Opamps for servos in tube amps - Christmas gift from LT, 12-12-12!

[Wavebourn]

Hi!

I got e-mail from LT today, looks interesting.

Since the chip draws low current, and slew rate is not very impressive, I would hesitate to spoil tube signal path by them, but it is a great candidate to be used in servos, when vacuum tubes are directly coupled, like in my Pyramids. The opamp has MOSFET inputs, and can run from up to 140V!

LTC6090 - 140V CMOS Rail-to-Rail Output, Picoamp Input Current Op Amp - Linear Technology

I ordered samples, it will be a great Christmas gift!

[TheGimp]

Interesting, but care must be exercised for the thermal pad.

[gingertube]

Quote:
"I got e-mail from LT today, looks interesting".

Ditto,
I posted the same link over at Ampage. You could use it for output tube drive (if its not against your religion) not just bias servo (10mA source/sink capability). Interest there was also in the simple 100W MOSFET Amp shown on p 19 of the datasheet. Only available in surface mount which is a bit of a bummer.
Cheers,
Ian

[Wavebourn]

My religion does not allow to use class B and 11V/uS opamps to drive tubes.

[ruben alstad]

i allways wondered about servos in tube amps,
but i had the bias in mind, a servo controlling a pot for the bias (fixed bias).
has anyone tried this?

[DF96]

Yes, people have tried this. If done the wrong way it acts like a CCS, fixing the average current instead of the quiescent current.

[Wavebourn]

I did that in class AB amp, with S/H and comparator: when signal level was above some level it switched off and kept voltage unchanged. When below, it opened the path closing servo loop.

[ruben alstad]

Originally Posted by DF96

Yes, people have tried this. If done the wrong way it acts like a CCS, fixing the average current instead of the quiescent current.

yeah of course, it would have to be a auto bias button or something alike, when there were no signal on the input you could press it and thus it would re bias itself. that's simpler in my opinion.

[DF96]

Wavebourn's method is better than a button. A button would require a very long-term hold (hours), or at least separate arrangements every time you switch on. At most his method need only remember the voltage for the length of a CD.

[Wavebourn]

Originally Posted by DF96

Wavebourn's method is better than a button. A button would require a very long-term hold (hours), or at least separate arrangements every time you switch on. At most his method need only remember the voltage for the length of a CD.

Less, actually: it had shorter time constant on soft volume than on high volume. No need for silence, about 5W for 100W amp is a good threshold to switch from 5 seconds to 300 seconds approximately. On 5W power idle and average currents are almost the same.