6HF5 sweep tube single ended amp

[Tony]

Hi all,

please look at scheme below and give me your thoughts, all inputs will be appreciated. choice of parts were based on what is available, i want to utilise my jnukbox fully.

thanks in advance.

[SY]

Screen drive from a MOSFET, I can't complain about that! You're going to be VERY limited on power, though- 60mA or so of idle is all that this tube will be able to stand.

You might consider either bypassing the 1.5K cathode resistor on the input stage or upping the value of the feedback resistors and returning the feedback directly to the cathode.

[Tony]

thanks sy,

i was thinking of byapssing the 1.5k cathode resistor of the 12at7 input.

what if i replace the input tube with a 6fq7 and up the plate power to about 2 watts? will i get higher drive to the screens?

thanks

[SY]

No, your MOSFET will handle the screen current needs. What you want from that first stage is lots of linear swing, and a 12AT7 is a good choice for that.

[Tony]

ok, so you think the input stage has enough voltage swing for the source-follower then?

[SY]

Mmmm, you should be able to swing 60V peak out of that first hole. A look at the output tube's curves will tell you if that's enough- most sweep tubes' data sheets have curves for control grid = 0V, but varying screen grid voltage.

I would strongly consider breadboarding the input stage through the follower to see if the high source Z of the input stage will cause you severe HF rolloff or not.

[Tony]

ok sy, i was hoping for a 150volt peak, that is why the active current source load on the 12at7 plate. guess i will have to increase plate volts, and change that active load to a mosfet type for increased voltage.

i do not have a scope, but i have a heatkit sine wave signal gen. will test it once breadboarded.

[SY]

150V peak is a challenge (and I don't know if you'll need that much). You'll probably want two stages to pull that off and still have enough gain left over to use feedback. My first inclination, if restricted to 9 pin miniatures, is to try 12AT7 coupled to 6CG7 or 7119. If you can use an octal, the second hole could very optimally be a 6SN7.

[Geek]

Originally posted by Tony
ok sy, i was hoping for a 150volt peak, that is why the active current source load on the 12at7 plate. guess i will have to increase plate volts, and change that active load to a mosfet type for increased voltage.

A mu-stage would cover a lot of bases for you perhaps?

[Tony]

gotcha, will test the circuit "as is" to find out hf roll offs situation!

thanks a lot.