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Old 13th November 2016, 03:14 PM   #861
piano3 is offline piano3  United Kingdom
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smoking-amp, now that I think I understand the circuit details a couple more questions:

are triodes really capable of driving the grids with low distortion?

the cathode and Rg1k connect to a relatively low voltage negative supply? does this supply need to be variable?

21HB5 is now $10
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Old 13th November 2016, 06:33 PM   #862
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"are triodes really capable of driving the grids with low distortion?"

One might want to put a follower in there for triode drivers. Rg2g1 and Rg1k in series, plus the g2 impedance in parallel would be the load. The g2 impedance certainly is not constant. With only a (Mosfet) follower gate to drive, the gain could be enormous for a pentode driver (maybe add a load R there).

"the cathode and Rg1k connect to a relatively low voltage negative supply? does this supply need to be variable?"

Cathode and Rg1k connect to 0V or ground. And the g2 idle voltage would be 0V also. (but going positive for signals) The driver stage (or CCS/VCS tail) needs to operate from -V on its cathodes to achieve that at its plate.

21HB5 is $6 here:
12A to 29LE3
Seeing as it was $1 before, I don't know why the price is up so much. ( I hope 40,000 of them didn't get sold lately! ) For a Sweep tube without a cap, the $4 6HJ5 would make more sense. For highest Watts per $, the $4 21LG6A would make sense, or 6CB5A or 26DQ5 or 6EX6. For more modest Watts, there are the 12GE5, 12GT5, 12JN6, all cap-less. 22JR6, 22JU6, 22KV6.

The present unity current gain Mosfet in the N Fdbk path (to eliminate PSRR issues) could be configured to send it's current down to the cathodes of the differential driver stage. Then use followers for the driver plates. This would put a lot more gain in the "local" N Fdbk loop. (lowering amplifier output Z considerably) That would linearize the driver stage too. (VCS Bias servo not shown, could be similar to the earlier version. Output g2 idles at 0V.)

Note: One little issue with these circuits so far: pull a driver tube out and the output tube fries! May want to make the 0V (g2) idle servo as a separate SS circuit, instead of using the driver.

..
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Last edited by smoking-amp; 13th November 2016 at 07:00 PM.
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Old 13th November 2016, 07:12 PM   #863
piano3 is offline piano3  United Kingdom
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Ah yes, I see; because every single amp I've ever built has been class A, I wasn't thinking of the g2-k idle voltage being zero.

I'd wondered before how best to send feedback to the cathodes of a differential pair; your mosfet configuration is really neat for this!

Probably every inexpensive tube mentioned here is now, unfortunately, going to be speculated upon. I am still stunned that someone bought 40,000 tubes with plate caps.
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Old 13th November 2016, 07:13 PM   #864
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That earlier Wavebourne Schade schematic could avoid the output tube frying issue (when a driver tube is pulled; shown below again, some mod needed). But the P Mosfet is essentially the driver stage, so it's really a Hybrid design. If one is OK with that, it should work well (although, I'm not convinced how stable that output bias is if the Mosfet heats up, some additional mods in order I think). Only one Mosfet, and it does all 3 things elegantly (Fdbk, buffer, driver), hard to ignore. Hats off to Anatoly!

"I am still stunned that someone bought 40,000 tubes with plate caps."
Must be for an OTL design. Maybe using a bar across the top of a long row of them, with the plate caps hidden inside.

Just for fun, I was thinking of gluing a fake plate cap with fancy cooling fins onto a 6HJ5.
Could dare people to touch it!
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Last edited by smoking-amp; 13th November 2016 at 07:41 PM.
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Old 13th November 2016, 08:46 PM   #865
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I'm a great believer in Pavlov's dogs-your prank might train someone to think that it was always ok to touch plate caps; that would certainly be the result in the (highly selective) girl's high school where I teach.

Talking of Anatoly (yes, he has many great ideas!) I seem to remember that his favourite tube, the GU50 fared pretty well in the crazy drive curves which was a bit surprising. I've lots of those as they are really common and cheap on this side of the Atlantic. No harm in trying it I guess (and indestructible!)
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Old 13th November 2016, 09:44 PM   #866
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You do have a point. Not a good idea to fake that.
At least only to an experienced knowledgeable group.

I would be sure to make some electrified rigid shaking motions while touching the fake plate caps, then act
-suspiciously- like "nothing" happened, "no really!". Some fake burn marks on the finger tips should clinch the deal. A little puff of smoke too, somehow. Hopefully keep anyone from trying the same anyway. They might never trust you again though, if you don't let them in on the joke.

--------

The GU-50 should be able to work in Twin drive, but it is going to require rather high g2 drive voltage I think. I had to graph the plate curves on the tracer with two pics, one with a floating DC supply to get the upper drive range.

Using a "big" driver tube like 12HL7/12GN7 (10 Watt) or a small Sweep like 6GF5/12GE5 might allow one to avoid the Mosfet follower buffered drive function for g2 drive.

..

Last edited by smoking-amp; 13th November 2016 at 09:55 PM.
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Old 18th November 2016, 12:01 AM   #867
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OT posts moved to the safety practice thread, working with tube is fraught will shock hazards....anyone working on tubes know this....
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Old 20th December 2016, 12:24 AM   #868
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Quote:
Originally Posted by smoking-amp View Post
21HB5 is $6 here:
12A to 29LE3
Seeing as it was $1 before, I don't know why the price is up so much. ( I hope 40,000 of them didn't get sold lately! )
Today someone on another forum posted an Asian amplifier manufacturer I hadn't hear of before, so I googled the name and found this: https://afroaudio.jp/products/detail...roduct_id=2111

Low and behold there are 16 12JF5's in this amp. Pretty sure this tube would have been on the dollar list at ESRC, but it's now listed at $14.00 ea. So I think it's safe to assume someone is buying up sweep tubes in large quantities for OTL amps.

jeff
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Old 20th December 2016, 04:59 AM   #869
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As far as I know 12JF5 was never on either of the $1 lists. (Vacuumtubes.net or ESRC)

It is functionally equivalent to the 12GE5, which WAS on the $1 lists. 12GE5 does not have a plate cap, while the 12JF5 does have one. 12GE5 lists for $3 now. It's also equivalent to the 6JN6 (except pin-out is different), 6JN6 mostly disappeared with the Red Board DCPP activity. If one were building a DCPP now, I would check out the pin compatible 21JV6 if no 6JN6 are to be found.

Somewhat odd to see the 12JF5 used for an OTL since it does not have a particularly high max current rating. They must have had a boat load of them.
And plate caps too? Guess they wanted that industrial look.
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Old 25th January 2017, 12:58 PM   #870
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Smoking-amp:
Some questions from a guy who don't have read all topics about power law:
In some measurements I perceived a upward-curved gm plot for pentodes. If straigh "inclined" one is a perfect square-law, and a horizontally one is a unity-law, a 3/2 law between that, so a curved upwards is a more than square law, perhaps a 2.5 power law (less than cube law), for example? Or is another rule? What is the aspect for a cube law gm graph?
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