Re: Big Video Amp
Posted: Thu Jul 13, 2017 5:09 pm
Step by step, inch by inch.
I changed the power supply for the opamps to be + 12 and -22 volts so I could go a little more negative on the output, and maybe run some screen voltage.
To accomodate that and keep the nice (for this) property of it all being cutoff with no input, I changed the offsetting resistors from 1k to 619 ohms. I also put a 100k resistor to the negative rail on the positive inputs (which are all tied together) so with nothing plugged in, it's at the absolute negative limit output of about -20.1 volts to the control grids. With the input shorted to ground, it goes up to -19.1v or so. Both are in cutoff...with +15v on the screens (so...depending, maybe I'll increase that voltage).
For now, not, as I may want to use kickback in the output circuit to get higher peak output voltages at some point - so complete cutoff gives me more of a switch than a linear amplifier, and with the arbitrary waveform generator I can adjust the offset to what I want anyway, should I want to run "more or less pure linear amp" (knowing full well that this isn't going to be the king of fidelity no matter what).
6kd6's are rated at 7kv peak plate volts. I've run them past 12 kv peaks in flyback with no issues if they're turned fully off. They make great switches and decent amplifiers.
During this testing, I had one "rogue" tube in there that didn't match the others, and the plate current in cutoff went to 3-4 ma. When I put in all matched Zenith tubes, it went down to the numbers on the schiz. (I prefer Sylvania for power tubes, so if anyone has a stash, let me know). I have some super-weird GE tubes that are two small sweep tubes in the same envelope in parallel....GE, don't get me started, I bet they're almost entirely unlike a real 6kd6, despite markings.
But, so far so good. I have an APAP machine blowing air into the chassis underside over the opamp heatsink I made, then out and up past the tubes...nice quiet blower for this (at one time we scored a bunch of these machines, reprogrammed them to their max output, and use them for cooling other fusor things too - nice and quiet as these things go).
I'm using one opamp per two tubes here as the specs say I can and it seems to work so far (I haven't tried AC real signal inputs yet, just DC bias tests). When trying to drive the control grids positive, they limit to around 5v instead of the 10v or so the opamp puts out in theory. The 47 +11 ohm resistor (effective) plus the opamp current limit agree with this.
Those extra 22 ohm resistors in each tube grid were just there (so I didn't change it, they were mechanically convenient) and used to be all that was there. I added the 47 ohm in series from the opamp output on the advice of the data sheet for driving big capacitive loads - a 6kd6 is supposed to be around 40pf input capacity, and here we have two - a pretty big load for this preamp. That 619 ohm offset resistor is a little more conductance than the 638 ohm calculated, but in tests it works right - the opamp input isn't a perfect virtual ground.
It's hard to believe nothing oscillates here (yet - now go back to your room, Murphy)...I did "the right things" but it usually also takes some luck when there's this much gain and this much bandwidth at the same time in the same box...
Belated note to self - "enable" is high true, pulled up to 5v or so internally. Short to ground to disable.
I changed the power supply for the opamps to be + 12 and -22 volts so I could go a little more negative on the output, and maybe run some screen voltage.
To accomodate that and keep the nice (for this) property of it all being cutoff with no input, I changed the offsetting resistors from 1k to 619 ohms. I also put a 100k resistor to the negative rail on the positive inputs (which are all tied together) so with nothing plugged in, it's at the absolute negative limit output of about -20.1 volts to the control grids. With the input shorted to ground, it goes up to -19.1v or so. Both are in cutoff...with +15v on the screens (so...depending, maybe I'll increase that voltage).
For now, not, as I may want to use kickback in the output circuit to get higher peak output voltages at some point - so complete cutoff gives me more of a switch than a linear amplifier, and with the arbitrary waveform generator I can adjust the offset to what I want anyway, should I want to run "more or less pure linear amp" (knowing full well that this isn't going to be the king of fidelity no matter what).
6kd6's are rated at 7kv peak plate volts. I've run them past 12 kv peaks in flyback with no issues if they're turned fully off. They make great switches and decent amplifiers.
During this testing, I had one "rogue" tube in there that didn't match the others, and the plate current in cutoff went to 3-4 ma. When I put in all matched Zenith tubes, it went down to the numbers on the schiz. (I prefer Sylvania for power tubes, so if anyone has a stash, let me know). I have some super-weird GE tubes that are two small sweep tubes in the same envelope in parallel....GE, don't get me started, I bet they're almost entirely unlike a real 6kd6, despite markings.
But, so far so good. I have an APAP machine blowing air into the chassis underside over the opamp heatsink I made, then out and up past the tubes...nice quiet blower for this (at one time we scored a bunch of these machines, reprogrammed them to their max output, and use them for cooling other fusor things too - nice and quiet as these things go).
I'm using one opamp per two tubes here as the specs say I can and it seems to work so far (I haven't tried AC real signal inputs yet, just DC bias tests). When trying to drive the control grids positive, they limit to around 5v instead of the 10v or so the opamp puts out in theory. The 47 +11 ohm resistor (effective) plus the opamp current limit agree with this.
Those extra 22 ohm resistors in each tube grid were just there (so I didn't change it, they were mechanically convenient) and used to be all that was there. I added the 47 ohm in series from the opamp output on the advice of the data sheet for driving big capacitive loads - a 6kd6 is supposed to be around 40pf input capacity, and here we have two - a pretty big load for this preamp. That 619 ohm offset resistor is a little more conductance than the 638 ohm calculated, but in tests it works right - the opamp input isn't a perfect virtual ground.
It's hard to believe nothing oscillates here (yet - now go back to your room, Murphy)...I did "the right things" but it usually also takes some luck when there's this much gain and this much bandwidth at the same time in the same box...
Belated note to self - "enable" is high true, pulled up to 5v or so internally. Short to ground to disable.