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Pentode section of 6M11
Pentode section of 6M11
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Old 9th April 2011, 02:40 PM   #1
rsumperl is offline rsumperl  United States
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Pentode section of 6M11
Default Pentode section of 6M11

Morning everyone,
I came up with a plate load of 34K for the pentode section of a 6M11 (http://tubedata.milbert.com/sheets/093/6/6M11.pdf). Is this right? It seems rather low. My B+ is ~500v, though I do have ~200v available.

Thanks,
Ray
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Old 9th April 2011, 11:12 PM   #2
rsumperl is offline rsumperl  United States
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Pentode section of 6M11
Ok, so I experimented:
470KΩ G1 to GND
750Ω Cathode to GND (0.8V)
1.5MΩ G2 to 380V (gave me ~40V)
220KΩ Plate to 380V (gave me ~90V)
Fed plate to amplifier input with a .1UF cap and shielded cable.
SO...with G1 grounded, quiet as if it was not there, unconnected or connected to a source, very hissy, no hum though. not sure why.
Tons of gain, WAY to much gain.
At this point, unless the hiss can be managed, this tube may not be an option. I did not try another tube, perhaps this one is defective. I believe my layout was fine.

Any inputs???

Ray
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Old 9th April 2011, 11:44 PM   #3
SY is offline SY  United States
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Pentode section of 6M11
Your gain is way high. With a gm of roughly 13mA/V, 220k plate resistor, gain = 13x220 = 2860! Try dropping the plate resistor to 20k or so, and scale the screen resistor accordingly (70-80k). That will give you a high-but-possibly manageable gain of 250 or so.
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Old 10th April 2011, 12:18 AM   #4
Miles Prower is offline Miles Prower  United States
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Quote:
Originally Posted by rsumperl View Post
Tons of gain, WAY to much gain.
At this point, unless the hiss can be managed, this tube may not be an option. I did not try another tube, perhaps this one is defective. I believe my layout was fine.

Any inputs???
It's definitely a high gain pentode, and it'll produce lots of gain even operating with such low voltages. That's gonna mean noise. The high impedances also don't help in that regard either. Did you remember to include grid and screen stoppers? The "hiss" you're hearing could be an artifact of high frequency oscillation. High gain pents, like transistors (also high gain devices) are susceptible to oscillation.

What are you using this for, and how much gain do you really need?

Your other options, if you don't need the gain, are to triode the pentode section, or make an anode follower out of it. NFB will both reduce gain and noise.
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Old 10th April 2011, 01:13 AM   #5
rsumperl is offline rsumperl  United States
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Pentode section of 6M11
I tried what SY mentioned, still lots of hiss. My thought was to use the pentode as a first stage of gain and then use the triodes as a phase inverter LTP. Do you think it's doable?

Ray
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Old 10th April 2011, 05:05 AM   #6
SY is offline SY  United States
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Pentode section of 6M11
The gm is really high- you could be oscillating.
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