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Re: TFA-204 designs

Posted by Thorsten (M) on March 24, 2005 at 10:55:40

In Reply to: TFA-204 designs posted by AnandR on March 24, 2005 at 06:45:10:

Hi,

> 1) Input stage using a WE396a and EXO-99 plate choke.
> Fully Cathode bypassed. I am wondering what bias I should
> look into so that direct coupling is possible.

Direct coupling has no different bias requirements than any other type of coupling.

> 2) Direct coupling between the input stage and 300B.

Cool. Be aware that this requires either 2 seperate supplies for driver and output stage or a single supply of (Driver Anode Voltage + (-Output Valve Bias) + Output Valve Anode2Cathode Voltage).

> 3) Output stage: Running the 300B very conservatively at
> 40-50mA maximum into a TFA-204. I am open to options on
> how to run this tube.

I liked the 300B in general best at 350V/60mA/5K load.

> Either fixed or cathode bias, but I need some help with
> this since I want to direct couple the driver and output
> stages.

If you direct couple the driver stage the bias is determined by the drivers anode voltage. If you want to use self bias then you must make the drivers anode voltage in effect the same as what you consider "ground" for the cathode bias resistor.

The DRD / Full Monkey circuit does that for a self bias circuit. What I call "full monkey" looks like this:

The fact that no values are listed is intentional. A vaersion which includes values and voltages is on my website but it is for a different set of vales and thus not relevant here.

Another option is the way the Komuro 300B Amp does direct coupling:

Here two seperate supplies are used. I would argue that if such a circuit is employed you might as well switch to a version of fixed bias for this, kinda like this concept (never build!):

> 4) BCP-16 Grid choke between the WE396a and 300B.

If you already use an anodechoke the gridchoke is superfluos. You only need it if using a coupling capacitor.

> 5) Maximum power output like 2.5-5 watts.

I'd go for 5 - 6W, which is what I got from the 300B @ 350V/60mA/5K.

> Let me know if you guys think the WE396a can swing it as a driver,

Depends. If you use 350V/60mA the bias will be such that the 396A will struggle with the swing at normal conditions. I'd probably tend towards a 417A or similar valve, but that is just me.

Ciao T

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