Hey Folks,
Looking at this kit:
https://diyaudioprojects.com/Tubes/6418-Tube-Phono-Preamp-Kit/
I was thinking if that would be possible to use a directly heated low voltage tube like 6418 from above in a hybrid jFet/tube configuration? One of the examples is here:
https://www.tubecad.com/2007/09/blog0119.htm
Best, M
Looking at this kit:
https://diyaudioprojects.com/Tubes/6418-Tube-Phono-Preamp-Kit/
I was thinking if that would be possible to use a directly heated low voltage tube like 6418 from above in a hybrid jFet/tube configuration? One of the examples is here:
https://www.tubecad.com/2007/09/blog0119.htm
Best, M
Thanks for the reply. I posted the kit schematics only as a sample of how this tube is used and its voltages (30V B+ and 1.25V at 10ma heater). The tube has low transconductance and high distortions. I was thinking the cascode will help with both--distortions and low mu. The heater is grounded on one side, so I was thinking of inserting the jFet to that side and then float the heater voltage. Since that voltage is pretty low, but has relatively high current those might contradict each other--I would like to run the jFet at some 0.5ma. Another possibility is to make a resistive divider from the drain to both grounded and hot leg of the heater... Trying to get opinions what would be the best solution here.
Thanks, M
The upper position of a cascode is very difficult for a directly-heated valve.
Firstly, the microphonics are going to be worse than a decent-quality indirectly heated bottle.
Secondly, the low gm is a disadvantage, and adds distortion.
I have experimented, and measured many versions of this kind of circuit, and I found that a simple, low cost NPN transistor (MPSA42, 2N6520) measures and sounds much better than any valve here.
OTOH, the second stage of a RIAA preamp can use all kinds of triodes in the lower position, including DHTs. But the NPN transistor is still better performing, and better sounding in the upper position, mainly due to gm.
Use a stiff (Low impedance) voltage source to bias the upper cascode device, whichever you use.
Firstly, the microphonics are going to be worse than a decent-quality indirectly heated bottle.
Secondly, the low gm is a disadvantage, and adds distortion.
I have experimented, and measured many versions of this kind of circuit, and I found that a simple, low cost NPN transistor (MPSA42, 2N6520) measures and sounds much better than any valve here.
OTOH, the second stage of a RIAA preamp can use all kinds of triodes in the lower position, including DHTs. But the NPN transistor is still better performing, and better sounding in the upper position, mainly due to gm.
Use a stiff (Low impedance) voltage source to bias the upper cascode device, whichever you use.