I'd not come across issues either until I looked at the outputs of each pentode in the .ac frequency sweep, and found they were a bit touchy WRT feedback (in the circuit I was trying it in). It was because the RHS negative triode always lagged, as the LHS tube is driven, then the RHS tube reacts. But the CCS helps everything.
I did find a decent article on this subject too, but I think I read it on a different PC (phone!) and haven't found it again yet
There are a few articles however on how to make the LTP far better.
@Zoran Than you, I will.
@TheGimp I am aiming to have as less resistance as possible in the cathode, that is the reason for 1R choice. Thanks for the rebias observation. The UL vs triode mode is not going to be selectable inside the same amplifier. The schematic/pcb will be done this way just so I can have both options.
@TheGimp I am aiming to have as less resistance as possible in the cathode, that is the reason for 1R choice. Thanks for the rebias observation. The UL vs triode mode is not going to be selectable inside the same amplifier. The schematic/pcb will be done this way just so I can have both options.
Thank you for this observation! The values for the resistors in the bias circuit were copied from another project, without checking the actual datasheet.
I will change them to match the 6F6G valves.
@stocktrader200 I don't know yet. As soon as the schematic will be adjusted I will proceed with a PCB design for building the prototype.
Would not be better to build a PCB after the prototype? It should be easy to wire this point-to-point.
The 6N7GT will be more challenging for the DC-coupled LTP input. At the given Va and Ia (122V, 3.3mA) the 6N7 requires ~-2V bias, while the 6SN7 needs ~ -4V, giving a bigger range of input voltages to play with before you run into grid current. Unless you are really set on the 6N7Gs I would go with a 6SN7.
I put 100k in my calculator and left the 220nF in and it moved the rolloff point from 2Hz to 7Hz. My opinion is that is a better place for it for LF stability. That said, I have no calculation on the change would make to lower the gain of the driver but my gut tells me it wouldn't drop it enough to matter unless it's on the low end of what's needed for full power as designed. Not enough info on the schematic.
Hello everyone!
The reason why I am thinking of a prototype PCB is mainly because I don't have a HV bench power supply. I can get cheap pcb if they're smaller than 100x100mm. I am thinking of making one for the HV PSU and another one for heater DC (for input stage and phase inverter) and the fixed bias voltage.
The amplifier schematic can be done on 2 boards as well. One pcb for input stage and phase splitter and another one for the output stage.
About adjusting the value of the caps between phase inverter stage and output stage, I get the same 7Hz rolloff -3dB. This can be changed during the test phase.
Considering the 6SN7 option for phase splitter, I think I might add that option too on a separate PCB.
The reason why I am thinking of a prototype PCB is mainly because I don't have a HV bench power supply. I can get cheap pcb if they're smaller than 100x100mm. I am thinking of making one for the HV PSU and another one for heater DC (for input stage and phase inverter) and the fixed bias voltage.
The amplifier schematic can be done on 2 boards as well. One pcb for input stage and phase splitter and another one for the output stage.
About adjusting the value of the caps between phase inverter stage and output stage, I get the same 7Hz rolloff -3dB. This can be changed during the test phase.
Considering the 6SN7 option for phase splitter, I think I might add that option too on a separate PCB.