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Old 4th June 2010, 02:46 AM   #111
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Originally Posted by UnixMan View Post
there's something I do not understand here. According to some datasheet I've found, it's Va(max) is "only" 750V. How could you run it at 1050V?
There are some ham radio rigs that run these tubes at a lot higher voltage in SSB / CW mode (e.g. the Kenwood 520-530). It will shorten the life of the tube but not as much as one would think. Forced air cooling has a lot to do with it.

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Old 4th June 2010, 09:42 AM   #112
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Hey guys,

Why are you even mentioning 6BQ6 or 6L6 in a thread about linear triode-strapped pentodes? 6L6 works great as Schade but triodestrapped it sucks. 6BQ6 seems even worse but maybe Schade can help there too? SE or PP is not the subject. Only the linearity of the tube characteristics interests me.

The only one Ive found so far is 813. 850V/5k as Millett uses looks OK when you check the curves. Ive got four 813s on the shelf waiting for the right chassis and OPTs.

http://www.pmillett.com/file_downloads/813%20curves.gif

Yet to find another an equal but lower power. Until then Schade rules.

Great thread anyway, with many interesting angles!
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Old 4th June 2010, 04:18 PM   #113
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Originally Posted by revintage View Post
Hey guys,

Why are you even mentioning 6BQ6 or 6L6 in a thread about linear triode-strapped pentodes? 6L6 works great as Schade but triodestrapped it sucks. 6BQ6 seems even worse but maybe Schade can help there too? SE or PP is not the subject. Only the linearity of the tube characteristics interests me.
We mentioned both actually.

12BQ6GTB in my amps with Schade work like a charm.

Also, I tried them with success as drivers (LTP) for Schaded Gu-50
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Old 4th June 2010, 08:12 PM   #114
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Theres been some talk about EL156 earlier in this thread. MJKs design together with this tube could be a real winner. Indications of 20W Pout, low THD and dominating 2nd order all the way up to just before clipping. 5k load, 450V Ua(525V B+), 100mA Ia and 280V Us. Must try, but maybe with a little more safety-margin!
EL156 looks like a winner. Load line below is pretty linear.

I would go with 4K plate load for 20W plate Po given 450V/100mA. Looks like 280V g2 is good scaling to 4K loadline also. The DCR of the OPT would be the determining factor for output resistance ;-) Tube only contributes 100 ohms effective Rp. 4K plate load would be 3K6 or 3K8 nominal plus DCR.

Point of reference: on my 5881WXT tester with 250V g2, the anode swing looks like it goes down to about 35V at clipping.
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Last edited by Michael Koster; 4th June 2010 at 08:15 PM.
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Old 4th June 2010, 08:39 PM   #115
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its not an output or a pentode for that matter, but the 12be6 pentagrid has some really nice looking triode curves. been wanting to try one as an input stage.
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Old 4th June 2010, 09:16 PM   #116
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Hey Michael,

Id still go for 5k. Besides have you had a chance to measure the chinese version? Enjoy the zipped *.asc, also attached the 156-model.
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Old 5th June 2010, 12:23 AM   #117
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Its not an output or a pentode for that matter, but the 12be6 pentagrid has some really nice looking triode curves. been wanting to try one as an input stage.
Dang, I must be getting old. I have hundreds of these things, and several dozen 5915's (a dual control heptode with nice pentode curves) and I didn't even think to try them in the red board. Yes, the pinout is compatible and this might even give us a new place to connect the Schade resistor (choose a grid!). My red board is back on the shelf for now, but next time I have it hooked up I will stuff some 6BE6's and 5915's into it.

The thinkers can find all sorts of reasons why this won't work, but I would rather just try it. There are 3 possibilities, it doesn't work, it works, or smoke and fireworks happen. At least two of the three would be interesting!
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Old 5th June 2010, 01:13 AM   #118
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An elder brother of my friend, when I was a kid, made an amp with 6i1p tube as a preamp. It is like ECH81 or 6AJ8. It sounded really very nice.
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Old 5th June 2010, 06:10 AM   #119
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Default SS Schadeode, A Linear "Triode"

Something I've wondered about that I now have the opportunity to test. Using the same testbed, I mocked up a SS version of a 6L6 using a low capacitance 1700V IGBT (6pf Crss) and a 200 ohm resistor in the source to limit the gm to about 5000 (200 ohms Rp effective, just like the 6L6 Schadeode I tested earlier).

I built it onto an octal plug so I can swap back and forth with only a change in feedback resistor to compensate for higher gate voltage of the MOSFET + Rs vs. grid voltage of the 6L6.

The distortion spectra are amazingly similar, with the SS version producing a little more 4th harmonic above 1W and a little less 3rd harmonic between 5 watts and clipping. Power output at clipping is the same 9W at the speaker terminals.

They also sound amazingly similar; the SS circuit is also neutral to slightly SET-like on vocals but with the same tight bass control and low end punch as the tube Schadeode.

Oops, forgot to show the gate protection zener on the IXGH6N170A. I included it from the start as it's probably needed; I don't want to find out...
Attached Images
File Type: png SSSchadeode-benchtest.png (6.1 KB, 1016 views)
File Type: jpg ssschadeode.jpg (67.0 KB, 915 views)
File Type: jpg 1w-ssschadeode.jpg (49.6 KB, 793 views)
File Type: jpg 100mW-ssschadeode.jpg (52.0 KB, 758 views)

Last edited by Michael Koster; 5th June 2010 at 06:16 AM.
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Old 5th June 2010, 01:23 PM   #120
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They also sound amazingly similar; the SS circuit is also neutral to slightly SET-like on vocals but with the same tight bass control and low end punch as the tube Schadeode.
I wonder if a good portion of that is the fact you are using an output transformer with each version. Most transistor amps not requiring it...
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