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Old 26th January 2021, 08:44 PM   #81
pmillett is offline pmillett  United States
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Hi George -

I'm thinking this topology should work well with some of the (relatively) modern transmitting tubes. 6146 and 4D32 come to mind. They aren't great in most conventional circuits I've tried. And like sweep tubes both have screen voltage ratings around half the plate voltage rating.

6146 can do 35 watts plate dissipation (ICAS). 4D32 is listed as 49 watts in AB1, but looking at the massive plate structure I have to believe it can do more.

I have both tubes sitting around, and will do some experiments. Maybe 500V/250V to start.

Pete
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Old 27th January 2021, 01:41 AM   #82
TheGimp is offline TheGimp  United States
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I started tweaking the driver using a 6CB6A. Simulations showed less than 1%thd at 100Vrms output.

I managed to tweak it to get less than 0.3%thd at 112Vrms out.

I have not tried multiple tubes yet, so this could be a fluke.
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Old 27th January 2021, 02:17 AM   #83
TheGimp is offline TheGimp  United States
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And, it is a noisy test setup with lots of flying leads.

And I blame it on George for giving me the idea for the "Tube Lab".

Although I actually used a similar setup when I was in "A" school in the NAVY back in 1971.
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Old 28th January 2021, 03:20 PM   #84
Tubelab_com is offline Tubelab_com  United States
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Quote:
Originally Posted by pmillett View Post
Hi George -

I'm thinking this topology should work well with some of the (relatively) modern transmitting tubes. 6146 and 4D32 come to mind. 6146 can do 35 watts plate dissipation (ICAS). 4D32 is listed as 49 watts in AB1, but looking at the massive plate structure I have to believe it can do more.
Pete
I haven't tried the 4D32 in UNSET yet, and tried a 6146 in anything recently.

I did stuff a pair of 4D32's in my Universal Driver push pull test amp. They made 200 watts from a pair in AB2 at the onset of clipping from 650 volts into 3300 ohms. Screen voltage was 250 volts. I tried them into 2500 ohms at the same voltage where they made 250 watts. Nothing was glowing, but the efficiency had dropped from the 3300 ohm load, so I'm guessing 200 to 250 watts per pair is the limit. I can get the same power from the 36LW6's which I have more of, so that's where I'm experimenting now.
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File Type: jpg 4D32_202W_x.jpg (416.7 KB, 494 views)
File Type: jpg 4D32_250W_noglow_x.jpg (318.1 KB, 480 views)
File Type: jpg 4D32_bigOPT_x.jpg (706.0 KB, 477 views)
File Type: jpg 4D32_DARK_x.jpg (437.4 KB, 89 views)
File Type: jpg 4D32Cranked_x.jpg (596.6 KB, 103 views)
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Old 4th February 2021, 01:56 PM   #85
Francois G is offline Francois G  United States
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UNSET is coming?
Hi George,

I’m following this UNSET thread with great interest. For hobby planning purposes, when do you anticipate the PCBs becoming available for purchase?
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Old 9th February 2021, 11:55 PM   #86
TheGimp is offline TheGimp  United States
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I finally got the oscillation on the negative peak eliminated.

I managed to get 15W at 10% THD out of a 6P41S.

15.125 W at 10% THD 3.04Vrms drive
4W at 1.8 % THD 1.36Vrms drive 3rd 10dB below 2nd
1W at 0.88% THD 0.64V drive

The schematic is not 100% accurate, I am using pots to adjust the bias of the two tubes.

Listening tests will be next.
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File Type: png LTspice_XVII_6C41S_Triode_Unset6CB6_Proto_.png (64.0 KB, 123 views)
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Old 15th February 2021, 08:50 AM   #87
ErikdeBest is offline ErikdeBest  Switzerland
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I put a very basic PP together. ECC81 loaded with gyrator (no unset here, just a led in the cathode), mosfet based concertina and the 250V P fets to feed the cathodes of 25DQ6Bs. For the moment the grids are at a fixed potential, but I want to use these for local feedback later on.

I used the zener in the P fet, just like shown here by George G2 using MOSFET ultralinear feed , but I could not get any current going through the circuit. I drew the P fets, and then it became clear that the gate is at a lower potential than the source, so with the zeners as drawn there can never be more than 0.7V between source and gate (and more is needed to turn them on). It saved the Fets, but I had to reverse the zeners to get current flowing.

Now going for the fine tuning!
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Old 18th February 2021, 10:36 PM   #88
gorgon53 is offline gorgon53  Finland
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Hi Tubelab,
have you tried E130L ? may need some damping resistors on some of the electrodes or ferrit beeds but should work otherwise, is a sturdy tube
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Old 24th February 2021, 12:54 PM   #89
Francois G is offline Francois G  United States
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UNSET is coming?
Could someone who is LTSpice-savvy please post the schematic for George’s simulation in his post #48. I assume that is his latest UNSET schematic that lead to Zintolo’s GU50 variation and TheGimp’s 6P41S.

I’m very eager to build a UNSET 36LW6; hopefully George will be able to offer a Tubelab PCB soon.
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Old 24th February 2021, 01:25 PM   #90
gillougillou is offline gillougillou  France
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« I’m very eager to build a UNSET 36LW6; hopefully George will be able to offer a Tubelab PCB soon.«

Me too !
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