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Old 15th April 2013, 08:09 PM   #1
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Default 30W sweep tube amp

Howdy, Folks:

While not new to tubes, I am relatively new to the DIY hobby of designing and building tube amplifiers.
I have been in the downhole oil/gas industry for several decades, designing high temperature analog and power electronics, with the added requirement of very high reliability (which flies in the face of the high temperature, fun, huh!!).
Wanted to do something challenging and a little different. After reading numerous vintage articles (Crowhurst and the likes), I have decided to go the UL route. After looking at many candidate tubes, the sweep and RF transmitter tubes started to make a lot of sense. Finally have settled on the 6CB5 (after seriously considering the 1625, 12V version of the 807). Had to design a simulation tool to investigate its transfer and plate curves for UL operation, that was fun in itself.
Anyway, nothing concrete yet except for the overall topology of full differential from the input to the output transformer, as much DC coupling as possible and lots of local feedback loops, with no loop covering more than one RC constant.
Solid state CCS don't bother me a bit, they will be used instead of pentodes (no concerns about heater/cathode potentials plus lower minimum voltage) and modern SMPS for heater and likely +B.
I'll be concerned more about proper circuit design, things like operating points, phase shifts, levels, in short, the basics all around.
While quite capable of designing and building my own output transformer, not really looking forward to that. I am attracted to a toroidal design (all things equal, best coupling, lowest core losses and magnetizing currents) and would welcome any comments on providers out there. I know of Plitron and the Dutch equivalent but not of any others.
Anyway, nothing concrete for now, no details but the floor is certainly open to any pertinent suggestions and thoughts and ideas and, most of all, encouragement I can get from the group.
Oh, yes, what are you kind folks' ideas on schematic capture/sketching programs? While I have PCAD, it has no tube libraries and am loath to create any, so if there is something already out there, I would much prefer not to have to invent anything.

Thanks!

Rene
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Old 15th April 2013, 08:42 PM   #2
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I'd worry about runing the 6CB5 in UL mode because of the limted voltage rating of the screen grid. If you ground the control grid and drove the tube via the screen, it would work just fine, and you could get a lot more than 30W from a pair. I have a nifty SE amp using the 6CD6GA in screen driven mode with Hammond 125ESE output iron. It is a surprisingly listenable amp for something so cheap.
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Old 15th April 2013, 09:56 PM   #3
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Thanks, that actually sounds cool. One of the things under consideration is the low G2 max voltage, as you mention. There are a couple of ways around that, among them pushing the screens a bit while running the plate on the low side, say both at 250V, a lot of power can still be had. Another is the use of separate screen and plate windings, more money on the transformer but solves the issue. Another approach I've been considering is using a conventional single tapped winding, raising the plate supply to 350 or so, then using zeners to drop the mean voltage for the screens back down to the 175-200V range. have not seen that done but would not be surprised to see that it has. Planning on staying AB1 so driver power is not an issue while screen drive for sure requires power.
I'd love to see a sketch or at least a more detailed textual description of your setup, again it sounds cool. If one properly understands G2 gm it would appear to me that there is no reason why one could not make a reasonably linear amp out of it. You say SE and I am more on the PP camp but the principle you used should be adaptable.
Any suggestions on a sketch/schematic capture program?
Thanks!
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Old 15th April 2013, 10:08 PM   #4
SY is offline SY  United States
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Look up the schematic of the Berning EA-230- it's a fine screen drive amp that is likely exactly what you're looking for. If I were going to change/update anything, I'd substitute some power MOSFETs for the cathode followers used to drive the screens.
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Old 15th April 2013, 10:17 PM   #5
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if you want to go UL with sweep tubes you have to get the OPT's with separate screen grid windings so you can feed it with lower voltages....
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Old 15th April 2013, 10:35 PM   #6
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Thanks, Tony. I am well aware of the screen G2 voltage limitations for sweep tubes, very similar to the issue with RF tubes. This is why I was thinking of the 3 options mentioned in the earlier post: one is to push them but not too much, run the plates at around 250 and still get a lot of power out of these guys: another is your suggestion, separate windings; another still might be to use zeners to get the G2 voltages down to withing spec. Lossy, yes, but should be effective.
I hope there will be a whole more posts from which we can all draw some good ideas.
And Sy, thanks for the tip, already looked it up, kind of neat. Like you, if I go that route, cathode followers will likely be replaced with MOSFETs. Can't tell what is the output tube type used?

Rene
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Old 15th April 2013, 10:37 PM   #7
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The Berning amp gets 30W with a pair of 6JN6s - one of what seems to be almost an infinite series of duodecar sweep tubes with 17-18W anode dissipation. There's nothing particularly magic about the 6JN6 - other tubes of similar rating would do just as well. They'd also be cheaper, as the "audiophile recognition factor" is not in effect. One benefit of using the smaller tubes is that there are lots of them with no anode cap. If you're going to take the trouble to mess with anode caps and plate wires flopping in the breeze, you might as well get some extra watts for your trouble. The 6CB5 could get you 60W with no trouble (Tubelab George would say lots more).
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Old 15th April 2013, 10:38 PM   #8
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It's 6JN6, but any of the 20-watt-class sweep tubes will work. The nice thing about this circuit is the amazing efficiency- it idles at (if memory serves) something like 3mA, so the output stage runs very cool and tube life/reliability is superb.
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Old 15th April 2013, 10:47 PM   #9
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Thanks, Wrenchone. Done the math a long time ago, well over 60W is indeed quite possible, just not sure I want to spend the money for the OPT but probably should go for 100W iron just for the margin, go for 50-60W out of the amp again just for the margin and personal satisfaction, even though in reality, I'll never use that much power. So, the engineer in me was being pragmatic and says 30W is plenty but it won't take much to tempt me to the dark side of a whole lot of power!
Plate caps and the T12 envelope (I'm actually after the 6CB5A) are just plain attractive to me and have the visuals I'd want. I don't mind dealing with their issues.
Any tips on a schematic drawing package?
Thanks again!
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Old 15th April 2013, 11:10 PM   #10
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Thanks, Sy. The docs I found do say idle current is indeed 3.3mA! I looked up the plate current family of curves for Eg1 at 0 and various G2 voltages (from which we derive the gm for G2). the G2 voltage at idle will be around 5 volts or so! Only thing is that all this smells of class B operation and not AB, being that close to cutoff when idling. If the output transformer is not super tightly wound for super low leakage inductance, there is the high potential for notch distortion. But, since at least 2 people have endorsed the Bearning, I must either be missing something, or else the OPT is indeed super-duper.
This screen drive approach has suddenly intrigued me enough to make me want to dig deeper. Can't be sure that's good or bad!
Any more thoughts?
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