Historical note.
I have no illusion that I was the first to "invent" MOSFET grid drivers, but I did come up with them independently. Back in the early internet, circa 2000, using any solid state device in a guitar amp was a heresy certain to get blasts of near-invective on the net. I responded to these blasts with the MOSFET Follies. http://www.geofex.com/Article_Folders/m ... tfolly.htm
It was just the germ of an idea, not including any of the refinements of looking at how +V and -V supplies, CCS versus resistor loading and grid dissipation would affect things. I really like seeing the idea fleshed out into accepted amp practice.
The point of a grid driver is to paper over the radical change in grid impedance at Vgk = 0 as the charts show. The local feedback of a source follower does this reasonably well because of the very high transconductance of the MOSFET, but to avoid a glitch when the grid starts drawing current, the MOSFET needs enough current through it to not notice the current shift away from its source load (resistor or CCS) into the grid, so it needs enough bias current to make the transition into the grid "small" compared to its idle current. This makes me think that a slight mod to the MOSFET circuit to give it more local feedback might cure some of the issues with +/- VS headroom. Something like arranging a PNP with its base driven from the MOSFET drain and its collector feeding the MOSFET source might be able to use a much smaller +Vs and still stay very linear on peak swings much less than 3x the output drive voltage.
Just speculating. Dang. Now I have to get out the circuit simulator again.
AB2 power supply rails?
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Re: AB2 power supply rails?
"It's not what we don't know that gets us in trouble. It's what we know for sure that just ain't so"
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Mark Twain
Re: AB2 power supply rails?
Thank you for your memories R.G.!
Your Mosfet Follies have been debated, misunderstood, ostracized and rarely implemented.
I found those pages really enlightening at that time, even if I dropped in the error of trying to emulate Marshall's CF with a IRF820.
So you are doing an hybrid Mosfet-BJT Darlington, with the low Ciss of the mosfet and the high power capability of the BJT (so current can be increased)?
At the same time the small Mosfet will have a very low Ciss and even without keeping +25V on toppositive swing, the variation will be so low not to affect the guitar tonal range?
Your Mosfet Follies have been debated, misunderstood, ostracized and rarely implemented.
I found those pages really enlightening at that time, even if I dropped in the error of trying to emulate Marshall's CF with a IRF820.
So you are doing an hybrid Mosfet-BJT Darlington, with the low Ciss of the mosfet and the high power capability of the BJT (so current can be increased)?
At the same time the small Mosfet will have a very low Ciss and even without keeping +25V on toppositive swing, the variation will be so low not to affect the guitar tonal range?
Re: AB2 power supply rails?
Has anyone successfully implemented AB2 with 6V6 power tubes?
If so, could you please elaborate on the maximum grid voltage you were able to achieve safely?
I also want to know what maximum peak cathode current can be achieved. The GE datasheet for the 6V6GT lists the peak cathode current as 105mA, although that is under the triode connection column. If that limitation is adhered to, it seems to be limiting what AB2 can achieve in pentode mode. In triode mode, it seems to give more options, but a high impedance load seems to be required. Thank you!
If so, could you please elaborate on the maximum grid voltage you were able to achieve safely?
I also want to know what maximum peak cathode current can be achieved. The GE datasheet for the 6V6GT lists the peak cathode current as 105mA, although that is under the triode connection column. If that limitation is adhered to, it seems to be limiting what AB2 can achieve in pentode mode. In triode mode, it seems to give more options, but a high impedance load seems to be required. Thank you!
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Stevem
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Re: AB2 power supply rails?
Here's part of a schematic I came across from Boozhound labs back in 06.
It's a simple build using all Octals and 6V6 outputs and after the shut down I have most of the parts now to build it.
Of most interest to me was his PI section that uses a Choke for the tail / Cathode resistor of the PI to extend the current drive to the outputs.
Boozhound called the circuit a Choke tail PI , I am thinking of having the front panel I have made for it say "the toodler AB2 model "
The Choke used is a Hammond 157G.
595 ohms.
30H
It's a simple build using all Octals and 6V6 outputs and after the shut down I have most of the parts now to build it.
Of most interest to me was his PI section that uses a Choke for the tail / Cathode resistor of the PI to extend the current drive to the outputs.
Boozhound called the circuit a Choke tail PI , I am thinking of having the front panel I have made for it say "the toodler AB2 model "
The Choke used is a Hammond 157G.
595 ohms.
30H
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When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: AB2 power supply rails?
Steve, that's interesting. I've seen choke biased drivers in hi-fi, as a constant current source. I think you would have to be careful with the placement of the choke with respect to (unwanted) coupling.
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Stevem
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Re: AB2 power supply rails?
Yes, I was planing on fully shielding it with .005" sheet copper that I have for just this reason.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
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Stevem
- Posts: 5144
- Joined: Fri Jan 24, 2014 3:01 pm
- Location: 1/3rd the way out one of the arms of the Milkyway.
1 others liked this
Re: AB2 power supply rails?
I will post up when I have it done, probably by mid Sept after my vacation.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
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Stevem
- Posts: 5144
- Joined: Fri Jan 24, 2014 3:01 pm
- Location: 1/3rd the way out one of the arms of the Milkyway.
Re: AB2 power supply rails?
So why would either of those two be safer?
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: AB2 power supply rails?
I looked at the 6550 datasheet and it quotes maximum cathode current to be 175mA. You must take that to mean DC and not signal peak? I'm trying to understand the limitations of AB2 and where they can be exceeded in a musical instrument amplifier.