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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
cerrem is offline cerrem  United States
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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
SY is offline SY  United States
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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
Yvesm is online now Yvesm  France
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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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