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Old 25th March 2013, 12:00 PM   #11
roline is offline roline  United States
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I took the start up 100k and just ran it to ground instead of V+. It takes a little longer to cold start but that is the desired effect. That should be a little kinder to the tubes.
Keep the ideas coming! No apparent change to the sound when up and running...
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Last edited by roline; 25th March 2013 at 12:07 PM.
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Old 25th March 2013, 02:16 PM   #12
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I can believe an output swing all the way up to 12V.

But downward, the plate can't go any lower than the
cathodes. And I hesitate to believe even with a VGS
drop, that source could ever dip much below ground.

Take you a look at the topology of suggestion #10.
You can see a little sand gain here means the plate
swing stays above the cathodes and within reason.
Yet the output can now travel the full span of both
rails...

No change to your current source and mirror, except
to flip the mirror around. Was just drawing shorthand...

Last edited by kenpeter; 25th March 2013 at 02:22 PM.
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Old 25th March 2013, 03:08 PM   #13
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Half the plate resistance would be seen by the gate (6K5/2).
So maybe I've added more sandy gain than I was thinking.
Perhaps reduce the 270K and 100K, to 27K and 10K???

Now 3K25 combined plate resistance to 27K sets the ratio.
About 8.3x gain added. The plate is never asked to dive
below the cathode at full swing would be the advantage.

10K pull down reduces feeds forward and back equally,
so has no effect upon gain ratio, only DC filament bias.

A 3K3 gate stopper (not shown in #10) would cut the
sandy gain boost in half ( to the original intended 4x).
And the plates would be able to swing some voltage,
which I think is necessary for Mu to work its magic.

Without this stopper, the triodes plates might be held
too closely to gate threshold, and in the less desirable
gm gain mode. I didn't see that when I drew #10...
Anyways, easily fixed...

Have I done anything stOOpid that would make the loop
unstable? If anything, thats where I usually go wrong...

Last edited by kenpeter; 25th March 2013 at 03:11 PM.
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Old 25th March 2013, 05:15 PM   #14
roline is offline roline  United States
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Measured.......
12.6 volts across the heater:
The cathodes are sitting at -9.3 volts
the +gate at -9V, -gate is at -7.9V
Plate V to Mosfet is at 4.6 volts, output at .75 so ~4volts Vgs
Taking the input +, the max output goes to 7.8 volts limit of the Mosfet Vgs
Taking the input -, the max output goes to -11.7volts, heater goes cold if DC and it stays there
AC coupled --So the max output swing is 19.5 volts PP on the wall-wart supply , just short of 24V.... 6.9Vrms into 20ohm cans=2.4 watts rms best case?? Again , just a headphone amp.........
With the heaters being the output current sink, it is not designed to swing rail to rail and be linear. The heaters can be replaced by a current sink if that is the desire, and the output current could be cranked up to drive an 8 ohm speaker. ( I did that with my other design)
I have to admit, this is a very unconventional design. I was pleasantly surprised that the idea worked at all!
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Old 26th March 2013, 02:27 AM   #15
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I had a tube fail cold start, not enough Mosfet current to warm it up, it just sat with 4 volts across it. So I added a small resister to B+ to raise the MosFet drive a couple volts. Problem tube solved.
Currently listening to Spiff...... excellent!
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Old 26th March 2013, 06:07 AM   #16
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How can those ground ref'd grids measure so negative?
And yet still slightly positive vs the cathodes...

Are forward grid currents pumping the grid bias down?
I guess with such low plate voltage, stray electrons have
to go someplace...

However you done it, if you measured as you say, its fine.

Last edited by kenpeter; 26th March 2013 at 06:10 AM.
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Old 26th March 2013, 11:19 AM   #17
roline is offline roline  United States
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I searched and found a graph on grid voltage vs plate current. At low voltages, the grid can be positive 4 to 6 volts which is so strange. I've never tried using a tube at such low voltage before. It is like you have to go positive to coax the electrons to leave the cathode where at high voltage you have to do everything to contain them, they want to leave real bad.
I read something about the real old car radio low voltage space charge tubes that had 2 grids, one was positive to coax the electrons to leave and then the 2nd grid controlled the flow.
I thought of another mod, a pot in the cathodes for offset adjustment, a simple change to try.
Sometimes reality is stranger than fiction.........
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Last edited by roline; 26th March 2013 at 11:35 AM.
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Old 27th March 2013, 04:25 AM   #18
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If we accept big leaky grid currents, whats the point of a rigid
current source below? Would a simple resistor do just as well?

If you add a little resistor under the filaments, such that it sees
a voltage swing similar to (but slightly less than) the input.
Then let the lower end of the tail resistor connect to this active
node, instead of directly to the negative rail.

Its kinda bootstappish. I dunno if this makes the tail to be a
genuine higher impedance, or just a feedback problem.

If so, then 6 components could maybe be replaced by just 2.

Last edited by kenpeter; 27th March 2013 at 04:42 AM.
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Old 27th March 2013, 04:49 AM   #19
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Like R10 in this old drawing of mine, same principal could be applied?

R10 above my bootstrap, but in your case would be below filaments.
Size to be slightly less than than 1/Gain, to stiffen the tail resistor
without going overboard and becoming a positive feedback. I drew
mine at exactly 1/Gain which is the threshold of becoming a problem.
If you try, I wouldn't advise cutting it quite so close.

Then maybe you don't need 6 parts active current source thats just
going to leak out the grids anyway...

You do seem one to forgive an imperfection in pursuit of minimalism.
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Last edited by kenpeter; 27th March 2013 at 05:00 AM.
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Old 27th March 2013, 11:14 AM   #20
roline is offline roline  United States
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The current sink was used to check the sensitivity to plate current to sound quality. I backed it down to 100uA and it still sounded good and then raised it up to 6mA and still sounded good. I have it set a 3mA. Original thought was 2mA/side 4ma total tail current.
If I could set the plate current by cathode to grid voltage your idea cold work. What I'm concerned about is the variation found in the tubes when subjected to this low voltage operating point. They are less predictable that when used at high voltage.
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