the coupling cap, which then takes some time to recover. It's not exclusive to pentodes but it is more severe, because they take much longer to recover than triodes (I think).

This kind of distortion can be counter-acted, to some extent, by using a high value grid stopper on the OP tube or it can be completely avoided by using DC coupling (i.e. no coupling cap at all).
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Old 9th June 2008, 02:02 PM   #5
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I wasn't aware of the recovery time difference. As far as I know, they're pretty much the same- it's the RC time constant which determines recovery time.

OK, you know the advantages of a follower (I think source followers are even better in that application, but I'd probably be disinvited from the European Triode Festival this year for saying that). The feedback question Douglas raised is probably not an issue- follower bandwidths are ridiculously good. One more advantage is the ability to drive appropriate tubes in AB2, which is good for another dB or two of power.

Disadvantage is parts count and heat. For a fixed-bias triode output stage, I'd use a follower driver whenever possible. For a fixed-bias pentode that won't be driven AB2, I'd probably just use a big grid stopper instead.
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Old 9th June 2008, 05:01 PM   #6
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Here is one version of such driver, from my Alligator amp (drives triode-strapped GU-50 in A2):

Click the image to open in full size.
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Old 9th June 2008, 05:47 PM   #7
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Another one:

Click the image to open in full size.

More:
http://www.dissident-audio.com/PP_6L6/Page.html

Yves.
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Old 10th June 2008, 12:07 AM   #8
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One more: Heathkit W6-A

http://www.triodeel.com/heathw6.gif
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Old 10th June 2008, 01:57 AM   #9
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Default Re: Direct-coupled cathode/source follower driver in PP?

Quote:
Originally posted by ray_moth
If not using transformer coupling, a direct-coupled CF or SF driver for a PP output stage would seem to offer a number of advantages. It can be used to prevent blocking distortion of pentodes, to supply sufficient drive current to overcome miller capacitance of triodes, or to serve as a low impedance source to drive Class AB2.

Seems like a good idea to use this type of driver in every PP design, but is there any downside to doing this?
A direct coupled cathode follower means another hole in the chassis.

And two more:

Vixen

Le Renard
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Old 10th June 2008, 04:43 AM   #10
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Ray,
I thoroughly RECOMMEND a Source Follower drive of output tube grids.
Advantages are, as you state, freedom from blocking distortion.

It also allows use of low Rg1 resistor values which helps with bias stability too.

Waveborn has obviously been reading Allen Wright. He is showing a MOSFET version of Allens SLCF. With triodes it is good to current source load the cathode follower (Constant Current mode operation) and Bootstrap the anode voltage (Constant Voltage mode operation).

With a MOSFET Source Follower I might consider the current source load (although I'm using simple resitive load with good results) but as long as you keep minimum Vgd to say +25V with maximum signal swing, then the bootstrap is totally un-necessary.

If you want to go the "No Sand" route, then use a pentode cathode follower with the screen bootstrapped to the output - with or without a current source load.

Cheers,
Ian
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Old 10th June 2008, 06:55 AM   #11
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Quote:
Originally posted by gingertube

Waveborn has obviously been reading Allen Wright. He is showing a MOSFET version of Allens SLCF.
No. Wavebourn has obviously a diploma of Engineer Designer and Technologists of Radio Electronics Equipment from famous TIASUR.

I just loaded a source follower on a CCS and bootstrapped it with one more source follower so the main SF sees nearly constant SD voltage and current. As the result, distortions caused by variations of transconductance and capacitances are minimal.

Obviously Allen Wright had similar purpose in mind...

Also, if I remember right, George (Tubelab.com) used something similar. Sometimes very similar tunes may be made of 12 notes available for composers. 12 notes. How many elements are available for electronics designers?

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Old 10th June 2008, 11:25 AM   #12
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Allen will be the first to tell you that it's not original to him. There's a pretty similar circuit in Horowitz and Hill. In tube form, it certainly was pretty common in oscilloscopes back in the '50s and '60s.
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Old 10th June 2008, 01:04 PM   #13
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SY, I should hace said that a pentode OP stage has a much harder time recovering from blocking than a triode stage because of the much greater amount of NFB used in the former case. The triode will clip but only momentarily.

I suppose what I really want to know is if there is any reason not to use direct coupled cf/sf drivers, apart from economy.
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Old 10th June 2008, 01:27 PM   #14
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No reason at all, beyond superstition.
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Old 10th June 2008, 02:15 PM   #15
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Quote:
Originally posted by ray_moth
. . .
I suppose what I really want to know is if there is any reason not to use direct coupled cf/sf drivers, apart from economy.
Build one, don't forget to insure that screens supply does not sag (easier if you do not use UL ) and you'll know the reason to use !

Listen to something with heavy percussions

Yves.
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Old 10th June 2008, 05:14 PM   #16
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Quote:
Originally posted by ray_moth
SY, I should hace said that a pentode OP stage has a much harder time recovering from blocking than a triode stage because of the much greater amount of NFB used in the former case. The triode will clip but only momentarily.
Screen grid is a good sensor of overload: I usually put Vactrol there to sense current peaks by it's LED, and reduce amplification factor by it's opto-resistor. Also, bright red LED's grab my attention to too much input level. Triodes don't offer such a convenience, but I drive them with 1'st grid current, and a source follower does that fine.
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Old 11th June 2008, 12:52 AM   #17
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hey-Hey!!!,
I have gotten good results with using a cascode pair of MOSFET's instead of a single every time I have needed such a thing. I even tried them as grounded source amplifiers. It seems to deliver the near-constant capacitance of a pentode without the need( and trouble ) of g2 current.
cheers,
Douglas
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Old 11th June 2008, 06:58 AM   #18
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Default #1

Hello ray_moth
Quote:
Originally #1 posted by ray_moth
If not using transformer coupling, ... to supply sufficient drive current to overcome miller capacitance of triodes, or to serve as a low impedance source to drive Class AB2.

