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Old 17th July 2008, 09:18 PM   #1
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Default Horz Sweep amp Pent or screen drive

As I am in the habit of doing I am looking ahead a project or two. I have read a bunch of threads on horizontal sweep tubes and I have yet to figure out which method of use would be the most desirable WRT ease of building with minimal test equipment and best performance.

I have on hand one each of
6JB6
6GE5
6JN6
6JS6
17JM6
17JN6
22JF6
33GY7A
36KD6
38HE7

and three
6LQ6

Given that the only one I have multiple copies of is the 6LQ6 I thought I might make a PP subwoofer amp to drive a tapped horn sub or something like that. A couple of these tubes seem to be virtually identical except for the socket used or wiring thereof but that seems like a really kludgy way to build a PP amp. Maybe a couple of SE mono blocks or something down the road.

The question is do I go pentode mode with regulated screen supply (Maybe a gas regulator tube) or screen drive with FET drivers? What are the advantages and disadvantages of each?

Since the 6LQ6s are so valuable I suppose I could sell them and buy a bunch of one of the other tubes. Or selling all of the other sweep tubes might net me enough to buy one more 6LQ6.

Another possibility might be some swapping with others who have a mismatched tube collection so that we can all end up with some matching sets.
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Old 18th July 2008, 12:51 AM   #2
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Default Re: Horz Sweep amp Pent or screen drive

Quote:
Originally posted by mashaffer
The question is do I go pentode mode with regulated screen supply (Maybe a gas regulator tube) or screen drive with FET drivers? What are the advantages and disadvantages of each?
Screen drive can give better linearity. However, it also has a higher drive requirement, and you may simply lose back in driver distortion what you gained in final linearity. I did an amp with the 6BQ6GTB horizontal deflection VT. Used PP pentode mode with active screen regulation. Worked great, and the 6BQ6GTB turned out to have a friendly harmonic signature: Mostly h3 with but a trace of h5. It didn't require much help from gNFB to clean up whatever open loop pentode nastiness was there.

Quote:
Maybe a couple of SE mono blocks or something down the road.
Doubtful that'll work. These HD VTs like to pull some big currents, and aren't likely to operate Class A1 without red plate destruction to hit the most linear part of the characteristic.
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Old 18th July 2008, 01:22 AM   #3
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Thanks Miles.

Quote:
Screen drive can give better linearity. However, it also has a higher drive requirement, and you may simply lose back in driver distortion what you gained in final linearity. I did an amp with the 6BQ6GTB horizontal deflection VT. Used PP pentode mode with active screen regulation. Worked great, and the 6BQ6GTB turned out to have a friendly harmonic signature: Mostly h3 with but a trace of h5. It didn't require much help from gNFB to clean up whatever open loop pentode nastiness was there.
Interesting to hear of your success. I note that McIntosh made a couple of well respected power amps using 6LQ6 in pentode mode so there is another data point favoring the pentode mode. I don't quite understand their driver and bias setup. As I read the schematic they are using something akin to a CF to drive the grids of the 6LQ6s but the drivers look a little funny. The plates of the two drivers are connected to a 470V B+ at the CT of an OPT winding and the cathodes to the the ends of the winding. I don't follow that at all. I don't get the ground reference for the cathode.

I had assumed either a very high performance CF or a SS follower for adequate current drive but I infer from what you are saying that voltage swing will need to be substantial as well due to lowish mu. Is that correct.

Quote:
Doubtful that'll work. These HD VTs like to pull some big currents, and aren't likely to operate Class A1 without red plate destruction to hit the most linear part of the characteristic.
Sounds like HDTs like PP. That's OK I guess. There are other candidates for midrange/fullrange amps. Use these guys for bass where they naturally fit the bill.
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Old 18th July 2008, 01:27 AM   #4
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Mac used a unity-coupled output stage, a pretty special circuit. Bootstrapping, special transformers, you name it. Draw some arrows and you'll see that all the DC currents in the transformer primary cancel out.

edit: There was a fine article about this topology in November 1959 "Audio." The author was Norman Crowhurst.
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Old 18th July 2008, 01:33 AM   #5
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Thanks SY. Pretty tricky are they hmmm...

BTW I seem to remember that you have a bit of experience with these things. Any thoughts to add.
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Old 18th July 2008, 01:39 AM   #6
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I use p-p screen drive on my amp, run very close to class B. Distortion is surprisingly low; the tubes act very much like zero-bias triodes. SE is a lot trickier because you'd need to run A2 if you want to screen drive.

Pentode mode for HD tubes tends to be pretty nonlinear, but as Miles said, using screen drive makes the driver stage a challenge. It's a fun challenge.
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Old 18th July 2008, 09:50 PM   #7
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Thanks SY. For a subwoofer or musical instrument amp it seems like a bit of non-linearity might not be a big problem as some GNF is not such a problem in those applications. Subs being driven by SS seems to work pretty well so Pentode mode with GNF should work pretty well also I would guess. It might also be fine for a full range amp in non-critical listening systems such as a garage or work room system.

What do you think of using gas regulator tubes to control the screen voltage in pentode mode?

mike
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Old 18th July 2008, 10:16 PM   #8
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You're asking someone who thoroughly believes that tubes shouldn't be used for subwoofers, and especially not pentodes.
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Old 19th July 2008, 06:34 AM   #9
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Quote:
Originally posted by mashaffer
What do you think of using gas regulator tubes to control the screen voltage?
Screen drive? A2? Gas Tubes??? What, are you crazy?!?

I wasn't very far along with this one when I set it aside...
Its got errors I am sure. Something I been meaning to debug
and finish. I have actual intentions of building, go figure...

Input stage sorta Lofty, intended to float up on to B+ noise.
Goin back down, its a FETron P-CH triode emulator, abusing
a screen driven Pentode as if it were an enhancement FET.
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Old 19th July 2008, 11:59 AM   #10
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Quote:
Screen drive? A2? Gas Tubes??? What, are you crazy?!?
Actually I was thinking gas regulator for screen supply in pentode mode.

SY, I would be interested in your thoughts on why pentodes are especially inappropriate for subwoofer duty. I am assuming a reasonable amount of GNF for the sake of Zout. Also do you not like tubes for Sub duty because SS does it easier or because tubes are in some way unable to do the job properly.

So if I do use them for subwoofer duty screen drive (which I take it is pretty much a triode) is what you would suggest?

As far as power goes my current 8 ohm 12" sealed sub is working fine with plenty of output with a plate amp rated at about 120W into 4 ohms and the next sub should be more efficient yet (tapped horn) so power shouldn't be an issue.

Would need an OPT with plenty of inductance on the primary but that should be doable especially since highs don't matter.

As far as it goes I haven't committed to doing sub with these. I could also get another 6LQ6 (or three of one of the cheaper ones) and use it for the regular woofer amp (60-200Hz).

P.S. SY, didn't you post a schematic of source follower driver for screen drive somewhere here?
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