reubster wrote:
Hi TS
Tubes with higher current capability than 12AX7s are usually preferred in cathode follower applications, but as your circuit parallels them it should be ok.
I wouldn't recommend a DC coupled cathode follower for hifi applications
But I love them and always use them in in guitar amplifiers.
In the DC cathode follower, the CF's grid is connected to the previous stages plate, so is sitting at well over 100V
This creates grid conduction at the CF's grid (it becomes forward biased like a diode) and this pulls current down through the previous stages plate.
The CF ends up biassed very close to cutoff - but it settles to a balance point.
When the signal voltage at the CF grid swings negative it all works ok.
When it swings positive, the CF grid conducts again and pulls additional current through the previous stages plate resistor which creates distortion.
If you implement an AC follower it will be better for hifi
Merlin has some excellent on AC & DC coupled cathode followers here:
http://www.valvewizard.co.uk/accf.htmlhttp://www.valvewizard.co.uk/dccf.htmlThanks Reubster, however I don't have as much knowledge as yourself about valve design etc...
A person I emailed came up with these modifications to the JP200 schematic (attached)
He used 2 x 12AT7's for the cathode follower positions (T3A and T3B) to lower output impedance...
....he also changed a few resistor values...
- Change R7A and B to 1kOhm
- Change R10A and B to 1.5 kOhm
- Change R14A and B to 100 Ohm
Lowering gain from 20 to 12dB:
- Change R5A and B to 33 kOhm
- Change R3A and B to 1 kOhm
- Replace C4A and B with silver mica 8 pF / 500 V and place a silver mica 22 pF / 500 V capacitor between grid and cathode of T1A and B to fight self oscillation
What do you think of this mod with regards to the attached schematic?