Seems like a good idea to use this type of driver in every PP design, but is there any downside to doing this?
A transformer does not solve these problems and Miller capacitance does not cause any problem in a PP design.
This is one advantage of push pull against single ended.

Quote:
Originally #1 posted by ray_moth
... a direct-coupled CF or SF driver for a PP output stage would seem to offer a number of advantages. It can be used to prevent blocking distortion ...
You'll get the blocking distortion at the grid of the cathode follower.
Thus it is not preventing blocking distortion.

Quote:
Originally #1 posted by ray_moth
If not using transformer coupling, a direct-coupled CF or SF driver for a PP output stage would seem to offer a number of advantages. ... or to serve as a low impedance source to drive Class AB2.

Seems like a good idea to use this type of driver in every PP design, but is there any downside to doing this?
There are a lot of good reasons not to go to AB2 mode, especially for triodes.
If you can live with the crossover distortion from AB1 to AB2 the follower is a good solution.

Kind regards,
Darius
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Old 11th June 2008, 07:55 AM   #19
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Hi Darius,

Quote:
A transformer does not solve these problems and Miller capacitance does not cause any problem in a PP design.
I don't follow that. Triodes have significant Miller AFAIK, whether they're SE or PP. It's pretty well negligible for pentodes, though.

Quote:
You'll get the blocking distortion at the grid of the cathode follower.
That can happen, true, but it's easy to avoid.

Quote:
There are a lot of good reasons not to go to AB2 mode, especially for triodes.
I had the opposite impression. Ithought that that triodes had more to offer by going into AB2, in terms of extra power, than pentodes.

In any case, I wouldn't want to run in AB2 for any length of time, but it does seem to offer a bit of useful headroom for transients. Better than ugly clipping, anyway.
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Old 11th June 2008, 08:15 AM   #20
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Default #19

Quote:
Originally posted by ray_moth
Hi Darius,

...
That can happen, true, but it's easy to avoid.
...
How?

Kind regards,
Darius
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Old 11th June 2008, 08:18 AM   #21
Yvesm is offline Yvesm  France
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Quote:
Originally posted by ray_moth
. . .
In any case, I wouldn't want to run in AB2 for any length of time, but it does seem to offer a bit of useful headroom for transients. Better than ugly clipping, anyway.
That's my faith !

TRUE AB2 calls for lower load and a much beefier psu !

Yves.
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Old 11th June 2008, 08:43 AM   #22
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Hello Yvesm,

Quote:
Originally #21 posted by Yvesm


That's my faith !

TRUE AB2 calls for lower load and a much beefier psu !

Yves.
I am sorry to tell you that this is not true.
First a transient is not able to change the charge of C2/C4
because the pentode has 82KΩ output resistance.
Second clipping takes place at the grid of V1B/V2B
instad at the output pentodes grid.

The advantage is more output power because of class AB2.
If you don't want AB2, you don't need the cathode followers.
No more headroom, you can choose between clipping at the grids
of the cathode follower and the grid of the output tube.

Kind regards,
Darius
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Old 11th June 2008, 10:55 AM   #23
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Default Re: #19

Quote:
Originally posted by oldeurope


How?

Kind regards,
Darius
Getting enough voltage swing, and for a grid-current capable cathode follower, delivering power is just like designing a stage for RC coupling. Account for the load, and draw the line across the curves... Now this line won't be a straight one, but putting it onto the proper set of curves( as in don't plan to drive an 813 grid 80V positive with a 12AX7 CF ) is not some black magic art.
cheers,
Douglas
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Old 11th June 2008, 02:17 PM   #24
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Default #23

Quote:
Originally #23 posted by Bandersnatch


Getting enough voltage swing, and for a grid-current capable cathode follower, delivering power is just like designing a stage for RC coupling. ...
cheers,
Douglas
Hi Douglas

Thanks, but I think we misunderstood.
My question is how to prevent "blocking distortion" caused by charging C2/C4
when the grid of V1B/V2B takes current. See schematic #7

Kind regards,
Darius
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Old 11th June 2008, 02:26 PM   #25
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Default Re: #23

Quote:
Originally posted by oldeurope


Hi Douglas

Thanks, but I think we misunderstood.
My question is how to prevent "blocking distortion" caused by charging C2/C4
when the grid of V1B/V2B takes current. See schematic #7

Kind regards,
Darius
hey-Hey!!!,
That is the answer I gave. Make sure, by the usual and traditional means that the stage never needs the positive grid drive. It is not a given that it will be required. At some point the amp will run out of power, that is inescapeable. It is desired that this power limit be one developed in the output stage( usually anyway ).

That requires setting the operating conditions and selecting device parameters to fill the need. You'd need to do the same thing if we had a MOSFET there, save the design would keep it away from gate-source saturation voltage, yes?
cheers,
Douglas
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