6sl7 cascode

[funkmeblue]

I am designing an amp with a 6sl7 cascode input and plan on elevating the heaters by connecting the heater center tap at my cathode bias 6l6 bias resistor. Is there any reason this will not work, something about heater to cathode ratio or what not. I have done this before just not with a cascode. Thanks for the help

[tubeswell]

You may not get a high enough heater elevation. Better way is to make a voltage divider from the HT/B+ rail (which can double as a bleeder for the filter caps). That way you can design it for the DC voltage you want. (You also need to 'decouple' the voltage divider knee with a decent cap to keep the DC voltage steady). I would aim for 60-80 volts elevation for a cascade

Example below:

[img:436:317]http://farm9.staticflickr.com/8325/8144 ... fd73_o.png[/img]

[funkmeblue]

Thanks, I have elevated the heaters that way as well. I was actually worried that I would be elevating them at too high a voltage for a cascode circuit at the bias resistor

[stephenl]

Just a suggestion, but you might like a 6SN7 better in cascode. Higher transconductance tubes work better in that configuration.

[funkmeblue]

I actually plan on trying both

[stephenl]

This works well with a 6SN7. Use .68uf cathode bypass cap- it makes ia nice input stage.

[funkmeblue]

Grid bias upper triode. That is sorta what I was wanting to try. Was checking out some of the circuits in Merlin's book. Thanks I will give it a try

[stephenl]

I posted regarding this somewhere recently... I tried this same cascode with fixed and self bias. Grid leak sounded the best to me.

[talbany]

6DJ8 (ECC88) is actually better suited for Cascode (150V on cathode) as it's designed for Cascode circuits..

Tony