gm70 driver tube tests

[Magz]

Ale,

What do you think of the mu-follower gyrator as the load for a single 6e5p to drive an 833 at 1500V and 250mA, in heavy A2? I'm working up an 833 amp and was instead thinking of using a CCS on top of the 6e5p followed by an AOT1N60 MOSFET with separate +/- supplies in order to ensure enough drive. Do you think the simple depletion mode mosfet gyrator could do the job? I'm a little doubtful...

[djn]

I will only be driving my JBL 2441 compression drivers with the GM70 so I don't think I will use more that 1wpc because the JBLs are 113db efficient.

[mogliaa]

I'm in London and there is an Audio Circle meeting every month if you ever manage to make it in town. Would be great if you can join us.
The 6e5p is a great valve indeed. I'm on a 100% DHT route otherwise I'd have chosen it as a driver for my amp.
Ale

[mogliaa]

Originally Posted by Magz

Ale,

What do you think of the mu-follower gyrator as the load for a single 6e5p to drive an 833 at 1500V and 250mA, in heavy A2? I'm working up an 833 amp and was instead thinking of using a CCS on top of the 6e5p followed by an AOT1N60 MOSFET with separate +/- supplies in order to ensure enough drive. Do you think the simple depletion mode mosfet gyrator could do the job? I'm a little doubtful...

The CCS is not an ideal load for an A2 driver. Your load impedance is reduced to low ohms when A2, so you want very load output impedance and high current capability. An IXYS 01N100D is rated 1kV and 100mA. What is the drive required? What is the estimated A2 grid current?
Check the 4-65a SE amp thread where Michael Koster explains extensively the gyrator driver in this topology
Ale

[enzoastro]

I'll check that out too. I know nothing about gyrators but would like to use one on the output of my phono stage (6e5p again!)
Audio group sounds good, perhaps you could let me know details? Thanks.

I was hoping to try 4p1l two stage amp with 4p1l preamp, it wont leave much gain headroom for quiet recordings but might be worth a go
Enzo

[Magz]

Originally Posted by mogliaa

The CCS is not an ideal load for an A2 driver. Your load impedance is reduced to low ohms when A2, so you want very load output impedance and high current capability. An IXYS 01N100D is rated 1kV and 100mA. What is the drive required? What is the estimated A2 grid current?
Check the 4-65a SE amp thread where Michael Koster explains extensively the gyrator driver in this topology
Ale

The voltage drive needed will be much less than the 4-65a since the 833 has a mu of 35, but the IXYS 01N100D may be current limited in this application, where I could be in grid current most of the time, hundreds of milliamps of it depending on where I set the operating point. That's why I'm thinking of implementing the CCS on top of the 6e5p, followed by an AOT1N60 mosfet stage (1N60: 700V, 1.3A, only 7.5ohm typical on resistance, low capacitance Crss = 1.8pF). The IXYS 01N100D has an on resistance up to 10X higher than the AOT1N60 and about the same reverse transfer capacitance.

Maybe I could use the AOT1N60 in a gyrator on the 6e5p and save the extra stage...I'll need to investigate further.

[palmas]

Originally Posted by Magz

The voltage drive needed will be much less than the 4-65a since the 833 has a mu of 35, but the IXYS 01N100D may be current limited in this application, where I could be in grid current most of the time, hundreds of milliamps of it depending on where I set the operating point. That's why I'm thinking of implementing the CCS on top of the 6e5p, followed by an AOT1N60 mosfet stage (1N60: 700V, 1.3A, only 7.5ohm typical on resistance, low capacitance Crss = 1.8pF). The IXYS 01N100D has an on resistance up to 10X higher than the AOT1N60 and about the same reverse transfer capacitance.

Maybe I could use the AOT1N60 in a gyrator on the 6e5p and save the extra stage...I'll need to investigate further.

The 833 is a devil to drive, unless you go for more than 3000V.
I was never satisfyed with the driver supplying enough current that the "diode for grid" demands.

[Magz]

Originally Posted by palmas

The 833 is a devil to drive, unless you go for more than 3000V.
I was never satisfyed with the driver supplying enough current that the "diode for grid" demands.

It seems you have experience with the 833? What types of drivers did you try?

Your description of the drive requirements above is what I was afraid of, and why I'm planning on a separate MOSFET source follower stage with super low impedance and lots of current on demand. Please share your experiences!

[palmas]

Originally Posted by Magz

It seems you have experience with the 833? What types of drivers did you try?

Your description of the drive requirements above is what I was afraid of, and why I'm planning on a separate MOSFET source follower stage with super low impedance and lots of current on demand. Please share your experiences!

Not that much experience with 833A. A few years ago a friend of mine had a collection of NOS 833A and we were building a PP with them. We used 1700V for plate, output trannie was custom built, ...
We tryed all configurations we could think about, using tubes and interstage transformers. Even with impedances as low as 22Ohm, the diode grid effect was visible.
We didnt try mosfet or any SS devices. Hope you can get a better driver than we did.

[Magz]

Thanks, Palmas!

I'll give it a try, anyway!