Rerurn to Romy the Cat's Site
In the Forum: Analog Playback
In the Thread: About EAR 834P Modifications
Post Subject: About voltage starvation of EAR-834P input stage.Posted by Romy the Cat on: 6/1/2008
 hagtech wrote: |
| First, it seems like the tubes are starved. They are running very low current. The 12AX7 is also at low voltage. I'm guessing they are running at about 0.3mA at 65V plate. This operating point is way down in the muck, where the transfer curves are anything but straight. This will result in a LOT of even harmonic distortion. Hey, maybe that's ok, maybe that's what makes this stage musical.
Here's some ideas to think about:
1) Rebias the tubes to higher current and voltage. I found the 12AX7 to like 1mA with 140V or more on plate. And 5mA for 12AU7. Unfortunately the B+ is pretty low, so you don't have much room to play with. |
|
There were many considerations lately about the subject of the fist 12AX7 running lower plate voltage. Jim thinks that the operation point is too low; Guy feels that it does the syrupy euphonic mess. I feel that it sound absolutely perfect. I asked Dima to analyze this subject and here are some of his confusions
Jim slightly mistaken considering that the first stage in default EAR-834P sits at 65V. I borrowed one and measured - it was 79V. Dima feels that lower operation point of the first stage is not big deal as the tube runs very low voltage on grid. Perhaps the specific harmonics of that low operation pint do crate some values of this phonostage. However, how low would it be in order the corrector do to have operational problems? According to Dima it all depends from the specific type of 12AX7 used. As Dima explained in some 12AX7 with low plate voltage some of the electron dispatching from cathode has no enough “juice” and some of them retire on grid. This creates small grid current. That might not be a big deal in other case but what Dima explained to me is that the grid current in this case acts like low impedance load that actually shunts the loading resistor. As the result the cartridge instead of 42K for instance sees 5K that is very-very low and that would provide a very strong HF roll-off for any cartridge.
Dima, made an experiment and some of the tubes that he tried had the grid current at 65V, at 80V not of them had the problem. When he had grid current he reported that his phonostage was rolling off (it was not phonostage itself but the severally over-dumped cartridge via the grid’s shunt) at 7kHz. Would it explain the Guy’s syrupy euphonic mess?
Anyhow, I measured my both old and new 834PT and not of them have retired on grid electrons. Dima proposed to measure with very high sensitively (at least .0001V) the voltage on the first stage grid. If there is zero then there the first tube has the grid shunt. My corrector has Telefunken 12AX7 and it begins to have voltage on grid at 52V. Some other tubes that I tried crated voltage on grid from 48V to 65V, interesting that one GE tube did not have the grid’s voltage even at 40V!!!
Anyhow, Dima proposed to run the first tube pate between 80V and 100V to be on the save side. The low voltage on plate is not the problem that I experience but I thought to post it as a warning for other people to understand the nature of a potential problem.
Rgs, The caTRerurn to Romy the Cat's Site