A few years ago, I made a 814 push pull amplifier like this:
step-up tranformer as input stage, driven by a small amplifier.
It was nice but today I would like to make a more "conventional" amp with a tube driver.
As I already have a pair of LL1660 10mA, I wonder if a 5687 (200V/10mA) can drive the 814's into A2 region or definitively not ?
From the datasheet, control grid at +30V eats 10mA of current, will it works ?
It may be better with a direct coupled CF or even better with solid state but as I have these 1660's, I am curious..
step-up tranformer as input stage, driven by a small amplifier.
It was nice but today I would like to make a more "conventional" amp with a tube driver.
As I already have a pair of LL1660 10mA, I wonder if a 5687 (200V/10mA) can drive the 814's into A2 region or definitively not ?
From the datasheet, control grid at +30V eats 10mA of current, will it works ?
It may be better with a direct coupled CF or even better with solid state but as I have these 1660's, I am curious..
Nice amp!
I did some experiments with Lundahl ITs to drive a pair of 6S4S (6B4G) into class A2 a couple of years ago.
A 6SN7 through a 1660 wasn't even close, but a 12B4A worked better. A trioded 6AV5 through a 1671 was the best but every combination had problems with severe ringning, a known problem that occurs when the grid current suddenly stops flowing throught the windings.
I did some experiments with Lundahl ITs to drive a pair of 6S4S (6B4G) into class A2 a couple of years ago.
A 6SN7 through a 1660 wasn't even close, but a 12B4A worked better. A trioded 6AV5 through a 1671 was the best but every combination had problems with severe ringning, a known problem that occurs when the grid current suddenly stops flowing throught the windings.
You can always use the interstage transformers to couple the gain stage to the source/cathode followers.
I had some success with the LL1660 wired for 2.25+2.25:1+1 and driven by a balanced pair of 12B4As, ignoring the airgap, but it still wasn't a very good A2 driver.
I ended up running the tubes in A1, but left room in the chassis for some mosfet followers between the ITs and the power tubes as a possible future upgrade.
I ended up running the tubes in A1, but left room in the chassis for some mosfet followers between the ITs and the power tubes as a possible future upgrade.
it struggled too much when asked to push into A2 drive or was it related to ringing ?
For this 814 project, g1 will eat probably about 10mA of current at +30V from the datasheet, but it is in tetrode mode with 300V vg2 so I don't know if it is the same in triode mode.
Attachments
Both. The Dcr is a bit too high and I couldn't get rid of the ringing without loading everything down with resistors and/or Zobel filters.
The LL1671 worked better, being a lower impedance device, but the ringing was still a dealbreaker.
The problem with A2 operation/grid current, as I see it, is not the current per see but the abrupt changes when crossing the 0V line. This is what makes transformers ring like churchbells and causes ugly notches in the waveform if the driving impedance isn't low enough.
Mosfets are the best when it comes to ignoring sudden changes in the load impedance, but I've also had good results using high gm tubes as cathode followers.